Quantum Entanglement and time travel

[ueit]

Your provided “mechanism” (?) is no evidence. Also, the WHAT, WHERE, AND HOW it gives it well short of being exact in any detail.

It is good evidence against your interpretation of EPR. I only need a logically consistent counterexample, not a TOE.

Plus, as you describe the “state of the universe” (whatever that may be) as stationary or changing deterministically; that “state” you require is well outside the bounds of the “Local” part of EPR.

The experiment is a part of that state and, as a result, it displays the symmetries embedded in that state.

That Non-Local character is similar to MWI selecting an appropriate “world” with its entire surrounding non-local character configured appropriately to justify EPR correlations. I do not ask that you two abandon your opinions as being wrong, just stop demanding that everyone accept you opinion(s) as the correct one!

Look, I think you are redefining the word "local" here. The sea level at your place being similar with the one at my place is hardly a proof for our ftl communication.

I don't demand anything from you but I expect you to back up your claims.
You reject my counterexample for not being detailed enough but this is hardly a reason. What you need to show is that it is either logically inconsistent or it violates accepted physics.

The two of you cannot both be right, so at least present convincing evidence to each other to demonstrate which is correct BM or MWI. They maybe you will save the entire scientific community that is wasting so much time and effort pursuing so many different things that do not agree with either of you.

In fact we are probably both wrong. This doesn't make you right, though.

I’ll disengage from this debate until I see ueit & vanesch concur on which is correct MWI or BM.

I cannot compete with vanesch on this, I'm only trying to understand this stuff.

Until then I choose to continue my own individual search for a true Local Theory (I agree, a long shot), without the distraction of these two Non-Local theories claiming to be local.

Can you provide us with some insight about what you are looking for? Does any of the existing theories/interpretations/hypotheses present any resemblance with your "true Local Theory"?

As far as I remember, your position is that both QM and GR are non-local. Given the fact that these theories are the best we have, can I ask you for what reason do you look for a theory that necessarily denies both of them?

 

[Careful]

You miss quote me again; I did not explain a “determinate” vs. “deterministic” universe! I said Einstein expected the HV in an EPR to provide “determinate” not problematic solution, and that is all he said. Without actually finding a HV, I’ve seen nothing to suggest that he jumped to the conclusion or opinion that the entire universe was “deterministic” or "determinate", and that all his discoveries and every thought had been predestined from the Big Bang or even earlier. I do not accept that all these posts we fuss over here are meaningless events already set by a predetermined deterministic universe and the idea that Einstein did is balderdash!

GR is a determinate theory, and are we not all using it for the entire universe as a good approximation ? It seems YOU are pretty religious about not having determinism.

Easy
A photon approaches a PDC at some point (unknown & undeterminable to the photon) ahead. Likewise the PDC cannot know or determine exactly when or it what specific condition all the variables that makeup the EM details of that photon. But when they do interact, all these unknown variables combine to produce two photons. The random direction of one photon within a well defined “cone” of options is perfectly matched by the direction of the second based on conservation laws. Likewise, the polarity of the two photons is set one V the H. These parameters are set at the LOCAL creation of the two photons and remain determinate and unchanging until there interact with something else in each of there unknowable paths. PLUS, going with them on that path is the HV also determinate and unchanging and provides the fixed and unchanging information required to account for “entanglement” correlations.

Sure that is more or less the most general scenario (in which you could account for conspiration, stochastic theories, as well as faster than light). I asked you for a specific non-deterministic, but determinate example and you did no such thing at all.

Now if you can show the exact & complete details of such a HV; only then can you make meaningful judgments to speculate on an interpretation that could define with precision all the unknown details about the PDC and the Photon as it approached, to ultimately generate the two photons. Until that can be done you cannot, and I do not believe Einstein ever presumed, that all the events in the universe that lead up to the conversion of that one photon turning into 2 photons was preset in a “deterministic universe”.

Why would I need an HV theory of entanglement prior to figuring out a model for a photon ; the latter could very well be found by relying upon well known physics. But again you say, I do not *believe* that Einstein ever presumed a deterministic universe (many people must misread Einstein then, including me, 't Hooft, ..).

That is an unfair, self-serving, and unsupported claim on Einstein’s real work.
And I do not consider that a small point.

Sure, sure, but the point so far is that you did not do anything but pointing out to others what some famous physicist did not say according to you. By the way, what observation is concerned, I guess determinstic theories are indistinguishable from determinate ones; so all this discussion seems to turn around religious tastes.

 

[RandallB]

I don't demand anything from you but I expect you to back up your claims.
In fact we are probably both wrong. This doesn't make you right, though.

Fine that means you think I’m probably right as the ONLY claim I made was it was wrong to consider BM Local or as haven been accepted as “correct”.

GR is a determinate theory, and are we not all using it for the entire universe as a good approximation? It seems YOU are pretty religious about not having determinism.

No not until someone resolves Lee Smolin’s points about GR being background independent. His new book is an OK read but his papers on the subject are easily found and should be understandable by most on this forum.

And I don’t know about religious, but I’m very well convinced that you and I have free will and are not just going though the motions of responding here in accord with some predetermined “deterministic universe”.
The rest of your comments are still so far off the point for me they are “not even wrong” so I can even comment.

Look guys as far as I’m concerned between careful ueit & vanesch I’m seeing three different versions of Non-local theories, each claiming to be “local” and as far as I can see by little more than that claim of being “local” as being the correct theory of reality. None of that has been shown as “accepted physics” in the view of the majority of prominent scientist.

Even at three against one I’ve not changed my mind, as the only thing I think I can agree with you guys on is that showing AND PROVING a local solution would disprove both “entanglement” and “superposition” and “time travel” as well. And that such a proof would displace all non-local theories. BUT AFAIC to do that, the standard for local needs to be much higher that any of you seem to be willing to accept. So allow me to depart this and I’ll stick with my standard for local, even if it might be impossible to satisfy - - that’s my problem.

 

[Careful]

No not until someone resolves Lee Smolin’s points about GR being background independent.

Euhhh, background independence has nothing to do with GR being determinate ! And neither do I consider it to be a crucial ingredient for a future theory of quantum gravity.

And I don’t know about religious, but I’m very well convinced that you and I have free will and are not just going though the motions of responding here in accord with some predetermined “deterministic universe”.

So, that confirms my point.

The rest of your comments are still so far off the point for me they are “not even wrong” so I can even comment.

Look guys as far as I’m concerned between careful ueit & vanesch I’m seeing three different versions of Non-local theories, each claiming to be “local” and as far as I can see by little more than that claim of being “local” as being the correct theory of reality. None of that has been shown as “accepted physics” in the view of the majority of prominent scientist.

You confuse since as long as I am here locality with causality, that's all there is to it. Bell didn't invent the terminology ``local causality'' for nothing you know.

Even at three against one I’ve not changed my mind

Bravo, an heroic act.

BUT AFAIC to do that, the standard for local needs to be much higher that any of you seem to be willing to accept. So allow me to depart this and I’ll stick with my standard for local, even if it might be impossible to satisfy - - that’s my problem.

Perhaps, you should ask wether you can find a local, determinate, realistic theory of photons first, using your notion of locality, such that Maxwell theory is recovered in a suitable limit. You might end up very dissapointed soon.

Careful

 

[vanesch]

That Non-Local character is similar to MWI selecting an appropriate “world” with its entire surrounding non-local character configured appropriately to justify EPR correlations. I do not ask that you two abandon your opinions as being wrong, just stop demanding that everyone accept you opinion(s) as the correct one!

The two of you cannot both be right, so at least present convincing evidence to each other to demonstrate which is correct BM or MWI. They maybe you will save the entire scientific community that is wasting so much time and effort pursuing so many different things that do not agree with either of you.

Both, MWI and BM are "correct" theories in the sense that they set up a logically coherent construction (at least to a level of informal rigor which is usual with physical theories). That's what I'm trying to tell you: logically, these theories exist. Whether they correspond to our universe is another matter of course (probably not !), but that's not the point. The point is that they exist, as theories, and they both agree with the experimental results which are under discussion here (EPR correlations). So their logical existence means that one cannot deduce logically, from these experimental predictions, any property which BM or MWI would not have, given that they logically exist.

von Neumann already committed this error with BM: he claimed that certain predictions of QM could never be obtained with a deterministic HV theory. BM does exactly that, so von Neumann's reasoning must have been wrong, irrespective of whether BM is or isn't "true" in nature.

In exactly the same way, MWI has an entirely local dynamics, and makes the same predictions of observation as does standard QM. As such, any claim that one can deduce, from these observations, that the dynamics must be non-local, is just as erroneous as von Neumann was in his case. Again, irrespective of whether MWI is "true" in nature or not.

So, logically, BM and MWI are both "correct", as is any well-constructed theory. This has nothing to do with whether it is the correct description of nature. Newtonian mechanics is also "correct".

 

[Demystifier]

Vanesch, I would very appreciate if you could answer to my post #82.
Thanks!

 

[vanesch]

OK, it is local for you. But is it local for itself? Is the whole Universe (the collection of All worlds of mwi) local? Is the split/branching of the Universe in a number of different copies a local event? And if you say that this question is irrelevant, isn't such a view of nature quite antropomorphic?

"The whole universe" in MWI is nothing else but the state vector, which evolves under unitary evolution. So the question of locality is related to this unitary evolution. It doesn't need to be so. For instance, the unitary evolution induced by, say, the Coulomb hamiltonian in NR QM certainly doesn't induce a local unitary evolution (in the same way as the Coulomb force in classical phase space doesn't induce a local phase flow, given that there is "action at a distance"). But if the interactions in the hamiltonian (which is the generator of unitary evolution) are local, then the unitary flow in Hilbert space is just as local as the phase space flow in classical mechanics, if the interactions are local. This can easily be verified by the fact that this unitary evolution can be written out in a Lorentz-invariant way.

The "splittings in worlds" in MWI is a pure observer-dependent concept: not for all observers, the universe is "split" in the same way. In fact, this splitting is simply the projection on the different "subspaces of awareness" by the observer (that is, those subspaces of hilbert space which correspond to different "states of awareness", which you can grossly imagine to correspond to different memory states which correspond to "observations"). "Worlds" do not have any objective ontological existence in MWI, independent of an observer.

Look at the wavefunction, where Alice has done a measurement and Bob has done a measurement, but they didn't talk to each other yet:

|psi> = |alice+> (u |bob+> + v|bob->) + |alice->(w |bob+> + x |bob->)

For Alice, there are 2 worlds, one in which she has seen the + outcome, and one in which she has seen the - outcome. Her local state description is |alice+> in one world, and |alice-> in another, and the |alice+> state has an overall weight of u^2 + v^2, while the |alice-> world has an overall weight of w^2 + x^2.
Note that these elements are in no way affected by what happens at Bob, as long as this is a local, unitary evolution. Whatever happens to the "bob" states, the weight of the Alice+ state will always remain u^2 + v^2.

We can re-write the SAME state vector from Bob's PoV:
|psi> = |bob+>(u |alice+> + w |alice-> + |bob-> (v |alice+> + x |alice->)

For Bob, there are also 2 worlds, one in which he has seen the + outcome, and one in which he has seen the - outcome. The bob+ state has an overall weight of u^2 + w^2 and the bob- state has an overall weight of v^2 + x^2.
Again, this local state description of Bob is independent of what happens at Alice: the weights of these two states will remain the same under any unitary transformation at Alice's place.

Note also that Alice's worlds have nothing to do with Bob's worlds.

Now, imagine they come together, and exchange their information. This alters of course the "state of awareness" for both alice and bob, but can only occur when they are in local contact.

We now have an evolved state:
|psi2> = u|bob++> |alice++> + w |bob+->|alice-+> + v|bob-+>|alice+-> + x |bob-->|alice-->

We now have an altered state for alice, which can be in 4 different states:
alice++, alice+-, alice-+ and alice--, with weights respectively u^2, v^2, w^2 and x^2.

This modification of alice's state description came about because of her LOCAL interaction with Bob, when he came to tell her his results. The unitary evolution which did this was local to the place of meeting.

Alice now lives in 4 worlds (and so does bob), and this time the worlds coincide between alice and bob (because they are in local interaction).

Notice the difference with a projection-based (and hence non-local) explanation of this story:
Before Alice or Bob performs a measurement, the state is:
|psi> = |alice0> (|a+>(u |b+> + v|b->) + |a->(w |b+> + x |b->)) |bob0>

(where we introduced the a-states, of the particle at Alice's place,and the b-states, of the particle at Bob's place).

Alice's weight is 1 for her state alice0.
However, the weight for the a+ state (at Alice's place) is (u^2 + v^2) while the weight for the a- state (at Alice's place) is (w^2 + x^2)

Let us first say that Bob performs a measurement, and his outcome is +:
this MODIFIES the statevector by projection:
|psi'> = |alice0> (|a+>(u |bob+>) + |a->(w |bob+>))/sqrt(u^2 + w^2)

and note that this CHANGES the weights of the a-states at Alice:
instead of a weight u^2 + v^2, we now have a weight of u/sqrt(u^2 + w^2) for the a+ state, and a weight w/(sqrt(u^2 + w^2) instead of (w^2+x^2) for the a- state.

So we see here that a state description of something at ALICE has been changed by a remote interaction (Bob's measurement). This is what is non-local in the projection postulate. It is the essence of the EPR effect.

Alice's state description has not been altered however. But of course, as Alice is going to interact locally with a, she will find a different state now than if Bob wouldn't have measured (with projection) on his side.

Alice's measurement will do nothing special to Bob's: imagine she finds +:
|psi"> = |alice+>|bob+>

This interaction doesn't do anything "non-local". Bob's state was +1 with weight 1, and this remains so. The culprit was the first measurement by bob, who ALTERED the state description of the a-particle remotely, by projection. This is the explicit non-local mechanism in "standard" projection-based quantum mechanics. Mind you that what introduces the alteration of the local state description is the projection: the fact of throwing away some terms in the wavefunction for ALL constituents, local or remote. The unitary evolution cannot do such a thing (if the interactions in it are local), because there is conservation of hilbert norm.

 

[Demystifier]

"The whole universe" in MWI is nothing else but the state vector, which evolves under unitary evolution. So the question of locality is related to this unitary evolution. It doesn't need to be so. For instance, the unitary evolution induced by, say, the Coulomb hamiltonian in NR QM certainly doesn't induce a local unitary evolution (in the same way as the Coulomb force in classical phase space doesn't induce a local phase flow, given that there is "action at a distance"). But if the interactions in the hamiltonian (which is the generator of unitary evolution) are local, then the unitary flow in Hilbert space is just as local as the phase space flow in classical mechanics, if the interactions are local. This can easily be verified by the fact that this unitary evolution can be written out in a Lorentz-invariant way.

The "splittings in worlds" in MWI is a pure observer-dependent concept: not for all observers, the universe is "split" in the same way. In fact, this splitting is simply the projection on the different "subspaces of awareness" by the observer (that is, those subspaces of hilbert space which correspond to different "states of awareness", which you can grossly imagine to correspond to different memory states which correspond to "observations"). "Worlds" do not have any objective ontological existence in MWI, independent of an observer.

OK, I am satisfied with the first part in which you explain why it is local.

But I still do not understand the second part, so I will ask you additional questions:
What is "observer"?
Is it necessarily a conscious being?
Can one electron be an observer?
Do observers have objective ontological existence in MWI?
Is the MWI interpretation above essentially the same as Rovelli's relational interpretation?
Is the projection on the different "subspaces of awareness" by the observer - nonlocal at least for that observer (even if there is no objective nonlocality)?
Does MWI misses a satisfying theory of the observer? If yes, does it mean that quantum mechanics is not complete? If yes, could that mean that a satisfying completion would require a sort of nonlocality?

 

[vanesch]

But I still do not understand the second part, so I will ask you additional questions:
What is "observer"?

This is going to turn philosophical again
The ultimate "reason of existence" of physics, and of all studying of nature, and even of all our thinking, is to explain our subjective experiences. Probably
you're not thinking of it that way, but if you give it some reflection, you will find out that all of our intellectual activity comes ultimately down to understanding our subjective experience.

Now, usually we think of that as a secondary problem, because deep down, we are somehow convinced that our subjective experiences are directly derived from our sensory inputs, which correspond to a unique "reality" in an obvious and evident way, and that we can put "ourselves" out of play in the business of describing that outside reality. But this is nothing but hypothesis. It is also possible that the relationship between "reality" and our subjective experience is far more complex than we imagine. (and then maybe not) This is the viewpoint of MWI, and it is essentially inspired by the clash between the obvious quantum description of, say, a human being (which can then be in several places at once, the superposition principle applying to him as well as to anything else), and our daily perception which doesn't allow for such phantasies. There are two ways out: 1) quantum theory doesn't apply to human bodies or 2) our daily perception doesn't correspond to "reality" as a whole. MWI takes on the last stance.

So what's an observer ? It is something having subjective experiences which tries to relate it to an objective world "outside" in this viewpoint. Otherwise, it is just a physical construction as any other.

You might also say: an observer is something which cannot accept being in a quantum superposition because it is convinced that his subjective experiences do not correspond to that.

Is it necessarily a conscious being?

In as far as consciousness is related to "the existence of subjective experience", yes.

Can one electron be an observer?

In as far as an electron has subjective experiences...

It is maybe necessary to make a distinction between "an observer" and a "measuring device". A measuring device is a thing that interacts with a system, and has memory states that correspond to "outcomes of measurement". If you apply quantum theory to a measurement device, there is not really a problem: you will find that the device will end up in a quantum state which is a superposition of "outcomes", in a way which mimicks the original quantum superposition of the system at hand. Nothing goes wrong here. What goes wrong is when we *subjectively observe* such a measurement device, that we only see it in one state. Now, this can be because it truly is in one state (in which case the entire quantum description goes wrong at this level), or, and that is the MWI viewpoint, it is because we are only subjectively aware of one of the states.

Do observers have objective ontological existence in MWI?

The physical structure related to an observer has objective existence in MWI (the "body"). However, the "observer" itself, not really: it are specific body states which "act as observer". You cannot know if a physical structure is "an observer" or not: you just declare it to be so or not. (in fact, in exactly the same way as you cannot really know if a person is conscious or is a zombie) This is the ill-defined part (but as subjective observation is ill-defined in any case, that's not a problem FAPP, but only in principle).

Is the MWI interpretation above essentially the same as Rovelli's relational interpretation?

Personally, I think so ! But others do not agree with me on that point.

Is the projection on the different "subspaces of awareness" by the observer - nonlocal at least for that observer (even if there is no objective nonlocality)?

No, because these subspaces (which identify clearly different bodystates, corresponding to clearly different experiences) are only related to a local bodystate. What an "observer" observes, are his bodystates, and nothing else. So this bodystate is of course local to the body, and observers attached to this body are only "aware" of this bodystate (actually, of components of it). It are bodystate conglomerates which define "different states of awareness" and the corresponding subspaces.

Does MWI misses a satisfying theory of the observer?

yes, on a level of principle. No, FAPP (for all practical purposes). That is to say, our differentiated "states of awareness" are so terribly coarse grained, that their precise definition doesn't really matter. But we bump here into the same difficulty as in Copenhagen QM, with the "quantum-classical" transition. Only, we removed the problem totally out of the "objective physics" part and entirely in the link between objective physics and subjective experience, which is in any case a very fuzzy domain (in principle, but not FAPP).

If yes, does it mean that quantum mechanics is not complete? If yes, could that mean that a satisfying completion would require a sort of nonlocality?

This has nothing to do with it. Concepts such as locality and so on are supposed to reign in the domain of pure physics, which is entirely well-defined in MWI (namely, the Schroedinger equation, period). What is fuzzy, is simply the details of the link between physical reality and subjective experience - which is in any case fuzzy on a very detaillistic level, but which is more than sufficient to talk about observations which have such a clear influence on our "state of awareness" that there is not much discussion - this is what is usually considered as "macroscopic observations".

 

[Demystifier]

Thanks vanesch, now I think I understand MWI (or at least your view of MWI) much better.

But let me also present a critical "summary" of MWI. In MWI, the conscious observer plays an important role. On the other hand, MWI does not contain a theory for that conscious observer.

In comparison, the Bohmian interpretation also does not contain a theory of conscious observers, but they do not play an important role in that interpretation. (Of course, the Bohmian interpretation contains other disadvantages.)

Do you agree?

 

[vanesch]

Thanks vanesch, now I think I understand MWI (or at least your view of MWI) much better.

But let me also present a critical "summary" of MWI. In MWI, the conscious observer plays an important role. On the other hand, MWI does not contain a theory for that conscious observer.

In comparison, the Bohmian interpretation also does not contain a theory of conscious observers, but they do not play an important role in that interpretation. (Of course, the Bohmian interpretation contains other disadvantages.)

Do you agree?

Yes! MWI is a desperate attempt to try to give ontological sense to the quantum formalism, as we know it, in a universal way, while not trying to hide behind "you shouldn't ask that question" kind of rethoric (which Copenhagen does, in a way), and without adding any formal elements. And, as you point out, you are then confronted to the relationship between "subjective experience" and "physical reality". By cleaning out the part of "physical reality" all the fuzzyness has been put in that relationship with "consciousness". One can call it a cop-out, and in a way it is so! But no other view on quantum theory does any better.
Copenhagen, with its "transition into a classical world" needs just as well preferred subspaces which correspond to classical worlds. Statistical views don't even attempt at trying to give any kind of ontological picture, but nevertheless take it for granted that the "projected states" make some kind of sense.
As to Bohmian mechanics, true, it works *slightly* better. What is clear, in BM, is the particle configurations. This is entirely Newtonian, so we "feel at home". However, what one forgets, in BM, is that there is a much fuzzier part to the ontology, namely the wavefunction. As I tried to argue, the wavefunction is an integral part of the dynamical contents of BM (and hence not just a kind of auxilliary variable introduced for convenience, which can be done away with). As, in BM, the wavefunction evolves exactly according to the Schroedinger equation, just as in MWI, part of the interpretational difficulties of MWI are in fact also inherited by BM. Indeed, why should a conscious being only be aware of its "particle" aspect, and not of its "wavefunction" aspect (which contains, remember, also all "worlds" of MWI)?
Granted, this can be solved by a simple postulate: "conscious beings are only aware of their particle positions". I will agree that of all views, BM clearly has the cleanest ontological position (although it is not so clean as one usually presents it). In fact, it is MWI + a token (the particle configuration) in a certain way, where the token indicates "the common world of experience" (namely the particle positions). The difficulty I have with BM is its relativistic non-invariance.

 

[ueit]

As to Bohmian mechanics, true, it works *slightly* better. What is clear, in BM, is the particle configurations. This is entirely Newtonian, so we "feel at home". However, what one forgets, in BM, is that there is a much fuzzier part to the ontology, namely the wavefunction. As I tried to argue, the wavefunction is an integral part of the dynamical contents of BM (and hence not just a kind of auxilliary variable introduced for convenience, which can be done away with). As, in BM, the wavefunction evolves exactly according to the Schroedinger equation, just as in MWI, part of the interpretational difficulties of MWI are in fact also inherited by BM.

I know we've discussed this before but I still do not understand your position. Of course, the quantum force is a part of BM's ontology just like the coulombian, strong or gravitational force. Your point was that it cannot be deduced from the particle configuration only, OK, we need to specify the system energy as well. So what? We have a function relating the quantum potential to some well defined properties of the system evolving in 3d space + time. No need for other worlds/dimensions, no need to ascribe reality to the Hilbert space itself. If we imagine this quantum potential as a kind of space geometry, only this geometry needs to be real in BM, nothing else.
In conclusion, BM's ontology requires, beyond classical parameters, a parameter defining the "space quantum geometry". The details of how we perform the calculation are irrelevant to that ontology. MWI, on the other side, requires that the mathematical formalism maps to some existing reality, BM doesn't need that.

Indeed, why should a conscious being only be aware of its "particle" aspect, and not of its "wavefunction" aspect (which contains, remember, also all "worlds" of MWI)?
Granted, this can be solved by a simple postulate: "conscious beings are only aware of their particle positions".

This is a very strange objection, indeed. We are certainly aware of the quantum potential in the same way we are aware about any other potential, by observing how particles move in its presence. That we cannot directly access this potential is not in any way different from the fact that we cannot observe an electric field without at least a charged object being present. If you maintain that BM needs that postulate then you need to ask classical mechanics to add postulates for the non-observability of every classical potential as well.

I will agree that of all views, BM clearly has the cleanest ontological position (although it is not so clean as one usually presents it). In fact, it is MWI + a token (the particle configuration) in a certain way, where the token indicates "the common world of experience" (namely the particle positions). The difficulty I have with BM is its relativistic non-invariance.

I have some questions related to the relativity issue.

1. BM doesn't look non-local from the point of view of the universal wave-function in the sense that no interaction needs to be transmitted ftl. The quantum potential evolves deterministically, regardless of particles' motion and the particles only interact locally. So, whyle BM requires an absolute reference frame it doesn't seem to conflict directly with relativity (which could be formulated, I think, on an absolute RF).

2. Is there a fully relativistic QFT yet (without the non-local collapse or with the collapse relativistically treated)?

3. Is MWI proven to have a mathematically rigorous relativistic extension? I've read some articles claiming that problems relating to the world splitting could appear.

Thanks.

 

[Rade]

Both, MWI and BM are "correct" theories in the sense that they set up a logically coherent construction (at least to a level of informal rigor which is usual with physical theories)...So, logically, BM and MWI are both "correct", as is any well-constructed theory...

Perhaps, as with the dual particle-wave nature of the photon, quantum reality (whatever it may be) is the union of all the experimentally "correct" theories--that is, there never has been nor will be a "single" correct theory--is what what you are saying here ?

 

[vanesch]

OK, we need to specify the system energy as well. So what? We have a function relating the quantum potential to some well defined properties of the system evolving in 3d space + time. No need for other worlds/dimensions, no need to ascribe reality to the Hilbert space itself. If we imagine this quantum potential as a kind of space geometry, only this geometry needs to be real in BM, nothing else.

But this "geometry" needs the exact wavefunction, and can only be deduced from that wavefunction, so the wavefunction is entirely part of the "ontology" of the theory. Now, you can make extra hypotheses, which come down to specifying a specific wavefunction (like, it must be in the ground state or something), but this limits severely the applicability of BM. In short, you cannot do BM when the initial wavefunction is not given.

You can compare this with the EM field in classical physics. The fact that there are non-trivial vacuum solutions to the EM field, means that these fields have an essential existence of their own. You cannot do electrodynamics without using the EM field (or the vector potential or anything of the kind).
You could get away with it if there were purely a Coulomb interaction, because from the configuration of the charged particles, the Coulomb interaction can be derived. The E-field would then simply be a convenience, but would not be an essential part of the dynamics (and hence, of the ontology). You could, if you wanted to, eliminate the E-field in all calculations.

Even the EM field could be partly eliminated, by using the retarded potential expressions. All EM waves emitted from other charges can be eliminated that way. But what you cannot eliminate, are the initial radiative conditions. There can be initial EM waves, unrelated to any charged source. This dynamical element cannot be eliminated, and hence, the EM field has an essential dynamics to itself, which means that any theory of the EM interaction must consider that there is an ontology to the EM field, and that it is not just an intermediate variable used for convenience.

In the same way, in BM, you NEED the initial wavefunction, because it cannot be derived from the particle positions. It is not an intermediate variable which could be eliminated at leisure. It is an essential component of the dynamical formulation of BM, and hence has an ontological existence.

In conclusion, BM's ontology requires, beyond classical parameters, a parameter defining the "space quantum geometry".

Well, that's an euphemism to say that you need the wavefunction...

The details of how we perform the calculation are irrelevant to that ontology.

Not really. You cannot eliminate it, it is not an intermediate quantity just introduced by convenience but which could be eliminated entirely.

MWI, on the other side, requires that the mathematical formalism maps to some existing reality, BM doesn't need that.

yes, it does so, for exactly the same reason: it is an essential part of the dynamics.

This is a very strange objection, indeed. We are certainly aware of the quantum potential in the same way we are aware about any other potential, by observing how particles move in its presence. That we cannot directly access this potential is not in any way different from the fact that we cannot observe an electric field without at least a charged object being present. If you maintain that BM needs that postulate then you need to ask classical mechanics to add postulates for the non-observability of every classical potential as well.

Well, in classical physics, the ontology consists of particles and fields (both of them). Together they specify the configuration space (or the phase space, if you want). It is hard to say which aspect of the point in conguration space is generating a certain subjective experience: are it purely the particle states, or are it the field states, or both ? Hard to say whether it is the EM field configuration in the brain of a creature living in a classical world which is giving it his memory states, or whether it are the particle configurations ! I would say that it is the entire state which does so. But in a classical setting, this is pretty irrelevant.

1. BM doesn't look non-local from the point of view of the universal wave-function in the sense that no interaction needs to be transmitted ftl. The quantum potential evolves deterministically, regardless of particles' motion and the particles only interact locally.

No, not really. In BM, the potential is function of the wavefunction AND the positions of the remote particles. This is the non-local element: the guiding equation:
<br /> \frac{dq_k}{dt} = \frac{\hbar}{m_k}\frac{Im\left[ \Psi^* \partial_k \Psi \right]}{\Psi^* \Psi}_{q_1,q_2...,q_N}<br />

The presence, in the generalised velocity for the k-th particle, of the generalized coordinates of the other particles at the same moment, makes this an explicitly non-local (and non-lorentz-invariant) expression.

So, whyle BM requires an absolute reference frame it doesn't seem to conflict directly with relativity (which could be formulated, I think, on an absolute RF).

Well, that's a contradiction in terms: any theory requiring an absolute reference frame is in conflict with the fundamental postulate of relativity. It is always possible to make it observationally in agreement with relativity, but it means, in that case, that one has introduced unnecessary elements which break explicit Lorentz invariance. This is the same with an ether theory, or with, say, the coulomb gauge fixing condition in the canonical quantization of QED.

2. Is there a fully relativistic QFT yet (without the non-local collapse or with the collapse relativistically treated)?

You cannot treat the collapse relativistically. What is done in QFT, is the calculation of matrix elements of the unitary evolution operator which transforms initial particle states in final particle states, for "large times". It is very similar to the U(t1,t0) operator which transforms |psi(t_0)> into |psi(t1)>, but taken in the limit where t1 goes to +infinity and t0 goes to -infinity. It is only for that case that there are approximative techniques.
This unitary operator is nothing else but the solution to the schroedinger equation, as usual. These complex numbers, squared, give the transition probabilities of the corresponding initial state in the final state.

Usually, this is done by calculating an approximation to an expression which is called a "path integral". Given that we calculate in this way, the probabilities for the transition from "long ago" into the "far future", this can be interpreted in any way you like. You can continue to consider the superposition of final states (whose coefficients are nothing else but the matrix elements calculated) a la MWI, or you can decide to project one out (in which case you do something non-local), a la Copenhagen.

3. Is MWI proven to have a mathematically rigorous relativistic extension? I've read some articles claiming that problems relating to the world splitting could appear.

The mathematical part of MWI is nothing else but standard unitary quantum theory. QFT is known not to be rigorously correct, but this is just a model as any other.

 

[Demystifier]

Yes! MWI is a desperate attempt to try to give ontological sense to the quantum formalism, as we know it, in a universal way, while not trying to hide behind "you shouldn't ask that question" kind of rethoric (which Copenhagen does, in a way), and without adding any formal elements. And, as you point out, you are then confronted to the relationship between "subjective experience" and "physical reality". By cleaning out the part of "physical reality" all the fuzzyness has been put in that relationship with "consciousness". One can call it a cop-out, and in a way it is so! But no other view on quantum theory does any better.
Copenhagen, with its "transition into a classical world" needs just as well preferred subspaces which correspond to classical worlds. Statistical views don't even attempt at trying to give any kind of ontological picture, but nevertheless take it for granted that the "projected states" make some kind of sense.
As to Bohmian mechanics, true, it works *slightly* better. What is clear, in BM, is the particle configurations. This is entirely Newtonian, so we "feel at home". However, what one forgets, in BM, is that there is a much fuzzier part to the ontology, namely the wavefunction. As I tried to argue, the wavefunction is an integral part of the dynamical contents of BM (and hence not just a kind of auxilliary variable introduced for convenience, which can be done away with). As, in BM, the wavefunction evolves exactly according to the Schroedinger equation, just as in MWI, part of the interpretational difficulties of MWI are in fact also inherited by BM. Indeed, why should a conscious being only be aware of its "particle" aspect, and not of its "wavefunction" aspect (which contains, remember, also all "worlds" of MWI)?
Granted, this can be solved by a simple postulate: "conscious beings are only aware of their particle positions". I will agree that of all views, BM clearly has the cleanest ontological position (although it is not so clean as one usually presents it). In fact, it is MWI + a token (the particle configuration) in a certain way, where the token indicates "the common world of experience" (namely the particle positions). The difficulty I have with BM is its relativistic non-invariance.

I agree with you.
However, what I try to do in my research on Bohmian mechanics, is to use the disadvantages of the Bohmian approach to promote it from an interpretation (which contains negative philosophical connotations) to a physical theory. For example, in some regimes, such relativistic non-invariance might have observable consequences.

 

[Demystifier]

Vanesch, I was thinking about the MWI and I concluded that a version of MWI that seems reasonable to me is the version which I will call
"MANY-WORLD SINGLE-MIND" interpretation!

What is this? According to this, the wave function never collapses and there are many coexisting branches of the wave function (MANY "WORLDS"). However, not all these branches enjoy the same rights. One (and only one) of them is picked up randomly as the one which corresponds to the reality percieved. There are no "paralel universes" in which other realities are realized. Only one reality, one branch, is the "right" one. In particular, this branch is subjectively perceived by our minds, but this branch has a preferred role even without our minds, so the theory is self-consistent even without a theory of mind or consciousness. It is not clear what the "mind" is, but at least it is clear that there is only one mind for each person (SINGLE MIND), not many copies with different hystories in different parallel universes.

It can also be compared with classical physics described by a deterministic equation of motion. The equation of motion containes many possible "worlds" corresponding to many solutions. They can be viewed as different "branches" of the equation of motion. But only one solution is picked up as the right one. There is no theory that predicts which solution (or which initial condition) is the right one, so, effectively, the right solution is picked up randomly.

Now in quantum mechanics we have two levels of the unpredictable. One is the choice of the right solution of the Schrodinger equation. The other is the choice of the right branch of that solution.

What do you think about such a version of MWI?
(You will probably note that this version combines some desirable features of MWI and BM and eliminates some undesirable ones.)

 

[vanesch]

What do you think about such a version of MWI?
(You will probably note that this version combines some desirable features of MWI and BM and eliminates some undesirable ones.)

It is non-local, unfortunately. The "synchronisation" of the different minds associated with physical structures (bodies) at space-like intervals, would require us to do something non-local (such as projection in Copenhagen, or the particle guide equation in BM).

 

[Demystifier]

It is non-local, unfortunately. The "synchronisation" of the different minds associated with physical structures (bodies) at space-like intervals, would require us to do something non-local (such as projection in Copenhagen, or the particle guide equation in BM).

I seems to me that it is not more nonlocal than the usual original (but not yours) form of MWI. This is probably the main motivation for a relational version of MWI introduced by Rovelli and advocated by you. Am I right?

But note that I was inspired by your note on the analogy between the block-time interpretation of time and MWI. In particular, the block-time interpretation combined with the self-consistency principle (which, in fact, is nothing but a tautology) automatically resolves the time-travel paradoxes, but it also requires a sort of nonlocality in order to prevent initial conditions that do not lead to globally self-consistent solutions. What do you think about that? (Recall also that this is actually a topic on time travel.)

 

[vanesch]

I seems to me that it is not more nonlocal than the usual original (but not yours) form of MWI. This is probably the main motivation for a relational version of MWI introduced by Rovelli and advocated by you. Am I right?

I don't know what other versions you are thinking about. I'm not aware of any MWI version where there is a "single world" which is shared by remote observers. A "world" is an observer-dependent concept, and the "slicing up of the wavefunction in different worlds" is also an observer-dependent concept, in all MWI versions I know of.

But note that I was inspired by your note on the analogy between the block-time interpretation of time and MWI. In particular, the block-time interpretation combined with the self-consistency principle (which, in fact, is nothing but a tautology) automatically resolves the time-travel paradoxes, but it also requires a sort of nonlocality in order to prevent initial conditions that do not lead to globally self-consistent solutions. What do you think about that? (Recall also that this is actually a topic on time travel.)

I'm affraid I don't see what you're aiming at...
The analogy between block-time and MWI I had in mind was the following:
in both cases, we seem to be subjectively experience something else but the "whole". In MWI, we don't seem to experience subjectively the entire wavefunction (with all its alternatives, in the form of superpositions of classical situations, which we call "worlds"), and in block-time, we seem subjectively to experience only "now" and not the entire "time dimension". But in both cases, the relationships between observations/events (in other words, the physics) are all right. In MWI, in each branch, you have a consistent set of observations (the Alice that saw "up" will be entangled with the "up" state of the particle etc...), and in "block time" you have the correct temporal relationships (between event A and event B, the two twins have observed a certain number of rotations of the hands of their clocks, and that number is exactly what is expected etc...)

So, true, in both cases there seems to be a fundamental difficulty in explaining our subjective experience (which makes some people reject the ideas of MWI or of block time), but in both cases all relational aspects between observations are ok. In both cases, the proposed "ontology" fits close to the formalism based upon the fundamental ideas of the theory (superposition and unitary evolution for MWI, 4-dim spacetime manifold for block time).

 

[Demystifier]

I don't know what other versions you are thinking about. I'm not aware of any MWI version where there is a "single world" which is shared by remote observers. A "world" is an observer-dependent concept, and the "slicing up of the wavefunction in different worlds" is also an observer-dependent concept, in all MWI versions I know of.

Then I probably misunderstood some versions of MWI. I thought that, in some versions, observers are irrelevant, while it is the whole universe itself that splits in many branches. Since the universe is a nonlocal object, such spliting would necessarily be nonlocal.

 

[vanesch]

Then I probably misunderstood some versions of MWI. I thought that, in some versions, observers are irrelevant, while it is the whole universe itself that splits in many branches. Since the universe is a nonlocal object, such spliting would necessarily be nonlocal.

This is indeed an often encountered misunderstanding of MWI. If it were the case, as you point out, MWI would have no particular advantage over Copenhagen "projection" because the magical "split" would be just as unexplained and mysterious as the "projection by observation" - and would imply just as non-local a happening. But it is an often-encountered misunderstanding, leading also to an often-encountered objection to MWI, which is that each time there is an entanglement of two electrons on Andromeda, my worlds would "split".

Branching only makes sense wrt an observer.

 

[Demystifier]

This is indeed an often encountered misunderstanding of MWI. If it were the case, as you point out, MWI would have no particular advantage over Copenhagen "projection" because the magical "split" would be just as unexplained and mysterious as the "projection by observation" - and would imply just as non-local a happening. But it is an often-encountered misunderstanding, leading also to an often-encountered objection to MWI, which is that each time there is an entanglement of two electrons on Andromeda, my worlds would "split".

Branching only makes sense wrt an observer.

I think that MWI people are responsible for that misunderstanding. They should call their interpretation MOI (Many Observer Interpretion) or something like that.

 

[vanesch]

I think that MWI people are responsible for that misunderstanding. They should call their interpretation MOI (Many Observer Interpretion) or something like that.

The original name (by Everett) was much better: relative state interpretation.

 

[tehno]

I wonder guys how you never get tired of such topics (like "time travel","exceeeding the speed of light" etc)?

-tehno

 

[Demystifier]

I wonder guys how you never get tired of such topics (like "time travel","exceeeding the speed of light" etc)?

Everyone has something that never gets him/her tired. We have "time travel", "exceeeding the speed of light", and stuff like that. And you?

 

[tehno]

I enjoy watching series of "Star trek" .

 

[vanesch]

I wonder guys how you never get tired of such topics (like "time travel","exceeeding the speed of light" etc)?

If this question really needs to be answered, I'll give you my PoV. We have accumulated a certain amount of knowledge, by past experiences, by our own personal experience and so on, and from this knowledge results some model of reality. In fact, according to different categories of experiences, we extract different models of reality: we have our "common sense everyday" model of reality (I know my home, my car, my family, some animals, some weather, ... which is a certain model of reality I have set up, since I was a child, of "the world") ; scientific experiments have lead to some scientific theories which also give us some (mathematical / formal) model of reality. Now some of these models are highly counter-intuitive (in other words, don't seem to conform to our common sense model of reality we have set up based upon personal experience). They are, nevertheless, just as good (or even better) models of reality as are our "naive" views from everyday life. If you take these models completely seriously, then you arrive at sometimes very strange predictions/concepts. One of it is time travel. Of course, it is an extrapolation, far beyond the data that were gathered in order to set up the mathematical model. Nevertheless, it is an intriguing possibility. So it is fun to explore it, on the purely theoretical side.

 

[drphysica]

This is reply 4 vincentm:
I would have to agree with most physicist who say that time travel is impossible. But you have to be careful when you say anything like that because time is actually a relative concept. However there are some mathematical prediction of time travel but it bring a lot of questions with paradoxes that we cannot comprehend. The thing is if "time travel" as you imagine is not the same as what I have in mind. The time travel as what I understand is a possible action however there is no such thing as paradoxes of going back in my past and killing my grandfather because what most of us call time travel is actually a travel to other space-time continuum all together. By warping time we actually travel to different universe and the funny thing is we do it all the time just cannot tell the difference. If anyone familiar with atomic clock experiment when one set of clocks put on a plane and taken around the world for a trip, when plane returns and the clocks compared it is apparent that time was warped by few nanoseconds during the plane trip and hence it an evident that time travel is indeed possible. Just now we arrived to the universe which is almost identical copy of the previous one. So you see it is possible with no paradoxes but it is not a time travel as you might think. :)

 

[vincentm]

Backwards time travel might be problematic for other reasons, but it's definitely allowed in GR (though a theory of quantum gravity may change this), and your arguments for why it's nonsensical don't work, for the reasons I tried to explain above.

So one can travel backwards in time via relativity. Sorry to ressurect this thread but i didn't understand most of the discussion here, perhaps a more laymen's type terminology is best for the purposes of my understanding.

 

[rewebster]

Quote Brian Greene :
"Despite years of debate, scientists still haven't completely ruled out the possibility of going back in time. "Many physicists have a gut feeling that time travel to the past is not possible," said Columbia University theoretical physicist Brian Greene. "But many of us, including me, are impressed that nobody's been able to prove that."


To me, this sounds like political double-speak as not to offend anyone


He could as well have been talking about the existence of unicorns.