Undergrad Does quantum mechanics obey causality?

[bhobba]

There is something to this idea. Indeed you will often see it said that classical special and general relativity are theories of causality. Spacetime gives rise to 2 notions of causality.

Indeed there is. Its actually a reasonably common view.

Its not what I agree with, but that means nothing.

When discussing foundational QM issues things are much simpler when an actual interpretation is specified. And we must move beyond this idea that interpretations are inherently better than others - all are equally valid. We can compare and contrast various interpretations and say what you prefer but beyond that isn't really science.

Thanks
Bill

 

[bhobba]

English is fundamental to foundational issues. Without English or natural, intuitive language, you cannot formulate mathematics.

Mostly I agree with Atty, but on this I cant.

My examples of 'prior', 'initial' and 'final' show its limitations and problems. As long as its understood what is meant things are fine, but beyond that its a big issue IMHO.

Thanks
Bill

 

[ShayanJ]

There is no wave-function collapse. So you don't bother you with it :-).

Just because the ensemble interpretation doesn't have collapse, doesn't mean it doesn't have the measurement problem!
As I explained in this post. The problem with ensemble interpretation is that it doesn't deal with single systems and so its doomed to be the interpretation that only makes it easy to use QM and not a fundamental interpretation that explains anything.
But if you reject collapse on the basis of dBB or MW interpretations, be my guest!

 

[bhobba]

so its doomed to be the interpretation that only makes it easy to use QM and not a fundamental interpretation that explains anything.

First you need to show it needs to explain anything. Nature may simply be like that, as IMHO it is - but of course that means Jack Shite.

Thanks
Bill

 

[ShayanJ]

First you need to show it needs to explain anything. Nature may simply be like that, as IMHO it is - but of course that means Jack Shite.

Thanks
Bill

Are you referring to improper mixtures just being the same as proper mixtures?
But even if we accept that, it doesn't solve anything. The ensemble interpretation's premise is that QM is only about an ensemble of identically prepared systems. So it certainly is not at the same level of dBB or MW that are about single systems. If one accepts the ensemble interpretation, then they either should embrace the fact that single systems somehow don't have the ability to evolve and have properties and only ensembles can(!), or they have to accept that there should be an underlying theory to QM which means an advocate of ensemble interpretation that wants to pursue questions on foundations of QM, is inevitably an advocate of hidden variable theories.

 

[bhobba]

Just because the ensemble interpretation doesn't have collapse, doesn't mean it doesn't have the measurement problem!

And indeed it does - but its the modern version ie why do we get any outcomes at all.

As I said above nature simply may be like that ie there is no difference between improper and proper mixed states or there may be an underlying explanation eg MW or BM. Until there is a way to decide experimentally its not really science - simply an expression of what you prefer.

Thanks
Bill

 

[bhobba]

Are you referring to improper mixtures just being the same as proper mixtures? But even if we accept that, it doesn't solve anything.

Yes.

Every explanation, every single one assumes some things. Assuming proper and improper mixtures are the same thing is no better or worse than the assumptions of BM, MW or others that explains it. You are perfectly entitled to prefer one over the other and explain why, but its not science which is based on experiment, not opinion.

I prefer ignorance ensemble because IMHO it gets to the heart of the issue. I have explained why but it doesn't mean anything in a fundamental sense - its just an interesting exercise in comparing and contrasting different interpretations.

Thanks
Bill

 

[ShayanJ]

Every explanation, every single one assumes some things. Assuming proper and improper mixtures are the same thing is no better or worse than the assumptions of BM, MW or others that explains it. You are perfectly entitled to prefer one other the other and explain why, but its not science which is based on experiment, not opinion.

I'm fine with that. That's not what I'm complaining about.
What I'm saying is, the ensemble interpretation is not a fundamental interpretation at the same level of dBB and MW. Its just for people who don't care about foundational questions and just want to use QM.

 

[bhobba]

I'm fine with that. That's not what I'm complaining about.
What I'm saying is, the ensemble interpretation is not a fundamental interpretation at the same level of dBB and MW. Its just for people who don't care about foundational questions and just want to use QM.

I gave what you said my like because I sort of agree with what you say.

What I don't agree with is your characterization as ensemble not being fundamental - it's no more fundamental, or not, as any other. Some simply explain some things others accept as fundamental.

I think the characterization as minimalist is a better way of expressing it. As John Baez says most (but of course not all) are simply arguments about the meaning of probability:
http://math.ucr.edu/home/baez/bayes.html

The difference between Copenhagen and Ensemble has to do with the difference between frequentest and Bayesian view of probability. My background is applied math and in that you mostly use frequentest (not always eg Actuaries often use Bayesian or even the decision theory approach of MW) which is more operational in nature.

Just as an aside while John is correct about the issues with the Frequentest view, there is more to it but that needs a whole new thread.

Thanks
Bill

 

[ShayanJ]

The false vacuum is responsible for creating space-time so obviously the concept doesn't apply to it. Ideas like this have been around for a while eg:
http://blogs.scientificamerican.com/guest-blog/is-all-the-universe-from-nothing/

Note - I am not in anyway an expert on such things - its just general knowledge such modern ideas exist.

Thanks
Bill

But as far as I know BICEP2's results were refuted, weren't they?
And aside from that, I don't understand this statement. When we do QFT, we put it on a background spacetime. How can a QFT explain the emergence of its background?

 

[bhobba]

But as far as I know BICEP2's results were refuted, weren't they?

Sorry don't know that one.

And aside from that, I don't understand this statement. When we do QFT, we put it on a background spacetime. How can a QFT explain the emergence of its background?

Not the false vacuum - and obviously so because it represents the birth of space-time. You don't have to interpret the parameters as anything - just some parameters.

But its not the only approach to emergent space-time eg string theory:
http://physics.stackexchange.com/qu...me-is-not-fundamental-but-should-be-considere

Again I am not expert. I am surprised its in anyway controversial though - from my perspective it's just general knowledge.

Thanks
Bill

 

[martinbn]

The false vacuum is responsible for creating space-time so obviously the concept doesn't apply to it. Ideas like this have been around for a while eg:
http://blogs.scientificamerican.com/guest-blog/is-all-the-universe-from-nothing/

Note - I am not in anyway an expert on such things - its just general knowledge such modern ideas exist.

Thanks
Bill

The "universe from nothing" is not the same thing as inflationary models. In the inflation case you don't have anything prior space-time.

 

[bhobba]

The "universe from nothing" is not the same thing as inflationary models. In the inflation case you don't have anything prior space-time.

Point taken. I was thinking of its explanation via the false vacuum.

Thanks
Bill

 

[vanhees71]

Just because the ensemble interpretation doesn't have collapse, doesn't mean it doesn't have the measurement problem!
As I explained in this post. The problem with ensemble interpretation is that it doesn't deal with single systems and so its doomed to be the interpretation that only makes it easy to use QM and not a fundamental interpretation that explains anything.
But if you reject collapse on the basis of dBB or MW interpretations, be my guest!

I'm always puzzled by the question, what the "measurement problem" is? The experimentalists around me have practical problems to solve when they want to measure various things accurately, but there's no real unsolved fundamental problem with measurements out there. They construct marvelous devices to measure things, and that's how the physical quantities are in fact defined, not by quantum theory (or any other theory for that matter). Theory has to describe (predict) what is (will be) measured, if it is a good theory (at least in some limited range of applicability). That's what quantum theory (in the minimal statistical interpretation) provides with an astonishing success. So I don't see, where there is a problem with it, particularly I don't see any "measurement problem".

 

[bhobba]

particularly I don't see any "measurement problem".

Nor do I. But I try to be 'unbiased' and understand others views.

Thanks
Bill

 

[ShayanJ]

I'm always puzzled by the question, what the "measurement problem" is? The experimentalists around me have practical problems to solve when they want to measure various things accurately, but there's no real unsolved fundamental problem with measurements out there. They construct marvelous devices to measure things, and that's how the physical quantities are in fact defined, not by quantum theory (or any other theory for that matter). Theory has to describe (predict) what is (will be) measured, if it is a good theory (at least in some limited range of applicability). That's what quantum theory (in the minimal statistical interpretation) provides with an astonishing success. So I don't see, where there is a problem with it, particularly I don't see any "measurement problem".

The problem is, how do we get a definite outcome in macroscopic experiments while the world is fundamentally quantum mechanical?
Interpretations are there to answer this question. MW and dBB seem to solve this. For ensemble interpretation the solution seems to be what bhobba suggests: improper mixtures are the same as proper mixtures. But the difference between ensemble interpretation and MW and dBB interpretations is that ensemble interpretation only works for ensembles. So my problem is, how do we get a definite outcome for a macroscopic experiment on an individual system?

 

[bhobba]

But the difference between ensemble interpretation and MW and dBB interpretations is that ensemble interpretation only works for ensembles. So my problem is, how do we get a definite outcome for a macroscopic experiment on an individual system?

Its a frequentest view. Ensembles is just one way of doing it.

But here really is not the place to discuss it.

Start another thread.

Thanks
Bill

 

[vanhees71]

Well, why do you get a definite outcome when throughing dice although these outcomes are random? Where is the problem with this everyday phenomenon? It's just a very common observable fact that outcomes of measurements are definite after they have occurred although they are not predictable and thus are described in terms of probability theory and statistics.

How do you think MW solves your apparent problem? It just adds "parallel universes" to the picture whose existence cannot be observed. It's an element of the interpretation which might be amusing to some philosophers. For physics it's irrelevant. The same holds for dBB: It adds unaobservable trajectories to the picture of interpretation.

Our measurement devices are constructed such as to give definite outcomes in individual experiments although these outcomes are not predictable since the corresponding observables of the quantum system have indefinite values if the system is not prepared in a way that they have definite values. QT also tells you that it is impossible to prepare a system in a state where all its observables take definite values. Through the interaction of a measurement device you get however always a definite pointer outcome, and this pointer outcome is interpreted as a definite value for the measured observable. Otherwise the apparatus is not taken as a good device to measure this observable. That's all.

 

[ShayanJ]

When I think in terms of Bayesianism, then everything falls into place and now I can accept the ensemble interpretation as an interpretation on equal footing as MW and dBB and so I'm OK with it now.

Well, why do you get a definite outcome when throughing dice although these outcomes are random? Where is the problem with this everyday phenomenon? It's just a very common observable fact that outcomes of measurements are definite after they have
occured although they are not predictable and thus are described in terms of probability theory and statistics.

This isn't a good analogy and actually betrays your point. This phenomenon is not strange to us because we know the randomness is only emergent and fundamentally the dice follows classical mechanics and so it has a definite outcome whether we can predict it or not.
But if you suggest this is a good analogy to justify the ensemble interpretation, it means you're suggesting exactly what I said, that you need a hidden variable theory to justify the ensemble interpretation.(A claim that I'm now taking back!)
As I said, I'm now OK with the ensemble interpretation if we use Bayesian probability theory and have no problem with it but this is a bad analogy for justifying it!

How do you think MW solves your apparent problem? It just adds "parallel universes" to the picture whose existence cannot be observed. It's an element of the interpretation which might be amusing to some philosophers. For physics it's irrelevant. The same holds for dBB: It adds unaobservable trajectories to the picture of interpretation.

I didn't say I like them, I just said they provide solutions!

Our measurement devices are constructed such as to give definite outcomes in individual experiments although these outcomes are not predictable since the corresponding observables of the quantum system have indefinite values if the system is not prepared in a way that they have definite values. QT also tells you that it is impossible to prepare a system in a state where all its observables take definite values. Through the interaction of a measurement device you get however always a definite pointer outcome, and this pointer outcome is interpreted as a definite value for the measured observable. Otherwise the apparatus is not taken as a good device to measure this observable. That's all.

The fact that part of a physical phenomenon is man-made doesn't mean physics doesn't have to explain it! I don't see how this can be an argument in favor of ensemble interpretation!
But don't bother coming up with further arguments because as I said, I'm now OK with it.

 

[vanhees71]

Physics doesn't explain but describes observed facts (and sometimes makes predictions of observable facts).

The only difference between classical deterministic theory and quantum theory is that in the former randomness is only due to a lack of information of the state of the system and in the latter it's that even the full knowledge of the state does not imply that all observables are determined. This is hard for many to believe, but that's how nature seems to be. Why should it be deterministic? There's no plausible reason for that to be a true property of nature, and as it seems, quantum theory is a more comprehensive description of nature than classical deterministic theories. There is no need for hidden variables that in some way restores determinism, because it's simply not observed in nature! To the contrary all the many experiments demonstrating the violation of Bell's inequality (taken together with the very persuasive assumption os locality of interactions) proof this view wrong. The apparent classical deterministic behavior of macroscopic systems is the emergent phenomenon, not the irreducible randomness according to QT!

 

[ShayanJ]

Physics doesn't explain but describes observed facts (and sometimes makes predictions of observable facts).

The only difference between classical deterministic theory and quantum theory is that in the former randomness is only due to a lack of information of the state of the system and in the latter it's that even the full knowledge of the state does not imply that all observables are determined. This is hard for many to believe, but that's how nature seems to be. Why should it be deterministic? There's no plausible reason for that to be a true property of nature, and as it seems, quantum theory is a more comprehensive description of nature than classical deterministic theories. There is no need for hidden variables that in some way restores determinism, because it's simply not observed in nature! To the contrary all the many experiments demonstrating the violation of Bell's inequality (taken together with the very persuasive assumption os locality of interactions) proof this view wrong. The apparent classical deterministic behavior of macroscopic systems is the emergent phenomenon, not the irreducible randomness according to QT!

On this issue, I'm completely with you. I really don't understand people who think common sense is a good judge for understanding phenomena that are not at all common to our sense.