Undergrad Does quantum mechanics obey causality?

[bhobba]

Correct me if I am wrong, but the mainstream dominating thinking in Quantum Mechanics (QM) since the 1930s is the Copenhagen Interpretation (CI) conceived mainly by Niels Bohr

Yes - but with our modern understanding of decoherence it has morphed a bit because it leaves a question open. That question is, since in Copenhagen, QM is a theory about observations that appear here in an assumed common-sense classical world, how does it explain such a world that it assumes in the first place. Great progress has been made in rectifying that blemish, but some issues remain. I think the modern form of Copenhagen would be Consistent Histories:
http://quantum.phys.cmu.edu/CHS/histories.html

It's a nice interpretation with a lot to like. For me however it has the feel of defining your way out of problems which is why I hold to the ignorance ensemble interpretation. It's just a minor variation on the ensemble interpretation applying it only to the mixed state after decoherence.

I think studying interpretations is very interesting, and sheds a lot of light on the formalism. Particularly interesting is seeing exactly what the formalism implies and doesn't. For example since we have interpretations without collapse such as MW the formalism doesn't have collapse, even though at first sight you think it does. But it must be borne in mind, and this is VERY VERY important, no interpretation is better hasn't any other - choice is purely a personal thing depending on what appeals to you. The other thing is, very difficult questions in QM such as if its random or not, are trivial in specific interpretations and IMHO are best approached that way - ie discuss them in various interpretations and not generally.

Thanks
Bill

 

[bhobba]

Wholeness and the Implicate Order, by David Bohm (The conceiver of the Pilot Wave model).

David Bohm was a great physicist. When he was being that he was very good, but when he wasn't - well you get writings like the above. Its nearly, but not quite, metaphysical mumbo jumbo that borders on junk science, not Bohm's finest hour.

Thanks
Bill

 

[bhobba]

So you classify the Born rule as interpretation? It's what's observed to happen in experiments. Geiger counters count and photoelectrons are collected, at random.

That's actually an interesting question.

You see the Born rule mentions probability, and as John Baez, correctly IMHO, asserts, many QM interpretations are simply arguments about the meaning of probability:
http://math.ucr.edu/home/baez/bayes.html

John's comment about frequentest probability, while correct as it stands, needs further fleshing out, but this is not the thread to do it.

Thanks
Bill

 

[vanhees71]

Do you call all interpretations religions?Even worse, in the minimal statistical interpretations there is not even such a state, because you cannot give the wave function a physical reality (otherwise you need some way to get rid of it).

Yes, everything that's not testable by observations/experiments is not physics and thus in some sense free to individual believes like religion.

Of course, a state is not described by "a wave function" but a statistical operator in Hilbert space. Wave functions are representants of states for a very small subset of situations, where nonrelativistic descriptions with a conserved particle number are applicable.

For me, the quantum-theoretical state is very real. It's first defined operationally as a (an equivalence class of) preparation procedure(s). Given the state, you have probabilistic information about the system. If you have even a pure state, it's the situation, where you have maximal possible knowledge about the system. Contrary to the situation in classical physics, this complete state determination does not imply the determination of all observables. This indeterminacy of many observables in a given state is a real property of nature.

I avoid to use the word "reality" in the context of physics, because this notion is spoiled by a plethora of different philosophical meanings, which usually are not even clearly defined. As I said, from a physics point of view, reality is everything that's obejctively observable or even quantitatively measurable.

 

[n01]

Of course, a state is not described by "a wave function" but a statistical operator in Hilbert space. Wave functions are representants of states for a very small subset of situations, where nonrelativistic descriptions with a conserved particle number are applicable.

Isn't that the same thing, only that you're committing the wavefunction to a determinate state by measurement or observation? This would be, by all intents and purposes, eliminating entanglement and nonlocality states of wavefunctions, yes?

 

[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.