Does quantum mechanics obey causality?

[vanhees71]

How do you come to this conclusion? I've not said anything along these lines. I just describe standard quantum theory in the minimal interpretation, which of course includes entanglement and long-ranged correlations associated with them. I don't like to call them less precisely "nonlocality" since this is often mixed up with non-local interactions, which are very problematic to say the least, and there's no evidence from observations that they are needed. The most successfull theory is the Standard Model of elementary particle physics which has incorporated the locality of interactions in its foundations. Nevertheless, of course, it includes the possibility for entangled states of systems with "parts" showing strong correlations. This is well-established nowadays (mostly realized with polarization-entangled two-photon states as can be prepared on demand by shining a laser on certain birefringent crystals in a process called parametric down-conversion).

 

[StevieTNZ]

This is a recently published article which may be of interest: http://phys.org/news/2016-08-quantu...e=menu&utm_medium=link&utm_campaign=item-menu

 

[Mentz114]

This is a recently published article which may be of interest: http://phys.org/news/2016-08-quantu...e=menu&utm_medium=link&utm_campaign=item-menu

I think this is it http://arxiv.org/abs/1602.02767

 

[bhobba]

I think this is it http://arxiv.org/abs/1602.02767

From the above:
'Explaining observations in terms of causes and effects is central to all of empirical science. Correlations between entangled quantum particles, however, seem to defy such an explanation.'

Scratching head. Of course it does. Correlations must be removed from a causal explanation of anything and that is all Bell is. As I often post, standard QM is not local - its in its very foundations because it obeys the Galilean transformations. In fact as chapter 3 of Ballentine shows its the only thing needed to deduce the dynamics. And, just as an aside, as Landau's beautiful book, Mechanics, shows it's all that is required even in classical mechanics - if you have not read that masterpiece please get a copy - you will not be disappointed (the real foundation is the principle of least action which follows from QM - strangely at first sight the basis of classical mechanics is QM). You must go to QFT for locality to be an issue and when you do that its based on the cluster decomposition property:
https://www.physicsforums.com/threads/cluster-decomposition-in-qft.547574/

For it to make sense it can't apply to correlated systems so they must be removed ie it does not apply to Bell type correlations.

One thing I have found out about physics since posting here is sometimes even professional physicists get confused about basic things. To be fair even I was confused about a lot of stuff before posting here - I look back and wince at some of the things I thought back then. But it does show you must be careful of what is written in professional papers.

Added later:
I have read a bit more of that paper. I sort of see what they are getting at, but it is a bit confused about fundamental things and that needs to be disentangled before getting to its 'meat' so to speak. I seem to recall it has been discussed here before, but if people really want to chat about it best to start a new thread.

To get back to the original question, that too has been discussed many times - the answer depends of what is meant by causality (Schrodinger's equation shows the state is causal) but the standard formalism is non committal about the cause of observational outcomes and is best discussed with reference to some interpretation otherwise in answering it you just go around in circles.

Thanks
Bill

 

[mikeyork]

The notion of causality is rooted in our perception of space-time. There is currently some discussion of whether space-time exists in reality or is something an observer creates to bring some sort of consistency to their perception.

 

[bhobba]

The notion of causality is rooted in our perception of space-time.

Scratching head.

Its rooted in cause and effect. It's usually specified by some kind of differential equation.

QM has Schrodinger's equation so initial state determines final state. In that sense its causal. But the act of observation - that is a matter of interpretation - the formalism is silent on it.

Thanks
Bill

 

[mikeyork]

Your mention of "initial" and "final" state implies time. Schrodinger's equation involves derivatives with respect to space and time since it describes evolution in time.

 

[bhobba]

Your mention of "initial" and "final" state implies time

Your logic is astray. I wasn't going to spell it out, but decided to. Differential equations have a set of parameters that can be in anything not just time or space. What they are determines other values - not just space and time. It'simply colloquial to call them initial and final.

Thanks
Bill

 

[mikeyork]

Your own example of Schrodinger's equation (the context in which you mention "initial" and "final" states) shows the space-time context explicitly. Differential equations show analytic continuity. I consider this is a different concept from causality. For instance causality need not be continuous, but describe discrete but related events. That context also implies some notion of time though, just not continuous co-ordinate time.

 

[bhobba]

Your language is astray

Fair enough.

Take the example of inflationary models. They are expressed in the language of differential equations 'prior' to the emergence of space and time. But here we run into problems with language because prior makes no sense without the concept of time. That's why math, not English, is the best language to discuss this.

Thanks
Bill

 

[mikeyork]

Our posts are crossing each other. Editing complicates things even more.

You wrote "prior makes no sense without the concept of time". This sounds very much like "causality makes no sense without the concept of time". So I think we agree once we get the language issues out of the way and in the absence of time-dependence focus on the math.

 

[bhobba]

Our posts are crossing each other. Editing complicates things even more.

Yes - I apologize for the editing I did before.

My point is that causality has with modern theories gone beyond notions of space time etc.

Thanks
Bill

 

[David Lewis]

Parent has two different candies. Two kids ask him for candies, one asks first and the other one later. Parent gives random candy to the kid who asks first and the other one to other kid. Basically either kid can't really influence which candy he will get and which candy will get the other kid by asking first or second. But just the same the two events are not independent.

What if the candies are not of one type or another until after a child sees what kind of candy he got?

 

[bhobba]

You wrote "prior makes no sense without the concept of time". This sounds very much like "causality makes no sense without the concept of time". So I think we agree once we get the language issues out of the way and in the absence of time-dependence focus on the math.

Yes. With these kind of foundational issues English is the enemy.

If, in terms of math, you can explain your context that would be fantastic. Hopefully I have done that - but feel free to ask for any clarification.

Thanks
Bill

 

[martinbn]

Fair enough.

Take the example of inflationary models. They are expressed in the language of differential equations 'prior' to the emergence of space and time. But here we run into problems with language because prior makes no sense without the concept of time. That's why math, not English, is the best language to discuss this.

Thanks
Bill

What do you mean by this? Inflationary models do not deal with anything 'prior' to space-time.

 

[mikeyork]

Bill, the QM context I see is that fundamental reality has no space-time variables. The observer introduces a co-ordinate space-time frame as a helpful tool in understanding the world.

One of the intriguing facets of the 20th century revolutions of both QM and space-time relativity is the key role of the observer. Prior to this, classical physics assumed reality was independent of the observer who could effectively see that reality directly in an objective space-time world. My view is that this key role of the observer, although apparently very different in each case, is actually no coincidence and that understanding the connection will pay dividends in terms of uniting QM and GR.

 

[bhobba]

What do you mean by this? Inflationary models do not deal with anything 'prior' to space-time.

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

 

[bhobba]

My view is that this key role of the observer, although apparently very different in each case, is actually no coincidence and that understanding the connection will pay dividends in terms of uniting QM and GR.

Got it.

Its not my view but most certainly it one of a myriad of views out there so to speak.

My view, for what its worth, is right at the foundation of everything is some striking symmetry.

Thanks
Bill

 

[n01]

Bill, the QM context I see is that fundamental reality has no space-time variables. The observer introduces a co-ordinate space-time frame as a helpful tool in understanding the world.

One of the intriguing facets of the 20th century revolutions of both QM and space-time relativity is the key role of the observer. Prior to this, classical physics assumed reality was independent of the observer who could effectively see that reality directly in an objective space-time world. My view is that this key role of the observer, although apparently very different in each case, is actually no coincidence and that understanding the connection will pay dividends in terms of uniting QM and GR.

Isn't the role of the observer overblown here, giving rise to such anthropomorphisms of the primacy of human observers on reality? I mean, take Schrodinger's cat for example. If a human observer is the essential feature in determining the state of the cat, then isn't another observer of the human observer required to determine that state further... ad infinitum, giving rise to a third man argument?

 

[mikeyork]

There is no need for anthropomorphism. Anything that interacts is an "observer" of what it interacts with, but not necessarily one with a space-time frame. The only special thing we humans are doing is adding a co-ordinate space-time frame. The state of the cat can be part of an objective reality unobserved by any human, but described by a human (lacking information as to what is in the box) as a superposition. But "alive" or "dead" are not space-time co-ordinates. The cat itself is an observer of the poison. When a human opens the box and finds it dead a skilled pathologist can even say when it died by effectively interrogating the cat.

 

[n01]

Can a single photon be considered an "observer"?

 

[mikeyork]

Can a single photon be considered an "observer"?

Why not? It acquires state data from whatever it interacts with.

 

[atyy]

The notion of causality is rooted in our perception of space-time. There is currently some discussion of whether space-time exists in reality or is something an observer creates to bring some sort of consistency to their perception.

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.

(1) classical relativistic causality (the causes of an event lie in its past light cone)

(2) signal causality (no classical information can be transmitted faster than light)

(1) requires a notion of reality, while (2) requires a notion of an observer. What Bell showed was that although quantum mechanics respects (2), it violates (1).

 

[atyy]

Can a single photon be considered an "observer"?

To himself, an observer exists. For me, a photon need not exist. Whether you can say a photon is an observer or not depends on whether you can become a photon and remain an observer.

The standard interpretation of quantum mechanics does rely on this notion of an observer. We would like to imagine that quantum mechanics has something to say about whether the universe existed before observers existed. Yet that is very problematic. This is the famous measurement problem of QM.

Attempts to remove the "observer" as fundamental in physics include Bohmian Mechanics and the Many-Worlds Interpretation.

 

[atyy]

With these kind of foundational issues English is the enemy.

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

 

[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?