Is there a fundamental flaw in our understanding of space and conservation laws?

[ttn]

So it seems that Bell + experimental confirmation (which is suggestively strong, but I'm not sure it is 100% watertight) makes us choose between:
- a philosophically satisfying concept, that "what we see, IS there, and ONLY that" OR
- the formal principle of relativity.

So, in response to certain experimental outcomes which you SEE, you conclude that you can't trust what you see.

Good logic.

 

[vanesch]

So, in response to certain experimental outcomes which you SEE, you conclude that you can't trust what you see.

Good logic.

It's always your same caricatural objection, where you implicitly change "is not all there is" into "cannot be trusted" or "deluded", hence enlarging implicitly the original statement's conclusion, in order to try to show it evidently wrong.

You change "I see the tip of the iceberg, hence there must be much more below the surface which I don't see" into "I see the tip of the iceberg, hence I must be deluded in my seeing", and from that, you obviously conclude that if you are deluded, then there was no reason to expect there to be an iceberg in the first place. But the two statements are different.

 

[ttn]

It's always your same caricatural objection,

Not so! It's often one of several, different caricatural objections instead!

where you implicitly change "is not all there is" into "cannot be trusted" or "deluded", hence enlarging implicitly the original statement's conclusion, in order to try to show it evidently wrong.

You change "I see the tip of the iceberg, hence there must be much more below the surface which I don't see" into "I see the tip of the iceberg, hence I must be deluded in my seeing", and from that, you obviously conclude that if you are deluded, then there was no reason to expect there to be an iceberg in the first place. But the two statements are different.

The difference is that the two parts of an iceberg are *parts*, and may actually claim therefore a kind of independent existence. Each part is actually *there*. So it's no logical contradiction to be aware of the part above water, and unaware of the part below water.

But superposed terms in a wave function are not like that, are not each individual "parts" which all have independent existence. If the wave function is all there is to describe the physical states of systems (which is the MWI position) and if the spin state of some spin half particle is

|psi> = a|+> + b|->

and you believe that the particle is definitely in the eigenstate |+>, your belief is *wrong*. Not "part truth", but just plain error/delusion. If you think the state is |+> when it is actually |psi>, you are exactly as wrong as you'd be if you thought it was |+> when it is actually |->.

In any case, to recap, the logical problem I'm pointing out goes as follows: you say we (humans) go into labs and notice certain things, like pointers definitely pointing to certain directions and not others, and infer from this [after getting orthodox QM and then fighting for a while over how to interpret it] that, in fact, those pointers were *not* pointing the way we thought we saw them pointing. At the end of the day, MWI requires we accept that the best way to interpret our having seen certain physical objects in certain positions, is to deny that those objects were in fact in those positions at all.

(In addition, it requires us to deny that there is even such a thing as physical objects moving and interacting in spacetime. ...which is pretty weird for a theory whose only reason for existing is: (allegedly) to save relativity in the face of the otherwise problematic Bell theorem. Wherever you look, MWI is swallowing itself by the tail, contradicting and undermining its own motivations and foundations.)

My earlier post may have constituted "drive by shooting", but it was not caricatural.

 

[vanesch]

But superposed terms in a wave function are not like that, are not each individual "parts" which all have independent existence. If the wave function is all there is to describe the physical states of systems (which is the MWI position) and if the spin state of some spin half particle is

|psi> = a|+> + b|->

and you believe that the particle is definitely in the eigenstate |+>

Why would I believe that, any more, than I would believe that all there is to the iceberg, is what is above the water ?

Because, mind you, if the state of the system is a |+> + b|->, I will NOT see just |+> or |-> ! I will in fact not see anything. The only way I could possibly say that it is in fact in the |+> state, is when I INTERACTED WITH IT, and when the actual state is a |me>|+> + b |mycopy> |->.
But that is, IMO, analogous to:
(I above the water) (the tip of the iceberg) + (Joe in his diver suit)(the rest of the iceberg)

So there's nothing wrong with me saying that I saw |+>. It is an (erroneous) extrapolation, however, to state that the particle IS ONLY |+>. It is |+> for me (and that is correct), while it is |-> for mycopy (and that's correct to, probably, though I have no way to know directly).

, your belief is *wrong*. Not "part truth", but just plain error/delusion. If you think the state is |+> when it is actually |psi>, you are exactly as wrong as you'd be if you thought it was |+> when it is actually |->.

As I said, this is not correct IMO. The particle cannot be in the state (a|+> + b|->) while I saw |+>. I must have entangled with it, otherwise I cannot have measured it.

In any case, to recap, the logical problem I'm pointing out goes as follows: you say we (humans) go into labs and notice certain things, like pointers definitely pointing to certain directions and not others, and infer from this [after getting orthodox QM and then fighting for a while over how to interpret it] that, in fact, those pointers were *not* pointing the way we thought we saw them pointing.

Again, no. They ARE pointing in the way we saw them pointing, FOR US, and they are pointing elsewhere for COPYUS. A quantum pointer is a pointer which can point in several directions at the same time (by definition!). This is not a delusion. The extrapolation which brings all the misery is to say that because they point that way for us (which is correct!), that's all there is to it, and they cannot point elsewhere for copyus. Although this is not directly observable (and this is the main incentive to discard this other part of reality), it can have indirect observational consequences in certain cases, such as in EPR-Bell.

This is IMO exactly analogous as saying that if I am on the surface, that all there is to an iceberg, is its tip, because it is all I see, and denying that Joe in his diversuit can see the rest of it. Now, of course the difference is that the observational consequences of Joe are much more tangible: he'll come at the surface, or I can put on my diver suit etc... while I'll never meet "copyus" directly. But apart from that, my observing just the tip of the iceberg, and my inference from other observations that there must be an invisible part below the surface, are very close to the derivation of the existence of the copyus with the other result.

At the end of the day, MWI requires we accept that the best way to interpret our having seen certain physical objects in certain positions, is to deny that those objects were in fact in those positions at all.

Again, not at all. If we *observed* those objects (which, if they are quantum objects, have miriads of positions at the same time) at a certain position, then that means that FOR US, a "classical shadow" of that object WAS in that position state - but that doesn't exclude that for copyus, another classical shadow of that object was indeed elsewhere.

(In addition, it requires us to deny that there is even such a thing as physical objects moving and interacting in spacetime.

Spacetime is (as long as we don't do quantum gravity) a background on which quantum objects are defined, but they do not need to have a single position! Only, their interactions with other quantum objects must be such that they can be *decomposed* over spacetime.
You could just as well require the EM field to have a specific position in spacetime. It doesn't. It is everywhere. However, interactions between the EM field and other systems also occur in a decomposed way over spacetime. This is what relativity is about. Spacetime is the background over which interactions can be decomposed.

 

[ttn]

So there's nothing wrong with me saying that I saw |+>. It is an (erroneous) extrapolation, however, to state that the particle IS ONLY |+>.

A particle can have only one state. If it's |+> it's not |psi> and vice versa. This is equivalent to the assertion that the wave function alone provides a complete description of the physical states. Someone who says "sure, the wave function is |psi> but *really* the particle is definitely spin-up" is someone who rejects the completeness doctrine (and hence rejects MWI).

It is |+> for me (and that is correct), while it is |-> for mycopy (and that's correct to, probably, though I have no way to know directly).

"is ... for" is nonsense. We're talking about what the actual physical state is here. This is (by definition) not something that can be "relative" to me/you/mycopy/whoever. If the state is one thing "for you" and another thing "for me" then what we're talking about isn't *the state*.

As I said, this is not correct IMO. The particle cannot be in the state (a|+> + b|->) while I saw |+>. I must have entangled with it, otherwise I cannot have measured it.

I don't think it changes anything for you to be entangled. It's the same point to say that the state is

a|+>|me> + b|->|mycopy>

and you believe the particle to be definitely spin up. It isn't.

Again, not at all. If we *observed* those objects (which, if they are quantum objects, have miriads of positions at the same time) at a certain position, then that means that FOR US, a "classical shadow" of that object WAS in that position state - but that doesn't exclude that for copyus, another classical shadow of that object was indeed elsewhere.

"Classical shadow" is just a made up term to cover what you really mean: that your belief doesn't correspond to the actual facts (it corresponds only to the "shadow", which isn't actually real, so, in other words, your belief corresponds to nothing... i.e., isn't true).

Spacetime is (as long as we don't do quantum gravity) a background on which quantum objects are defined, but they do not need to have a single position! Only, their interactions with other quantum objects must be such that they can be *decomposed* over spacetime.
You could just as well require the EM field to have a specific position in spacetime. It doesn't. It is everywhere. However, interactions between the EM field and other systems also occur in a decomposed way over spacetime. This is what relativity is about. Spacetime is the background over which interactions can be decomposed.

This really confuses me. Are you saying that the wave function (which is all there is, according to MWI) is a field on spacetime? You know that's not true. It's a field on some giant-dimensional configuration space (-time). And if the wave function is all your theory posits, there is literally nothing in space-time (and so I can only assume you don't bother positing any empty spacetime, either). Maybe you should take a look at the fantastic paper here:

http://www.arxiv.org/abs/quant-ph/0603027

MWI is like "bare GRW" in the relevant respect (except it's even worse, since it's lacking in the non-linear terms which make bare GRW at least a step toward a solution to the measurement problem).

As to the EM field not existing in spacetime, I don't know what you're talking about. Sure, the field as a whole is in no one particular place (duh), but still such a field is a field on 3-space such that one can consider the field values at each point a "local beable" (in the sense of Bell). What pray tell are the local beables in MWI? That is, what/where is the matter -- the stuff we see moving and interacting in 3-space?

 

[vanesch]

A particle can have only one state. If it's |+> it's not |psi> and vice versa. This is equivalent to the assertion that the wave function alone provides a complete description of the physical states. Someone who says "sure, the wave function is |psi> but *really* the particle is definitely spin-up" is someone who rejects the completeness doctrine (and hence rejects MWI).

Again, I think you do not realize the difference between the state:

a |+> + b |->

on one hand,

and

a |u> |+> + b |v> |->

on the other.

It would be an error to claim that in the last case, the state of the particle is

a |+> + b |->

without any reference to u or v.

And indeed, in quantum theory, a subsystem doesn't necessarily have a state, only the overall system has. If you insist on giving a "state" to a subsystem, you have to specify also the relative states of the other subsystems in order for this to make sense, and that is exactly what we do. One of those subsystem states is "me", and in relation to me (which is part of u, say), the state of the particle is |+>, while in relation to v, the state of the particle is |->, and the particle, BY ITSELF, has no overall state. It is certainly NOT a |+> + b |->.

"is ... for" is nonsense. We're talking about what the actual physical state is here. This is (by definition) not something that can be "relative" to me/you/mycopy/whoever. If the state is one thing "for you" and another thing "for me" then what we're talking about isn't *the state*.

Well, apart from you not liking that, I don't see any fundamental argument why each individual subsystem should necessarily need to have one and unique state. It is a basic axiom of quantum theory that it doesn't, and that only the state of the overall system has a state. This sounds maybe strange and maybe against our intuition, but relativity of simultaneity is also one of those strange notions, which is against our intuition, but therefore not logically false.

In the same way, there is no logical requirement (but only an intuitive one) for each subsystem to have its own independent ontological state independent of the state of other subsystems. In classical physics, this is the case, and in quantum physics, this isn't.
So, if you INSIST on giving a state to a particle in quantum physics, you have to describe it relatively to the states of other subsystems.

I don't think it changes anything for you to be entangled. It's the same point to say that the state is

a|+>|me> + b|->|mycopy>

and you believe the particle to be definitely spin up. It isn't.

Exactly: it isn't "spin up", it is only "spin up" to me. That is, relative to me, if I insist on giving it a unique state, I have to say that it is "spin up", knowing that relative to mycopy, it is "spin down". Now, there is a consistency, in that if there is a third subsystem, it will be in such a state, relative to me, that it is in consistency with the particle being in "spin up" to me.
And given that in most cases, I will not hear anything anymore from "mycopy", I can, for EASINESS, now assume that the whole particle state is "spin up", because all I will ever observe, all other states that will ever be relative to me of other subsystems, will be consistent with the "spin up" state of my particle. So this is why I have the impression that ALL THERE IS, now, is this particle in a spin up state, because all the other "sub states" of sub systems that will be relative to me, will be consistent with this particle being in the spin up state.

"Classical shadow" is just a made up term to cover what you really mean: that your belief doesn't correspond to the actual facts (it corresponds only to the "shadow", which isn't actually real, so, in other words, your belief corresponds to nothing... i.e., isn't true).

Well, my *belief* does correspond to the actual facts, namely that the particle has SEVERAL "classical shadows", and that I see one of it. But my observation corresponds to only one. In what way is a "shadow" not real ? Again, let us not forget what is our discussion:

I claim that we only see PART of reality, but that part is there, all right, only there "is more than meets the eye", while you claim that what we see is simply delusion, and hence not thrustworthy. You have to make that claim in order for you to be able to make the argument that MWI is bogus, namely that it is some theoretical construct which takes as basic tenet that observations are bogus - while of course it has been set up to explain observations (as are all scientific theories).

There's a big difference between our claims of course, because claiming that "what is observed is not all there is" is not an argument that would allow your reasoning to hold. Indeed, it is not unthinkable, that from the PARTIAL INFORMATION we get by our observations, we can nevertheless DEDUCE the existence of a more involved reality than what is ONLY observed - indeed, that is my point. There wouldn't be any logical inconsistency, any more than there would be a logical inconsistency in the deduction, from our sole observation of the tip of an iceberg, and some other observations (like Archimedes' law), that we can DEDUCE that there must be a much bigger part of the iceberg which we don't see. Extrapolating from the observed to the (more involved) real is not, by itself, logically inconsistent.

However, in order for your argument to hold, you need to require that observations are BOGUS, are *erroneous*, and are not just "a part of reality" in MWI.

Well, I claim that this is not the case, and that, what we observe are parts of reality, which are good enough for us to allow to derive the "invisible" part, after some thinking. Now, we can give different names to these parts, but "shadows" are quite convinient, no ? With one difference: all the shadows make up the total state description of the object, which is not the case of an optical shadow.

This really confuses me. Are you saying that the wave function (which is all there is, according to MWI) is a field on spacetime?

It is better to switch to the Heisenberg picture for that, and then the field operators are indeed a field on spacetime, but an operator-valued field.
You can then "go back" to the Schroedinger representation, and this will bring you extra structure to the wave function's evolution. In other words, spacetime has nothing to do with the *state* of a system, but with its *evolution*. Indeed, spacetime being a "bag of events", it is a structure of *interaction* and not of "the state" of nature.

You know that's not true. It's a field on some giant-dimensional configuration space (-time).

Yes, but so is the state in classical particle dynamics: it is a point in a giant phase space. However, the FLOW in that phase space is such, that one can find back the illusion of some spacetime, although the actual state is an element in a huge-dimensional phase space. Now, the difference between classical physics and quantum physics, is that one can distill more easily individual substates for subsystems (take slices in the phase space which correspond to subsystems), while the quantum state cannot be sliced up in a juxtaposition of individual states.
But again, it is the specific structure of the evolution of that state which gives you the illusion of spacetime "as a place to be", while it is just a structure of interactions ; just as in classical phase space.

Let's not forget that spacetime is the SET OF EVENTS. Not the "set of places to be". Events are interactions.

As to the EM field not existing in spacetime, I don't know what you're talking about. Sure, the field as a whole is in no one particular place (duh), but still such a field is a field on 3-space such that one can consider the field values at each point a "local beable" (in the sense of Bell). What pray tell are the local beables in MWI? That is, what/where is the matter -- the stuff we see moving and interacting in 3-space?

Uh. I don't see the EM field as "consisting of local beables" ! Imagine you write the EM field in a Fourier decomposition: where are your local beables now ? The state of the EM field is a point in a very big phase space! However, its *interactions* are such that it gets a spacetime structure, and that's all that is needed for us to think that that we see a "space of 3 dimensions" around us, because what we see is a result of interactions.
Now, indeed, one can, in classical field theory, if one wishes, slice up the phase space of the EM field into "subspaces" which correspond to "pieces of EM field" - these subspaces correspond to your local beables, I suppose.
As I said, this cannot be done in quantum theory, because "the subsystem state" doesn't exist there.

Indeed, in quantum theory, there are no local beables. What is local (that is, what gives a structure of spacetime) are interactions between subsystems.

 

[vanesch]

Maybe you should take a look at the fantastic paper here:

http://www.arxiv.org/abs/quant-ph/0603027

MWI is like "bare GRW" in the relevant respect (except it's even worse, since it's lacking in the non-linear terms which make bare GRW at least a step toward a solution to the measurement problem).

I'm reading this paper now. I'll continue, but I already have a serious problem with the following:

To appreciate the concept of PO, it might be useful to regard the positions of particles,
the mass density and the flashes, respectively, as the output of BM, GRWm and GRWf,
with the wave function,in contrast, serving as part of an algorithm that generates
this output. Suppose we want to write a computer program for simulating a system
(or a universe) according to a certain theory. For writing the program, we have to
face the question: Which among the many variables to compute should be the output
of the program? All other variables are internal variables of the program: they may
be necessary for doing the computation, but they are not what the user is interested
in.

That is what I call "sneaking out". We now divide "ontology" in two parts, namely "primitive ontology" and "algorithmic ontology". All the nasty stuff is put into "algorithmic ontology", which doesn't have to respect certain rules, while what we need, is just the "primitive ontology", which can then respect the rules.

But if such a thing is allowed, then I don't see why we bother with any interpretational stuff at all: call "observations" the PO, and quantum theory the "algorithmic ontology" and we have the standard "shut up and calculate" interpretation (which is different from Copenhagen, btw). The whole point in providing a rigorous ontology for the formalism of quantum theory is to be able to give a precise ontological picture in which NO "algorithmic" part is foreseen, but in which all elements of the ontology are treated on the same level.

If I'm allowed to play that game too, then I can say that the PO of MWI are the different states of my awareness (different body states which correspond to different awarenesses), and the whole wave function stuff is just an algorithm which generates probability distributions over my different states of awareness. It produces perfectly what I am aware of then.

In a certain way, I'm not surprised that GRW and Bohmian mechanics turn out to be related. They have similar advantages and weaknesses: the cleavage in the "ontology of the world" ; the relativistic invariance of the results, but not of the internal workings (which are done away with by putting the non-relativistic part in the "algorithmic" section, so that we shouldn't care about it) etc...

(reading on...)

EDIT:
After reading further, I think that the difficulty is this:

Whoever suggests that matter exists not in 3-space but in 3N-space
must bridge the gap between an ontology in 3N-space and the behavior of objects in
3-space, to provide an account of which, it would seem, is an important purpose of a
fundamental physical theory.

I think it is this basic assumption which is giving troubles with quantum theory. After all, matter doesn't need to exist in some kind of 3-dim space, as long as OBSERVATIONS are consistent with such an IMPRESSION. This is, again, like in classical physics, if we take it as ontologically correct that the world is 6N-dimensional, but that the interactions are such, that it looks like a 3-dim space with SEVERAL points in it. In that case, the impression of a 3-dim space is just a consequence of a special requirement on the Hamiltonian flow. There is moreover, no need for "extra" ontology in this hamiltonian vision on things, because our very measurement procedures would also be defined as a function of this requirement on the hamiltonian flow. The only thing we would need to specify (again), is to what points in phase space correspond what subjective impressions (in other words: what configuration of "brain-particles" - which is a part of the phase space corresponds to me having the visual impression of "a green frog at 2m from me, slightly to the right", for instance).

Now, in classical physics, it doesn't give any formal PROBLEM to reformulate mechanics, instead of on a 6N dim space (which we could consider as ontologically real), as a "mechanics of several points" on a 3-dim space. So for people having such desire, they can do so. But in quantum mechanics, it DOES give some formal problem, and insisting, such as the authors of the paper, of wanting to have an ontology which consists PURELY of local beables in space, will necessarily clash at one point or another with the formalism of quantum theory. In fact, they even have to split their ontology into two parts, a PO one which can be "local-beablisised", and an "algorithmic ontology" which doesn't.

So I think that the difficulty comes from a requirement which is not necessary, namely, the requirement that "ontology" (or at least, the speakable part of ontology) must live, as local beables, on spacetime. I think the correct way of seeing things from a quantum formalism viewpoint, is not to put any requirement on what kind of mathematical object "ontology" must be, and to see spacetime as a source of symmetries of the ontology (the ENTIRE ontology).

EDIT 2:

I finally reached the conclusions of the paper. I re-iterate my comments above: I think it is a "cheap trick" to separate the "ontology of the world" into two different ontologies. Or, the ontology of the world is entirely, always and with no exception, represented by local beables on spacetime, or this isn't the case. Now, the first point in the conclusion seems to be that this ought to be the case. Fine. But next, they talk about the wavefunction in hilbert space. I object to that. In as much as this wavefunction has some ontological character, according to the first point, it must live on spacetime (which it doesn't). I think that from that point on, one can stop. There's no point in requiring PART of the ontology to live on spacetime, and changing names into "primitive ontology" and "algorithmic ontology" is a verbal game. If some part of the ontology doesn't need to live on spacetime, then there's no point in requiring that ANYTHING lives on spacetime. Spacetime can then become a requirement of symmetry in that other place, where the ontology lives (say, Hilbert space) ; and from that symmetry can follow our *impression* of spacetime.

But once we're there, there's no point in fiddling with the standard quantum formalism, which does exactly that.

If we're allowed to algorithmic "deux ex machina", then quantum theory is good enough an algorithm to calculate outcomes in the lab, which we can then take as "primitive ontology".

In other words, all this is a lot of work for no formal results. The aim of interpretative work is to help understand the formalism, not to fiddle with it. We can deduce no new fundamental principle from all this, which might guide us in, say, having an idea about how to deal with the borderline between quantum theory and GR.

 

[ttn]

Again, I think you do not realize the difference between the state:

a |+> + b |->

on one hand,

and

a |u> |+> + b |v> |->

on the other.

It would be an error to claim that in the last case, the state of the particle is

a |+> + b |->

without any reference to u or v.

And indeed, in quantum theory, a subsystem doesn't necessarily have a state, only the overall system has. If you insist on giving a "state" to a subsystem, you have to specify also the relative states of the other subsystems in order for this to make sense, and that is exactly what we do. One of those subsystem states is "me", and in relation to me (which is part of u, say), the state of the particle is |+>, while in relation to v, the state of the particle is |->, and the particle, BY ITSELF, has no overall state. It is certainly NOT a |+> + b |->.

I understand all this just fine. You're still just missing the point: if the wave function is all there is, and if the wave function is something like

a|+>|I believe +> + b|->|I believe ->

and I *think* the particle has state |+> and I *think* I believe it has state +, i.e., I *think* the combined state is

|+>|I believe +>

then I am just wrong. I am wrong about the particle, wrong about myself, and wrong about the combination.

MWI forces me to accept that I am almost always wrong about everything in this way. Included in the "facts" I am wrong about, are all of those "facts" which made me believe in the quantum algorithm in the first place. So MWI asks me to accept that the best way to interpret the physical meaning of that algorithm, is to accept that I was deluded into accepting it in the first place... and yet somehow still I should still accept it.

Paraphrasing Monty Python, you may call this logic, but I call it crap.

In the same way, there is no logical requirement (but only an intuitive one) for each subsystem to have its own independent ontological state independent of the state of other subsystems. In classical physics, this is the case, and in quantum physics, this isn't.
So, if you INSIST on giving a state to a particle in quantum physics, you have to describe it relatively to the states of other subsystems.

It has nothing to do with the separability of states. It has to do with whether there is anything actually posited by the theory which corresponds to my beliefs -- i.e., whether the theory leaves any room for beliefs (including beliefs about how certain experiments came out in the 1920's) to be true.

Well, my *belief* does correspond to the actual facts, namely that the particle has SEVERAL "classical shadows", and that I see one of it. But my observation corresponds to only one. In what way is a "shadow" not real ?

You introduced the term "classical shadow" so you should define it, not me. I took it as clear, though, that you weren't meaning to posit these "classical shadows" as things that really exist *independent* of the wave function. They are just a certain "narrow subjective perspective" on what is already in the wave function, right? But then you haven't actually addressed my point. If all that really exists is the wf, then what really exists doesn't correspond to my beliefs about what exists. Or if you are positing some new beables here (the "classical shadows") you better flesh it out a bit and give some dynamical laws for how those things behave and interact.

There's a big difference between our claims of course, because claiming that "what is observed is not all there is" is not an argument that would allow your reasoning to hold.

You give up the whole game when you say "what is observed is not all there is". The fact is, "what is observed" directly contradicts the posited nature of "all there is." That's the point I keep raising above. What is observed is a world with entities with certain definite properties. Yet according to MWI the real world is nothing like that. Unless, of course, you think that the plus signs in superpositions mean "and/or" or some such. But only naive people who believe in the "ignorance interpretation of superposition" accept that -- not people who accept the completeness doctrine.

Your whole case here hangs on interpreting one term in a superposition as a "part" of the whole. Right? The problem is, that just isn't what the word "part" means (which is why your iceberg analogies don't work). It might work, granted, if MWI actually meant "many worlds". Then the one term would describe a certain physical reality that is realized in one universe, with other terms also describing realities in other universes. Then it would be true that the one term is a part, and hence that *belief* that the state of things is as described by that one term is a "partial truth" (like your iceberg analogy) rather than just false.

But I think you know the problems with interpreting terms in a superposition as living in different worlds, literally. So you're stuck.

It is better to switch to the Heisenberg picture for that, and then the field operators are indeed a field on spacetime, but an operator-valued field.

You want the beables of the theory to be mathematical operators? I don't even know what that means.

You can then "go back" to the Schroedinger representation, and this will bring you extra structure to the wave function's evolution. In other words, spacetime has nothing to do with the *state* of a system, but with its *evolution*. Indeed, spacetime being a "bag of events", it is a structure of *interaction* and not of "the state" of nature.

So... do you accept that there is matter (e.g., tables, chairs, planets) in 3-space?

That's probably what this comes down to. I say: if you don't accept that, you are just crazy and not a serious (or, at least, psychologically healthy) physicist. And if you do accept that, you have a problem because such stuff is nowhere to be found in MWI.

Yes, but so is the state in classical particle dynamics: it is a point in a giant phase space. However, the FLOW in that phase space is such, that one can find back the illusion of some spacetime, although the actual state is an element in a huge-dimensional phase space. Now, the difference between classical physics and quantum physics, is that one can distill more easily individual substates for subsystems (take slices in the phase space which correspond to subsystems), while the quantum state cannot be sliced up in a juxtaposition of individual states.
But again, it is the specific structure of the evolution of that state which gives you the illusion of spacetime "as a place to be", while it is just a structure of interactions ; just as in classical phase space.

I'm sorry, but this is just silly. Yes, it's possible to formulate classical particle dynamics in phase space. But each particle actually exists in 3-space, yes? So there is no problem here like the one faced by MWI.

Uh. I don't see the EM field as "consisting of local beables" ! Imagine you write the EM field in a Fourier decomposition: where are your local beables now ?

Imagine you take the Fourier transform of the mass density of a tree. Oh my god! The tree is gone! There are just an infinite number of abstract waves each spreading across the entire universe!

The state of the EM field is a point in a very big phase space!

But we're not talking about that phase space. The EM field lives in 3-space. As do trees. Period. If you don't accept that, we can't possibly have a serious discussion.

Indeed, in quantum theory, there are no local beables. What is local (that is, what gives a structure of spacetime) are interactions between subsystems.

If what you mean by "quantum theory" is orthodox or Copenhagen, you're wrong. There are local beables: classical objects (including especially apparatuses). Those are just *posited* by the theory as the local beables. Bohr then said that's *all* that really exists physically ("there is no quantum world...").

This theory of course suffers from the measurement problem, essentially because it never says what is "classical" and what isn't, i.e., it's "unprofessionally vague and ambiguous." But at least it has the virtue of accepting that there are real physical things moving around in 3-space!

 

[vanesch]

I understand all this just fine. You're still just missing the point: if the wave function is all there is, and if the wave function is something like

a|+>|I believe +> + b|->|I believe ->

and I *think* the particle has state |+> and I *think* I believe it has state +, i.e., I *think* the combined state is

|+>|I believe +>

then I am just wrong.

Indeed ! That's why I don't believe that!
It is not because I SEE a car over there, that I believe that there is car over there ! I only believe that there must be a term

|I see a car over there> |car is over there>

in a superposition.

I am wrong about the particle, wrong about myself, and wrong about the combination.

Absolutely. If you keep your beliefs the way you state them, then you are indeed wrong. But that is your problem, not mine.

MWI forces me to accept that I am almost always wrong about everything in this way.

Again, only if you believe that what you see, is all there is. If you believe that what you see, only corresponds to a term amongst others (which you won't see), then there's no problem, right ?

It has nothing to do with the separability of states. It has to do with whether there is anything actually posited by the theory which corresponds to my beliefs -- i.e., whether the theory leaves any room for beliefs (including beliefs about how certain experiments came out in the 1920's) to be true.

Well, often with initially wrong beliefs, one arrived at the right answers ! It is not because many astronomical observations were made based upon totally erroneous beliefs about astrology, that all that work (including Kepler's) was wrong.

You introduced the term "classical shadow" so you should define it, not me. I took it as clear, though, that you weren't meaning to posit these "classical shadows" as things that really exist *independent* of the wave function. They are just a certain "narrow subjective perspective" on what is already in the wave function, right? But then you haven't actually addressed my point. If all that really exists is the wf, then what really exists doesn't correspond to my beliefs about what exists.

Again, that depends on your beliefs. If you believe that the entire reality is what you observe, then indeed, there's a problem.

Or if you are positing some new beables here (the "classical shadows") you better flesh it out a bit and give some dynamical laws for how those things behave and interact.

No, it are those parts of the term of the wavefunction, corresponding to other subsystems, which is entangled with that part of the body wavefunction from which emerges *my* subjective experience (and not the potential subjective experience of my copy, for instance).

They are "pieces of wavefunction", selected by two things:
1) the separation of "my body" and "the rest of the universe"
(which gives us a more or less unique decomposition of the overall wavefunction in different terms, which correspond to different "worlds" according to me)
2) my subjective experience emerges from ONE of these terms (randomly selected by the Born rule) - eventual other subjective experiences emerge from copies of myself, but I don't experience that.

This completely selects the unique term in the wavefunction, which consists of two factors: a body state, and a "rest of the universe" state. What I observe is of course directly, my body state, and indirectly, the "rest of the universe" state which is compatible with this body state.

Now, because of the nature of the interactions between "my body" and "the rest of the universe", by decoherence, each different term contains state vectors of subsystems which look very much like classical systems. I observe ONE such classically-looking term, which gives me the impression that things are localised in space, and have more or less well-defined momentum.

You give up the whole game when you say "what is observed is not all there is". The fact is, "what is observed" directly contradicts the posited nature of "all there is." That's the point I keep raising above. What is observed is a world with entities with certain definite properties.

Well, these properties seem to be well defined, but in MWI, that's explained by the decoherence (that is, the quick split into different entangled terms) of states which are NOT so, in many cases (but not in all! That's how we found out about quantum theory in the first place!). This comes about because of the specific structure of INTERACTIONS, which needs to respect this famous spacetime structure. In other words, our IMPRESSION of living in a spacetime, only comes about, because interactions between different subsystems have to be organized in such a way that they are "labeled by spacetime" and can interact only "locally".

Yet according to MWI the real world is nothing like that.

Indeed, but it also explains why we get the IMPRESSION that things are so.

Your whole case here hangs on interpreting one term in a superposition as a "part" of the whole. Right? The problem is, that just isn't what the word "part" means (which is why your iceberg analogies don't work). It might work, granted, if MWI actually meant "many worlds". Then the one term would describe a certain physical reality that is realized in one universe, with other terms also describing realities in other universes. Then it would be true that the one term is a part, and hence that *belief* that the state of things is as described by that one term is a "partial truth" (like your iceberg analogy) rather than just false.

Yes, and in what way is this not so ? By "part", I mean: "mathematical part". There's a quite definite way in which the overall wavefunction is sliced up into terms (namely, the "my body/rest of the universe" decomposition). If we now accept that my subjective experience emerges from one of those "my body" components, then the component corresponding to "rest of the universe" in that term is well-defined. I call that "my world", but there's no reason to assume that the other terms in the overall wavefunction are qualitatively different, so I can call them "their worlds".

But I think you know the problems with interpreting terms in a superposition as living in different worlds, literally. So you're stuck.

No, I don't know those problems... each of these terms (mathematical parts) of the wavefunction can live as a wavefunction all by itself (because of the linearity of the time evolution operator).

So... do you accept that there is matter (e.g., tables, chairs, planets) in 3-space?

In quantum theory ? No. Of course not. However, I can accept that I make observations which give me the impression that there are chairs, planets and so on in 3-space. And I also understand that I can set up a classical model in which there ARE tables and chairs in 3-space, which can, most of the time, also predict my impressions correctly. In that classical theory (in its Newtonian version), of course, I accept that there are tables and chairs and planets in 3-space. But not in quantum theory.

Remember that what is to be considered "real" is depending on the theory in which one works. With each physical theory goes a different ontology (= hypothesis about what's real).

That's probably what this comes down to. I say: if you don't accept that, you are just crazy and not a serious (or, at least, psychologically healthy) physicist. And if you do accept that, you have a problem because such stuff is nowhere to be found in MWI.

Half. In MWI, there are statevectors (the components of that part of the wavefunction which is supposed to describe the chair and so on which are in "my" term of the wavefunction) which come very close to a mathematical model which corresponds, what will concern my impressions, to a classical description of the chair and so on. So, given that there is a mathematical piece of the ontology which I could bring into correspondence with "chair in 3-dim space", in as far as that piece is well-defined, I could give some form of "existence" to that "chair in 3-dim space". A bit like you can give some form of existence to a character in a film, or even better, to icons on your computer screen.
And in as far as I can make that approximation, I can then just as well work with the classical model, in which I can say that there ARE chairs and planets and so on in 3-space, because there, there is no problem in considering them "genuinly real".

I'm sorry, but this is just silly. Yes, it's possible to formulate classical particle dynamics in phase space. But each particle actually exists in 3-space, yes?

In the Newtonian view on classical mechanics, yes. In the Hamiltonian view, no. I have no problem with considering that the world is actually 6N dimensional (is a single point in a 6N dimensional space - hey, maybe there are even OTHER points, corresponding to other worlds, in that same phase space), if it is organized in such a way that I get the impression that there are different particles in a single 3-dim space. I even consider that a good exercise to get a feeling for quantum theory.

Imagine you take the Fourier transform of the mass density of a tree. Oh my god! The tree is gone! There are just an infinite number of abstract waves each spreading across the entire universe!

Exactly. However, the way these waves interact with my "body waves" is such that I get the subjective impression of seeing a tree. I have no difficulty in principle with such an approach. However, it would not be a representation of reality in which I could easily deduce what brain states corresponds to my "subjective impressions". The "states corresponding to my subjective impressions" would be rather defined in a complicated way.

But we're not talking about that phase space. The EM field lives in 3-space. As do trees. Period. If you don't accept that, we can't possibly have a serious discussion.

I do not require this as an absolute prerequisite, indeed. At a certain point in time, it was not a bad idea (because inspired by our intuition). But I think we are now far beyond that. The only thing we still need, is that somehow we get a subjective IMPRESSION of 3-space. And I think that this comes about because of the specific structure of interactions, which needs to be a representation of spacetime.

This theory of course suffers from the measurement problem, essentially because it never says what is "classical" and what isn't, i.e., it's "unprofessionally vague and ambiguous." But at least it has the virtue of accepting that there are real physical things moving around in 3-space!

I think that that is the reason why it has so many "paradoxes".
You cannot keep the superposition principle, and require things to be localised in space. However, you CAN keep the superposition principle, and require interactions to be "indexed on spacetime".

 

[vanesch]

I moved our discussion to a new thread, because it started to hijack the "OK Corral..." thread.

 

[MaverickMenzies]

I was wondering if I could interrupt this discussion with a question about MWI?

According to MWI everything is quantum and has as associated state space and that "measurements" are just entangling unitary operations between systems (one of which will be the system of interest and the other will be an observer).

I can see how this works in the case of measuring an observable of a system but how does it work when measuring a POVM?

 

[vanesch]

I can see how this works in the case of measuring an observable of a system but how does it work when measuring a POVM?

I'm only acquainted in a rudimentary way with POVMs. I'm not sure that they are physically relevant in this discussion, in that in MWI, we're supposed to use the pure state of the entire universe. As such, the only measurements which are physically executable should be of the projective kind, no ?
But, as I said, I probably don't know enough of this to give any meaningful answer...

 

[ttn]

I don't have time to get into this at great length (and it's probably pointless anyway since we've had this same discussion a thousand times), but a few brief comments:

This completely selects the unique term in the wavefunction, which consists of two factors: a body state, and a "rest of the universe" state.

But neither of those factors actually represents the state of the relevant degrees of freedom. That's the whole problem. The factor representing what you call "a body state" in the one term (evidently the one you subjectively experience) refers to the same physical degrees of freedom (the same particles or whatever) as some factors in other terms. *That* is why it is invalid to say your subjective experience actually corresponds to the real state of those (or any) degrees of freedom, in MWI. Those particles (or whatever) simply are not in the state you experience them to be in.





In my opinion, you essentially concede the whole argument to me when you say:

In quantum theory ? No. Of course not. However, I can accept that I make observations which give me the impression that there are chairs, planets and so on in 3-space.

That is: really, according to MWI (though not quantum theory, as you suggest, since that *presupposes* the real existence of the classical world) there are not actually any such things as tables and chairs and planets in 3-space. And yet, every possible bit of *empirical evidence* that could in principle be claimed as supporting MWI, is of this form: a table or chair (or pointer) was at some particular place at some time. Thus my criticism of MWI: it asks us to accept that the best way to interpret the fundamental physical meaning of those pointers and chairs having done what we saw them do, is to deny that those things even exist. And that, frankly, is just dumb.

Remember that what is to be considered "real" is depending on the theory in which one works. With each physical theory goes a different ontology (= hypothesis about what's real).

Yes, sure. That was the point I had in mind in referring you to the paper I cited a few posts back. I actually agree with your criticism that the division between "primary" and "secondary" ontology is a bit dubious. Either something is posited by the theory to really exist physically, or not. There is no half-way house between reality and unreality. So we agree there. But my point in pointing out the paper was to help you see that, if a theory has none of what the authors call "PO" -- i.e., if it doesn't posit anything that can be thought of as familiar *matter* moving in 3-space -- the theory is literally about nothing, or at least nothing physical. It may still of course be about "subjective experience" and may concoct some crazy story about why/how we are *deluded* into believing in matter moving in 3-space, but that isn't physics, it's some kind of bizarre philosophical/psychological craziness.

 

[vanesch]

But neither of those factors actually represents the state of the relevant degrees of freedom. That's the whole problem. The factor representing what you call "a body state" in the one term (evidently the one you subjectively experience) refers to the same physical degrees of freedom (the same particles or whatever) as some factors in other terms. *That* is why it is invalid to say your subjective experience actually corresponds to the real state of those (or any) degrees of freedom, in MWI. Those particles (or whatever) simply are not in the state you experience them to be in.

This is because you have a preconceived notion of what are "degrees of freedom", which are in blunt contradiction with the superposition principle (and the same happens to your notion of 3-dim space). A degree of freedom in quantum theory is something which can take on several values at once, but of which only one can be observed. This is why we have the impression that it has a precise value. If you INSIST on it having only one precise value in order for it to be real, you have denied the very founding principle of quantum theory. But in its very definition, it also includes why we have the IMPRESSION that it can take on precise values!

A degree of freedom in QM corresponds by definition to a set of orthogonal basis vectors in a (factor) hilbert space, EACH INDIVIDUAL VECTOR corresponding to a POSSIBLE OBSERVATION. Now, saying that the state of a (sub)system will be a vector in this space, means exactly that it will be in a state that corresponds to MANY DIFFERENT POSSIBLE OBSERVATIONS at the same time. So, by its very construction, it is impossible to equate that what is observed, to the state itself. The whole theory is build upon this concept, that the actual state is a mathematical combination of the different possible "states of observation".

Now, observations are ultimately "subjective experiences". So what the superposition principle essentially says is that, whatever are the "degrees of freedom" on which subjective experiences emerge, they can only emerge on a specific component (in Hilbert space). They cannot emerge on the entire state vector, because that state vector has to be, by postulate, a superposition of "states of subjective experiences". Hence, *by postulate* it is build in from the start that, if quantum theory is going to have a psycho-physical link, it must be "component-wise".

So, all we call "degrees of freedom", "classical objects" etc... must hence be seen component-wise. A "single quantum degree of freedom" corresponds as such to MANY "classical states" at once (by hypothesis, in the basic postulate of quantum theory). Insisting that the FULL STATE ought to be the observed state is hence killing the initial postulate from the start, which REQUIRED the full state to be a combination of potential alternative and mutually exclusive observable states. From the moment that you could observe "the full state", you made an error in the construction of the hilbert space, which was supposed to contain *combinations of mutually exclusive observational states*.

In my opinion, you essentially concede the whole argument to me when you say:



That is: really, according to MWI (though not quantum theory, as you suggest, since that *presupposes* the real existence of the classical world) there are not actually any such things as tables and chairs and planets in 3-space. And yet, every possible bit of *empirical evidence* that could in principle be claimed as supporting MWI, is of this form: a table or chair (or pointer) was at some particular place at some time. Thus my criticism of MWI: it asks us to accept that the best way to interpret the fundamental physical meaning of those pointers and chairs having done what we saw them do, is to deny that those things even exist. And that, frankly, is just dumb.

Again, in quantum theory is constructed around the principle that mutually exclusive "observational states" are the "real state". So we START by assuming that the "real state" of the universe is unobservable! But its components are, each individually, consistent observational states. This is the very starting point of quantum theory.
We shouldn't be surprised, then, that what we observe, is one of those components ; moreover, if these components can lead, most of the time, individual lives and have an almost 1-1 correspondence with classical theories (a different classical state for each of the components), then to me that's just fine: it explains why we *observe* essentially a classical world.

if a theory has none of what the authors call "PO" -- i.e., if it doesn't posit anything that can be thought of as familiar *matter* moving in 3-space -- the theory is literally about nothing, or at least nothing physical. It may still of course be about "subjective experience" and may concoct some crazy story about why/how we are *deluded* into believing in matter moving in 3-space, but that isn't physics, it's some kind of bizarre philosophical/psychological craziness.

I agree with you with most of what you write, but we clearly disagree upon that last statement. The whole purpose of science is to explain our subjective experiences. If it can do that, based upon a limited number of principles, then that's perfect for me. There is no reason why there should by any 1-1 link between a "reality" and my observations. This could be, but this could just as well not be. Classical physics makes the hypothesis that this is the case, and quantum theory STARTS by assuming that this is NOT so (it is the content of the superposition principle!)

All, beyond our subjective experiences, is nothing else but hypothesis in any case. We sometimes have strong intuitive convictions that our observations "are real", and in as much as this is a helpful hypothesis, you can make that hypothesis. As I'm trying to point out, in MWI, your observations are "as real as is possible within the framework" (because that very framework takes as basic postulate exactly that "reality" is a combination of mutually exclusive observations).

 

[ttn]

This will be my last comment on this thread. I really don't have time to continue with it.

Vanesch, you basically admit that the whole point of MWI is simply to give an account of "subjective experience." Everything else -- the existence of chairs, tables, planets, atoms, matter in 3-space, everything -- can be dispensed with so long as the subjective experiences come out correctly. Indeed, you sloganize that this is the proper and only function of science. Well, perhaps Ptolemy and Ernst Mach would agree with you there, but many scientists wouldn't. Many scientists actually accept (yes, I know, it's shocking) that the external 3-D world full of material entities moving and interacting is *real*, and that there is a *difference* between theories like Ptolemy's and theories like Copernicus's, even though they (arguably) make all the same predictions for "subjective experience".

You mentioned the "psycho-physical link". I think that's the key point here. Interestingly, Bell sometimes uses the phrase "psycho-physical parallelism" to mean the kind of view I indicated in the previous paragraph -- that what we experience actually exists, that our "subjective experience" of chairs and tables actually corresponds to really-existing chairs and tables. To me (and to Bell) that is the kind of thing that we learn when we're about 1 year old, and that literally everything else we do subsequently (I mean "do" here in cognitive terms) is based on. For example, note the absurd circularity in the following: I claim to have gone into a laboratory and done some experiments and concluded, based on the outcomes of those experiments, that there are no such things as tables, chairs, or pointers. But that is just absurd. That the positions of certain pointers in the lab were observed to be such-and-such can *never* be taken as evidence that those pointers don't exist. It's a logical contradiction. And what I'm pointing out here is that MWI is doing exactly this -- undercutting itself by denying the reality of the very sorts of things which manifest all the purported evidence for MWI.

As you know, I grant that in a certain sense, MWI *can* successfully account for our (or really just *my*) subjective experience. It's just that that "experience" is then nothing like what I always thought "experience" was supposed to be. My experience is not just some "inner theater", but an actual *perception*, an actual *recognition* of really-existing external things. This is what all 2 year olds know, but what MWI denies. And it means that MWI is not really a theory about physics in the normal sense (a theory about the hidden/underlying workings of little bits of matter or fields or particles or whatever, attempting to account for the directly observed behavior of things like tables and chairs and pointers). It is rather closer to a theory about brains in vats, except that even that grants too much psycho-physical parallelism to it, because according to MWI there really aren't any brains or vats (at least not in the way we've seen them and conceptualized them). So MWI is more like Leibniz's theory of monads or something -- there are these (or more likely just this one) consciousness(es) which have a certain stream of "subjective experience" fed into them, not by any familiar material things like evil scientists living outside the vat, but by some ineffable "stuff" living in some totally unfamiliar "space". And then the crucial dynamics of the theory (the part where you, vanesch, differ somewhat from the other, stupider, MWI advocates in the world, the part having to do with a "consciousness token" flowing downstream through the branches in accordance with some stochastic Born rule type law) has really nothing whatsoever to do with anything remotely physical. What we have always meant by "physical objects" are literally nowhere to be found in MWI, except inside the mind of the poor soul having all these "subjective experiences." And as I've stressed (and as you've agreed, though only by twisting the words a bit to hide the strangeness) all those experiences are literal delusions, since (eg) when he thinks there's a solid table in front of him which is brown and has 4 legs and exists in 3-space, none of those things are true. There is no such thing with no such properties in no such space.

As Bell once said in a different context, this (the boundary between systematically delusional subjective experience and some totally unfamiliar pseudo-physical "world") is a very uncomfortable place to try to do physics. Normal, rational, scientific people should recognize this and admit that MWI is simply not on the right track (in the same way and for the same reasons that they recognize that solipsism is not a good scientific way of making sense of some puzzling observations like retrograde motion or dark matter).


Anyone else who is lurking and agrees with my thinking here is invited to continue the good fight against MWI. :zzz:

 

[vanesch]

This will be my last comment on this thread. I really don't have time to continue with it.

That's what I tell myself each time I engage in such kind of discussion too

Vanesch, you basically admit that the whole point of MWI is simply to give an account of "subjective experience."

Yes. It is the ultimate goal of all forms of intellectual (and even practical) activity, no ? But it takes some philosophy to see this, that's true. After all, why do we conceptualize such a thing as a chair ? It is because we've had repeated combined sensations, both directly and indirectly, which are all rather well explained by making the hypothesis that there is such a thing as a chair. In other words, the only reason to assign some form of ontology to a concept such as a chair, is that it helps us explain and organize our sensations. "Reality" is a hypothesis which helps us organize our sensations.

Everything else -- the existence of chairs, tables, planets, atoms, matter in 3-space, everything -- can be dispensed with so long as the subjective experiences come out correctly.

Yup. The important thing is: *can* be dispensed with. It is not a philosophical necessity. Now, I agree as much as you do (just to prove my mental sanity which you are now doubting seriously, I'm sure ) that it would be nice if we could keep it. But, when interrogating myself, I don't see why that should be an absolute necessity.

Indeed, you sloganize that this is the proper and only function of science. Well, perhaps Ptolemy and Ernst Mach would agree with you there, but many scientists wouldn't. Many scientists actually accept (yes, I know, it's shocking) that the external 3-D world full of material entities moving and interacting is *real*, and that there is a *difference* between theories like Ptolemy's and theories like Copernicus's, even though they (arguably) make all the same predictions for "subjective experience".

Look at the very definition of a scientific theory: AGREEMENT between PREDICTION (of observation!) and ACTUAL OBSERVATION. If you realize that ultimately, an observation is nothing else but a subjective experience (whether you look at the display, or read the computer printout, or ...), then you see that the very definition of science is to find a "hypothesis of reality" (a model) which generates the subjective experiences in agreement with what we actually experience. The only other requirement we put on that model is that it is based upon as few principles as possible (Occam).

You mentioned the "psycho-physical link". I think that's the key point here. Interestingly, Bell sometimes uses the phrase "psycho-physical parallelism" to mean the kind of view I indicated in the previous paragraph -- that what we experience actually exists, that our "subjective experience" of chairs and tables actually corresponds to really-existing chairs and tables.

Uh ! Then he has mis-read von Neumann, who was the first to realize that this plays a role in the interpretation of quantum theory, in his very book on "mathematical foundations of quantum mechanics". This is btw why I find von Neumann much deeper than Bohr on the subject.
It doesn't mean at all what you write: it means that all subjective experiences should have a "physical carrier" of some kind. Maybe I misread that, but to me, it only means that it is the physics, through some or other mechanism, which "generates" the subjective experience. It means that there must be some aspect of physics which "contains" our experiences.

To me (and to Bell) that is the kind of thing that we learn when we're about 1 year old, and that literally everything else we do subsequently (I mean "do" here in cognitive terms) is based on.

Indeed, and it needs some work to get rid of it. In the same way as, say, relativity of simultaneity. That's also something you learn when you are 2 years old.

For example, note the absurd circularity in the following: I claim to have gone into a laboratory and done some experiments and concluded, based on the outcomes of those experiments, that there are no such things as tables, chairs, or pointers.

Yes. However, you forget to add: I did some experiments and concluded, that there are not really such things as tables and chairs (in the way they are usually conceived), but that there is something different, which, when observed, will give me the impression of there being tables and chairs, which will, most of the time, also behave as tables and chairs the way I learned when I was 2 years old.

But that is just absurd. That the positions of certain pointers in the lab were observed to be such-and-such can *never* be taken as evidence that those pointers don't exist.

You attach too much importance to "exist". "Exist" is a hypothetical concept which only serves to help us organize our sensations.

Again, reformulated: the OBSERVED POSITIONS of things which look to me like pointers, gave me evidence that all observations cannot correspond to uniquely existing pointers (but to a more complicated concept, which, most of the time gives me the impression that there are pointers pointing somewhere).

It's a logical contradiction. And what I'm pointing out here is that MWI is doing exactly this -- undercutting itself by denying the reality of the very sorts of things which manifest all the purported evidence for MWI.

Let me tell you why this is not logically impossible.
Consider (a bit like the cavemen in Plato - which is exactly the kind of situation MWI presents) that all your life you've been attached to some kind of device which keeps your head pointed at a movie screen, and that all you've ever seen and heard were projections of movies on that screen. Clearly, you will have a "picture of reality" which corresponds to an imaginary world - which corresponds in many respects to a kind of world which looks like our classical world (you have seen people, cars, airplanes etc...). Now, one day, they show you a movie in which they film a guy who has been looking at a movie screen for all his life, then they show you, in the movie, that there is a movie projector, etc...
And suddenly, you realize that this might very well be your situation ! So, from looking at stuff on a screen (which you previously thought, was "reality"), you suddenly came to realize that you're probably just looking at a screen, and that this is not "reality". You now also understand most of what "happened in your "observation of reality". But now you "know" that real reality, is a movie theater, a projector, and a poor guy who's forced to look just at the screen.

As you know, I grant that in a certain sense, MWI *can* successfully account for our (or really just *my*) subjective experience. It's just that that "experience" is then nothing like what I always thought "experience" was supposed to be.

Right ! But that's always a strange experience. Nevertheless, even an old guy like Plato thought of that.

My experience is not just some "inner theater", but an actual *perception*, an actual *recognition* of really-existing external things. This is what all 2 year olds know, but what MWI denies.

Yup. As I said, it takes more than a 2-year old to grasp that concept.

And it means that MWI is not really a theory about physics in the normal sense (a theory about the hidden/underlying workings of little bits of matter or fields or particles or whatever, attempting to account for the directly observed behavior of things like tables and chairs and pointers). It is rather closer to a theory about brains in vats, except that even that grants too much psycho-physical parallelism to it, because according to MWI there really aren't any brains or vats (at least not in the way we've seen them and conceptualized them). So MWI is more like Leibniz's theory of monads or something -- there are these (or more likely just this one) consciousness(es) which have a certain stream of "subjective experience" fed into them, not by any familiar material things like evil scientists living outside the vat, but by some ineffable "stuff" living in some totally unfamiliar "space".

That's exactly it, right. See, it is an old idea. But you have also a kind of problem in classical physics, so in any case, you need some kind of "conversion gate" between your subjective world and the "physical world" (at least, from a dualist perspective) The only difference between classical and quantum physics, is that the conversion gate is slightly less "transparant", and that hence the physical world is less familiar (in subjective terms).

And then the crucial dynamics of the theory (the part where you, vanesch, differ somewhat from the other, stupider, MWI advocates in the world, the part having to do with a "consciousness token" flowing downstream through the branches in accordance with some stochastic Born rule type law) has really nothing whatsoever to do with anything remotely physical. What we have always meant by "physical objects" are literally nowhere to be found in MWI, except inside the mind of the poor soul having all these "subjective experiences."

Yes and no. Indeed, a crucial part is given by what you say, but there is also a part which is "objectively physical". And that is where I disagree with the following:

And as I've stressed (and as you've agreed, though only by twisting the words a bit to hide the strangeness) all those experiences are literal delusions, since (eg) when he thinks there's a solid table in front of him which is brown and has 4 legs and exists in 3-space, none of those things are true.

Well, there IS a mathematical object, somewhere deep into the overall wavefunction, which DOES correspond to that table. Or better: which corresponds to the relationship between you and the table. That's why I say that what you see is PART of reality (but not "part" in "piece of volume" of course: part in the sense of part of the mathematical structure).

There is no such thing with no such properties in no such space.

Sorry, there IS something within the ontology of the world that corresponds to it. That's what I'm repeating now already a few times. In as far as you have a certain liberty in calling what is "real" (after all, it is just an organizing principle of sensations), you could call it real too.

As Bell once said in a different context, this (the boundary between systematically delusional subjective experience and some totally unfamiliar pseudo-physical "world") is a very uncomfortable place to try to do physics.

I agree. It would have been simpler to live in a classical world. It would even have been nicer to live in a Newtonian world.

Normal, rational, scientific people should recognize this and admit that MWI is simply not on the right track (in the same way and for the same reasons that they recognize that solipsism is not a good scientific way of making sense of some puzzling observations like retrograde motion or dark matter).

The problem with saying a priori what are "good" and what are "bad" principles, is that one might make an expensive mistake by adhering to a principle which is not of this world, no matter how intuitively attractive it may sound. As you see, I don't adhere to MWI for MWI's sake. I adhere to it as a view on quantum theory, because the theory generates good explanations of observations, and respects in the most fundamental way the fundamental postulate of quantum theory (which is the superposition principle). It wouldn't come to my mind to go and fiddle with a FORMALISM (which works well) which is based upon some principles (no matter how crazy at first sight) just to satisfy my intuitive desires of what ontological principles a theory should satisfy. The formalism IS the ontology, for me. So I twist and turn my views on ontology in order to make them fit the mathematical formalism, and not the other way around. If that exercise pushes me in the corners of unintuitiveness, well, then so be it. I am of the opinion that a good formalism has more to teach us about nature than our intuition, so whatever the formalism tells us, must be a better guess at the ontology of the world than whatever we can intuitively dream of.

To conclude, I can very well accept that some people don't like MWI. As I've pointed out several times, I only adhere to it because I think it is the "most natural" interpretation of the *quantum formalism*. I find it highly unintuitive too. However, it has the advantage that it explains quite naturally (because of its close ties to the formalism of course) certain "paradoxes" which seem to occur in quantum theory, when one tries to fit it upon a "classical world". As such, this stops me from lying awake at night about the EPR paradox or something of the kind. If it gives you more conceptual problems than it solves, then MWI is not for you. But for me, it works fine.

However, I do not accept your critique of the *logical contradiction* you try to force upon MWI, by saying that "MWI is based upon observations of which it tells you are bogus". I repeat my objection: we make observations. We may think that they correspond to "something real" or not, that's a separate matter. But in the end we need a story which tells us what our observations should look like. If that story tells us correctly what our observations will look like, EVEN IF FOR THAT IT MAKES THE HYPOTHESIS that reality is in fact quite different, then that is a good story. So it is not true that MWI tells us that the observations we made, which led us to set up MWI, are bogus. It only tells us that reality is different, but from that different reality, it can correctly derive the observations.

This is a bit like "parameter fitting". Imagine that I have a list of pairs of numbers. Now imagine that I come up with a calculational model which can generate those pairs of numbers, but that for that, I have to feed it with parameters which don't look at all like the pairs of numbers. This is nevertheless a good model. It is not because the parameters I "fit" do not look like the pairs of numbers "I observe" that this is a logical contradiction. This is the same in MWI: it is not because the reality that is postulated doesn't look at all like the observations that are performed, that it is a bad model to explain those observations.

 

[setAI]

Anyone else who is lurking and agrees with my thinking here is invited to continue the good fight against MWI. :zzz:

there is nothing good about fighting against the universe simply because you have aesthetic problems with it

 

[Doc Al]

there is nothing good about fighting against the universe simply because you have aesthetic problems with it

Perhaps there's a universe where you are the one trying to convince everyone of the silly extravagance of the MWI.

 

[vanesch]

Perhaps there's a universe where you are the one trying to convince everyone of the silly extravagance of the MWI.

Is this then a collision of worlds, where SetAI and I are copies ?

 

[Doc Al]

I'll know the original vanesch by the twinkle in your eye! (But my copy might not! )

 

[Schrodinger's Dog]

there is nothing good about fighting against the universe simply because you have aesthetic problems with it

No indeed, fighting against a flawed understanding is worthwhile, fighting against a philosophy based on it is ultimately fruitless. Which is probably why Venesch has an air tight hypothesis.

I think MWI is as frustrating and ultimately unprovable as some of the theorems/theories from which it derives, thus I can accept it as a good picture of what is happening, and a way of explaining certain paradoxes, but at an intuitive level it leaves me cold, as it is ultimately a purely philosophical, semantic invention, and may well be based on false assumptions.

That said my real reason for not liking it is probably the same as the Copenhagen Interpretation, the difference is, that against intuition CI appears to be true experimentally; MWI is just pretty much building a philosophical structure on what I hope is in essence a lack of real comprehension of what we are seeing, it's a little insubstantial. Worthy as a thought experiment, but ultimately leading us nowhere but back to the problems we started with.

Since it lacks practical application,such interpretations seldom evolve unless there tenets are proved to be inaccurate or incomplete. From what I have seen it's a bit of a dead end theory, or a deck of cards with itself at the top.

 

[vanesch]

I think MWI is as frustrating and ultimately unprovable as some of the theorems/theories from which it derives, thus I can accept it as a good picture of what is happening, and a way of explaining certain paradoxes, but at an intuitive level it leaves me cold, as it is ultimately a purely philosophical, semantic invention, and may well be based on false assumptions.

I also am of that opinion, btw

 

[Schrodinger's Dog]

I also am of that opinion, btw

Well I guess we'll just have to agree to agree

By the way it's nice to be on the other side of an attack of MWI, it's nice to watch the broadsides form an observers perspective and also nice to see someone who knows a lot more than I do having a go with his musket and a cutlass in his teeth very interesting discussion, good work.

 

[vanesch]

there is nothing good about fighting against the universe simply because you have aesthetic problems with it

Just to make things clear: although I always seem to defend MWI, my aim is not to "convert" people to it or anything. I can perfectly well understand that people don't like it. However, the discussion in this thread was a specific point.

ttn's argument (which he gives each time he shows up in a thread where MWI is mentioned) is essentially that "MWI shoots itself in the foot", and I wanted to deconstruct *that specific argument*. I think it is not a logical or philosophical necessity, as ttn seems to claim it, that MWI shoots itself in the foot. That, by itself, doesn't say anything about the validity or merit or not of MWI, but the specific argument ttn uses, is IMO, wrong.

The argument goes as follows:

- after a lot of observations and experiments, finally, people arrived at quantum theory.
- when looking at the formalism of quantum theory, some people interpreted it in the MWI sense.
- when looking in MWI what it means, "to make an observation", you realize that (and here the retoric verbiage comes in) "observations do not correspond to reality" or "observations are bogus"
- hence there was no reason, in the first place, to write down quantum theory which has been inspired by centuries of (bogus) observations.

As such, ttn concludes that MWI just rendered "without value" the very observations which were at its origin.

The trick resides in a tacit assumption that an explanation of observations needs to give them a status of unique reality in order for them "to have value" on which we can base any theory. But that is an unnecessary assumption.

An explanation of observations only needs to explain observations. It doesn't need to give any particular status to them. This happens many times: that one realizes that "there is more than meets the eye", just based upon "what meets the eye".

As such, the argument that, based upon observations, we came to the conclusion that there "is" something else, but that this generates those observations, we must necessarily have it wrong, is IMO a wrong argument.

Moreover, but that is a different argument in fact, in MWI, there IS some form of reality given to the observations (the component of stuff in "my" branch). But even if it weren't, that wouldn't be an objection.

All of this is of course not an argument in favor (or against) MWI. It is just an analysis of an erroneous objection to MWI.

 

[Thrice]

Since it lacks practical application,such interpretations seldom evolve unless there tenets are proved to be inaccurate or incomplete. From what I have seen it's a bit of a dead end theory, or a deck of cards with itself at the top.

That's probably the best criticism of it. It's only an interpretation.

 

[vanesch]

That's probably the best criticism of it. It's only an interpretation.

Well, that depends. An interpretation can or can't, prepare the mind for a change or extrapolation.

See, an MWI'er will have no problems accepting, say, strictly unitary attempts at quantum gravity, while someone in the GRW view will insist on deviations from unitarity (and a Bohmian may wonder why one would like to include GR into quantum theory in the first place) as well as deviations from GR.
Someone in the "shut up and calculate" approach will have no specific desire, one way or another.

The way one pictures a theory also influences what are its "crucial" lessons and what can be disposed off more easily.

 

[ttn]

The only way to interpret the quantum-mechanical computation < (sigma_1.a)(sigma_2.b) > as a suggestion for a local mechanism, would be when we interpret (sigma_1.a) as the OUTCOME at Alice, and we interpret (sigma_2.b) as the outcome at Bob. But that's almost too crazy to consider. And some people leave out the "too"
It would mean that there is no objective result at Bob, and no objective result at Alice, which would be a list of {+1,-1,-1,...} The result would be "an operator" and NOT a +1 or a -1.

If you are mentally capable of stretching your imagination so far as to claim that there IS no outcome at Alice, that looks like a -1 or a +1, but that it is an operator, AND ONLY IN THAT CASE, then you CAN interpret the quantum expression as being of the kind < F.G >. This is the MWI view on things, and the only way to keep a local mechanism compatible with the quantum-mechanical predictions. Moreover, the local mechanism is then given exactly by the formal expression that was thought not to stand for any mechanism (but "just a calculation"). But one should really realize the stretch of imagination that is needed for that case: there is no objective outcome at Alice which takes on a +1 or -1 value

Isn't this just what you were denying on the "MWI" thread? There is no objective outcome at Alie which takes on a +1 or -1 value. Yet (as you say just next) Alice (and Bob and anyone else interacting/corresponding/watching) observes either a +1 or a -1. So what Alice observes doesn't correspond to the actual objective situation.

Nevertheless, to be able to make this crazy view compatible with the obvious fact that Alice HAS SEEN a +1 or a -1, the trick is to consider that there are now TWO ALICEs, one who has seen +1 and another who has seen -1. The "overall state" of Alice is then described not by a +1 or a -1, but by, exactly, an operator which is sigma_1.a.

This tries to have it both ways. Either there are two alices after the "interaction" (her "measurement" of the spin of her particle) or there is still just one. (I think this is equivalent to the question of whether the branches in the wf refer to a superposed state in one universe, or two non-superposed states living in two distinct parallel universes.) But you don't get to have it both ways. So pick your poison: if there are really two Alice's, then the burden is on you to provide an account, in the theory, of this bifurcation -- exactly when does it happen, i.e., under what conditions? But this is notoriously difficult/unsolvable. It is essentially the orthodox "measurement problem" over again, since the obvious answer (the world splits into two copies when she makes a "measurement") contains an undefined and vague term which can't be cashed out by the theory. And if you choose the other way -- that there is really only one Alice in some complicated superposed/entangled state -- then you really can't help but agree with the criticism I keep making, namely that her actual physical posited state doesn't match what everyone experiences her state to be. (Which then leaves the theory quite open to the objection I've been making, that it undercuts itself by asking us to classify as "delusion" all of the things which might possibly be cited as evidence for the theory.)

When this "superposition of Alices" meets the "superposition of Bobs" (he will suffer a similar fate), then upon meeting, they will get together in SEVERAL DISTINCT COUPLES Alice/Bob (this time described by (sigma_1.a)(sigma_2.b) ). The statistics of this set of couples is then given by the quantum-mechanical expectation value < >, and gives us the correct correlations.

This is what the formalism suggests. It is also the IMO only way in which a local mechanism can be preserved. But it is of course totally crazy. That's MWI.

With all due respect, I think it's stupid to say the formalism suggests this. Part of the formalism you start with is the orthodox measurement axioms which are there for a very important reason: to account for the fact that measurements and other sorts of observations yield definite outcomes (e.g., that pointers always point somewhere, cats are either alive or dead, etc.). It's true that there is another part of the formalism (the Sch eq) which is linear. But to look at those together and think "this formalism as a whole is really suggesting that we drop the idea of measurements having definite outcomes, and hoist in its place a loony bin story about multiple copies of Alice living in parallel universes that are simultaneously different and the same as our universe" is just silly. It shows, at best, that one is looking at the equations with serious blinders on, which make one forget all of the rest of the knowledge that establishes the context and meaning for those equations (such as that the Sch eq is a dynamical equation for something that is a very spooky mathematical entity whose physical status is tentative or completely unknown -- as contrasted to our knowledge of the classical entities described in the measurement axioms, about which we are totally familiar and as completely certain as one could be of anything in empirical science).


Finally, if he is reading, I have a question for JesseM: I notice that you keep formulating Bell's Theorem as saying that any theory respecting (a) perfect correlations when Alice and Bob measure along the same axis and (b) "local realism", must make predictions satisfying the inequality. May I ask what you mean by "local realism"? In particular, what does the word "realism" add? Bell never used the phrase "local realism" but always insisted (correctly, I think) that the theorem follows from just plain "locality" or "local causality". He often refers to it as the "locality inequality". Presumably you think that Bell was wrong here, and that, in fact, some additional assumption ("realism") is needed to arrive at the inequality. Could you elaborate on what exactly you think that assumption is?

 

[vanesch]

Yet (as you say just next) Alice (and Bob and anyone else interacting/corresponding/watching) observes either a +1 or a -1. So what Alice observes doesn't correspond to the actual objective situation.

Right. We're not going to go over that _again_, are we ?
Indeed, in MWI, what Alice observes doesn't correspond to the actual objective situation. So ? It does correspond to her subjective observation.

Either there are two alices after the "interaction" (her "measurement" of the spin of her particle) or there is still just one. (I think this is equivalent to the question of whether the branches in the wf refer to a superposed state in one universe, or two non-superposed states living in two distinct parallel universes.) But you don't get to have it both ways.

You DO get it both ways in MWI, for the following reason.
Alice stands for two different concepts here. "Alice" first stands for the quantum-mechanical degrees of freedom which correspond to a physical structure which is her body. THIS "Alice" appears in a superposition. And THEN there is "Alice", the subjective experience, which lives according to each element of the basis "Alice observables" - basis vectors of the quantum-mechanical degree of freedom which is "Alice body", because we take it that an Alice subjective experience finds its origin in an Alice body.
However, to a single quantum-mechanical Alice body can correspond several distinct Alice experiences. This is the whole idea. Alice experiences are the classical basis vectors on which an Alice quantum body is spanned.
If several basis vectors are solicited in the state of the quantum body, then several subjective experiences are present. So, several subjective "Alices" inhabit one single objective Alice body, but that body is a quantum body, while the subjective experiences are only open to classical experiences. So from the PoV of a "subjective" Alice, these different terms are inhabited by copies of "subjective alices", and can thus be qualified as being "different worlds", while there is only one objective alice (quantum body), which is given by the entire set of its different components in the wavefunction.

The way this emerges from the formalism is exactly as the relationship between "position of a particle" and a quantum particle. The classical degree of freedom "position of a particle" (x,y,z) occurs with many different values in the quantum degree of freedom "position of a quantum particle". So you can say that "different classical positions" inhabit a single quantum position.

In the same way, we consider subjective experiences to emerge from classical body states, and a quantum body can hence be such that it is "inhabited" by many classical body states (with their associated subjective experiences).

So pick your poison: if there are really two Alice's, then the burden is on you to provide an account, in the theory, of this bifurcation -- exactly when does it happen, i.e., under what conditions? But this is notoriously difficult/unsolvable. It is essentially the orthodox "measurement problem" over again, since the obvious answer (the world splits into two copies when she makes a "measurement") contains an undefined and vague term which can't be cashed out by the theory.

The different "subjective alices" correspond to the basis vectors from which "subjective alice experiences" emerge. This is indeed not given by the theory, but only hinted at by decoherence. It seems indeed appropriate to take them to be those basis vectors of body states which have a certain stability under time evolution, probably a necessary condition for something such as a subjective experience to emerge.

And if you choose the other way -- that there is really only one Alice in some complicated superposed/entangled state -- then you really can't help but agree with the criticism I keep making, namely that her actual physical posited state doesn't match what everyone experiences her state to be.

Yes, but I don't see that as a problem, as in detail explained in the other thread. There's no problem in having somethings "objective state" not to be equal to "how one observes that state", because the last statement is one of a relationship between a (subjective) observer and that something, while the first is only about the something itself. As long as the entire system of relationships between different observations is coherent, there's no way of objecting to such a view.

(Which then leaves the theory quite open to the objection I've been making, that it undercuts itself by asking us to classify as "delusion" all of the things which might possibly be cited as evidence for the theory.)

I think we've sufficiently argued why that argument is not valid.

With all due respect, I think it's stupid to say the formalism suggests this. Part of the formalism you start with is the orthodox measurement axioms which are there for a very important reason: to account for the fact that measurements and other sorts of observations yield definite outcomes (e.g., that pointers always point somewhere, cats are either alive or dead, etc.). It's true that there is another part of the formalism (the Sch eq) which is linear. But to look at those together and think "this formalism as a whole is really suggesting that we drop the idea of measurements having definite outcomes, and hoist in its place a loony bin story about multiple copies of Alice living in parallel universes that are simultaneously different and the same as our universe" is just silly. It shows, at best, that one is looking at the equations with serious blinders on, which make one forget all of the rest of the knowledge that establishes the context and meaning for those equations (such as that the Sch eq is a dynamical equation for something that is a very spooky mathematical entity whose physical status is tentative or completely unknown -- as contrasted to our knowledge of the classical entities described in the measurement axioms, about which we are totally familiar and as completely certain as one could be of anything in empirical science).

Well, in as much as you consider these classical entities as "totally familiar and completely certain", you can consider the basis of subjective Alice states so too.

In the MWI formulation, I do not forget the measurement axioms (as some MWI-ers try to do, which I think is a mistake). Only, they are differently interpreted. Instead of saying that there IS an outcome of a measurement, these axioms (in this view) describe you the RELATIONSHIP between an observer and its subject. Indeed, "Alice saw a +1" is a shortcut. What you REALLY know, is: "I was subjectively aware of an interaction with something I call Alice, and where I heard that Alice claim she saw +1".

As such, there's no way for me to know that alice (the body) really had an outcome, only that MY OBSERVATION OF ALICE was in agreement with a claim that she saw +1. I have no way of knowing whether or not a different Alice subjective experience (also associated to the same alice body) saw something different. The reason for this is that due to decoherence, each time I am in contact with another body, things seem to split up in exactly such a way that a "subjective experience basis state" of mine also corresponds exactly to a SINGLE "subjective experience basis state" of hers. I never seem to be intangled with TWO subjective experience basis states of her, UNLESS WE ARE SPACELIKE SEPARATED (as in the EPR experiment) - but then I don't observe anything of what she's measuring. From the moment we are "in contact", we are in such a state that a 'subjective experience basis state' of hers is always entangled with just one 'subjective experience basis state' of mine. It is THIS particular effect, which is brought forth by decoherence (and that is genuine, objective physics!), which makes that one can never distinguish between "I saw that Alice saw that ..." and "Alice saw that...". If the subjective experience I live in, saw alice see something, then the subjective experience she is in, ALSO saw that same thing. And then we both agree over that. And we both think that IT IS A GENUINE OUTCOME.

The measurement axioms tell me about this relationship: between what I subjectively experience, and what is the state of the whole system.

 

[JesseM]

Finally, if he is reading, I have a question for JesseM: I notice that you keep formulating Bell's Theorem as saying that any theory respecting (a) perfect correlations when Alice and Bob measure along the same axis and (b) "local realism", must make predictions satisfying the inequality. May I ask what you mean by "local realism"? In particular, what does the word "realism" add? Bell never used the phrase "local realism" but always insisted (correctly, I think) that the theorem follows from just plain "locality" or "local causality". He often refers to it as the "locality inequality". Presumably you think that Bell was wrong here, and that, in fact, some additional assumption ("realism") is needed to arrive at the inequality. Could you elaborate on what exactly you think that assumption is?

I was mostly just using "local realism" because wm was using it to describe his own ideas about local hidden variables, and I wanted to make sure that he was clear on the fact that such local hidden variable explanations can be ruled out according to Bell's theorem. But in my mind I was vaguely thinking that "realism" referred to the assumption that every experiment would have a unique outcome, unlike in the many-worlds interpretation which can still be seen as respecting locality. Anyway, I tried to spell out the specific assumptions about physics I thought were essential to deriving the inequality in post #153:

And we make the following 3 assumptions about the laws of physics:

4. no violations of locality allowed
5. no possibility of future events affecting past ones, or "conspiracies" in the initial conditions of the universe that could create a correlation between an experimenter's choice of measurement setting and the properties of the object/signal sent to them by the source even when the source sends them out before the experimenters make their choices
6. each experiment yields a single definite result (no splitting into multiple copies with measurement)

I don't know if Bell ever noticed the loophole that locality can be preserved if you allow each experimenter to split into multiple copies--if he did, does anyone know which paper he addressed this in?

 

[vanesch]

I don't know if Bell ever noticed the loophole that locality can be preserved if you allow each experimenter to split into multiple copies--if he did, does anyone know which paper he addressed this in?

He did. Actually in his book "Speakable and unspeakable..." he mentions the MWI approach, which he dislikes. However, I don't recall from the top of my head whether he realized that locality could be preserved. Let us not forget that Bell was favorable to Bohm's approach (and those people seem to have an avid disliking of MWI ).

BTW, this is not MY PoV. I do like Bohmian mechanics, as it is also conceptually enlightning. (the very fact that it was a counter example to something a mastodont as von Neumann thought was impossible, illustrates the feat).
However, I also like relativity, and both go badly together.

 

[ttn]

He did. Actually in his book "Speakable and unspeakable..." he mentions the MWI approach, which he dislikes. However, I don't recall from the top of my head whether he realized that locality could be preserved.

He doesn't discuss MWI much. That in itself is telling. It's included as one of the six in his article "six possible worlds of qm", which is a fantastic article that anyone reading this thread should look at. Quite possibly it's the very best single-paper introduction to the various possible interpretations of QM and what the motivation for each is.

Interestingly, Bell classifies MWI as the "romantic" counterpart to the de Broglie - Bohm pilot wave theory. One gets the flavor of Bell's assessment of MWI from comments like this

"What happens to the wave [in the 2 slit experiment] where there is no flash? In the pragmatic approach the parts of the wave where there is no flash are just discarded... and this is effected by rule of thumb rather than by precise mathematics. In the pilot wave picture the wave, while influencing the particle, is not influenced by the particle. Flash or no flash, the wave just continues its mathematical evolution... even where it is 'empty' (very roughly speaking). In the MWI also the wave continues its mathematical way, but the notion of 'empty wave' is avoided. It is avoided by the assertion that everywhere that there *might* be a flash... there is a flash. But how can this be, for with one electron surely we see only one flash, at only one of the possible places? It can be because the world multiplies! After the flash there are as many worlds (at least) as places which can flash. In each world the flash occurs at just one place, but at different places in different worlds."

and

"It is easy to understand the attraction of the three romantic worlds [which are the Copenhagen view, the view that it is at the mind-matter boundary that wave function collapse takes over from linear Sch evolution, and the MWI] for journalists, trying to hold the attention of the man in the street. The opposite of a truth is also a truth! Scientists say that matter is not possible without mind! All possible worlds are actual worlds! Wow!"

Later he writes: "The 'many worlds interpretation' [the scare quotes perhaps implying that he, like vanesch and I, think this is a misnomer?] seems to me an extravagant, and above all an extravagantly vague, hypothesis. [The vagueness he is referring to is probably the preferred basis problem, which he talked about a few paragraphs earlier] I could almost dismiss it as silly. And yet... It may have something distinctive to say in connection with the [EPR puzzle], and it would be worthwhile, I think, to formulate some precise version of it to see if this is really so." I assume by the last bit he means what we all recognize in some form or other, that MWI purports to be the only local theory that is "consistent" with the EPR/Bell/Aspect data. (I put "consistent" in scare quotes because the theory isn't technically consistent with the data in the ordinary sense, but rather purports to explain why we would have a subjective delusion to *think* the data were what we thought they were, even though, in fact, those experiments didn't actually have those Bell-inequality-violating outcomes.)

Perhaps one has to be a bit of a Bell connoisseur to appreciate this, but a lot of the sentences above (particularly those involving exclamation points) exude a kind of subtle dismissiveness that Bell almost certainly felt toward MWI. To him, it simply couldn't be taken seriously.

Let us not forget that Bell was favorable to Bohm's approach (and those people seem to have an avid disliking of MWI ).

Indeed. The final sentences of the 6 worlds paper read: "In my opinion the pilot wave picture undoubtedly shows the best craftsmanship among the pictures we have considered. But is that a virtue in our time?"

The last sentence is somewhat obscure, but I'm pretty sure he is just bemoaning how few other sensible people (i.e., those "favorable to Bohm's approach") he found in physics.

BTW, this is not MY PoV. I do like Bohmian mechanics, as it is also conceptually enlightning. (the very fact that it was a counter example to something a mastodont as von Neumann thought was impossible, illustrates the feat).
However, I also like relativity, and both go badly together.

Whereas relativity goes well with MWI? Please. Only if you gut it within an inch of its life.

Anyway. Vanesch, I must say I miss the old days when we actually had interesting discussions. I guess nothing's changed and we've been there, done that, so it feels like a rehearsed performance with nothing to learn. But your latest reply above seems to give the impression that you think I haven't understood something about your arguments against my objection to MWI. But that's not true. I understand your position and arguments perfectly. It's just that I don't think you actually refute my argument. Instead, with practically everything you say (which is how this got started again, yes?) you admit exactly what I assert about MWI. So... I'm happy to agree to disagree, but don't be pretending that the reason I disagree is because I haven't yet grasped your cogent arguments, OK?

 

[ttn]

He did. Actually in his book "Speakable and unspeakable..." he mentions the MWI approach, which he dislikes. However, I don't recall from the top of my head whether he realized that locality could be preserved. Let us not forget that Bell was favorable to Bohm's approach (and those people seem to have an avid disliking of MWI ).

Oops, I meant also to ask: does anyone else remember Bell discussing MWI in any other papers? I honestly can't think of any paper other than 6 worlds where he even discusses it. As I said before, I think that is telling.

 

[ttn]

I was mostly just using "local realism" because wm was using it to describe his own ideas about local hidden variables, and I wanted to make sure that he was clear on the fact that such local hidden variable explanations can be ruled out according to Bell's theorem. But in my mind I was vaguely thinking that "realism" referred to the assumption that every experiment would have a unique outcome, ...

I think it's a mistake to pick up the terminology of other people who either (a) don't know what they're talking about or (b) kind of half know but don't understand the issues well enough to scrutinize the appropriateness of the terminology and hence themselves just pick up on the terminology that everyone else uses.

In this particular case, I think it's demonstrable that none of the possible things that one might think might be meant by "realism" are actually premises in Bell's derivation. Many people think the "realism" means some kind of hidden variable assumption (that each particle possesses pre-set values for how several distinct possible spin measurements will come out if performed... "instruction sets" as Mermin calls them). But this is wrong. You don't need that as an assumption to get to an inequality. (Bell often cites the EPR argument *from* locality *to* such hidden variables as "part one" of his two part argument, the second part of which is just the derivation of the inequality from the hv's. The point is, it's a two step argument, but the only actualy logical input/premise is locality. And anyway, as you know, you can get the CHHS inequality without ever assuming such deterministic hv's. It follows straight away from locality alone.)

You mention that maybe "realism" means the anti-MWI assumption that experimenters don't split into multiple copies. I don't think it's necessary to bring in anything like that as an additional assumption. What the Bell inequality constrains is the correlations between the *actual outcomes* of experiments done by Alice and Bob. To whatever extent you deny that those experiments have definite outcomes, you aren't finding an explanation *for those correlations*, but rather concocting a rationalization for rejecting (as delusions or subjective fantasies or whatever) what we thought were the real correlations. So it really doesn't make sense to make a fuss about this qua special extra assumption.

What else could it mean? Who knows. People who use it seem to slide back and forth between it meaning "deterministic hidden variables" and "there's an external physical reality", and too frequently they infer the falsity of the second from the various proofs of the impossibility of the former. Once you see that for the equivocation that it is, it's shameful and stupid. But the terminology "local realism" only helps to perpetuate the confusions that lead to this. So, I would recommend dropping it and being more precise. A good place to start would be reading Bell's numerous articles and paying attention to his careful choice of terminology!

I don't know if Bell ever noticed the loophole that locality can be preserved if you allow each experimenter to split into multiple copies--if he did, does anyone know which paper he addressed this in?

Loophole??! That's like saying there's a loophole in the measurement of the speed of light because maybe, although we *saw* that toothed wheel rotating and measured its rotation rate to be such and such, *really* we're brains in vats and that was just a delusion fed to us by evil scientists and really light moves at 3 miles per hour. Such a thing is *way* too "silly" and involves way too much *dismissing* of the empirical data in question, to be called a "loophole".

 

[JesseM]

You mention that maybe "realism" means the anti-MWI assumption that experimenters don't split into multiple copies. I don't think it's necessary to bring in anything like that as an additional assumption. What the Bell inequality constrains is the correlations between the *actual outcomes* of experiments done by Alice and Bob. To whatever extent you deny that those experiments have definite outcomes, you aren't finding an explanation *for those correlations*, but rather concocting a rationalization for rejecting (as delusions or subjective fantasies or whatever) what we thought were the real correlations. So it really doesn't make sense to make a fuss about this qua special extra assumption.

It seems to me that this is really a philosophical argument. I could reply that each copy of Alice and Bob *does* see a definite outcome, and likewise, once they have time to communicate, they also see a definite outcome for whatever copy they ending up sharing the same "world" with, so the correlations they see are perfectly "real" (if the universe had a different rule for 'mapping' copies of Alice to copies of Bob, the correlations seen by a typical copy could be different, so the correlations are a consequence of objective physical laws rather than something subjective). It's true that there are multiple versions of Alice and Bob in each experiment, but offhand I don't see why this is any more of an objection to the notion of "definite outcomes" than the fact that, in a spatially infinite universe, there is sure to be another region of space somewhere where there is are exact physical duplicates of Alice and Bob who share identical past light cones up until the moment of measurement, at which point they get different results than "our" Alice and Bob. You could say "yes, but that's not really the same experiment, it happened in a different region of space", but can't a many-worlds advocate say that the other "copies" of Alice and Bob are in a different region of Hilbert space? Of course, this is also a philosophical argument about what we mean by the words "real" and "definite outcome", but my point is that your denial that these words can be applied to the MWI is equally philosophical.

Loophole??! That's like saying there's a loophole in the measurement of the speed of light because maybe, although we *saw* that toothed wheel rotating and measured its rotation rate to be such and such, *really* we're brains in vats and that was just a delusion fed to us by evil scientists and really light moves at 3 miles per hour. Such a thing is *way* too "silly" and involves way too much *dismissing* of the empirical data in question, to be called a "loophole".

But the brain-in-the-vat scenario involves dismissing the possibility that our sensory experiences tell us anything about the laws of nature, or that they are genuine "empirical data". In the MWI interpretation your experience isn't a "delusion" in the same way, it's just limited to a subset of everything that is "really going on" (much the same is true of the Bohm interpretation, where you will never have access to the full information about all the hidden variables). Scientists in the MWI can discover the mathematical form of the laws of quantum mechanics through experiment, just as they can in single-universe interpretations.

And when you say the MWI involves dismissing empirical data, what is that data exactly? Do you think there's any empirical data we see that we would not expect to see if the MWI were in fact true? I suppose this question is a little ill-defined because of problems relating the MWI to our observed empirical probabilities, like the preferred basis problem...my point is just that if we have some general notion of physical systems splitting into multiple copies all the time in some lawlike way, there's no reason to expect such a universe to necessarily look any different 'from the inside' than a non-splitting universe with probabilistic laws. So, if some sort of splitting-universe solution can in principle explain the correlations in the Aspect/EPR experiments in a lawlike way (as opposed to a 'conspiratorial' way like the brain-in-a-vat scenario) without violating locality, there is no good reason for dismissing this as a possible "loophole", even if one's personal philosophical views incline one to consider it very implausible.

 

[vanesch]

The following quotes are probably a correct analysis (I accept your authority as a Bell connoisseur) that Bell (as many other people) had a serious emotional disliking for MWI. It is - as I often said - the main (and often only) objection people have against MWI. Bell was amongst them.

Perhaps one has to be a bit of a Bell connoisseur to appreciate this, but a lot of the sentences above (particularly those involving exclamation points) exude a kind of subtle dismissiveness that Bell almost certainly felt toward MWI. To him, it simply couldn't be taken seriously.

Indeed. The final sentences of the 6 worlds paper read: "In my opinion the pilot wave picture undoubtedly shows the best craftsmanship among the pictures we have considered. But is that a virtue in our time?"

The last sentence is somewhat obscure, but I'm pretty sure he is just bemoaning how few other sensible people (i.e., those "favorable to Bohm's approach") he found in physics.

Whereas relativity goes well with MWI? Please. Only if you gut it within an inch of its life.

Well, MWI goes as well with relativity as quantum theory in general. That means, with SR, things are more or less fine ; with GR, in as much as the background is fixed, it is fine, and in as much gravity must be considered as a quantum phenomenon, nobody knows.

But at least with SR, we can cope, and the proof is that EVERYTHING in the formalism of quantum theory remains Lorentz-invariant.

Anyway. Vanesch, I must say I miss the old days when we actually had interesting discussions. I guess nothing's changed and we've been there, done that, so it feels like a rehearsed performance with nothing to learn.

Well, that is because your contributions are only statements where you want to argue about the silliness of MWI - arguments which have been analyzed, and which nevertheless come back each time when you post about it.

But your latest reply above seems to give the impression that you think I haven't understood something about your arguments against my objection to MWI. But that's not true. I understand your position and arguments perfectly. It's just that I don't think you actually refute my argument. Instead, with practically everything you say (which is how this got started again, yes?) you admit exactly what I assert about MWI. So... I'm happy to agree to disagree, but don't be pretending that the reason I disagree is because I haven't yet grasped your cogent arguments, OK?

This cannot be, because you've never countered my argument which is the following:

Observations don't need to correspond to "reality". In fact, the concept of reality has not much to do with observations: observations are just that: observations. The role of a reality-hypothesis is to explain-describe-predict what kind of observations one can make (and to give a kind of "mechanism-behind-the-screens" that is responsible for us making observations).

So:
1) IF one has such a reality-hypothesis which can explain our observations, then that seems to be an acceptable reality-hypothesis (as it is its main objective and reason in the first place to make it)

2) With some thinking, it is not impossible to set up a reality-hypothesis which is not in a 1-1 relationship with what is observed. This is of course a bit harder than if one takes there to be a 1-1 relationship, but there is no deep logical or philosophical reason why reality can only be what is observed.

Now, your argument is exactly this:
that MWI suffers from the following defects:

1) although you recognize that MWI can explain all of our observations, you consider it not to be an acceptable reality-hypothesis because these observations are not "real" (or not all that is real or whatever).

2) You claim that, MWI saying that observations do not correspond precisely, in a 1-1 way, with the reality hypothesis in MWI, it has invalidated the very observations on which it was build - as if, from those observations, we were not allowed to deduce, with some thinking, such a hypothesis.

As such, this argument is not an argument which makes MWI (logically) self-contradictory, which you claim.

Because there is a difference between disliking a certain reality-view (such as MWI, or Bohmian mechanics or whatever), and claiming that it is self-contradictory. The argument you put forward does not show MWI to be self-contradictory. My argument against Bohmian mechanics (that it is not lorentz-invariant in its inner workings), doesn't render it self-contradictory either.

We both have different priorities, which lead us to different preferences. My preferences are clear: I want to accept everything, and I am ready to twist and turn my reality-hypothesis in such a way to arrive at the following:
have the most succinct set of general, universal principles, from which to deduce as much formal machinery as possible. I think that is the essence of Occam's razor (others interpret this differently). The reason for that is that I think that it is the most productive way of doing physics: have a limited number of principles (no matter how counter-intuitive), from which the machinery follows.

As such, because apart from the postulates of relativity, I don't have any principle from which to derive all the experimentally confirmed predictions of relativity, I do not want to let it go. I wouldn't mind trading it for another principle which gives you the same observed results, but I don't know of any.
So if we drop it, we would have to put in by hand different things in such a way as to make things come out as if a certain principle held. THIS is the kind of thing I want to avoid.
Another postulate I like a lot is the superposition principle, because starting from that postulate, almost all of the hilbert space machinery of quantum theory follows. Two postulates: superposition + relativity, and we can derive almost the entire formal machinery of modern physics! I find that very powerful, and I don't want to mess with this, as long as we don't have an equally powerful machinery in one way or another.
You only need a few other principles, and we fill in the structure entirely.


You, on the other hand, have a priority for "what we see is equal to the reality hypothesis". Now, that could be a good principle, only, you cannot derive anything formally from that principle. So for me, that is only very low on my list of desires, but you put it on top.
Putting that on top, you have to put in by hand almost all of modern physics. What you get for free is something like Newtonian mechanics, and now you have to fill in, by hand: the entire quantum-mechanical machinery of the wavefunction (that is not based upon any kind of principle: it is just put in by hand to make things come out equivalently with quantum mechanics), and you have to put in relativistic effects (Lorentz ether theory effects) in order to mimic relativity.

Maybe that is a good approach. It is certainly not an erroneous approach, but I consider it much less powerful. You cannot derive the entire Bohmian structure from a few principles. You have to plug in things by hand to make them come out that way. But, you have, that is to be recognized, a much clearer and simpler "reality view".

Hence we have two different preferences. You put "reality = observation" on the forefront, I put "small set of principles generate formalism" on the forefront. Each his liking.

Now, if I may add one comment: I think that Bohmians are such avid MWI-dislikers, because MWI has taken out a major argument they thought they had: they thought that there was a loophole-free argument to put relativity aside. They thought that it was shown, once and for all, that the main criticism of Bohmian mechanics (namely it not being Lorentz-invariant), was a dead argument, as Bell's theorem showed that quantum predictions were incompatible with relativity. Quantum predictions without Bohmian mechanics. If quantum mechanics ITSELF killed relativity, then, of course, the way was cleared for Bohmian mechanics, and its main criticism fell on the floor. Relativity was untenable if quantum theory was to be right. And hence, no more requirement for Lorentz invariance.

And then, we had these crazy MWI-ers which came up with a view on quantum mechanics which circumvented Bell's theorem, and kept quantum theory local ! Ok, they sacrificed a lot in the battle, that is true, BUT the water-tight argument that quantum mechanical predictions were in any case not compatible with relativity was now with a hole in, called MWI. It became an option, to reject relativity or not.

 

[vanesch]

I agree entirely with what JesseM writes here

MWI is *a* view on how *a formalism* works. That is, one had added a kind of "reality-hypothesis" to the formalism, and MWI does this in a minimalist way. Whether one finds this plausible or not, whether one finds it in agreement with one's philosophical convictions or not is a different matter. But one cannot deny that this view exists.

There is in fact not much use in attacking such a view. One can find it useful or less useful.

It seems to me that this is really a philosophical argument. I could reply that each copy of Alice and Bob *does* see a definite outcome, and likewise, once they have time to communicate, they also see a definite outcome for whatever copy they ending up sharing the same "world" with, so the correlations they see are perfectly "real" (if the universe had a different rule for 'mapping' copies of Alice to copies of Bob, the correlations seen by a typical copy could be different, so the correlations are a consequence of objective physical laws rather than something subjective). It's true that there are multiple versions of Alice and Bob in each experiment, but offhand I don't see why this is any more of an objection to the notion of "definite outcomes" than the fact that, in a spatially infinite universe, there is sure to be another region of space somewhere where there is are exact physical duplicates of Alice and Bob who share identical past light cones up until the moment of measurement, at which point they get different results than "our" Alice and Bob. You could say "yes, but that's not really the same experiment, it happened in a different region of space", but can't a many-worlds advocate say that the other "copies" of Alice and Bob are in a different region of Hilbert space? Of course, this is also a philosophical argument about what we mean by the words "real" and "definite outcome", but my point is that your denial that these words can be applied to the MWI is equally philosophical. But the brain-in-the-vat scenario involves dismissing the possibility that our sensory experiences tell us anything about the laws of nature, or that they are genuine "empirical data". In the MWI interpretation your experience isn't a "delusion" in the same way, it's just limited to a subset of everything that is "really going on" (much the same is true of the Bohm interpretation, where you will never have access to the full information about all the hidden variables). Scientists in the MWI can discover the mathematical form of the laws of quantum mechanics through experiment, just as they can in single-universe interpretations.

And when you say the MWI involves dismissing empirical data, what is that data exactly? Do you think there's any empirical data we see that we would not expect to see if the MWI were in fact true? I suppose this question is a little ill-defined because of problems relating the MWI to our observed empirical probabilities, like the preferred basis problem...my point is just that if we have some general notion of physical systems splitting into multiple copies all the time in some lawlike way, there's no reason to expect such a universe to necessarily look any different 'from the inside' than a non-splitting universe with probabilistic laws. So, if some sort of splitting-universe solution can in principle explain the correlations in the Aspect/EPR experiments in a lawlike way (as opposed to a 'conspiratorial' way like the brain-in-a-vat scenario) without violating locality, there is no good reason for dismissing this as a possible "loophole", even if one's personal philosophical views incline one to consider it very implausible.

 

[ttn]

The following quotes are probably a correct analysis (I accept your authority as a Bell connoisseur) that Bell (as many other people) had a serious emotional disliking for MWI. It is - as I often said - the main (and often only) objection people have against MWI. Bell was amongst them.

I think you misunderstood. Bell's (and my) disliking of MWI is not "emotional", or at least not fundamentally emotional. It's true, I have negative emotions about MWI -- but that's the *effect* of my thinking it is a bad theory by professional physics standards, not the other way round. I am quite certain the same is true for Bell. He thought that MWI couldn't be taken seriously as a scientific theory, not because he arbitrarily "hated" it, but because he thought carefully about what scientific theories are supposed to do. He hated it then because it failed to do those things and instead tried to play some pseudo-scientific mindgame.

Well, MWI goes as well with relativity as quantum theory in general. That means, with SR, things are more or less fine ; with GR, in as much as the background is fixed, it is fine, and in as much gravity must be considered as a quantum phenomenon, nobody knows.

But at least with SR, we can cope, and the proof is that EVERYTHING in the formalism of quantum theory remains Lorentz-invariant.

I believe there is more to SR than Lorentz invariance. For example, I can trivially write down a fully Lorentz invariant version of Bohmian mechanics, simply by putting in an "ether" (preferred frame) but then giving some made-up Lorentz invariant dynamical law which that ether is supposed to satisfy. It's exactly the same as having air for sound waves to propagate in: the fact that there is a "preferred frame" for sound waves (in which, e.g., they propagate isotropically) doesn't violate Lorentz invariance, because the preferred frame is made real by positing some *matter* (the air) which defines that frame, and that is fully consistent with Lorentz invariance so long as one can give some Lorentz invariant laws for that new matter. So, do the same thing for the ether -- instead of regarding the "preferred frame" as somehow fundamental to the structure of spacetime, you just say there's this new kind of matter which obeys Lorentz invariant laws, but there's some one frame in which the matter is "at rest".

You see that's possible, right? So would you therefore regard such a theory as fully consistent with SR? I think most people (including myself) wouldn't. It's cheating, somehow. And the "somehow" is what you need to flesh out to understand why "consistency with SR" and "Lorentz invariance" are not precisely the same thing.

But that wasn't even really the point I was making. As we've already covered, in MWI, there is no such thing as matter moving and interacting in 4-D spacetime. That exists only as a delusion in the minds of the "subjective conscious observers"
which also play such an important role in MWI. So it's very odd to say that the theory is fully special relativistic, when what SR is all about is the structure of that 4D spacetime. Or the same point a slightly different way: Lorentz invariance is *fundamentally* about the transformation properties of 4-D spacetime coordinates, i.e., the coordinates of events in 4D spacetime. But according to MWI there *are* no events in 4D spacetime.

So the point is that MWI is "consistent with SR" in about the same way it's "consistent with the data acquired in the Aspect experiment." In both cases, the theory provides a kind of fairy tale which helps you understand how you could come to be "validly deluded" into thinking the things in question... but that is not exactly the same as a theory *explaining* the things in question... at least not in the normal, historically practical sense of those terms.

But please take this as merely a recap of the same things I've been saying over and over again, which I know you don't accept. So don't feel the need to argue against them yet again, or accuse me of failing to understand your arguments.

Well, that is because your contributions are only statements where you want to argue about the silliness of MWI - arguments which have been analyzed, and which nevertheless come back each time when you post about it.

I of course would say the same thing in reverse: despite my having made quite clear why it's insane to take MWI seriously, you nevertheless come back to it each time I poke my head in this forum to see what's going on. (And no doubt you're doing so *between* those times as well, if you believe in such a thing as "what's happening when I'm not looking")...

Observations don't need to correspond to "reality". In fact, the concept of reality has not much to do with observations: observations are just that: observations. The role of a reality-hypothesis is to explain-describe-predict what kind of observations one can make (and to give a kind of "mechanism-behind-the-screens" that is responsible for us making observations).

You equate "observation" with "subjective inner-theater impression" or some such. I take observation to mean that something (read: some*thing*) was actually *observed*. If you see a pointer pointing left, you saw *a pointer pointing left*. (This is indeed a philosophical question, as JesseM has said. I'm in no way ashamed about that -- and anyways, it's not like *my* position on that philosophical question is any more philosophical than *your* position on that same philosophical question. So it's silly to think that you could argue against my position by labeling it "philosophical". It's just like the stupidity of people who say that Bohmian Mechanics is pointless/unscientific because it is "philosophical", which I guess is based on the idea that Bohm has the "preposterous, philosophical, unscientific" idea that there is actually an external physical world that it is the job of physics to describe... Anyway...)

This is exactly the difference between our views here, and it is a philosophical one. I think it is settled -- long before we get to any advanced topics like the sub-structure of atoms or how to interpret Schroedinger's equation -- that when we perceive we are actually perceiving a real physical world of physical objects external to our bodies. Those external physical objects (and/or the "matter" they're made of) are real. That is just completely *settled* and non-negotiable. The job of physics is then to understand in further, microscopic detail, what the properties of these entities/matter are, how they interact, what smaller pieces they are made of and how those interact, etc.

For you, as I understand it and by contrast, all of that stuff is left as completely negotiable. For you physics, isn't basically about explaining the hidden underlying structure of directly perceivable entities (or even directly perceivable attributes of those entities, such as their positions), but rather it is about explaining how we might come to have certain "subjective inner theater impressions". That is, you want physics to simultaneously address questions such as "why are atoms about an angstrom across?" and "is there really an external world that my experience is experience *of*?" As you know, I find this bizarre. But, for whatever it's worth, that's at least what our basic philosophical difference is.

Now, your argument is exactly this:
that MWI suffers from the following defects:

1) although you recognize that MWI can explain all of our observations, you consider it not to be an acceptable reality-hypothesis because these observations are not "real" (or not all that is real or whatever).

No, that's not right. I don't recognize that MWI can explain all of our observations. I think it fails to explain them. Instead of explaining them, it attempts to explain them *away* -- in the sense of telling a story about why I shouldn't have taken those apparent observations to be genuine observations. But as I explained above, this is because we disagree fundamentally about what "observation" means in physics. What I do recognize is that MWI can account for (I can't quite bring myself to use the word "explain" here) all of our (or, rather, my) "subjective experience". The problem is, so can solipsism or brain-in-vat theory. And they do it in essentially similar ways. And yet those latter are properly rejected by reasonable people as not being scientific, or at least as not able to be taken seriously in a scientific way. And that is also true of MWI, in my opinion, and for exactly the same reasons.

Because there is a difference between disliking a certain reality-view (such as MWI, or Bohmian mechanics or whatever), and claiming that it is self-contradictory.

That's not the relevant set of taxa here. Solipsism isn't self-contradictory. And "disliking" in a purely emotional sense has nothing to do with it. Professional judgments determine (or should determine) one's emotions, not vice versa. The relevant criteria are how well various theories live up to the professional standards for serious scientific theories. For example, evolution is a good scientific theory, creation science is not. The atomic theory of matter is good, solipsism is not. And the point isn't just that "good = true". There are all sorts of theories that turned out to be false, but which were nevertheless entirely reasonable as candidate theories for a time -- e.g., Ptolemy's epicycles theory is wrong but wasn't crazy or unscientific like "creation science" or solipsism or astrology.

But I fear there's no point getting into a discussion of what, exactly, these standards are, since we can't even agree on very fundamental philosophical points (which are hierarchically prior even to a discussion of proper standards for assessing candidate theories in science) such as whether the table I see in front of me is really there.

The argument you put forward does not show MWI to be self-contradictory. My argument against Bohmian mechanics (that it is not lorentz-invariant in its inner workings), doesn't render it self-contradictory either.

I agree. My argument wasn't attempting to show that MWI is self-contradictory. Rather, it's that any *case* for MWI (which is a scientific case in the sense of citing the results of various crucial experiments) will have to be self-refuting, since the theory the case is supposed to be a case for, says that those cited experiments never actually had the results we "subjectively experienced" them to have.

Now, if I may add one comment: I think that Bohmians are such avid MWI-dislikers, because MWI has taken out a major argument they thought they had: they thought that there was a loophole-free argument to put relativity aside. They thought that it was shown, once and for all, that the main criticism of Bohmian mechanics (namely it not being Lorentz-invariant), was a dead argument, as Bell's theorem showed that quantum predictions were incompatible with relativity. Quantum predictions without Bohmian mechanics. If quantum mechanics ITSELF killed relativity, then, of course, the way was cleared for Bohmian mechanics, and its main criticism fell on the floor. Relativity was untenable if quantum theory was to be right. And hence, no more requirement for Lorentz invariance.

This definitely has an element of truth, though the way you say it makes it sound like it's a deliberately dishonest thing -- like we (Bohmians) see that MWI is standing in the way of what would otherwise be a really good argument for Bohm, and so we viciously attack it because we don't want to face the truth that our argument is actually no good. You could put the same point more fairly if you stated it this way: Bohmians seem to understand Bell's theorem better than most (and btw Bell counts as a Bohmian here!). They recognize that there's no "hidden variables" assumption, no "realism" assumption, no "determinism" assumption, etc. If you grant that experiments actually have definite outcomes, and then if you require relativistic local causality, you get the inequality. And since we know, with more certainty than we know practically anything else in science, that experiments *do* have definite outcomes, it follows that we have to *give up* relativistic local causality. And that is nice, because it shows why the earlier-perceived barrier to accepting Bohm's version of QM, is actually no barrier at all. So it allows you to have all the *virtues* of Bohm's version (and anyone who has studied it honestly knows that there are *many virtues*), with no cost at all. But then these weirdo MWI people come along and say -- oh, but you can save relativistic local causality if you drop the requirement that experiments have definite outcomes (and drop also the whole relativistic worldview of physical events in 4D spacetime). And that's true, but come on!? Who could take that seriously?! It's about the equivalent of those annoying weirdos who come along and say "you don't have to believe that your grandparents were monkeys, if you just accept that jesus christ is your savior and he created the world 6,279.8 years ago on a tuesday." It's of course true in some sense that you can avoid the relevant conclusion you don't like if you accept that nonsense, but who could possibly accept that and think that what they were doing was *science*?

Well, OK, maybe I slid over into slanting it just ever so slightly the other way there...

 

[Thrice]

You guys know any good resources on this? Wouldn't mind contributing, but I realize I don't know this anywhere as well as you do.

 

[vanesch]

He thought that MWI couldn't be taken seriously as a scientific theory, not because he arbitrarily "hated" it, but because he thought carefully about what scientific theories are supposed to do. He hated it then because it failed to do those things and instead tried to play some pseudo-scientific mindgame.

However, for coming to that conclusion, he ADDED extra requirements on what a scientific theory is supposed to be like (and had to discard others).

The one, only and single non-disputable criterion for a scientific theory is:
it should correctly predict observations.

All the rest is open to matters of personal taste up to a point, and the case can be made for each of them. But, no theory can claim to be a scientific theory if it doesn't correctly predict observations. This is necessary.

The requirement, however, that elements of observation should be elements of ontology in a specific way, is a totally arbitrary extra requirement, but the fundamental requirement which made Bell (and Bohmians) conclude about their distaste for MWI. There is NO ABSOLUTE REQUIREMENT for observations to be corresponding in a simple and evident way with an element of ontology. Now, of course, it would render things simple. So I can very well understand a DESIRE for this requirement, but it is nothing more than this: a desire.

In the same way, absolute simultaneity could be an extra requirement of the same kind, it would be nice to have it, but there is no reason to take this as an absolute sine-qua-non requirement for a scientific theory.

There is a requirement that *I* put forward as an extra requirement, and that is: one should aim for a MINIMUM of universal principles from which a maximum of formalism can be derived. Now, again, this is not an absolute requirement, however, it is a very pragmatical one. Indeed, without such a requirement, there is no limit upon the possible alternatives, with as an extreme, just a list of all events in the universe, without any "law". So amongst all possible variations on scientific theories which are empirically equivalent, I think this requirement (which is not absolute) is a good guide. It is essentially Occam's rasor.

I believe there is more to SR than Lorentz invariance. For example, I can trivially write down a fully Lorentz invariant version of Bohmian mechanics, simply by putting in an "ether" (preferred frame) but then giving some made-up Lorentz invariant dynamical law which that ether is supposed to satisfy.

Indeed, as you outline, if you introduce some physical stuff, the ether, as living on spacetime, you could formulate the theory in a lorentz-invariant way, or in the SR way (which, to me, is the same: SR says that all physical entities should be functions over the spacetime manifold, and that means that their coordinate representations should satisfy Lorentz-invariant mappings. Lorentz transformations are nothing else but the transition maps between orthogonal coordinates on the spacetime manifold).

It's exactly the same as having air for sound waves to propagate in: the fact that there is a "preferred frame" for sound waves (in which, e.g., they propagate isotropically) doesn't violate Lorentz invariance, because the preferred frame is made real by positing some *matter* (the air) which defines that frame, and that is fully consistent with Lorentz invariance so long as one can give some Lorentz invariant laws for that new matter.

The problem is that Lorentz-invariance should then require that when the air is set in motion, things change. You hence have to introduce a genuine beable which is that "ether". But nothing observable has ever been related to that "ether". In order for that beable to make sense (and to use your OWN requirements of having observations to correspond to reality), there should be something physically observable to that ether. The air is observable. We can put it in motion, and study the sound waves in moving air.

But the ether is just an invention to make the theory fit a required symmetry which it doesn't have. You are inventing beables in order to simulate effects which would otherwise be derivable from a simple principle. One should propose experiments which demonstrate the physical reality of that "ether", and you should also demonstrate how it comes that other stuff interacts in such a way with the ether, as to make everything appear as if it didn't, but underwent some specific transformation in such a way as NOT to respect Lorentz invariance in its inner workings, but nevertheless in observable phenomena.

Because this is the problem with introducing an ether. If it is physically present (such as the air in your example), then there should be (unless very strange conspiracy) observable effects which are not Lorentz invariant without that ether. In other words, one should be able to MEASURE that specific reference frame in which the ether resides. In the same way as sound experiments can give you the rest frame of the air.

So, do the same thing for the ether -- instead of regarding the "preferred frame" as somehow fundamental to the structure of spacetime, you just say there's this new kind of matter which obeys Lorentz invariant laws, but there's some one frame in which the matter is "at rest".

Yes, yes. See, you are doing worse than MWI-ers. You are inventing totally unobserved physical things of a totally different kind, with totally unknown interactions in a conspirational way (the ether) in order for your stuff to comply to some principle you've actually killed, but which you need in order to be able to require compatibility with phenomena which ARE observed (SR effects).

You see that's possible, right? So would you therefore regard such a theory as fully consistent with SR? I think most people (including myself) wouldn't. It's cheating, somehow. And the "somehow" is what you need to flesh out to understand why "consistency with SR" and "Lorentz invariance" are not precisely the same thing.

Consistency with SR means that the ontological elements of the theory should be defined over spacetime. Strictly speaking, making the hypothesis of an ether, as some physical stuff, which fills spacetime, is consistent with SR. The above proposal of an ether is not a violation of SR, it is a violation of Occam's rasor. One needs to introduce a lot of extra postulates, of which the only aim is, to restore some broken symmetry.

But that wasn't even really the point I was making. As we've already covered, in MWI, there is no such thing as matter moving and interacting in 4-D spacetime. That exists only as a delusion in the minds of the "subjective conscious observers"
which also play such an important role in MWI. So it's very odd to say that the theory is fully special relativistic, when what SR is all about is the structure of that 4D spacetime. Or the same point a slightly different way: Lorentz invariance is *fundamentally* about the transformation properties of 4-D spacetime coordinates, i.e., the coordinates of events in 4D spacetime. But according to MWI there *are* no events in 4D spacetime.

Well, this is a misunderstanding. In MWI, there is a 4D spacetime all right. However, it is not the place where MATTER lives. It is the place where the operators live, which determine the interactions (which determine the unitary evolution). The Schroedinger picture makes this less evident, but in the Heisenberg picture, this is quite clear.
On one hand, you have Hilbert space, and on the other hand, you have spacetime over which operators over that Hilbert space live. All operators over hilbert space are indexed over spacetime, in such a way, that they transform under a (projective) representation of the Lorentz group.
So the spacetime manifold is there all right. Only, we thought that we had maps from R into that spacetime (which we call particles), or that we had tensor fields over spacetime (classical field theory), and it turns out that we have field operators over spacetime. In other words, spacetime exists as the basis space for a vector bundle in which the fibres are sets of operators over Hilbert space. So the structure is more complicated than originally anticipated, but spacetime is there all right.

Now, in specific cases, all this machinery looks a lot like a set of mappings from R into spacetime (in which case we restore classical relativitic particle dynamics) ; and in some cases it looks like tensor fields over spacetime (classical relativistic fields). But the structure is much richer.

What makes this "relativistically correct", is that the basis space is still 4D spacetime.

The funny thing is that people don't invent this *just for the sake of saving relativity with no observational consequences*. On the contrary, this kind of structure is full of predictions which can be verified. Gauge invariance is closely related to the above vision, and has also been a powerful principle from which a lot of OBSERVATIONALLY CORRECT STUFF has been derived.

In other words, the unitary part of quantum theory, as it shouldn't surprise anyone, is ENTIRELY relativistically compatible.

I of course would say the same thing in reverse: despite my having made quite clear why it's insane to take MWI seriously, you nevertheless come back to it each time I poke my head in this forum to see what's going on.

You classify as "insane", incompatible with some extra, and to a point arbitrary, extra requirements you've put upon physical theories. This is exactly what I call "emotional".

If I say that Bohmian mechanics is not, in its spirit, compatible with SR, and that that is one of the reasons why I do not prefer it, despite quite some merits, this is not an emotional statement. If I would say that Bohmians study some insane idea, because it is not compatible with my preference to keep relativity, I'm being emotional.

You dislike MWI for its lack of 1-1 relationship between ontology and observations. That's your good right. But qualifying MWI as "insane" because it is not compatible with this extra requirement of yours, is an emotional statement.

You equate "observation" with "subjective inner-theater impression" or some such.

It will be hard to convince me of anything else ! Can YOU tell me what is an observation, totally independent of any subjective inner-theatre impression ?

I take observation to mean that something (read: some*thing*) was actually *observed*. If you see a pointer pointing left, you saw *a pointer pointing left*. (This is indeed a philosophical question, as JesseM has said. I'm in no way ashamed about that -- and anyways, it's not like *my* position on that philosophical question is any more philosophical than *your* position on that same philosophical question. So it's silly to think that you could argue against my position by labeling it "philosophical". It's just like the stupidity of people who say that Bohmian Mechanics is pointless/unscientific because it is "philosophical", which I guess is based on the idea that Bohm has the "preposterous, philosophical, unscientific" idea that there is actually an external physical world that it is the job of physics to describe... Anyway...)

It is a philosophical act to make an ontology-hypothesis.
Choosing to equate observation with ontology is ONE possible answer one can prefer. It is called "naive realism". Refusing to make an ontology-hypothesis is called solipsism. Doing something else, is, well, making yet another ontology-hypothesis.

No answer to this question is "more" or "less" philosophical than another.

This is exactly the difference between our views here, and it is a philosophical one. I think it is settled -- long before we get to any advanced topics like the sub-structure of atoms or how to interpret Schroedinger's equation -- that when we perceive we are actually perceiving a real physical world of physical objects external to our bodies. Those external physical objects (and/or the "matter" they're made of) are real. That is just completely *settled* and non-negotiable.

Saying that is adhering strongly to a specific philosophical vision, which is called "naive realism". In as much as philosophy goes, nothing is settled.

It is the refusal to re-consider that position, which is, I repeat, an entirely philosophical position, which can lead one astray.

My position is that philosophical questions should be answered in such a way as to interfere minimally with what we have formally, and which allow us to build theories on a small set of principles. I think also that "naive realism" is a nice thing to have, if we can, but if another answer to the question seems more appropriate in order to set up a theoretical framework, then so be it.

The job of physics is then to understand in further, microscopic detail, what the properties of these entities/matter are, how they interact, what smaller pieces they are made of and how those interact, etc.

You see, you are IMPOSING a philosophical viewpoint upon the workings of a scientific theory.

For you, as I understand it and by contrast, all of that stuff is left as completely negotiable. For you physics, isn't basically about explaining the hidden underlying structure of directly perceivable entities (or even directly perceivable attributes of those entities, such as their positions), but rather it is about explaining how we might come to have certain "subjective inner theater impressions". That is, you want physics to simultaneously address questions such as "why are atoms about an angstrom across?" and "is there really an external world that my experience is experience *of*?" As you know, I find this bizarre. But, for whatever it's worth, that's at least what our basic philosophical difference is.

Yes, that is exactly true. I base this onto two points:
1) the only thing we really know, observe etc... are our "inner theatre impressions". All the rest is hypothesis.
2) we should minimise the number of principles, on which to derive a maximum of powerful formal machinery in order to allow one to predict what are going to be these inner theatre impressions.

Now, ONE possible solution could be that we make the hypothesis that there is a 1-1 relationship between our impressions, and actual ontology. In other words, that we take, as a working hypothesis, "naive realism". It is amazing up to what point this works ! It is amazing up to what point one can actually make simply 1-1 hypotheses, and use this as quite a consistent way of organizing one's sensations. In other words, it is quite amazing how we can simply assume that there is an object such as a chair, and that our sensory impressions (auditive, visual, sensory...) are compatible with that one single hypothesis. So this seems indeed a good idea. Classical mechanics is for a large part based upon that.
However, certain observations have shown the limit of that working hypothesis. As such, I have no difficulty taking on another working hypothesis. After all, it was almost too nice to be true, that there were simple things which were actually there, and which could simply explain these impressions!

No, that's not right. I don't recognize that MWI can explain all of our observations. I think it fails to explain them. Instead of explaining them, it attempts to explain them *away* -- in the sense of telling a story about why I shouldn't have taken those apparent observations to be genuine observations.

There are no "apparent observations". Observations (inner theatre experiences) are just as "real" as anything. Hell, it is the only thing we know there to be! But observations are a RELATIONSHIP between an inner theatre, and some hypothetical real world. That relationship is real, but the hypothetical real world ITSELF doesn't need to be equal to the relationship.

But as I explained above, this is because we disagree fundamentally about what "observation" means in physics. What I do recognize is that MWI can account for (I can't quite bring myself to use the word "explain" here) all of our (or, rather, my) "subjective experience". The problem is, so can solipsism or brain-in-vat theory.

No. Solipsism has no predictive power, nor does "brain-in-vat" theory, if you don't know the intentions of the evil scientist. It is the only reason, btw, not to adhere to them: the hypothesis doesn't bring in any organizational power of our sensations. MWI does. In MWI, you get just the same predictive power as in "standard" quantum theory. It helps one make predictions.

And they do it in essentially similar ways. And yet those latter are properly rejected by reasonable people as not being scientific, or at least as not able to be taken seriously in a scientific way.

I wouldn't say that solipsism is rejected on a scientific basis ! Only, it doesn't bring in anything useful. If it would, one should consider it. But solipsism essentially says: "subjective experiences happen", period. It is not the basis of any *prediction* of those subjective experiences, and it is on that account that it isn't of any use for scientists.

The relevant criteria are how well various theories live up to the professional standards for serious scientific theories. For example, evolution is a good scientific theory, creation science is not.

Yes, but the only reason for that is that creation science cannot explain certain observations.

The atomic theory of matter is good, solipsism is not.

Again, because from solipsism, you cannot derive many predictions.

But I fear there's no point getting into a discussion of what, exactly, these standards are, since we can't even agree on very fundamental philosophical points (which are hierarchically prior even to a discussion of proper standards for assessing candidate theories in science) such as whether the table I see in front of me is really there.

Well, because you already take on philosophical answers as non-negotiable even before the question is examined.

 

[vanesch]

continued...

I agree. My argument wasn't attempting to show that MWI is self-contradictory. Rather, it's that any *case* for MWI (which is a scientific case in the sense of citing the results of various crucial experiments) will have to be self-refuting, since the theory the case is supposed to be a case for, says that those cited experiments never actually had the results we "subjectively experienced" them to have.

Again, again. MWI is not self-refuting, because it doesn't claim that cited subjective experienced results weren't subjectively experienced. There is no need for the results to "ontologically be there" in order for a theory to say how results can be subjectively experienced.

The only need there is, is that subjectively experienced results (which are seen as a relationship between a subjective internal theatre and another, objective world) must FOLLOW AS PREDICTIONS for subjectively experienced results. You couldn't, indeed, start from observations, build a theory around that, and then come back with predictions that would be different, concerning those (subjective) observations, than the ones one started with.

Observations being the result of a relationship between a "subjective theatre" and "a postulated reality", there is no need for observations to correspond to that postulated reality ON ITS OWN.
We only need to build a postulated reality, AND a relationship between a subjective theatre and that postulated reality, so that the relationship comes out all right. It is of course a handicap to only possesses one side of that relationship, but with sufficient thinking, one can come to such an overall scheme. A guiding principle in doing so, is to try to limit the number of postulates to a minimum.

This definitely has an element of truth, though the way you say it makes it sound like it's a deliberately dishonest thing -- like we (Bohmians) see that MWI is standing in the way of what would otherwise be a really good argument for Bohm, and so we viciously attack it because we don't want to face the truth that our argument is actually no good.

Yes.

If you grant that experiments actually have definite outcomes,

Bzzt. Input of a philosophical requirement, which is often labeled "the realism" requirement. Very sensible, indeed. But nevertheless, an extra requirement, namely of "naive realism".

And since we know, with more certainty than we know practically anything else in science, that experiments *do* have definite outcomes,

We don't know that AT ALL, and we could never scientifically prove this, given that it is a philosophical position. If it were scientific, it could be falsified. You can never falsify even solipsism in a scientific way.

And that's true, but come on!? Who could take that seriously?! It's about the equivalent of those annoying weirdos who come along and say "you don't have to believe that your grandparents were monkeys, if you just accept that jesus christ is your savior and he created the world 6,279.8 years ago on a tuesday." It's of course true in some sense that you can avoid the relevant conclusion you don't like if you accept that nonsense, but who could possibly accept that and think that what they were doing was *science*?

The only way for preferring the non-creationist viewpoint is that it can explain more, with less input. If there wouldn't have been the fossile record, and there wouldn't have been other illustrations of evolution, but if on the other hand, we regularly saw appearances of the angel Gabriel and so on, then it would be scientifically more sound to accept the creationist viewpoint. Because it would THEN have a higher degree of prediction than that silly theory of evolution for which no single observational element was ever presented.

However, the creationist viewpoint is not entirely insane a priori. There could have been a creation 6000 years ago. Only, they need to put in A LOT of extra hypotheses, and each new dinosaur bone that is dug up, needs a new hypothesis on their side to explain it.
In other words, to keep to their cherished philosophical/religious viewpoint, they introduce *a lot of extra hypotheses* which seem moreover to be quite conspirational (like, God put those dinosaur bones in the ground, so as to "make us think that evolution was a principle, and hence to test our faith that it isn't, even though it seems to explain observations").

As such, for people, desperately wanting to adhere to a religious/philosophical viewpoint, creationism is a good viewpoint. It gives them ease of mind. Their preferred philosophy is saved. They have sacrificed about all predictional value of their theory in order to do so, and have to put in a lot of observational results by hand, but at least, their view on how the world "should be" is saved...

And they feverishly ATTACK a competing viewpoint, which discards their favorite philosophical viewpoint, but has, with much less input of principles, a much higher yield of predictive value: evolution.

Mmmm...

 

[MaverickMenzies]

I have another (probably pointless) question concerning MWI.

In the case of a pure entangled state between our quantum system and observer, MWI provides an interpretation for

|spin-up> |a> + |spin-down>|b>,

in that each term of the superposition should be treated as a separate copy of the universe. How does this interpretation deal with mixed entangled states? For example, suppose we include an environment in our system and then unitary evolutions give

|spin-up>|a>|e1> + |spin-down>|b>|e2> ,

and if i trace out the environment then i am left with:

rho = tr(|e1><e1|) |spin-up>|a><spin-up|<a| + tr(|e2><e1|) |spin-down>|b><spin-up|<a| + tr(|e1><e2|) |spin-up>|a><spin-down|<b| + tr(|e2><e2|) |spin-down>|b><spin-down|<b|

This is the state shared between the system and the observer. How do we interpret this state within MWI?

 

[vanesch]

I have another (probably pointless) question concerning MWI.

In the case of a pure entangled state between our quantum system and observer, MWI provides an interpretation for

|spin-up> |a> + |spin-down>|b>,

in that each term of the superposition should be treated as a separate copy of the universe. How does this interpretation deal with mixed entangled states? For example, suppose we include an environment in our system and then unitary evolutions give

|spin-up>|a>|e1> + |spin-down>|b>|e2> ,

and if i trace out the environment then i am left with:

rho = tr(|e1><e1|) |spin-up>|a><spin-up|<a| + tr(|e2><e1|) |spin-down>|b><spin-up|<a| + tr(|e1><e2|) |spin-up>|a><spin-down|<b| + tr(|e2><e2|) |spin-down>|b><spin-down|<b|

This is the state shared between the system and the observer. How do we interpret this state within MWI?

There are two answers to this.

The first is, that there will always be a split, which is binary:
"observer body" versus "rest of the universe".

So if the state is:
|spin-up>|a>|e1> + |spin-down>|b>|e2> ,

and the observer body is described by |a> and |b> then we have that the split between the observer body and the rest of the universe takes on the form:

|a> (x) |spin-up>|e1> + |b> (x) |spin-down>|e2> ,

(where I put (x) to emphasize the tensor product split between "observer" and "rest-of-universe").

So to the observer body degree of freedom correspond two distinct terms, |a> and |b> which are supposed to be corresponding to two different "subjective experiences" which emerge from these states.

The "rest of the universe" is now the union of the spin system and the environment. But state |a> will be compatible with both an environment e1, and a spin up state, while state |b> will be compatible with both an environment e2 and a spin down state.

Now, the nice thing about decoherence is that in this kind of decomposition, the "classical body states", together with the "classical environment states", together with the "classical pointer states" all come together in different terms. (ok, there is a caveat to this: we shouldn't think of basis states here, but as subspaces in Hilbert space, sufficiently compatible with classical, coherent states: there can still be wild oscillations within these subspaces).

So it is a bit as with the definition of inertial frames in Newtonian physics: although the definition is, strictly speaking, somewhat circular (inertial frames are rest frames of particles on which no forces act, and particles on which no forces act are particles which move in a uniform way in inertial frames...). Classical body states are states which appear in decohered systems of the above kind, and states are decohered when the different contributions are factors of classical states... But FAPP, there is no ambiguity of what is a classical pointer state, or a classical body state.

The statistical ensemble of "subjective observer states" is then given by the different classical bodystates that occur in the decomposition, with probability measure given by the hilbert norm squared of the term in which it occurs.

The second answer is this:

The use of the partial trace already pre-supposes the application of the Born rule, in other words, when doing this, we already accept the passage to a statistical mixture *as observed* by one of its components.

So we are already talking about the statistical ensemble of "subjective observer states" in a way.

Now, for any two environment states |e1> and |e2> which respect themselves, we have that < e1 | e2 > is essentially 0. From this follows that the reduced density matrix of "observer+spin system" is given by:

rho = tr(|e1><e1|) |spin-up>|a><spin-up|<a| + tr(|e2><e1|) |spin-down>|b><spin-up|<a| + tr(|e1><e2|) |spin-up>|a><spin-down|<b| + tr(|e2><e2|) |spin-down>|b><spin-down|<b|

= tr(|e1><e1|) |spin-up>|a><spin-up|<a|
+ tr(|e2><e2|) |spin-down>|b><spin-down|<b|

= |spin-up>|a><spin-up|<a|
+ |spin-down>|b><spin-down|<b|

In a "basis of classical body states" (here, |a> and |b>), this is a diagonal density matrix (it is always the case, when tracing out the environment, under the assumption of full decoherence).

It comes down to having a statistical ensemble of "subjective experience" (in 1-1 relation with classical body states), in which we have:

50% of the |spin-up>|a> state, and
50% of the |spin-down>|b> state.

In other words, this generates (without surprise) the same statistical ensemble of "subjective observer states" as in the previous approach.

 

[MaverickMenzies]

Interesting. Thank you for your answer.

 

[ttn]

Another question in the same spirit as the above... Suppose the state is

|1>|A1>|B0> + |2>|A2>|B0>

where, in order, the three factors refer to degrees of freedom pertaining to

* some physical system (like maybe the spin of a particle being up=1 and down=2 or whatever)

* the state of one observer ("Alice", with A1 = Alice's body/brain is in the state where she observes spin up, etc.)

* the state of some other observer who isn't observing or paying attention or even nearby (Bob). Note that the Bob part of the state is the same in the two superposed terms.

So here's the question: vanesch always talks about the "counting of terms" as being based on orthogonal "body states" of the observer. But which observer? If the state of the universe is the state above, is there just one universe, or two? Or is it "two for Alice" and "one for Bob"? Or what?

PS -- this is the kind of thing Bell had in mind when he noted that MWI is "extravagantly vague". It moves all of the physics into a very weird place (some boundary between "subjective mental impressions" and a "physical world" that is nothing like the physical world we know) and is *thereby* extremely difficult to actually formulate rigorously.

 

[vanesch]

Or is it "two for Alice" and "one for Bob"?

You've got it. Two for Alice, one for Bob.

Worlds are an observer-dependent concept in MWI.

This is exactly how MWI can weasel out of Bell's theorem.

However, in order for one to maintain such a state, there can be no interaction between "alice" and "bob", and the only way to make one sure about this, is to have them at spacelike separations. Sooner or later, via a common environment, they will entangle, and then both sets of worlds (those from Alice PoV and those from Bob PoV) will coincide.

 

[ttn]

You've got it. Two for Alice, one for Bob.

Worlds are an observer-dependent concept in MWI.

This is exactly how MWI can weasel out of Bell's theorem.

Yes, good, I understand that, but it's good to get something out in the open so clearly for once!

However, in order for one to maintain such a state, there can be no interaction between "alice" and "bob", and the only way to make one sure about this, is to have them at spacelike separations. Sooner or later, via a common environment, they will entangle, and then both sets of worlds (those from Alice PoV and those from Bob PoV) will coincide.

Right.

I've basically made the point I want to make, which is that you have to take all of the "different worlds" and "splitting" and all that (which *sounds* like it's supposed to be what the *dynamics* of MWI is fundamentally *about*) with a giant grain of salt. All of that is really just talk that can't be taken seriously/physically. And then it's not so clear what is left that *can* be taken seriously/physically. It shows nicely how subjective/internal/solipsist this gets. Incidentally, doesn't what you said above also raise some questions about Lorentz invariance? Not any real ones now that we know all the talk about "splitting worlds" and such is just empty talk and not something that is to be taken seriously by the theory. But anyway, if the physical effects that get Alice and Bob entangled (later) propagate at some slow speed (like the speed of sound, which is probably about right for many such situations) then what some MWI people might (erroneously) think of as "facts" that the theory says something about (like how many worlds there are at some time) aren't really facts at all, because they aren't Lorentz invariant. So, another nice illustration of why you can't take any of this seriously -- to do so is to spoil the (alleged) consistency of MWI with relativity.

Anyway, I didn't really want to discuss this. I just thought it would be a nice way to bring something important out into the open, which was a minor tweak of the other question that got asked. I'll hopefully find some time later to respond to your other post on the more philosophical stuff.

 

[mgelfan]

Interesting discussion ... sort of.

MWI is a most unattractive way of, er, looking at things. It certainly doesn't explain any of the stuff that it purports to explain. It seems to be saying that since the quantum processes involved in the generation of experimental observations (data) are pretty much unknown (ie., there's a measurement problem and an accompanying wave function interpretation problem), then let's just interpret the superposition of all possible outcomes as meaning that all possible outcomes actually happened (even though that interpretation doesn't actually mean anything). MWI also supposedly solves the locality/nonlocality problem.

Now, the measurement problem really is a problem in that physicists really don't know much about measurement (or emission, for that matter) processes at the quantum level. But I think that we can agree that the MWI solution doesn't provide any new knowledge of such processes.

The locality/nonlocality problem hasn't yet attained the status of being a real problem (everything seems to behave as if locality were the reigning standard ... and no superluminal anything has been produced for our consideration), so maybe its solution by way of MWI is premature.

ttn's idea (and demonstrations) that MWI is silly on several different fronts is more appealing than vanesch's adherence to MWI -- which adherence doesn't promise to help his (vanesch's ... or anybody else's for that matter ) understanding of physics or of the world. Of course, if MWI could be used to improve our knowledge and facility regarding, say, high T_c superconducting, or dark energy, etc., then I'll have to reconsider.

 

[ttn]

The one, only and single non-disputable criterion for a scientific theory is: it should correctly predict observations.

Sure, but that doesn't really help given our philosophical disagreement over the meaning of "observations". You say MWI does correctly predict our observations, and I deny this. So it doesn't really help move the discussion forward to just assert that what I mean by "observation" adds something to what "observation" really means. It's already clear that we disagree, and not clear (to anyone but you) that you are right.

The requirement, however, that elements of observation should be elements of ontology in a specific way, is a totally arbitrary extra requirement, but the fundamental requirement which made Bell (and Bohmians) conclude about their distaste for MWI. There is NO ABSOLUTE REQUIREMENT for observations to be corresponding in a simple and evident way with an element of ontology.

It's a "totally arbitrary extra requirement" to think that there are such things as pointers and that they point? Come on. That's the kind of talk that wins you friends among other freshman in a philosophy 101 class maybe, but it isn't the kind of talk that serious physicists take seriously. We know there are pointers, and that they point... and we're now onto working on more interesting questions.

Indeed, as you outline, if you introduce some physical stuff, the ether, as living on spacetime, you could formulate the theory in a lorentz-invariant way, or in the SR way (which, to me, is the same: SR says that all physical entities should be functions over the spacetime manifold, ...

So, the wf in MWI is not "physical"?

The problem is that Lorentz-invariance should then require that when the air is set in motion, things change. You hence have to introduce a genuine beable which is that "ether". But nothing observable has ever been related to that "ether". In order for that beable to make sense (and to use your OWN requirements of having observations to correspond to reality), there should be something physically observable to that ether. The air is observable. We can put it in motion, and study the sound waves in moving air.

Your parenthetical comment shows that you completely misunderstand what I think about "observation". It's not that reality = observation (meaning, whatever you observe is real *and* everything that's real is directly observable). I take the word "observation" quite literally. To observe something is to *see* it, literally. I'm observing a table right now. And now my cat. I've never directly observed an atom. Nor an ether. Some things (like tables and cats) can be known to exist because we observe them. Other things we know exist only through a chain of inference. So don't think that just because some hypothetical entity (like say an ether, an atom, dark matter, whatever) is unobservable, therefore my philosophy requires me to disbelieve its existence.

But the ether is just an invention to make the theory fit a required symmetry which it doesn't have.

I agree. Don't forget I said from the beginning that I didn't think this scheme was really a way to "relativize" Bohm's theory. It's obviously cheating, in roughly the way you describe. We agree there. I only raised it to point out that there is more to "consistency with relativity" than mere Lorentz invariance of the defining equations.

Consistency with SR means that the ontological elements of the theory should be defined over spacetime.

So, pray tell, which are the ontological elements (defined over spacetime) in MWI?

Strictly speaking, making the hypothesis of an ether, as some physical stuff, which fills spacetime, is consistent with SR.

That's a bizarre statement since basically the whole point of SR was to try to account for certain things *without* positing an ether (which bothered Einstein because it was arbitrary and unobservable). So you're saying (for example) the Lorentz ether theory is consistent with SR?

The above proposal of an ether is not a violation of SR, it is a violation of Occam's rasor. One needs to introduce a lot of extra postulates, of which the only aim is, to restore some broken symmetry.

I agree that that's a serious problem for the "relativistic Bohm theory" I proposed. It'd be more honest to just posit the ether and say "this conflicts with relativity" and let the chips fall where they may.

Well, this is a misunderstanding. In MWI, there is a 4D spacetime all right. However, it is not the place where MATTER lives. It is the place where the operators live, which determine the interactions (which determine the unitary evolution).

But, as you say, the operators are a human mathematical tool, right? Nobody thinks they somehow are (or correspond to or describe) anything *physical*, any *matter*. So *is* there any matter in MWI? Is there anything else, anything physically real, that lives in 4D spacetime?

And if not (and given your comments above) how can you say MWI is consistent with SR?

On one hand, you have Hilbert space, and on the other hand, you have spacetime over which operators over that Hilbert space live. All operators over hilbert space are indexed over spacetime, in such a way, that they transform under a (projective) representation of the Lorentz group.
So the spacetime manifold is there all right. Only, we thought that we had maps from R into that spacetime (which we call particles), or that we had tensor fields over spacetime (classical field theory), and it turns out that we have field operators over spacetime. In other words, spacetime exists as the basis space for a vector bundle in which the fibres are sets of operators over Hilbert space. So the structure is more complicated than originally anticipated, but spacetime is there all right.

Yes, the 4D spacetime is "there" in the theory, no doubt. But the question is: what, if anything, is "there" in the 4D spacetime? Nothing, so far as I can tell. But that's hardly consistent with our precious observations. (or SR)

Now, in specific cases, all this machinery looks a lot like a set of mappings from R into spacetime (in which case we restore classical relativitic particle dynamics) ; and in some cases it looks like tensor fields over spacetime (classical relativistic fields). But the structure is much richer.

What makes this "relativistically correct", is that the basis space is still 4D spacetime.

Not what you said before!

You classify as "insane", incompatible with some extra, and to a point arbitrary, extra requirements you've put upon physical theories. This is exactly what I call "emotional".

But what you call an arbitrary extra requirement is simply that physical theories should be able to account for our direct observation of physical objects! To deny that *is* insane (he says emotionlessly).

You dislike MWI for its lack of 1-1 relationship between ontology and observations.

No, you misunderstand, as I explained above.

It is a philosophical act to make an ontology-hypothesis.

Wow, you just defined most of physics out of existence. So when Copernicus said that the Earth really goes around the sun, he was doing philosophy, not physics. When Maxwell and Boltzmann argued that there are unobserved little particles (atoms), they were doing philosophy, not physics. When Rutherford inferred from his scattering data that these atoms have a nuclear structure, he was engaging in a "philosophical act", not doing physics. And so on and so on.

Choosing to equate observation with ontology is ONE possible answer one can prefer. It is called "naive realism". Refusing to make an ontology-hypothesis is called solipsism. Doing something else, is, well, making yet another ontology-hypothesis.

I'm not a naive realist.

You see, you are IMPOSING a philosophical viewpoint upon the workings of a scientific theory.

True, I am. I'm considering it settled (by philosophy, prior to any discussions of physics) that there are cats and tables and pointers, that the things we directly observe are actually there.

What's ironic is that you are also "imposing a philosophical viewpoint" -- it's just that you (wrongly) think your philosophical viewpoint is "obviously right" or "scientific" or some such, so all you see is everybody else's philosophical viewpoint. It's like: "everybody but me speaks with an accent!"

Yes, that is exactly true. I base this onto two points:
1) the only thing we really know, observe etc... are our "inner theatre impressions". All the rest is hypothesis.

I completely disagree with that.

There are no "apparent observations". Observations (inner theatre experiences) are just as "real" as anything.

I meant by that to distinguish between your understanding of "observations" and mine. "Apparent observations" are what *I* call what you call "observations" in MWI -- they are merely "apparent" because (as I describe it) they are delusional, because there is no really existing *object* of the observation (the way there is when I veridically observe a table or cat).

 

[ttn]

continued...

Again, again. MWI is not self-refuting, because it doesn't claim that cited subjective experienced results weren't subjectively experienced. There is no need for the results to "ontologically be there" in order for a theory to say how results can be subjectively experienced.

But if somebody mentions (I dunno, say) the results of the Davisson-Germer experiment as evidence for their theory, that only goes anywhere if I understand them to mean the *actual* realized results of that experiment (as opposed to the "subjective fantasy" that I or someone else had about the results of those experiments). It's only evidence if that's what actually *happened*. Dreams aren't evidence for scientific theories. And so, given that, such evidence can never be evidence for MWI since MWI asks us to reject the assumption that all the experienced stuff actually happened. Accepting MWI renders what might otherwise have been evidence (for MWI or anything) into non-evidence (for MWI or anything). So any such argument as "this and that experiment had thus and so outcomes, so you ought to believe in MWI" must necessarily fail.

We don't know that AT ALL, and we could never scientifically prove this, given that it is a philosophical position. If it were scientific, it could be falsified. You can never falsify even solipsism in a scientific way.

We can never scientifically prove that there are tables and cats? I think you seriously misunderstand "scientific proof". Scientific proof is what we resort to when we can't just directly perceive something!

 

[vanesch]

Yes, good, I understand that, but it's good to get something out in the open so clearly for once!

I always claimed that "worlds" is an observer-dependent concept (one should say "many subjective worlds, one objective one" ; or in other words: a statistical ensemble for subjective experiences, one single objective world). That's why all those "splittings" and so on are not objective phenomena. They are subjective phenomena.

I've basically made the point I want to make, which is that you have to take all of the "different worlds" and "splitting" and all that (which *sounds* like it's supposed to be what the *dynamics* of MWI is fundamentally *about*) with a giant grain of salt. All of that is really just talk that can't be taken seriously/physically.

Why not ? That's not an argument. From the start, in MWI, we state that there is a difference between "subjective experience/world/observation/..." and objective ontology, but that there is a rule to find the statistical ensemble describing the subjective experience/world/observation.

Now, it seems that each time you rediscover that basic assumption, that you restate how impossible/not serious/... this must be. But since we took it as a starting position, you will find it scattered all over the place.

It shows nicely how subjective/internal/solipsist this gets.

Yes, but that was said from the start.

But anyway, if the physical effects that get Alice and Bob entangled (later) propagate at some slow speed

Nope, that will essentially be light speed. It is sufficient that a single photon makes "contact" between both environments of Alice and Bob, for them to get entangled. The single most interaction that can leave a "classical trace" *in principle*.

(like the speed of sound, which is probably about right for many such situations) then what some MWI people might (erroneously) think of as "facts" that the theory says something about (like how many worlds there are at some time) aren't really facts at all, because they aren't Lorentz invariant.

But that's the point. Given that most decoherence occurs via the EM field (for instance, entanglement with the cosmic microwave background), this is essentially "at light speed".

Also, there isn't to be any lorentz-independent notion of "how many worlds" there are, given that it is observer dependent, in the same way as a coordinate system of the rest frame of an observer is, well, observer dependent.