A electron can exist in everywhere ?

ZapperZ

Er... you missed the point here. They are starting with the premise that has been argued that says that a quantum state is DIFFERENT from a real physical state, that a quantum state merely contains the state of our knowledge of the system (see Ref. 1-8, which is the "proof"). It is from this that they proceed to show that if this is true, then such an assumption will produce a result that contradicts quantum mechanics.

And again, isn't that what they were tackling?

But is this what you meant by the quantum state being nothing more than a "codification of the knowledge" of the system? This is what I was addressing when I produced such a reference, to show that if one were to argue that a quantum system is nothing more than a reflection of our knowledge and CAN be different than the physical system, then one CAN arrive at a TEST to differentiate between the two! They can be distinguished from one another! And I will include the reference on the test of quantum contextuality that I cited earlier as added evidence.

On the other hand, if that is not what you meant, and you think there is a third way to look at this, then this doesn't apply to you, because that is not what is being addressed here.

Zz.

bhobba

I am scratching my head a bit here. I thought I was pretty clear. What I wrote previously was: 'A system state is the codification of what the outcome would be if you were to observe a quantum system, but what it is 'doing' or 'existing' when it is not being observed the theory is silent about'.

Again from the paper you linked to:
'The argument depends on few assumptions. One is that a system has a 'real physical state' not necessarily completely described by quantum theory, but objective and independent of the observer. This assumption only needs to hold for systems that are isolated, and not entangled with other systems. Nonetheless, this assumption, or some part of it, would be denied by instrumentalist approaches to quantum theory, wherein the quantum state is merely a calculational tool for making predictions concerning macroscopic measurement outcomes.'

My position is precisely this denial - namely it is an instrumentalist view with the reality being the system and observational apparatus - the state merely codifies the result of what would happen, statistically, if you were to observe it. The state is not 'real and physical' any more than the probabilities assigned to a the sides of a coin are real and physical - it does not exist in any real sense, simply as a tool to predict the outcomes of observations. This is the view of Ballentine etc in the Ensemble interpretation, although I personally incorporate decoherence into it but that is another issue.

Thanks
Bill

ZapperZ

Well I'm scratching my head as well, because it appears that you're straddling both sides of the fence.

Let's be clear here. If the sides of the coin is the way that this paper has described, then we now have a way to test it. This is very much what Bell did with a test for local interaction as stated in EPR. His test moved it from being merely a matter of philosophy to actual physics that can be tested.

This paper, with the caveat of its assumption, is doing just that, and proposed for the first time a test to distinguish one from the other. It may not be the final form of such test, the same way Bell test has evolved into stricter and more stringent tests, but it is at least a start that will drag this debate from philosophy into physics.

Now, if your world view is different than from either of these, and presumably from your arguments, you can't distinguish one from the other, then you are back into the philosophy realm where we are going to argue based on a matter of taste! That may be what you wish to do, but that isn't the point of this discussion.

Zz.

bhobba

I am equally bemused by your position. My view is the system state is not like the sides of the coin which have a real physical existence - it is like the probabilities we assign to the sides of a coin that codifies knowledge of the long term outcomes of flipping the coin. It is not a 'real physical state' of the coin - but a codification of knowledge about the coin. To be specific I reject that the system must a priori be described by a real physical state - what we call the state simply tells us about long term statistical outcomes of observations. It, or something similar it depends on, may be real - but a priori does not have to be.

The issue is not philosophical - I am generally a bit anti philosophy - but a simple bit of common sense. Knowledge of something real and physical like the expected outcome of observations on a system is not the same as the outcomes themselves. The outcomes are real and physical - no denying it - but knowledge of what the expected value of those outcomes are over a long sequence of observations is not a 'real physical state' to use the words of the paper you linked to.

Balentine states the second axiom of QM as follows (although I personally prefer deriving it from the assumption of additivity of expectation values as was done by Von Neumann long ago - or even by Gleasons Theorem - but that is another issue) - if R is an observable then there exists a positive operator p of unit trace, called the state of the system, such that the expected value of R E(R) = Tr(pR).

Since the expected value of the outcome of many observations is simply knowledge about the system precisely why do you consider the state p a 'real physical state' or indeed that the system must depend on some such? Do you also consider the probabilities assigned to each side of the coin real and not just a calculational device to predict the proportion of long term outcomes? Do they in turn must depend on something real and physical (of course in this case it depends on all all sorts of other things like how hard the coin is flipped but in principle it does not have to - this is my key point)? Of course I can't prove you wrong and indeed there could be a long drawn out philosophical discussion on what exactly real is that IMHO would be pointless (just like I believe a lot of philosophical stuff is - which is why I am a bit anti philosophy) but to me this is pretty much common sense.

I suppose after thinking about it a bit you for some reason believe a quantum system must be described by something that exists external to us and exists in a real physical sense - I do not agree that is a priori so. It is very easy to be fooled by something like the coin flipping of the classical world. If we knew all the details of how it was flipped then in principle we could predict the outcome and so think the probabilities we assign to it depend in some imperfect way on something real - which in that case it does. But the quantum world a priori does not have to be like that.

Regarding issues of taste - isn't that exactly what a choice of QM interpretation amounts to? And the point of answering a question like the OP posted is that the answer depends on interpretation?

Thanks
Bill

bohm2

The PBR no-go theorem rules out a ψ-epistemic interpretations (see links).

1. If one assumes that there is some reality that underlies the quantum state (probability distributions over the ontic states)it is ruled out.
2. If one assumes that there is no deeper underlying reality, then PBR has no effect.

Does Ballentine's ensemble approach you favour subscribe to position 1 or 2?

Can the quantum state be interpreted statistically?
http://mattleifer.info/2011/11/20/ca...statistically/

http://www.physicsforums.com/showthread.php?t=551554

San K

if probability (wave-function) is quantized, then the electron probability cloud has a limited boundary

beyond a certain range (of time-space) the probability of finding an electron will drop to zero

interestingly (per some assumptions/hypothesis) even quantum entanglement would then have a limit/range

bhobba

It is silent on it, and deliberately so. You can choose either. Einstein favored the Ensemble interpretation because it whispered the first alternative in your ear (and it most certainly does) but whispering and being so are two different things.

In discussing issues of principle I personally choose position 2 because a lot of the philosophical waffle goes out the door - but that does not mean I rule position 1 out - I simply hold that view until we know something more definite from experiment. To me its easier that way - but what is one persons easier is another's I can't stand it.

By philosophical waffle I mean issues such as what position does an electron have when not observing it, and how does it instantaneously change to another state when you observe it - they become non issues.

Thanks
Bill

bohm2

Then, I'm guessing that PBR not only rules out Einstein's ensemble interpretation but also Lee Smolin "real ensemble interpretation":

A real ensemble interpretation of quantum mechanics
http://arxiv.org/pdf/1104.2822v1.pdf

bhobba

No it doesn't. Einstein would have loved the PBR because it pointed to a deeper reality that the ensemble was an approximation to ie the out here is QM was not complete so the underlying theory that is really in charge is different from QM hence PBR does not apply. In Smolin's theory the ensembles are literally real so the state is real anyway.

Thanks
Bill

bohm2

If you read the papers linked above, Leifer argues that PBR rules out an option favoured by Einstein:
It is argued in this paper that this is the view Einstein endorsed:
Einstein, incompleteness, and the epistemic view of quantum states
http://arxiv.org/pdf/0706.2661v1.pdf

Such a model is ruled out by PBR.

bhobba

I have read what Leifer says and think he is correct but is drawing the wrong conclusion - or rather just because it is sick does not mean the theory (under that interpretation) still can not be used.

The PBR theorem only applies to standard QM - not to some hypothetical theory that QM may be the approximation of like classical thermodynamics is the approximation of quantum thermodynamics. This was Einsteins belief - QM was not incorrect - just incomplete and he would have thought the PBR theorem is simply a symptom of that incompleteness. Classical thermodynamics has problems such as the black-body radiation problem quantum theory fixes up. There are similar issues with Electrodynamics such as the infinite self energy of the field and acasual runaway solutions in some circumstances. But again it is not an issue because it is only an approximation to QED that resolves the problems (but of course has others) - in virtually all practical situations classical Electrodynamics is applied to no problems arise - but to be sure it is in fact a sick theory. Einstein would be smiling merrily and looking over to Bohr who really couldn't care less because like me he thought the state, although fundamental, was simply knowledge about the system and not something real and physical.

In fact one version of the Ensemble interpretation accepts Einsteins take and that when you make an observation it selects a pre existing outcome despite Kochen-Sprecker. It invokes some as yet unknown theory to resolve it. Conceptually actually a very neat solution. It has but one problem - what is this theory it is an approximation to - damn that pesky detail.

Funny how nothing really has changed on the QM interpretation front - each side simply retreats to their impregnable positions - hmmm - I seem to recall posting that before.

Thanks
Bill

bohm2

I think Bohr would have been happier with the PBR theorem than Einstein. The reason, being that PBR leaves anti-realist approaches unscathed whereas it eliminates certain ontic models where ψ is epistemic/probabilistic. Not that there are any theorems that are likely to eliminate anti-realistic approaches. Having said that, there are other models that are also not affected by PBR or Bell's: superdeterministic ones.

bhobba

Yea Bohr may have liked PBR.

But really its this 'realist' view of the world that is the issue - for some of us its just so deeply ingrained its hard to shake and why I think there will always be disagreement.

Early on I was in the realist camp but after a while its baggage became too much and I jumped ship - long before PBR BTW.

Thanks
Bill