I Is QM incomplete?

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Hi
I have heard that QM is the most accurate and scientifically tested theory to date.
I have also read in many of these threads that it is silent when it comes to unobserved/unmeasured quantum systems?
Is this not a contradiction in terms?

I can say with almost 100% probability that the hubble comet will return every 86 years , from classical Newtonian laws without looking at the comet.
I accept that I will only know that it has arrived when I look for it at a certain position and time.
But at least I can predict that it will be there with 100% certainty.

QM seems to accept that , it is impossible to predict with certainty( or at least with very high probability)
where and when to measure microscopic quantum objects such as electrons and photons.

So we use probability and interpretations which give us the next best guestimate?
Surely our universe is well defined and not best described on probabilistic and accidental events as
some physicists are led to believe.

I would like to know the current views on the subject and has there been any progress in this line of thought.

Johan
 

DrChinese

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Hi
I have heard that QM is the most accurate and scientifically tested theory to date.
...
So we use probability and interpretations which give us the next best guestimate?
Surely our universe is well defined and not best described on probabilistic and accidental events as
some physicists are led to believe.

I would like to know the current views on the subject and has there been any progress in this line of thought.
You touched on a lot of topics! I will pass on a few comments. :smile:

Yes QM is quite a well-tested theory, and much thought has gone into precisely the issues you ask about. Generally, quantum objects do not possess well defined properties at all times. For example, a particle with well-defined momentum lacks a well-defined position. Yes, this is counter-intuitive, but there are very strong reasons to accept this. The EPR (1935) and Bell (1964) papers discussed the completeness issue. QM is not considered incomplete (although that is something of an open question).

Experimental evidence of the past 30 years has done nothing but confirm QM. However, in that time, many attempts at alternative approaches have been soundly ruled out. Specifically, the so-called local realistic theories have been excluded by experiment. But this is a big subject, quite fascinating, and one which lends itself to a lot of study.
 

phinds

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Surely our universe is well defined and not best described on probabilistic ... events...
Uh ... surely it IS described by probabilistic events (at the quantum level). Do you seriously believe that the thousands and thousands of serious physicists who study this stuff have it wrong?

EDIT: I see Dr. Chinese has beat me to it.
 
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.
Do you seriously believe that the thousands and thousands of serious physicists who study this stuff have it wrong?
No not wrong just incomplete, I am not disputing QM great achievements...but

If quantum objects at the microscopic level can only be defined in terms of probabilistic properties and we postulate this , but our everyday macro(classical) experience of reality has well defined properties.
Then the two cannot both be fundamental, one must be derived from the other?

Why can we not just assume that all objects Quantum or not , have well defined properties , and that we just do not have the technological means
to measure all these properties down to the quantum scale yet.
Is there sufficient proof today which rules out this line of thought. Does the Bell inequality experiments really rule out all loop holes?

Bell, Aspect, Quantum erasure , they all seem to imply that you cant accept Local reality because of instantaneous 'collapse ' of the unobserved
entangled photon , that only gets its state once the other is measured.

Why can we not say that all the quantum objects in the universe , are all well defined and , somehow entangled or connected . And that we just don't have the means to measure them yet without disturbing them or getting our measuring instruments entangled/connected to them ?

Surely we cannot just be the product of concurrent 'accidental' events as the great Murray Gell Mann once quoted.
There must be an underlying order that our universe obeys.

The action is not spooky but rather not measured accurately or cleverly enough.


Regards
Johan
 

phinds

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If quantum objects at the microscopic level can only be defined in terms of probabilistic properties and we postulate this , but our everyday macro(classical) experience of reality has well defined properties.
Then the two cannot both be fundamental, one must be derived from the other?
Actually, they can and are. Quantum level is REALLY REALLY small and at the macro level things average out.

Why can we not just assume that all objects Quantum or not , have well defined properties , and that we just do not have the technological means
to measure all these properties down to the quantum scale yet.
We can make that assumption but we would be totally wrong as shown by experiment after experiment.

Does the Bell inequality experiments really rule out all loop holes?
I'll leave that one to @DrChinese

Why can we not say that all the quantum objects in the universe , are all well defined and , somehow entangled or connected . And that we just don't have the means to measure them yet without disturbing them or getting our measuring instruments entangled/connected to them ?
The Heisenberg Uncertainty Principle is not a measurement problem it is a description of nature.
 
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The Heisenberg Uncertainty Principle is not a measurement problem it is a description of nature.
Very profound statement for me , since our description of nature is based on the measurements we make?

For example, can one measure a pure quantum object state accurately , that is X meters wide with an measuring instrument that is 2x meters wide ?
Yes but we interfered with the smaller object so much that it will now seem to behave differently.Can this not be why Bell's inequality is violated?

Until of course we find that measuring device that is 0.01x wide and we do not interfere with the quantum objects properties.
I do see the problem which eventually arises, how do we measure the 0.01x object accurately!
Which delves into the plank scale and string theory. So is there lower limit. Plank scale? String theory?

As a local realist , I will say when the theory has to many 'outs' that cannot be falsified or tested .E.G. Many worlds theory.
Then one can shut up and calculate many weird outcomes that have a positive probability of occurring , but in reality never will.

Just my intuitive opinion.

Fascinating subject , thanks for your comments.
 

Nugatory

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I have heard that QM is the most accurate and scientifically tested theory to date.
I have also read in many of these threads that it is silent when it comes to unobserved/unmeasured quantum systems?
Is this not a contradiction in terms?
No, because all tests of all theories involve observing/measuring something to see if the result agrees with the predictions of the theory. If you aren't measuring/observing, you aren't testing the theory; and if the theory correctly predicts the result of every single experiment that ever has been performed, that's as good as testing ever gets.
I can say with almost 100% probability that the hubble comet will return every 86 years, from classical Newtonian laws without looking at the comet.
I accept that I will only know that it has arrived when I look for it at a certain position and time.
But at least I can predict that it will be there with 100% certainty.
Did you mean Halley's comet, which has a 75 year period and was last seen in 1986? If so, that's also what quantum mechanics predicts about the comet - so where's the problem?
QM seems to accept that , it is impossible to predict with certainty( or at least with very high probability)
where and when to measure microscopic quantum objects such as electrons and photons.
That's not quite what QM accepts. We can make arbitrarily accurate measurements of the position or momentum of a particle; we're limited only by the resolution of our measurement devices, and that's no different in classical physics. Of course quantum mechanics is interestingly different from classical physics, but misunderstanding these differences is a poor starting point for any serious discussion.

Quantum mechanics does predict that quantum objects will behave differently from classical objects. For example, QM predicts that we might measure the position of an electron and find it to be one side of a potential barrier; and then later measure its position and find it on the other side of the potential barrier. Classical mechanics predicts that such a thing cannot happen, but experiments show that it does. Thus, both classical mechanics and quantum mechanics correctly predict the behavior of Halley's comet, but only QM correctly predicts the behavior of electrons.
Surely our universe is well defined and not best described on probabilistic and accidental events as some physicists are led to believe.
You may feel in your heart and gut that the universe cannot be probabilistic at its core but there's no proof either way, and without a plausible candidate theory there's nothing to discuss within the scope of Physics Forums.

I'm leaving this thread open because there is room for answering questions and correcting misconceptions about what QM does say. However, further discussion along the lines of "Why cannot we say....?" will likely get the thread closed.
 
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Did you mean Halley's comet, which has a 75 year period and was last seen in 1986? If so, that's also what quantum mechanics predicts about the comet - so where's the problem?
Apologies , yes I meant Halleys comet. Thanks for the correction


No, because all tests of all theories involve observing/measuring something to see if the result agrees with the predictions of the theory. If you aren't measuring/observing, you aren't testing the theory
One more clarification please Nugatory.

If an observer observes Halleys comet through some telescope or measuring device , on its next pass , is the observer becoming entangled in some way with the Comet. Is the observer changing the macro state of the comet if we strictly define the comet as a average' of many quantum microstates that have experienced decoherence
 

Boing3000

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As a local realist,
Being a realist is ok. But local has been proved wrong, just by observing the universe behaviors, and using logic.
I will say when the theory has to many 'outs' that cannot be falsified or tested .E.G. Many worlds theory.
MW is not a theory at all. It is an interpretation. A quite reasonable one. If you don't like it, don't use it.

Then one can shut up and calculate many weird outcomes that have a positive probability of occurring , but in reality never will
Actually weird outcome with low probability do occurs all the time, tunneling being one of the classical example, the sun shines because of it.
 

ZapperZ

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The claim that QM is "incomplete" implicitly implies that there ARE physics theories that are "complete".

Can you give me an example of such complete theories?

Zz.
 

DrChinese

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Does the Bell inequality experiments really rule out all loop holes?
Yes. Does everyone accept that answer? No.

I consider it ridiculous that Bell tests are held to a standard that no other experiment is held to. Who talks about loopholes in tests of c? The truth is that every Bell test confirms the predictions of QM. IMHO, belief in Bell test loopholes is really a display of stubbornness and one's contrarian viewpoint.

@phinds I hope I didn't equivocate on this. :smile:
 
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If an observer observes Halleys comet through some telescope or measuring device , on its next pass , is the observer becoming entangled in some way with the Comet. Is the observer changing the macro state of the comet if we strictly define the comet as a average' of many quantum microstates that have experienced decoherence
No - its entangled with its environment of which the observer is just one small part, and can certainly be ignored in your telescope example. That why classical things are there whether you are looking or not - its always being looked at by its environment.

Now this is an I level thread. If you are at the I level then you can understand the following book - which IMHO is THE book on this stuff:
https://www.amazon.com/dp/3540357734/?tag=pfamazon01-20

It clearly explains what we do know, what we do not know, and exactly what interpretations explains what and what interpretations simply say - that how it is.

Note every theory, interpretation etc, QM or not, makes assumptions. Providing they lead to exactly the same experimental results it just a matter of taste which you prefer.

Thanks
Bill
 

phinds

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Yes. Does everyone accept that answer? No.

I consider it ridiculous that Bell tests are held to a standard that no other experiment is held to. Who talks about loopholes in tests of c? The truth is that every Bell test confirms the predictions of QM. IMHO, belief in Bell test loopholes is really a display of stubbornness and one's contrarian viewpoint.

@phinds I hope I didn't equivocate on this. :smile:
I think you nailed it :oldbiggrin:
 
93
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The claim that QM is "incomplete" implicitly implies that there ARE physics theories that are "complete".
Why would it imply that ?
There is no complete theory , as far as I know.
 
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No - its entangled with its environment of which the observer is just one small part, and can certainly be ignored in your telescope example. That why classical things are there whether you are looking or not - its always being looked at by its environment.
This is how I understood it as well.
Does it follow that if all photons were emitted from some source that they too are in some way entangled with the environment, whether we measure/detect them or not.

regards
 

Demystifier

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The claim that QM is "incomplete" implicitly implies that there ARE physics theories that are "complete".

Can you give me an example of such complete theories?

Zz.
Good point! For any theory one can find questions that the theory doesn't answer. Whether those questions are interesting and relevant is subjective and depends on one's curiosity profile.
 
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There is no Gödel's incompleteness theorem in physics and the reason is pretty straightforward. For all we know, there is only a finite number of experiments that can be done. List them all and you have a complete physics theory. There is no way to demonstrate that the universe is infinite with finite perception. But on the other hand you could actually attempt to list them all and always find more things which need to be considered. We have no consensus on a theory for Quantum gravity.

The MW interpretation is local and deterministic, so QM can in principle get away with the probabilistic nature.
 
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Does it follow that if all photons were emitted from some source that they too are in some way entangled with the environment, whether we measure/detect them or not.
Well if other things are about for it to be entangled with of course.

Thanks
Bill
 

Lord Jestocost

The feeling that quantum theory is incomplete is merely an outcome of our “classical” psychological predispositions. “Nature acts as it acts” and is not interested in our expectations “how it ought to act”. We know that there is something wrong with our classical ideas and conceptions of physics and we do not see how to set it right, viz. the feeling of “incompleteness” is a reference to our limited mind: We have no language at hand to understand the conceptual status of quantum theory.
 
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I don't agree that MWI is local. See Sec. 3.1 of
https://arxiv.org/abs/1703.08341
Hi Demystifier. I have read this this view from you a few times, but I don't quite understand what it means. Of course the wavefunction as a whole isn't in 3D space, but it seems the claim made by Deutsch and Hayden is it can be written in a form which is fully decomposed into systems localised (in 3D space) and where the real factual situation of system A is independent of what is done with system B, which is spatially separated from the former.

Are you disagreeing that this has been shown or do you not consider that the meaning of local?
 

ZapperZ

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Why would it imply that ?
There is no complete theory , as far as I know.
Then what's the problem with QM? Why did you single it out with a question like this?

If no theory is ever "complete", then QM will also never be "complete". The question is moot and has no answer.

Zz.
 

Demystifier

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Hi Demystifier. I have read this this view from you a few times, but I don't quite understand what it means. Of course the wavefunction as a whole isn't in 3D space, but it seems the claim made by Deutsch and Hayden is it can be written in a form which is fully decomposed into systems localised (in 3D space) and where the real factual situation of system A is independent of what is done with system B, which is spatially separated from the former.

Are you disagreeing that this has been shown or do you not consider that the meaning of local?
I am disagreeing that this has been shown. In particular, the authors seem to make some ontological conclusions from Heisenberg picture, but this is inconsistent with MWI which postulates ontology in the Schrodinger picture. Even though the two pictures are observationally equivalent, in MWI they are not ontologically equivalent.
 

atyy

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Then what's the problem with QM? Why did you single it out with a question like this?

If no theory is ever "complete", then QM will also never be "complete". The question is moot and has no answer.

Zz.
The point is that QM is intrinsically incomplete - it is incomplete even before comparison with data.

Other theories (eg. classical special relativity) are extrinsically incomplete - they are incomplete after comparison with data.
 

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