Problems with Many Worlds Interpretation

[Hurkyl]

Perhaps the simplest way to sum up MWI-esque interpretations is the rule "add no postulates simply to acquiesce to what is perceived when a simpler set of postulates is not falsified."

I have no idea how you manage to equate this with "extreme faith in postulates" or "radical rationalism".

Your phrasing is odd, leaving me to suspect you aren't describing MWI-esque interpretation at all. It's usually stated something to the effect of "don't add postulates about something already explained by the other postulates -- especially when your new postulate is that the theory is wrong, but only in an effectively unobservable way".

 

[Fra]

If I may add one thing to this list corresponding to my perspectibe

The good empiricists would say "we need to do more experiments" to get more information to help them understand what's going on.

The good rationalists would say "we need to look for new theories that can explain these new results, and still reproduce what we already know!"

A good information processing gamer who understand a theory as an interaction tool would say:

- We use the CURRENT theory to tell use WHICH experiments to make. Because making experiments is nothing else but choosing your actions. It does not make sense to make every possible experiment. Some experiments are wisest not to make because they threaten our existence. And which this I don't mean human activities, I mean that a system can act "in contradiction" to it's environment in such a way that it gets destabilised.

- The feedback from experiements are nothing buth the environments backreaction onto the observing system(which here is the gamer and scientist all in one). Now the environment will exert on the system a selective pressure to adjust their theory(which rules ther action just like the hamiltonian does) so as to maximize it's chances of survival and continued existence. Those that fail to LEARN, will get outcompeted. So the results of the experiments is used to IMPROVE our theory.

In this picture, it's irrelevant ot ask of a theory is "wrong", becase except for trivial cases a theory is of course wrong most of the time in fact for the simple reason that it's always improving.

So the issue is to howto update and revise the thery in the light of falsification, or equivalencenty in the light of threatening backreaction from the environment. The game is "adapt or die". Soon one realizes that the theory becomes unified with the theory of howto
revise the theory itself; thus this completely does away with the platonic timeless idea of "physical law".

So a theory should not postdict historical observations; a theory should tell us what to expect from the future, and thus which experiments to make. There is a close development between theory and experiment as there always was in physics, but it was never formalized in the way I suggest. Look at the colliders like LHC - such a massive choice isn't made for a random reason. It's dictated by current theory. This is how things work, but why no one takes this seriously enough to see what it implies for the theory itself is amazing. Apparently we need a nutcase to consider that.

/Fredrik

 

[Ken G]

I have no idea how you manage to equate this with "extreme faith in postulates" or "radical rationalism".

Because that exact principle is what underlies the MUH. Now, would you, or would you not, describe MUH as "extreme faith in postulates"?

Your phrasing is odd, leaving me to suspect you aren't describing MWI-esque interpretation at all. It's usually stated something to the effect of "don't add postulates about something already explained by the other postulates -- especially when your new postulate is that the theory is wrong, but only in an effectively unobservable way".

Except that would be quite an incorrect way to frame MWI. There is something that is very clearly not explained in the postulates-- why, if the evolution is unitary, do we perceive an eigenvalue whenever we do a measurement? MWI has nothing to say on that issue, it completely punts it.

 

[Ken G]

The good empiricists would say "we need to do more experiments" to get more information to help them understand what's going on.

The good rationalists would say "we need to look for new theories that can explain these new results, and still reproduce what we already know!"

You are simply confusing "empiricists" and "rationalists" with "experimental physicists" and "theoretical physicists." However, it is perfectly possible for an experimentalist to be a rationalist, and a theorist to be an empiricist.

(and, of course, they would ideally operate synergistically, rather than totally independently, and many would be both)

Obviously experiment and theory are synergistic in science. I'm talking about the philosophical bent that motivates CI and MWI, which has nothing to do with anything you are saying right now.

 

[Eqblaauw]

ok, if you don't like the authority argument and think you have a genius mind that's above all that, (you're the Newton, and the rest is the church) than this message means nothing, that being said, I currently conduct a poll, I've mailed over 70 prominent physicists all having quantum mechanics as (at least) one of there main specialities, none of which I knew their ideas in advance, I asked the following question:

a: I subscribe to the MWI that contains many parallel universes that differentiate during every 'quantum event' (meaning that there are many perhaps an infinite number of copies of everyone that inhibits earth)

b: I subscribe to the MWI that contains many parallel universes that split during every 'quantum event' (meaning that there are many perhaps an infinite number of copies of everyone that inhibits Earth constantly being created)

c: I don't subscribe to a or b, because I think they are both false

d: I don't prefer any of the above

this where the answers (I admit that it's a very small number of answers) (the only answer that I don't mention in the following results is one a, but only to him I didn't added ' meaning that there are many- infinite copies of everyone, so I don't know what he thinks about that) but you can count that as one a if you like

- c/d,
- c,

- d,
- c/d
- c
- d
- c

- c

and one physicist says: I'm sorry but I can't do anything with this, I can't make comments about some theory that doesn't have empiric consequences, it's a believe, and has nothing to do with physics

I can't name any names, because I promised I wouldn't do so unless they would give me there permission, and I didn't asked that directly to any of the people in the poll, but they all earned there marks,

it may be a bit early, but it seams david raub was either misquoted, misleading, or a fraud

 

[Hurkyl]

Because that exact principle is what underlies the MUH. Now, would you, or would you not, describe MUH as "extreme faith in postulates"?
Except that would be quite an incorrect way to frame MWI. There is something that is very clearly not explained in the postulates-- why, if the evolution is unitary, do we perceive an eigenvalue whenever we do a measurement? MWI has nothing to say on that issue, it completely punts it.

Where the heck did you get that idea? Well, maybe I should ask first what you actually mean by your question.

We, of course, don't "perceive an eigenvalue"; we perceive experimental outcomes and consistency of observations. You already agreed that this is consistent with mixed states without insisting on any sort of objective collapse.

The starting point of MWI is that entirely through unitary evolution (two subsystems becoming entangled, and jointly decohering), a mixed state can be produced. Suitable entanglements wold be called measurements, and the decoherence ensures that (for all practical purposes) this description remains stable and reproduces the classical picture.

 

[Hurkyl]

I'm talking about the philosophical bent that motivates CI and MWI, which has nothing to do with anything you are saying right now.

It looked like you were trying to find classical analogies to the details of the interpretation, rather than saying anything on topic about the philosophies. And as I was trying to point out, it is far too soon in your hypothetical for such questions to be seriously considered.

The philosophical bent that motivates my preference for MWI (for me, anyways) is that science is the best way we have for "understanding how the universe behaves". So long as science gives right answers, when science and intuition conflict, science wins. One should disdain the idea of contorting science to conform to one's intuition, philosophy, or prior knowledge.

That's what kills CI for me -- it forces the science through contortions. Among the various variations, the worst contortion is "oh, QM isn't telling us about how the universe behaves. It's just telling us how to manipulate our information." I say it's the worst, because even when taken at face value, it actively opposes the idea that science is the best way we have for "understanding how the universe behaves." But in practice, it's not meant as face value and is really being used to rationalize not thinking about questions of interpretation rather than being used in a constructive manner.


What's an example of it being used in a constructive manner? I mentioned earlier the idea that (in classical mechanics) if we take physical propositions and the logical inferences we can make with them to be the important thing, this leads directly to the idea of setting up a mathematical universe^* whose truth values are subsets of phase space.

I asked on math overflow if there were any corresponding notions for quantum mechanics. The answers were interesting, although I haven't really digested them yet.


*: Meant in the sense of "collection of mathematical objects satisfying the axioms of set theory" rather than "things that are real"

 

[Ken G]

Where the heck did you get that idea? Well, maybe I should ask first what you actually mean by your question.

You didn't answer the first question, about MUH.

We, of course, don't "perceive an eigenvalue"; we perceive experimental outcomes and consistency of observations.

I would agree that is pretty obvious. Yet those outcomes are designated by what name in the theory of quantum mechanics? Eigenvalues. This hardly seems controversial.

You already agreed that this is consistent with mixed states without insisting on any sort of objective collapse.

Yes, it is consistent with mixed states, but we can still admit that we have a perception there, can we not? Can we not even agree that when you read a dial on a measuring device that you are perceiving something? And that something is called an eigenvalue of the operator?

The starting point of MWI is that entirely through unitary evolution (two subsystems becoming entangled, and jointly decohering), a mixed state can be produced.

Actually, that is the ending point of the theory of quantum mechanics. The starting point of MWI is what to make of that mixed state.

 

[Fra]

What's an example of it being used in a constructive manner?

Is ANY _pure_ interpretation supposed to be constructive?

I for one has said I think that the interesting things is where we try to extend current theory, and interpretations just show us directions.

In that sense, how is MWI constructive in solvin open questions? does it suggest any research directions that aim to solve issues in QG, and unification?

I think I've already added how my view is supposed to be constructive, but while it's rooted in CI thinking, it's much more. But then, if we don't add more... what's left? How can an interpretation be constructive?

/Fredrik

 

[Ken G]

It looked like you were trying to find classical analogies to the details of the interpretation, rather than saying anything on topic about the philosophies.

All interpretations are philosophy, so I can't see what distinction you are trying to draw.

And as I was trying to point out, it is far too soon in your hypothetical for such questions to be seriously considered.

How could it be too soon? If that were the physics of the day, that is what would require interpretation (for any but the "shut up and calculate" types, of which I've never actually seen a true example).

The philosophical bent that motivates my preference for MWI (for me, anyways) is that science is the best way we have for "understanding how the universe behaves".

Yes, I get that, you have a philosophical bent which describes what you view the mission of science to be. I'm trying to get you to see that your philosophical bent has a well-known name, it is called rationalism. What's more, your rationalism is relatively untempered, as is generally true for anyone who takes MWI seriously.

One should disdain the idea of contorting science to conform to one's intuition, philosophy, or prior knowledge.

That is the claim I am trying to get you to see the inconsistency in. Because that is just exactly what you are doing when you adopt MWI, you are contorting science to conform to your rationalistic intuition that the truth corresponds to the most straightforward interpretation of the mathematical postulates, rather than the empiricist intuition that the truth corresponds to the most straightforward interpretation of our consistent perceptions. Rationalism sees observations as the only ways we can check theories despite their limitations, empiricism sees theories as approximate and limited yet unifying ways to understand observations.

That's what kills CI for me -- it forces the science through contortions. Among the various variations, the worst contortion is "oh, QM isn't telling us about how the universe behaves. It's just telling us how to manipulate our information."

But don't you see, what you call a "contortion" is very simply an axiom of all empiricism. Indeed, those who tend to see science as primarily an empirical discipline would say that all any theory can ever do is "tell us how to manipulate our information." The very fact that you see that mindset as a distasteful contortion is the clearest possible proof that you are a die-hard rationalist. I don't think there's any crime in that, because I think the "rationalist hat" is where all theories come from. I merely point out the historical record about the dangers of taking that rationalistic hat too literally.

What's an example of it being used in a constructive manner? I mentioned earlier the idea that (in classical mechanics) if we take physical propositions and the logical inferences we can make with them to be the important thing, this leads directly to the idea of setting up a mathematical universe^* whose truth values are subsets of phase space.

There's no doubt it's fruit for useful discussions and even insights-- as long as one does not take it too literally, for that requires entering into a form of naivete.

 

[Hurkyl]

You didn't answer the first question, about MUH.

I don't have anything to say about MUH. *shrug* There's a erason why I

I would agree that is pretty obvious. Yet those outcomes are designated by what name in the theory of quantum mechanics? Eigenvalues. This hardly seems controversial.

An eigenvalue is just a number, a label; there is nothing all that special about it. The particular number isn't even all that special -- measuring "3X+7" is effectively the same thing as measuring "X". You've seemed to put some emphasis on it specifically being an eigenvalue, which I don't understand.

Each outcome corresponds to some subset of the state space of the measuring device. Each observer perception corresponds to some subset of the state space of the observer.

A measurement is an interaction between the measuring device and a system. If the system is in an eigenstate, the measuring device transitions^* from some state in the "unmeasured" class to some state in the appropriate class. (this is enough to work out how it should behave for any state, so long as the system and the measuring device start out essentially independent of each other)

*: This is true in the thermodynamic sense -- things could still go wrong with very low probability

 

[Ken G]

You've seemed to put some emphasis on it specifically being an eigenvalue, which I don't understand.

I put no emphasis on the label "eigenvalue," I put emphasis on two things:
1) we perceive definite outcomes of observations
2) the theory of quantum mechanics has a name, and a prescription, for referring to those definite outcomes, and that name is eigenvalue and that prescription involves associating measurements with operators. So #1 is what we perceive, and #2 is how we attempt to understand and unify those perceptions. No interpretations yet.

Each outcome corresponds to some subset of the state space of the measuring device. Each observer perception corresponds to some subset of the state space of the observer.

That depends on what you mean by the "state space of the observer." If by that, you mean, all the things that the observer is perceiving, then I agree with you. If you mean by that a one-to-one correspondence with the state space of the measuring device, that is exactly what I am saying an empiricist would never allow. The observer state is a state of perception, not a subset of a larger space that is not perceived. Empiricism would always view such a larger space as comprising of angels doing a nice little dance on a pin.

 

[Samshorn]

I wasn't aware that SR was more easily falsified than LET though, because they both make all the same predictions. I felt SR was favored more on Occam's razor-- if you don't need an aether, why have one? But if SR was proved wrong, and there is an "aether frame", then Lorentz' version would be wrong too.

It’s true that the Lorentzian ether interpretation and the spacetime interpretation are empirically equivalent for all presently known phenomena, but they stand differently in regard to falsifiability in the face of any future phenomena that might be discovered. In the etheristic interpretation, Lorentz invariance represents a large number of independent coincidental facts: electromagnetism happens to be Lorentz invariant, the strong nuclear force happens to be Lorentz invariant, mechanical inertia of every known elementary particle happens to be Lorentz invariant, and so on. There is no conceptual link between these (once the electromagnetic view of the world was ruled out), so for any new class of phenomena that might be discovered, the ether interpretation really gives no warrant to believe it would be Lorentz invariant. The ether can be given whatever properties it needs to conform with any new facts. Indeed this was Lorentz’s professed reason for continuing to prefer his interpretation. He said we shouldn’t relinquish the language of an absolute rest frame, because we might need it some day. In contrast, the spacetime interpretation takes all those coincidences and removes them from the individual phenomena, and accounts for them in terms of the Minkowskian structure of spacetime itself. In this interpretation, any new particle or interaction that might be discovered tomorrow is constrained to be (at least locally) Lorentz invariant. The only way the spacetime interpretation is viable is if (local) Lorentz invariance is universal and complete. If it fails for any phenomenon, then the single unified spacetime interpretation fails, and we must go back to treating the Lorentz invariance (or lack thereof) of each phenomena as an independent fact, as it is in the ether interpretation. It’s in this sense that the spacetime interpretation is much more exposed to falsifiability than is the etheristic interpretation. Think of all the new phenomena, interactions, and particles that have been discovered subsequent to 1905, not to mention the increase in the range of parameters explored by experiment. Any one of these new classes of phenomena or observations might have been found to violate (local) Lorentz invariance (think CERN neutrinos...) and rendered the spacetime interpretation unviable, but none of them did – not even the entanglement aspects of QM. But the Lorentzian framework would not have been invalidated by whatever might have been found.

I'm not saying that there is something special about quantum postulates, I'm saying there is something special about postulates in the first place. The empiricist view is that all theories are approximate, so none of their postulates should be assumed to hold precisely.

But it isn’t necessary for any particular set of postulates to hold precisely in order for someone to espouse an MWI interpretation, is it? I think we agree that one could have an MWI of virtually any theory, so revising the postulates of QM wouldn’t necessarily drive someone away from MWI.

The question of whether we insist that our current theory is precisely correct, or whether we acknowledge that our current theory may be subject to future revision, is separate from the question of whether a many-worlds interpretation is useful or sensible. I think the espousal of MWI doesn’t necessarily commit someone to the position that the postulates of QM are precisely correct, nor does it imply that they would have to abandon MWI if/when the postulates of QM were revised.

Since MWI is never falsifiable, it is only the motivation for it that is falsifiable (the aesthetic structure of the postulates).

I agree that if the experiential theory (in this case, QM) changed, it is conceivable that the revised theory might give people less – or more – motivation to think in terms of MWI. For example, Deutch thinks we could perceive the other worlds after all, and he would take this as proof positive of the reality of the many worlds. This would entail a revision of QM, but would actually (according to Deutsch) increase our motivation for MWI. Of course, one could also imagine changes to QM that would make MWI seem less plausible. So the fact that the postulates of QM might be revised at some point doesn’t really argue univocally either way, for or against MWI, because as a conceptual framework it has none of the rigidity of, say, the spacetime interpretation of special relativity. It is much more akin to an ether theory, that could accommodate any set of observations.

It's very hard to say just what it is exactly about QM, rather than any other theory based on postulates, that supports an MWI approach, when no other theory ever did (though it could be argued that Parmenides, 2500 years ago, did in a sense suggest a similar theory, but it was pre-scientific).

I agree that it’s interesting to consider what motivates the MWI approach, and whether some experiential theories give more motivation than others, or if it is just an accident of fashion. Throughout history, people have contemplated the notion that the world of our experience is just one of many “possible worlds”, along with the ancient idea that every mathematical form has physical meaning. Ideas like this have been around forever. Remember Leibniz discussing what sounds a lot like the principle of least action (his Born rule), asserting that we live in “the best of all possible worlds”, which is quite similar to how many modern advocates of MWI conceive of things, i.e., possible worlds rather than actual worlds.

The notion of multiple possible worlds (and counterfactual definiteness, etc) arises in any theory based on differential equations. Newton and his contemporaries were acutely aware of this “deficiency” in any such theory, and some even argued that Newton’s “mathematical principles” were non-scientific and vacuous, complaining that a theory such as Newton’s tells us nothing that is not already implicit in the initial conditions, which must be put in by hand. It’s difficult for us to grasp this today, but when the idea of a differential equation representing a physical theory was new, it was not immediately accepted as even accomplishing anything. The equations of Newtonian mechanics may be said to imply a giant phase space, within which our universe is represented by a single trajectory, so it is a superb descriptive tool, but the theory doesn’t tell us WHICH trajectory in this enormous phase space is THE trajectory of THE universe, which is what many people regarded as the main task of a theory of natural philosophy. In Newton’s theory we have to specify a full set of initial conditions, sufficient to fully define everything of interest (per Laplace’s vision of determinism). This specification provides nearly ALL the information of the theory, and yet it is purely ad hoc and separate from the differential equations that supposedly define the theory. This could easily lead someone to eschew the task of specifying THE trajectory, and simply take the equations themselves as the entire theory, which entails ALL possible trajectories within the phase space. This certainly yields a more symmetrical and less ad hoc interpretation... but it's also sterile and pointless.

We certainly encountered lack of complete information in theories before, like in thermodynamics. I think it was the discovery of a fundamental limit on information, the HUP, that is the real source of MWI...

I agree that the inherent uncertainty in QM is what leads to the extra motivation for MWI. The key difference between Newtonian mechanics and quantum mechanics (for motivating MWI) is that in Newtonian mechanics the equations of motion range only over the set of observable states, so there is no ambiguity when translating from the variables in the equations to the measures of our experience, whereas in quantum mechanics the equations of motion range over the set of superpositions of observable states. People tried hard to avoid this, and to come up with equations that range only over observable states, but eventually it came to be seen as an impossible task. Quantum phenomena apparently cannot be modeled effectively except by equations that range over superpositions of observable states. This means that an extra step is required when going from the variables of the equations of motion to an actual observed outcome. This is the “collapse of the wave function”, in accord with the Born rule, and the special challenge for interpreting quantum mechanics is how to conceive of this extra step, selecting just one trajectory through phase space, and calling it OUR trajectory. But this is not really as different as it might seem from the extra step that is required in Newtonian clockwork mechanics, i.e., the stipulation of initial conditions, which essentially represents a collapse from the set of all possible trajectories down to the single trajectory that we experience. The difference is that Newtonian mechanics requires only one “collapse” step, because for any given exact specification of the state, the equations of motion give exact specifications of all subsequent (and prior!) observable states. Hence we never need to repeat the collapse step, and we can set aside the single collapse step and not worry about it too much. In contrast, because of the inherent uncertainty in QM, after stipulating an initial state, the equations of motion lead to a superposition, and even if we stipulate an observation and update our state, the equations of motion again lead to a superposition, and so on. Thus we encounter the need for collapse repeatedly, making it more difficult to ignore. But it isn’t a qualitative difference, it’s just a quantitative difference, relative to the collapse required in Newtonian mechanics.

Ultimately I think a MWI is no more useful for QM than it is for Newtonian mechanics. We can always imagine that the world of our experience is embedded in a larger structure of other worlds, but if this imagined embedding doesn’t tell us anything new about the world of our experience, then it’s pointless.

 

[Ken G]

I understand and agree with much of what you are saying. These are the points I found particularly insightful:

The ether can be given whatever properties it needs to conform with any new facts. Indeed this was Lorentz’s professed reason for continuing to prefer his interpretation. He said we shouldn’t relinquish the language of an absolute rest frame, because we might need it some day.

Any one of these new classes of phenomena or observations might have been found to violate (local) Lorentz invariance (think CERN neutrinos...) and rendered the spacetime interpretation unviable, but none of them did – not even the entanglement aspects of QM. But the Lorentzian framework would not have been invalidated by whatever might have been found.

Yes, I take your point now about "flexibility" as not necessarily such a good thing for an interpretation to have.

Throughout history, people have contemplated the notion that the world of our experience is just one of many “possible worlds”, along with the ancient idea that every mathematical form has physical meaning. Ideas like this have been around forever. Remember Leibniz discussing what sounds a lot like the principle of least action (his Born rule), asserting that we live in “the best of all possible worlds”, which is quite similar to how many modern advocates of MWI conceive of things, i.e., possible worlds rather than actual worlds.

Yes, it's hard to come with an idea that hasn't been anticipated in some way!

It’s difficult for us to grasp this today, but when the idea of a differential equation representing a physical theory was new, it was not immediately accepted as even accomplishing anything. The equations of Newtonian mechanics may be said to imply a giant phase space, within which our universe is represented by a single trajectory, so it is a superb descriptive tool, but the theory doesn’t tell us WHICH trajectory in this enormous phase space is THE trajectory of THE universe, which is what many people regarded as the main task of a theory of natural philosophy. In Newton’s theory we have to specify a full set of initial conditions, sufficient to fully define everything of interest (per Laplace’s vision of determinism). This specification provides nearly ALL the information of the theory, and yet it is purely ad hoc and separate from the differential equations that supposedly define the theory. This could easily lead someone to eschew the task of specifying THE trajectory, and simply take the equations themselves as the entire theory, which entails ALL possible trajectories within the phase space. This certainly yields a more symmetrical and less ad hoc interpretation... but it's also sterile and pointless.

Excellent points. I have long recognized that physics is lacking a "theory of initial conditions", but I hadn't appreciated what a seed change it was to have to think in terms of differential equations for those who were used to seeking the "proper place" of things, the "natural outcomes" that ought to transcend initial details.

This is the “collapse of the wave function”, in accord with the Born rule, and the special challenge for interpreting quantum mechanics is how to conceive of this extra step, selecting just one trajectory through phase space, and calling it OUR trajectory. But this is not really as different as it might seem from the extra step that is required in Newtonian clockwork mechanics, i.e., the stipulation of initial conditions, which essentially represents a collapse from the set of all possible trajectories down to the single trajectory that we experience.

Indeed, it is no different at all if one adopts deBB-B, it is more or less the driving purpose of deBB-B to maintain that classical structure. How ironic, the same element that was so unsatisfactory to so many when the differential equation concept was originally introduced, is now the driving feature of one of the main interpretations of QM!

Ultimately I think a MWI is no more useful for QM than it is for Newtonian mechanics. We can always imagine that the world of our experience is embedded in a larger structure of other worlds, but if this imagined embedding doesn’t tell us anything new about the world of our experience, then it’s pointless.

This is also my main objection to MWI, and it seems to center on our desire to make "the world of our experience" the primary target of doing science. That's why I stress the fundamental empiricism in our stance here-- I can easily see that those who want to make "the world of mathematical concepts" the primary target of science, the rationalists, would not share our emphasis here.

The one point you made that I still disagree with is:

The question of whether we insist that our current theory is precisely correct, or whether we acknowledge that our current theory may be subject to future revision, is separate from the question of whether a many-worlds interpretation is useful or sensible. I think the espousal of MWI doesn’t necessarily commit someone to the position that the postulates of QM are precisely correct, nor does it imply that they would have to abandon MWI if/when the postulates of QM were revised.

I don't think MWI-enthusiasts like MWI because of its many-worlds element, which you are quite right they could preserve in any theory, they only like it because of the simplicity and generality of the unitarity postulate, and the many worlds just come along for the ride. If unitarity were broken in observations, I think the rationalists in the MWI camp would jump ship immediately. They would of course look for a new set of postulates with the same unifying power, as physics requires, and taking those new postulates literally (as rationalists are apt to do) might lead them to adopt even more bizarre constructs than the many worlds to embrace the new postulates. But in this sense, the MWI camp actually has a less flexible and more falsifiable stance than either CI or deBB-B-- if unitarity were broken, they would consider MWI to be falsified, whereas CI already thinks unitarity is broken in the act of perception so it wouldn't care much if it were broken in the other postulates, and deBB-B is happy with anything that preserves strict determinism, regardless of whether it is unitary or not. It is harder to falsify empiricism or determinism, when they are taken as axioms, than it is to falsify unitarity, which is the crux of MWI.

Still, I don't think minimal flexibility is the most important design characteristic of a scientific interpretation. It's an important one, as is Occam's Razor, but to me the most important one of all goes something like "don't use the postulates of a theory to build a world view, use observations for that", because the postulates change but the observations don't. So I don't object to MWI when it is framed as "an interesting way to look at the theory of quantum mechanics", I object to it when framed as "a surprising aspect of reality."

 

[nazarbaz]

The real issue in this debate is the status of the mathematical language in scientific research. Does any chunk of coherent mathematics have the right to make ontological claims ? Or is it properly the mission of science to decide what mathematical models fits the data and would enable us to understand the true nature of matter and the way it is made, even if it is continually delayed ?
The relation between the subject and the object is the cornerstone of all the theories of knowledge since Kant. Although I am not sure we can call it a "dichotomy", since we all aim to gain some knowledge from the practice of science or thought. Science is a set of methodologies to deal with the observable world, not to admire coherent mathematical hypothesis.

 

[Ken G]

Spoken like a true empiricist! And I agree that "dichotomy" might be too strong a word for the subject/object separation, it's more like a "necessary pretense" in order to apply the axioms of the scientific method. In my opinion, when science finally figures out a good way to include the physicist in the physics, it will be a revolution akin to the development of the scientific method itself. It could be argued that the scientific method as we know it today really stemmed more from the demonstrable empiricism that is so central to medicine, rather than from more rationalistic natural philosophy, and perhaps the new physics, that includes the physicist, will stem from neurology or psychology, rather than physics itself. That's just speculative of course-- at the moment, I don't think we really have any kind of reasonable way to avoid the subject/object separation that is the cornerstone of empiricism.

 

[Hlafordlaes]

I've been staying on the sidelines, careful to not break the flow with my less-than-knowledgeable input, but I just have to ask:

If MWI purports that for every collapse event a new universe is spawned, why would our own universe seem to have a long history starting with a BB? Seems contradictory.

Or if MWI considers that there are multiple worlds that are all sort of similar sharing their part of each outcome (each observed eigenvalue), how on Earth could the worlds be coordinated over time? How could each have a "me" flipping a coin or observing a collapsed value, when after some n number of generations of collapses each world would have vastly differing observers and observation events? This view seems impossible.

Neither makes any sense to me, so perhaps one of you gents can straighten me out.

Late edit: Broke two rules above: never post when exhausted, and read the darn wiki first. Sorry; pls ignore.
...

PS Ken G, here's hoping I get the time to start a thread here on d'Espagnat's "The Quantum Theory and Reality" and see how that meshes/messes with your fine empiricist outlook (which I'd like to subscribe to, but gets fuzzy when dear Bernard raises his objections, iirc). Mebbe you can start the thread, seeing as how you don't have to run upstairs and serve beers like I do.

 

[Ken G]

If MWI purports that for every collapse event a new universe is spawned, why would our own universe seem to have a long history starting with a BB?

I wouldn't say a new universe is spawned, it's just that the one universe fragments more and more with time (by "fragment" I mean "spawns incoherent islands that have no further influence on each other"). So it's rather like a river forking over and over-- it still all traces back to the same upstream watershed. We are like an ant floating on a log that takes, seemingly at random, a certain fork each time, but the rest of the river is still there, and all branches share a consistent history up until they forked off.

How could each have a "me" flipping a coin or observing a collapsed value, when after some n number of generations of collapses each world would have vastly differing observers and observation events?

There wouldn't necessarily be any "me" in many of the forks, if I died in some of them, or if I changed so much that it was no longer recognizable as me. Indeed, it seems to me that "I" would only ever be in one of the worlds anyway, but that depends on what is meant by "me"-- this is one criticism I have of MWI, it requires a "model of me" that sounds an awful lot like CI-type collapse if you're not careful. MWI goes to a lot of trouble to maintain unitarity in the universe, but it can never maintain the unitarity of "me", unless "I" am just a projection of something else. How can a projection think for itself? What happens to "I think therefore I am"? There's very little left in MWI of anything I can call "me" without invoking the very collapse that MWI seeks to avoid. This certainly doesn't make MWI wrong, but it shows why I can call it a "radical" form of rationalism.

Indeed, my latest insight into all this interpretation business is that interpretations are kind of like gedankexperiments. Many people seem to think gedankenexperiments are a way to achieve understanding of reality without leaving their armchairs, but you can't understand reality that way, you can only understand a particular theory that way. Gedankenexperiments are ways of probing theories, only real experiments can probe reality. All the same, they are very valuable for understanding theories, and no one would ever "object" to any gedanken on the grounds that a different one is "better." When we look at our theories, we should look at them from all angles, and so we should address all the gedankenexperiments, as well as all the viable interpretations. We just shouldn't imagine we are learning about reality when we do that-- we are learning about our theories.

To the extent that the theories work well, however, we must be accessing some kind of truth about reality via the theories, but it's not a world view we are getting, it's just some kind of taste of the truth, or insight that fits in our heads rather than the actual truth. Maybe every interpretation has some kind of valid lesson about reality to teach us, even though they only help us understand what our theories are saying. Most likely only one (or zero) will survive the next theory, but even so the lesson of each is learned all the same, and captures some shadow of the reality or it wouldn't have been a valid interpretation of a good theory in the first place.

 

[Hlafordlaes]

I wouldn't say a new universe is spawned, it's just that the one universe fragments more and more with time (by "fragment" I mean "spawns incoherent islands that have no further influence on each other"). So it's rather like a river forking over and over-- it still all traces back to the same upstream watershed. We are like an ant floating on a log that takes, seemingly at random, a certain fork each time, but the rest of the river is still there, and all branches share a consistent history up until they forked off.

OK, but aren't the branches happening in a considerably large number of events all across the cosmos, or is the idea only related to observers "forcing" a collapse? If the former, isn't all of spacetime forking at extraordinarily frequent intervals? As for the later, this seems related to the MWI counter-argument against it violating conservation of energy, but in either case it seems to my poor imagination that such a vast replication of matter and energy couldn't go on for long, at least not until an third inexhaustible source from which all branching draws substance were found. (Meaning it seems unlikely; no suggestion of things religious.)

There wouldn't necessarily be any "me" in many of the forks, if I died in some of them, or if I changed so much that it was no longer recognizable as me. Indeed, it seems to me that "I" would only ever be in one of the worlds anyway, but that depends on what is meant by "me"-- this is one criticism I have of MWI, it requires a "model of me" that sounds an awful lot like CI-type collapse if you're not careful. MWI goes to a lot of trouble to maintain unitarity in the universe, but it can never maintain the unitarity of "me", unless "I" am just a projection of something else. How can a projection think for itself? What happens to "I think therefore I am"? There's very little left in MWI of anything I can call "me" without invoking the very collapse that MWI seeks to avoid. This certainly doesn't make MWI wrong, but it shows why I can call it a "radical" form of rationalism.

I think I agree, methinks.

Indeed, my latest insight into all this interpretation business is that interpretations are kind of like gedankexperiments. Many people seem to think gedankenexperiments are a way to achieve understanding of reality without leaving their armchairs, but you can't understand reality that way, you can only understand a particular theory that way. Gedankenexperiments are ways of probing theories, only real experiments can probe reality. All the same, they are very valuable for understanding theories, and no one would ever "object" to any gedanken on the grounds that a different one is "better." When we look at our theories, we should look at them from all angles, and so we should address all the gedankenexperiments, as well as all the viable interpretations. We just shouldn't imagine we are learning about reality when we do that-- we are learning about our theories.

To the extent that the theories work well, however, we must be accessing some kind of truth about reality via the theories, but it's not a world view we are getting, it's just some kind of taste of the truth, or insight that fits in our heads rather than the actual truth. Maybe every interpretation has some kind of valid lesson about reality to teach us, even though they only help us understand what our theories are saying. Most likely only one (or zero) will survive the next theory, but even so the lesson of each is learned all the same, and captures some shadow of the reality or it wouldn't have been a valid interpretation of a good theory in the first place.

Well, this last is something you've been pointing out a bit, and it certainly is pertinent to those like me who wish for proper world views, when all we have to rely on are the facts on the one hand and our fancies on the other. Certainly at a basic level, of course, the simple theories required for survival and evolution work well enough we may be confident of a close mapping to reality. As for much of the esoteric rest, everything is defined by everything else and all definitions become circular after a while, so a complete world view can only be something perhaps as simple as "it is."

There's a cautionary tale, as well, in steering clear of overarching theories than span too many domains, as is often the case in social science and, er, politics and religion (Color commentary, not intended to divert the thread.)

 

[Ken G]

OK, but aren't the branches happening in a considerably large number of events all across the cosmos, or is the idea only related to observers "forcing" a collapse?

There isn't any collapse, there's just decohering islands. Basically, there isn't enough information in the initial state to maintain all the coherences that would be required as more and more subsystems interact, so coherences are restricted to tiny subspaces called "worlds." When there's no coherences, there's no crosstalk, so there's no influences either, the branches don't affect each other. But they also cannot perceive each other, they are angels on each other's pins. And yes, the number of such branches is truly spectacular.

As for much of the esoteric rest, everything is defined by everything else and all definitions become circular after a while, so a complete world view can only be something perhaps as simple as "it is."

Yes, it's hard to imagine we can really do better than that. It's fine to "run with the ball", but we must also keep in mind the dangers of taking ourselves too seriously.

 

[BruceW]

There isn't any collapse, there's just decohering islands. Basically, there isn't enough information in the initial state to maintain all the coherences that would be required as more and more subsystems interact, so coherences are restricted to tiny subspaces called "worlds." When there's no coherences, there's no crosstalk, so there's no influences either, the branches don't affect each other. But they also cannot perceive each other, they are angels on each other's pins. And yes, the number of such branches is truly spectacular.

Yep, that's mostly right. But there is always cross-talk and different branches always affect each other. The important thing is that once decoherence has happened, this cross-talk is extremely small, and the effect of other branches also becomes extremely small.

So what kind of experiment do we need to distinguish MWI from CI? This depends on what causes the non-unitary collapse in CI. If you define non-unitary collapse to happen when a 'classical measuring apparatus' is entangled, then to disprove CI we simply need to get classical objects in two different branches to affect each other. (In other words, interference between classical objects). This is technologically incredibly difficult.

This same argument goes if we define non-unitary collapse to happen when any human makes a measurement. In this case, we would need to make an interference experiment involving a human (even harder to do, and we may never have the technology to do this).

 

[nazarbaz]

I wouldn't say a new universe is spawned, it's just that the one universe fragments more and more with time (by "fragment" I mean "spawns incoherent islands that have no further influence on each other"). So it's rather like a river forking over and over-- it still all traces back to the same upstream watershed. We are like an ant floating on a log that takes, seemingly at random, a certain fork each time, but the rest of the river is still there, and all branches share a consistent history up until they forked off.
There wouldn't necessarily be any "me" in many of the forks, if I died in some of them, or if I changed so much that it was no longer recognizable as me. Indeed, it seems to me that "I" would only ever be in one of the worlds anyway, but that depends on what is meant by "me"-- this is one criticism I have of MWI, it requires a "model of me" that sounds an awful lot like CI-type collapse if you're not careful. MWI goes to a lot of trouble to maintain unitarity in the universe, but it can never maintain the unitarity of "me", unless "I" am just a projection of something else. How can a projection think for itself? What happens to "I think therefore I am"? There's very little left in MWI of anything I can call "me" without invoking the very collapse that MWI seeks to avoid. This certainly doesn't make MWI wrong, but it shows why I can call it a "radical" form of rationalism.

Indeed, my latest insight into all this interpretation business is that interpretations are kind of like gedankexperiments. Many people seem to think gedankenexperiments are a way to achieve understanding of reality without leaving their armchairs, but you can't understand reality that way, you can only understand a particular theory that way. Gedankenexperiments are ways of probing theories, only real experiments can probe reality. All the same, they are very valuable for understanding theories, and no one would ever "object" to any gedanken on the grounds that a different one is "better." When we look at our theories, we should look at them from all angles, and so we should address all the gedankenexperiments, as well as all the viable interpretations. We just shouldn't imagine we are learning about reality when we do that-- we are learning about our theories.

To the extent that the theories work well, however, we must be accessing some kind of truth about reality via the theories, but it's not a world view we are getting, it's just some kind of taste of the truth, or insight that fits in our heads rather than the actual truth. Maybe every interpretation has some kind of valid lesson about reality to teach us, even though they only help us understand what our theories are saying. Most likely only one (or zero) will survive the next theory, but even so the lesson of each is learned all the same, and captures some shadow of the reality or it wouldn't have been a valid interpretation of a good theory in the first place.

That sounds a bit like a postmodernist view of science with some ideas close to Thomas Kuhn concept of paradigm and Michel Foucault's (or Bachelard's) epistemological nick. These people freaked out when they hear about "progress" and "truth" in science.
I use these words with precaution too but I can't take these guys seriously since accumulation is an effective modality in the constitution of knowledge. To contest that, you have per exemple to prove that the structure of DNA is an "interpretation" with no links to the previous achievements of biology. They urge science to give them a "theory of everything" right now, which is a pretty violent demand, or they deny it the right to seek for truth. Curious, because they think of themselves as anarchists and libertarians. What they are trying to do is to split definitively the scientific experience, that relates subjects to objects, and to think of it as a mere creation of a theoretical subjectivity, since, for them, the world is unknowable per se, by nature.
These views are useful in some fields but their pretention to cover all the realm of thinking, and especially science, is dangerous. They deal the same way with truth, morality and aesthetics. Which is intellectually very suspicious.

 

[mitchell porter]

A few comments:

1) CI does not have to involve "collapses" caused by "measurement devices". In fact the whole idea of wavefunction collapse as anything other than an epistemic process, just like the "collapse of a probability distribution" when you acquire new information, is alien to CI as originally intended - although many people now speak casually about wavefunction collapse as an actual process, a side effect of the de facto realism regarding wavefunctions that has arisen.

Nonetheless, if you refer e.g. to Schrodinger's original article in which he introduces the cat thought-experiment, he clearly distinguishes between two interpretations of the wavefunction's meaning: one as physical state, the other as "expectation-catalog", i.e. something that tells you probabilities about reality, not something that is reality itself.

If we define CI as the view that observables are what's real and that wavefunctions are not real things, then interference effects involving macroscopic devices will not falsify it. The wavefunction of a macroscopic device will still just have been the mathematical construct you use to compute its probable behaviors, and not the object itself, and you will still have zero empirical evidence of there being anything more than one world.

2) Most of these discussions of MWI overlook the fact that there is no derivation of the Born probabilities within MWI, and in fact if you count the worlds you get the wrong probabilities. I suppose it is a sign of the low level at which these discussions occur. They revolve around fuzzy intuitive ideas of "worlds splitting" and thereby increasing in number, or of "wavefunctions collapsing" when someone looks at them.

The fact is that if you do assume a wavefunction to be reality, and then chop it up into worlds in the obvious way, then all worlds are equally real, they all occur once, all outcomes are equally probable, and this is in conflict with experiment! Under other circumstances, MWI would be regarded as falsified by this observation. Casual advocates of MWI just assume that the usual quantum prescription for probability (the Born rule) also applies to their interpretation, but there's no reason why it should, unless worlds are actually duplicated in proportion to the associated probability, and this is a position that almost no-one (perhaps actually no-one) seriously advocates.

The reason is simple. If all outcomes are realized, then probabilities of possible events refer to their actual frequencies of occurrence in the multiverse. If all worlds occur once, then all possible events occur only once, and they are all equally probable, in contradiction to experiment. So if A empirically occurs twice as often as B, it had better be realized in the multiverse twice as often. This is why a really effective version of MWI would involve duplication of worlds: it's the obvious way to get the frequencies right.

Among actual physicists, advocacy of MWI usually means advocacy of a "no collapse" interpretation which doesn't involve a particular division of the wavefunction into worlds. Weeks ago in this thread I had some discussions with Hurkyl and another no-collapse advocate, in which I endeavored to show that this no-collapse interpretation is actually vacuous. It's just a slogan; it doesn't actually describe a picture of objective reality in which we can say which part of the wavefunction corresponds to observed reality. If you try to do that, it inevitably leads back to the naive version of MWI, which has problems, not just with obtaining the correct empirical frequencies, but also with relativity. It's not that it makes the wrong relativistic predictions (as I was just explaining, it already makes the wrong predictions if you just count the worlds), but no-one has yet managed to describe how to divide up a wavefunction in a way that doesn't involve a preferred set of spacelike hypersurfaces, i.e. an objective notion of simultaneity.

3) The debate over the meaning of quantum mechanics is therefore just full of confusions and illogic. Once, everyone believed in CI, but it seems that a lot of people had a simple "uncertainty" interpretation in their minds, according to which the real ontology is just like classical physics, except that the fields are fluctuating randomly and their statistics happen to match up with the quantum predictions. But from Bell's theorem forwards, work on "quantum foundations" has shown that this would require highly unusual correlations in the random fluctuations.

Also, Schrodinger's distinction between the two interpretations of the wavefunction has been lost on many physicists, who advocated CI while also speaking about wavefunctions as if they were real, and so the tendency to say that collapse of the wavefunction is a real physical process sprang up. This has encouraged the rise of the Many Worlds interpretation, but as I have attempted to indicate, that interpretation has a lot less going for it than people imagine. Forget the "weirdness" of multiple worlds, and so on. The important point is that it does not explain the probabilities, and the probabilities are the whole of the empirical content of quantum mechanics! But this point is hardly appreciated except among the few people who work seriously on trying to make MWI work as a physical theory - and the avantgarde there are pursuing the Deutsch-Wallace dodge of trying to derive probabilities from "multiverse decision theory" rather than from the counting of the worlds. In other words, unable to explain how their multiverse would produce the probabilities that we actually experience in the world, they have decided to redefine the very concept of probability in terms of "rational behavior in the multiverse". Then they confuse themselves with baroque exercises in quantum game theory, eventually surreptitiously slip in an assumption equivalent to the Born rule, and announce that they have managed to justify MWI.

All of that is in any case a sideshow; the majority of physicists who believe in MWI adopt the no-collapse interpretation, which is also only tenable if you avoid thinking about it too clearly. Basically, it involves using the Schrodinger equation for everything, treating wavefunctions as physical states rather than sources of probabilities for observables, and relying on decoherence to make sense of everything. But decoherence is simply a shift in the pattern of probabilities. The world we see is always particular - one specific outcome. If you want to say that this world is part of an uncollapsed wavefunction, then you must say that the various parts of the decohered wavefunction are all just as real as what we see, and this leads inevitably back to "naive MWI" and its problems in justifying the Born rule.

4) For people who want to know what's really going on: the answer is that we don't yet know. It could be that quantum probabilities are what you get in a universe in which causal chains go from future to past as well as from past to future, and perhaps quantum mechanics results from subatomic closed timelike curves, or from the self-consistency of a physics in which particles can zigzag in time as well as in space. But no-one has actually proven that this is where it comes from. The transactional interpretation, which is the best known of these zigzag-in-time interpretations, is also more concept or hype than mathematical substance, just like many worlds. It's just that Many Worlds has about 100 times as many believers as the transactional interpretation. But this is due only to sociology and historical contingency; many worlds has zero advantage over transactional interpretation in terms of evidence.

In my opinion, the true path to explaining quantum mechanics lies through arcane technical developments in the foundations of the most advanced physical theories we actually use. For example, the unfolding motivic and twistorial revolutions in quantum field theory, which have recently been promoted by Nima Arkani-Hamed among others. These are initially formal changes, a new calculus in which to perform the calculations, but they are deconstructing and reconstructing the whole conceptual framework, in a way which looks highly nonlocal from the ordinary space-time perspective. It is still impossible to say what the natural concepts to accompany the new formulation of QFT will be, but I will point out that when Penrose developed his twistors he was hoping to explain QM - they were supposed to be an extension of spin networks, which offer a combinatorial derivation of quantum angular momentum, that would also include spatial extension.

But they languished neglected for 30 years until Witten combined them with string theory and showed that the twistor string could explain some mysterious formulae (the Parke-Taylor amplitude) from gauge field theory. That was the start of the "twistor uprising" and it is still unfolding, largely hidden from public view. That is where the real explanation of QM is most likely to be found. Perhaps it will draw upon some concepts found in one of the already discussed interpretations (Bohm, Everett, Bohr...), but as I have been emphasizing, many of those interpretations are not conceptually self-sufficient; they borrow the Born rule from ordinary QM, unable to justify it within their own framework.

The new QFT, since it is starting life as an alternative calculational framework, does not have that problem, and what remains to be done is to round it out and to then find its natural interpretation.

 

[Hurkyl]

This could easily lead someone to eschew the task of specifying THE trajectory, and simply take the equations themselves as the entire theory, which entails ALL possible trajectories within the phase space. This certainly yields a more symmetrical and less ad hoc interpretation... but it's also sterile and pointless.

I wanted to make a comment on this specifically. We have a few hundred more years of understanding about logic and reason -- and ironically the 'sterile and pointless' better models how we actually do things.

The main thing that we can do today that we couldn't do back then is to consider the idea "Okay, we have this mathematical structure -- what sort of reasoning can be done internally to the structure?"

A corresponding philosophy is that that is what is actually important from a scientific theory: it tells us how to make inferences. It tells us things like "From X and Y, you can infer Z". Talking about "THE trajectory" was never the important thing; it was simply a means to the end of determining what inferences could be made.

 

[Hurkyl]

2) Most of these discussions of MWI overlook the fact that there is no derivation of the Born probabilities within MWI,

I've seen some and derived one myself -- e.g. that the weights on the mixture correspond to frequentist probabilities in the limit of infinitely many iid trials. But that aside, there doesn't have to be a derivation anyways.

and in fact if you count the worlds you get the wrong probabilities.

As well you should, since the counting measure has nothing to do with the probability measure.

The reason is simple. If all outcomes are realized, then probabilities of possible events refer to their actual frequencies of occurrence in the multiverse. If all worlds occur once, then all possible events occur only once, and they are all equally probable,

This is not an argument, this is somewhere between wordplay and willful ignorance.

 

[Ken G]

So what kind of experiment do we need to distinguish MWI from CI? This depends on what causes the non-unitary collapse in CI. If you define non-unitary collapse to happen when a 'classical measuring apparatus' is entangled, then to disprove CI we simply need to get classical objects in two different branches to affect each other. (In other words, interference between classical objects).

Actually, I don't think that would suffice. If QM predicts such interference exists, no matter how small or hard to detect, I find it pretty likely that such interference would be found in any experiment sensitive enough to detect it (though other new physics might appear also, at that level of sensitivity). In other words, if we go into the "middle domain" of "quasi-classical" systems, do any of us doubt that we will see clearly how decoherence happens, and when decoherence is incomplete? The classical realm is a realm that is understood quite well via the process of very-nearly-complete decoherence. All CI does is say that "reality" begins once decoherence is fully complete, and reality only exists to the extent that complete decoherence is meaningful. ("There is no quantum world"-- Bohr.)

The reason it does that is not because there must be complete decoherence, it is because complete decoherence is all we can perceive-- our brains are simply not built to "grok" superpositions of outcomes. It is very likely a limit of what we even mean by "perception" that it involves complete decoherence, just as it is very likely a limit of what we even mean by an "experiment" that it involves a subject/object separation. It is not the absence of a tiny bit of incomplete decoherence in a classical measurement that forces the CI interpretation onto us, just as it is not the complete absence of subject/object interactions that forces the scientific method onto us. It is instead the presence of the need for the axioms of empiricism that do both those things-- we must have something demonstrably definite to anchor our conclusions to or it's more like opinion than like science.

So an experiment to distinguish CI from MWI must be an experiment that proves quantum mechanics wrong, which we are motivated to look for regardless of what we think of those interpretations. The differences in CI and MWI are just useful for thinking of ways to find the cracks in QM itself, but watching the decohence occur the way QM predicts it would is not going to falsify CI, we still have the same basic issue that CI is empiricist, and empiricism must follow a direction of logic that starts at the fully decohered result. If one experiment shows only partial decoherence, then CI says that experiment is simply not finished yet.

Perhaps a different example will help see what I'm arguing. Take tunneling. What if there was a debate between people who felt wave functions were real, and people who thought they were just instructions for making predictions. The first camp could point to tunneling, and say, "well there's no classical analog to tunneling, so if we can just find an example where it happens in reality, then we would be falsifying the perspective that wavefunctions aren't real." Then they find tunneling, and claim victory. Trouble is, the other camp just says, "how does that prove wavefunctions are real? All you've shown is they can be used to make tunneling predictions. We already thought of them that way, you haven't told us anything we didn't already know."

Same for an experiment that shows "influences" between the "worlds"-- the empiricist CI person just says "so what, it just means quantum mechanics is making a correct prediction. That doesn't mean those other worlds are real, I would have to be perceiving them for them to be real, and I already know that I do not perceive them, unless you can build an apparatus that allows me to perceive them." It's not just that it's technologically impossible, it's that it isn't perception in the first place-- it is not the way our brains evolved to function, and the empiricist believes we are always stuck inside the world that our brains give us, no matter what postulates you like. Physics is what we can say about nature, not about nature herself-- Bohr again.

 

[Ken G]

That sounds a bit like a postmodernist view of science with some ideas close to Thomas Kuhn concept of paradigm and Michel Foucault's (or Bachelard's) epistemological nick. These people freaked out when they hear about "progress" and "truth" in science.

Not necessarily. We don't have to choose between either saying that there is no "progress" in science, or that the ultimate goal is a "TOE." I don't think either of those views have much of a clue about the progress of science as it has been, and I don't see any reason to expect the progress of science to change suddenly now. Instead, I'd say these lessons of the history of science are pretty hard to deny:
1) scientific progress looks like gradually improving accuracy achieved by radically different ontological pictures, and
2) every generation thinks their own ontological picture is "on the right track", regardless of how completely it differs from previous ontological pictures.
Given these two seemingly uncontroversial facts, I think we can conclude that scientific accuracy converges, but scientific ontology does not. The main problem with postmodernist science is that it is generally not done by scientists, but it does have some useful "cautionary" remarks to make, as you say.

 

[Ken G]

If we define CI as the view that observables are what's real and that wavefunctions are not real things, then interference effects involving macroscopic devices will not falsify it. The wavefunction of a macroscopic device will still just have been the mathematical construct you use to compute its probable behaviors, and not the object itself, and you will still have zero empirical evidence of there being anything more than one world.

Exactly, CI is automatically the interpretation used by empiricism, and is not falsifiable in any context where quantum mechanics works. Even then, it would only be the quantum mechanics that was falsified, not the basic tenets of CI. We would have to find observations that cannot be consistent with empiricism, and that is paradoxical. Thus CI-type thinking will never be falsifiable within anything we now consider to be science. All the same, what we consider to be science today could eventually be returned to its more rationalistic roots someday.

2) Most of these discussions of MWI overlook the fact that there is no derivation of the Born probabilities within MWI, and in fact if you count the worlds you get the wrong probabilities. I suppose it is a sign of the low level at which these discussions occur. They revolve around fuzzy intuitive ideas of "worlds splitting" and thereby increasing in number, or of "wavefunctions collapsing" when someone looks at them.

I think you are building a bit of a straw man here, which may have been Hurkyl's objection too. The crux of MWI is the unitarity of the time evolution of a closed system, not the splitting of worlds (despite its name!). MWI is not actually responsible for the Born rule, that rule must be viewed as part of QM proper, and MWI is merely a way to interpret QM proper such that the unitarity of closed systems is taken as inviolate regardless of what observations suggest when the observer is regarded as part of the closed system. That's why it is a rationalist approach rather than empiricist, it is a fundamentally different way to think about what science is doing. The weights of the splitting, given by the Born rule, is something very different-- we can't ask MWI to supply those any more than we ask CI to do that, the Born rule is just part of the meaning of the wave function regardless of which direction one takes the logic (from wave functions to the observations, as MWI does, or the other way around, as CI does).

3) The debate over the meaning of quantum mechanics is therefore just full of confusions and illogic.

I don't see any illogic, I see just the age-old debate between rationalism and empiricism. You understand that old debate, I presume, so I don't see why you think the modern debate is anything different, even if the core issues do sometimes get confused with verifiable predictions the theory is making.

Also, Schrodinger's distinction between the two interpretations of the wavefunction has been lost on many physicists, who advocated CI while also speaking about wavefunctions as if they were real, and so the tendency to say that collapse of the wavefunction is a real physical process sprang up.

Yes, I think there is a lot of confusion about what CI is really saying, and indeed many who criticize it criticize a naive version of it, rather than its empiricist roots. But I think the same is true of MWI-- many who criticize it don't recognize that it is not empiricism, so using empiricist thinking will always create issues with MWI. One must first adopt the rationalist credo that reality is what we can make sense of by thinking about it-- that the map is the territory because maps are all that we do. The empiricist counters that perceptions are all that we do, and maps are informed by perceptions.

I think I would have to say that maps and perceptions are together all that we do, so we should probably think of interpretations simply as ways of looking at our theories from different angles. No one would suggest there is a "right angle" to see an elephant from, so if taken in this spirit, we should understand all the interpretations. My issue is simply with not recognizing that when we marry a particular one, we are marrying a set of philosophical choices about science that are not shared by all scientists. That is the only important thing to remember.

But they languished neglected for 30 years until Witten combined them with string theory and showed that the twistor string could explain some mysterious formulae (the Parke-Taylor amplitude) from gauge field theory. That was the start of the "twistor uprising" and it is still unfolding, largely hidden from public view.

This is indeed well out of the public view, or even the view among physicists quite frankly, so thank you for drawing attention to it, even if its significance is currently far from clear.

 

[Samshorn]

If unitarity were broken in observations, I think the rationalists in the MWI camp would jump ship immediately…. CI already thinks unitarity is broken in the act of perception …

Doesn’t your second statement cast doubt on your first? I think we agree that, at least from an empirical standpoint, unitarity is already abundantly falsified, and yet this doesn't cause anyone to jump ship. It’s not easy to imagine what additional observations could possibly “break unitarity” for those who - in the face of all the empirical evidence to the contrary - have adopted unitarity as a first principle, and who are willing to posit an infinity of unobservable worlds in order to preserve it.

Maybe you have in mind the idea that the part of quantum mechanics presently believed by everyone to be unitary – namely, the Schrodinger evolution - might be found to fail, and need to be modified in such a way that the state evolution was no longer precisely unitary, and you think such a “change of postulates” threatens the motivation for MWI. Without knowing the details of the hypothetical modification, it’s hard to comment on how it would affect people’s enthusiasm for MWI. However, bear in mind that the quo ante postulates of quantum mechanics already included the non-unitary projection postulate, which was included to match observations, but this didn’t prevent MWI advocates from simply claiming we can do without it – even though from an empirical standpoint we obviously can’t.

Regardless of our observations, one can always bifurcate our theory into a unitary part and a non-unitary part (as we do in traditional quantum mechanics), and then someone who adopts unitarity as a first principle can rationalize away the non-unitary part (as the MWI advocate does for quantum mechanics), with the understanding that the “unitarity” applies only to some posited unobservable meta-world in which the world of our experience is embedded as just an infinitessimal and non-unitary part. But this is silly. Suppose the conservation of momentum was found to be violated in physics, but nevertheless we really really like the principle of momentum conservation, so we ‘preserve it’ by saying that objects sometimes exchange momentum with unobservable objects from other parallel universes, so in the overall meta-universe, including these other parallel universes, we have conservation of momentum! Yay! Obviously no observation can “break conservation of momentum” for us if we are willing to invoke other worlds like this, but just as obviously our satisfaction with having maintained momentum conservation in this way would be absurd. The same applies to the “unitarity” of MWI. (As an aside, in the early 1900s Poincare and Lorentz actually did discuss the prospect of needing to invoke interactions with an unobservable entity in order to maintain conservation of momentum according to the ether interpretation of electrodynamics.)

But I would say this is all somewhat of a red herring - see below.

I don't think MWI-enthusiasts like MWI because of its many-worlds element, which you are quite right they could preserve in any theory, they only like it because of the simplicity and generality of the unitarity postulate, and the many worlds just come along for the ride.

I think you were nearer the truth in your previous message, where we agreed the main motivation (at least technically) for MWI in QM is the uncertainty principle and its consequences, not a yearning for unitarity per se. If some new phenomena came to light, violating existing QM, and people decided the best way fix the theory was to modify the Schrodinger equation to make it non-unitary (rather than modifying the Born rule or the projection postulate, which I think is what would actually happen, but never mind), would this necessarily invalidate the uncertainty principle, eliminate the fundamental quantum of action (h=0), or make all observables commute? Unless we do all these things, we will still have a theory that entails some kind of “state reduction” or "collapse" in the mapping from the variables in the Schrodinger equation to the measures of our experience, and it is the desire to rationalize the palpable non-classical features of this "collapse" - rather than a yearning for unitarity as an abstract niceity - that motivates MWI (over and above the motivation that already exists for non-quantum theories).

Admittedly I have no idea what a revised quantum mechanics with a non-unitary Schrodinger equation would look like. Maybe it really would eliminate uncertainty and imply that all observables commute (perhaps on some deep level that isn’t readily perceivable), in which case we're just back to something like the Newtonian theory… but I have a hard time imagining such a theory accounting for all observations, and even in that case there would still be motive and opportunity to commit MWI – just as there is for Newtonian mechanics.

So, I still think over-exposure to falsification is not a problem of MWI. In fact, quite the opposite, on both counts. Exposure to falsification is good, but MWI doesn’t have any. The main problem with MWI is that it would be sterile and pointless, even it succeeded in giving a satisfactory account of observations – which it doesn’t. To illustrate that, look at your recent comment in another post:

…one universe fragments more and more with time (by "fragment" I mean "spawns incoherent islands that have no further influence on each other"). So it's rather like a river forking over and over-- it still all traces back to the same upstream watershed.

This is a fairly typical prosaic description of MWI, but there are problems. To give just one example, remember that Schrodinger’s equation is time-symmetrical, and yet you say that (according to MWI) the universe evolving according to this equation “fragments more and more with time”. In which direction does this fragmenting increase? Shouldn’t we actually have worlds fusing back together just as much as we have them fragmenting? But then a given stream would be fed by infinitely many different pasts, as well as fragmenting into infinitely many different futures. This has led some prominent MWI advocates to adopt the view that the number of self-coherent worlds is actually constant, and they just are partitioned into shifting equivalence classes. Our lack of awareness of the many futures of our current equivalence class of worlds may be explained away by noting that we aren’t aware of our futures (for some reason), but surely we are aware of our past. Indeed, some MWI advocates (e.g., Deutsch) contend that MWI implies it actually IS possible for an observer to gain awareness of their multiple, mutually exclusive, histories.

One might argue that it’s unfair to expect MWI to resolve the “arrow of time” problem, because it is also an open problem for other interpretations, but I would answer that it is an especially acute problem for MWI, because without a clear and explicit resolution of the arrow problem, we can’t even really say that MWI gives any intelligible account of observations at all. MWI isn’t really an interpretation of quantum mechanics, it is an idea for an interpretation of quantum mechanics.

We are like an ant floating on a log that takes, seemingly at random, a certain fork each time, but the rest of the river is still there, and all branches share a consistent history up until they forked off.

Careful with the ant and the log there... you are smuggling in the very dualism that MWI seeks to exclude on principle. According to MWI, there is no log or ant, there is only water. You are nothing but one particular streamline in the flow. I realize this is inconsistent with your concept of empiricism, which is wedded to dualism, but that's precisely what MWI most vehemently denies. The main motivation for MWI isn't a yearning for unitarity, it is an abhorance of dualism (which of course underlies the kind of empiricism that you've described).

 

[BruceW]

If we define CI as the view that observables are what's real and that wavefunctions are not real things, then interference effects involving macroscopic devices will not falsify it. The wavefunction of a macroscopic device will still just have been the mathematical construct you use to compute its probable behaviors, and not the object itself, and you will still have zero empirical evidence of there being anything more than one world.

Exactly, CI is automatically the interpretation used by empiricism, and is not falsifiable in any context where quantum mechanics works. Even then, it would only be the quantum mechanics that was falsified, not the basic tenets of CI. We would have to find observations that cannot be consistent with empiricism, and that is paradoxical. Thus CI-type thinking will never be falsifiable within anything we now consider to be science. All the same, what we consider to be science today could eventually be returned to its more rationalistic roots someday.

Good, this is how I would define CI as well. In other words, we only use the non-unitary collapse after all other calculations have been done, to show us what the end-user sees. (and I agree it would then make all the same predictions as MWI).

But in so many places on the web and in books, CI is defined differently, so that non-unitary collapse happens whenever any classical measuring apparatus makes a measurement.

In am now wondering if your 'CI' and my 'MWI' are the exact same thing...