Sidney Coleman's opinion on interpretation in his Dirac lecture

[Demystifier]

Let's say yes. Why does it matter?

Because if you can prove it, you have solved the problem of measurement in QM and deserved the Nobel prize.

 

[martinbn]

Because if you can prove it, you have solved the problem of measurement in QM and deserved the Nobel prize.

But it is irrelevant for this thread!

 

[Demystifier]

But it is irrelevant for this thread!

On the contrary, it's crucial to it, for the reasons I and others explained several times.

 

[martinbn]

On the contrary, it's crucial to it, for the reasons me and others explained several times.

No, this thread is about what Coleman's done in the paper. Not about what you and others think Coleman should have done.

 

[Demystifier]

No, this thread is about what Coleman's done in the paper. Not about what you and others think Coleman should have done.

I argued that Coleman assumed interpretation B, according to which the observer does not distinguish different straight trajectories. It was you who asked me additional questions about the difference between interpretation B and interpretation A, and now you object that it is not relevant to the thread.

 

[martinbn]

I argued that Coleman assumed interpretation B, according to which the observer does not distinguish different straight trajectories. It was you who asked me additional questions about the difference between interpretation B and interpretation A, and now you object that it is not relevant to the thread.

You introduced the distiction between A and B, and insist that it is relevant, but you havent even tried to exlpain why.

 

[PeterDonis]

The problem is that Coleman's operators L and D are never actually measured, they are only "talked about".

What's the point of even talking about them if they don't correspond to any actual measurements we make?

The point of the argument was supposed to be to convince us that Coleman's interpretation, where there's just unitary evolution all the time, no other dynamics, explains why the world looks the way it does to us. How can any such explanation be valid if it doesn't even look at the operators that represent how we find out the way the world looks to us?

That said, I don't think Coleman intended for his operators L and D to only be "talked about". I think he intended them to represent actual things we do to find out how the world looks to us. I think he intended L to represent "the track in the cloud chamber is a definite straight line", and D to represent "we have a solid belief that the track in the cloud chamber is a definite straight line".

Those are indeed parts of how the world looks to us. The problem is that they are only parts, and they don't explain other parts. For example, the track in the cloud chamber isn't just some unspecified, but definite, straight line, which is all that L represents--it's some particular straight line, pointed in a particular direction. And we don't just have a solid belief that the track in the cloud chamber is some unspecified, but definite, straight line, which is all that D represents--we have a solid belief that it's some particular straight line, pointed in a particular direction.

Coleman doesn't address those other parts of how the world looks to us at all. And his L and D operators, if they are supposed to represent all of how the world looks to us regarding tracks in cloud chambers and our beliefs about them, simply, well, don't. I think he simply failed to consider that.

 

[Demystifier]

For a change, let me now try to analyze the Coleman's argument by using the charity principle https://en.wiktionary.org/wiki/principle_of_charity namely, in light of its best, strongest possible interpretation.

First, I point out that at page 10 he says:
"But if I have a linear superposition of eigenstates of the particle with respect to the operator L, each of which is an eigen state with eigenvalue +1, then the combination is also an eigenstate with eigenvalue +1. So this also has straight line tracks in it."
He uses the plural, "tracks", so he is not saying that a single track can be associated with the combination. Thus, at this point he is not wrong.

Second, slightly earlier at the same page he says:
"... if you started out with the particle in a narrow beam it would of course make a straight line track along that beam."
I think this is correct and crucial for the whole argument. It would be wrong if there was no interaction with the molecules in the chamber, but the locality of interactions together with the momentum conservation law makes it essentially right.

So my positive interpretation of the Coleman's argument is that it is just a version of the decoherence argument. The state of the system is a mixture of different straight tracks, each in a different direction from the center of the chamber. Of course, from this Coleman cannot explain how only one of those straight tracks in the mixture is chosen, or in other words, he cannot explain how a definite single outcome appears. But he does not even claim that he can do that. On the contrary, if he is defending a version of the Everett interpretation, he does not even want to claim that.

So from this positive point of view, my only objection would be that he was not perfectly clear what he has achieved and what he has not. Some of the claims he made can be confusing and misleading. There are much better explanations of decoherence and Everett interpretation in the literature. But his crucial ideas, when interpreted according to the charity principle, are correct.

 

[PeterDonis]

he cannot explain how a definite single outcome appears. But he does not even claim that he can do that.

Yes, he does claim that he can explain how it appears that a definite single outcome occurs. From the last paragraph of the paper:

"[P]eople say the reduction of the wave packet occurs because it looks like the reduction of the wave packet occurs, and that is indeed true. What I’m asking you in the second main part of this lecture is to consider seriously what it would look like if it were the other way around—if all that ever happened was causal evolution according to quantum mechanics. What I have tried to convince you is that what it looks like is ordinary everyday life."

By "ordinary everyday life" he evidently means "it looks like the reduction of the wave packet occurs". But he does not explain how it looks like that. He says that the state of the system is a mixture of multiple straight line tracks--it's an eigenstate of "the track is a straight line", but with no particular straight line track picked out--and the state of the observer is a mixture of observations of multiple straight line tracks--it's an eigenstate of "I observed some straight-line track", but with no particular straight line track picked out. But that's not what "ordinary everyday life" looks like. In "ordinary everyday life", we observe one particular straight line track--it looks like reduction of the wave packet occurs, that all but one of the multiple tracks goes away--and he hasn't explained how that happens--not even how it can look like that happens, whether it "actually" does or not.

 

[Demystifier]

By "ordinary everyday life" he evidently means "it looks like the reduction of the wave packet occurs". But he does not explain how it looks like that.

Maybe he does not explain it very well, but since now I'm in the charitable mood, I tend to interpret it as an implicit assumption of an Everett type of explanation of an apparent reduction without the true reduction, or something in that spirit.

 

[PeterDonis]

tend to interpret it as an implicit assumption of an Everett type of explanation of an apparent reduction without the true reduction

But his analysis does not do that. An Everett type of explanation makes crucial use of the entanglement between the measuring device (or observer) and the measured system. That is what produces the appearance of state vector reduction--the fact that each branch of the entangled wave function contains a state of the measuring device (or observer) that measures (or observes) the state of the measured system that is also in that same branch. Coleman recognizes that this entanglement exists (it's at the bottom of his slide 14), but he doesn't include it in his analysis at all.

And all Coleman's talk about the L and D operators has nothing to do with an Everett type of explanation of apparent state vector reduction. Even if I try my best to be charitable, I can't see any reason to even talk about all that stuff at all, if he's trying to argue for an Everett type interpretation. The only reason I can see for all that talk, even charitably, is to avoid having to talk about an Everett type interpretation, because he doesn't like that type of interpretation--but he also doesn't like actual collapse. He's trying to find some kind of middle ground between the two--but there isn't one.

 

[herwinda pramudya]

Maybe Coleman was right when he said he made no original contribution—
because what he did was not to add new content,
but to show that we have been reading in the wrong direction all along.

pines-demon quoted from Sidney Coleman's Dirac Lecture "Quantum Mechanics in Your Face" (https://arxiv.org/abs/2011.12671)

and martinbn added "the full quote" of what Sidney Coleman said before (and refers to here) to make things clearer:


Sidney Coleman's opinion on interpretation in these two quotes is:

• Thus our aim is NOT “the interpretation of quantum mechanics.” It is the interpretation of classical mechanics.
• The problem is not the interpretation of quantum mechanics. That’s getting things just backwards. The problem is the interpretation of classical mechanics.

However, more quotes are needed to capture Coleman's position in his Dirac lecture:






Sidney Coleman's position in these quotes is:

• I have made no original contributions to this subject.
• Zurek ... actually raised a question one can talk about.
• In order to ease into this, I’d like to begin with an analysis of Neville Mott.
• Now people say the reduction of the wave packet occurs because it looks like the reduction of the wave packet occurs, and that is indeed true.

The connection to the annecdote about Wittgenstein definitively was an original contribution. This raises the question whether it was the only original contribution. Another likely original contribution is his ambiguous reframing: "The problem is the interpretation of classical mechanics."

But I don't think Sidney Coleman was intentionally lying when he denied having made original contributions. The scientists mentioned in the quotes above are Neville Mott, Hugh Everett and Wojciech Żurek. (Other scientists are mentioned in: "Some of the things I’ll say about probability later come from a paper by Jim Hartle, and one by Cambridge’s own Eddie Farhi, Jeffrey Goldstone, and Sam Gutmann.") So I wonder whether his position can be justified as a non-original selection from published works of Mott, Everett and Żurek, or whether it must be considered as an original position, which was never peer-reviewed or even worked-out properly.

Maybe Coleman was right when he said he made no original contribution - because what he did was not to add new content,
but to show that we have been reading in the wrong direction all along.

 

[gentzen]

The only reason I can see for all that talk, even charitably, is to avoid having to talk about an Everett type interpretation, because he doesn't like that type of interpretation--but he also doesn't like actual collapse.

Exactly, that is the impression I got.

 

[Demystifier]

If he doesn't like the Everett type interpretation, then why (at the bottom of page 7) does he say "The position I am going to advocate is associated with Hugh Everett in a classic paper."?

 

[PeterDonis]

If he doesn't like the Everett type interpretation, then why (at the bottom of page 7) does he say "The position I am going to advocate is associated with Hugh Everett in a classic paper."?

He says "associated with". Why not just "is"? Because actually, no, the position he describes is not Everett's relative state interpretation--because it leaves out the crucial piece of that interpretation that I described. (Not to mention that Everett's paper doesn't talk at all about these L and D operators, nor does Everett claim that it's sufficient to just explain why the observer thinks some definite outcome happened, but has no idea which one. Everett at least took on the entire problem.)

As I said, I think he's trying to find some kind of middle ground between many worlds and actual collapse. I don't think there is one, at least not along the lines he's attempting.