Revisiting the Completeness of Quantum Theory: A Scientist's Perspective

In summary, the conversation revolves around the question of whether or not quantum theory is complete. Some argue that it is complete, as it can accurately predict the outcomes of experiments within its scope of application. Others argue that it is not complete because it cannot accurately predict the exact location of particles in experiments. The concept of probabilities also arises in this discussion, with some questioning whether a truly complete theory should rely on probabilities. Ultimately, it is a complex and ongoing debate with no definitive answer.
  • #1
computerphys
128
0
I am sorry if this issue has been already addressed previously in this forums, but I have been looking for old threads on the subject and I haven't found any specific to the matter. If anyone knows about any, please let me know and sorry again.

Otherwise, this is my question: "Is Quantum theory complete?"

If I am not wrong, this was Einstein's point in the famous letter to Bohr in 1920. According to Bohr quantum theory is complete because what is to supposed to be left in the theory (hidden variables) is also left in the very reality. Ok. I understand it, and I already know and accept that there are important reasons to debunk hidden variables.

Nevertheless, I still have a doubt. When a measurement takes place an unpredictable value is obtained (under certain circumstances of uncertainty). How can we say that quantum theory is complete and at the same time that quantum theory cannot predict accurately the outcomes of a measurement? This is the very point I don't understand. Reality is given us the accurate outcome, whilst quantum theory is not, so reality is in fact "more complete" than quantum theory!

Call me "retro" and "stubborn" but I sincerely still think Einstein was right about this point, but at the same time I am ready to open my mind to reasonable statements about the completeness of Quantum Theory.

Thanks in advance.
 
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  • #2
Define what you mean by "complete".
 
  • #3
We have to know what reality is and how reality is, then we will be able to say if qm is a complete description of reality. If there is no underlying reality, qm is complete. But such questions should be relegated to God(Nostradamus comes as a second option).
 
  • #4
Dickfore said:
Define what you mean by "complete".

I am going to try it: a complete theory means to me that every outcome of a given experiment (*) could be accurately calculated using that theory.

* under the scope of application of that theory.

I assume that the position of a free electron, for example is inside the scope of quantum theory.
 
  • #5
computerphys said:
I am going to try it: a complete theory means to me that every outcome of a given experiment (*) could be accurately calculated using that theory.


This is not enough. You need to elaborate if "accurately calculated" includes probabilities or not.
 
  • #6
GeorgCantor said:
If there is no underlying reality, qm is complete.

First of all, thanks for the answer, GeorgCantor, I am kind of a fan of your posts, by the way.

I still don't understand how can we see QM as complete, even in the case that there is no underlying reality. My concern is that a simple quantum experiment outcome is showing certain information that the theory is unable to render, as for example the exact location of an electron.

So, if the theory is unable to render the outcome of an experiment, would be correct to say the theory to be complete?

Thanks!
 
  • #7
computerphys said:
I am going to try it: a complete theory means to me that every outcome of a given experiment (*) could be accurately calculated using that theory.

* under the scope of application of that theory.

I assume that the position of a free electron, for example is inside the scope of quantum theory.

Then it is complete. QM predicts that the outcomes of a measurement of an observable are the eigenvalues of the operator associated with that observable.
 
  • #8
GeorgCantor said:
You need to elaborate if "accurately calculated" includes probabilities or not.

No probabilities. The outcome of an experiment is not a probability, but a value. The outcome of a complete theory should also be a value, not a probability. Sorry if I am too naive.

The same way that thermodynamics is not a complete theory of the microstates, could we say the same about QM?

Thanks!
 
  • #9
computerphys said:
First of all, thanks for the answer, GeorgCantor, I am kind of a fan of your posts, by the way.

I still don't understand how can we see QM as complete, even in the case that there is no underlying reality. My concern is that a simple quantum experiment outcome is showing certain information that the theory is unable to render, as for example the exact location of an electron.


You can't know the location of the electron simultaneously with the momentum of the electron.


So, if the theory is unable to render the outcome of an experiment, would be correct to say the theory to be complete?

Thanks!


The theory would be complete even if there exists a deterministic underlying reality(to which we wouldn't have access).
 
  • #10
computerphys said:
No probabilities. The outcome of an experiment is not a probability, but a value. The outcome of a complete theory should also be a value, not a probability. Sorry if I am too naive.


Then this universe isn't for you :smile:. The probability "thing" comes as a result of the wave nature of matter. Waves aren't well localized in space.


The same way that thermodynamics is not a complete theory of the microstates, could we say the same about QM?

Thanks!


If you push it that far, there isn't ever going to be a complete theory of anything.
 
  • #11
computerphys said:
No probabilities. The outcome of an experiment is not a probability, but a value. The outcome of a complete theory should also be a value, not a probability. Sorry if I am too naive.

The same way that thermodynamics is not a complete theory of the microstates, could we say the same about QM?

Thanks!

Hmmm. It seems you have objections to the probabilistic interpretation of the wavefunction. I will refer you to Sakurai's introductory section.

Let's look at polarization of light. An unpolarized light beam passes through a polarizer. As a result, the light becomes linearly polarized in the direction of the polarizer's optical axis. Then we put another polarizer at an angle different from 90o to the first one. Some light still passes, but it is now polarized in the direction of the second polarizer's axis.

But, then, we use yet a third analyzer. Its axis is parallel to the first one's. The question is: Will some light still pass through?
 
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  • #12
It's definitely not complete in the sense that it's a unified theory of everything.

But of course, no theory is complete like this, or we'd have a unified theory of everything and science would be dead and applied science would be the new thing.

I don't personally think a TOE is even possible, because:

You're trying to model reality. Reality consists of a lot of stuff (virtually infinite bits of matter and interactions between that mater). Anytime you want to model that reality, you have to use stuff to do it (whether it's your neurons, pencil and paper, computers, or actual 3d models) and those models aren't going to have anywhere near the infinite resources that reality have, so they're always going to be missing information.

All you can do is scale your model from general to specific, taking on the flaws of one, or compromising for a little of the flaws of both for a more balanced model.
 
  • #13
Dickfore said:
Hmmm. It seems you have objections to the probabilistic interpretation of the wavefunction

I think I haven't, honestly. The theory says that there could be several eigenvalues with given probabilities and I accept (even believe) that it is due to the very nature of the world and not a flaw of QM.

Nevertheless, my concern is the information that an experiment give us (a single, accurate and certain value) but the theory does not (theory gives us values and probabilities). How can QM explain that information?

QM cannot tell us which one of the eigenvalues is going to be detected. QM cannot predict completely the event happening when doing the measurement. The event is that certain eigenvalue is the outcome, but not that a set of eigenvalues with certain probabilities are the outcome.

The fact is that we measure a value, whilst QM only renders sets of values and probabilities.

Facts don't match completely with QM predictions. Is that correct?

Thanks!
 
  • #14
Dickfore said:
Then it is complete. QM predicts that the outcomes of a measurement of an observable are the eigenvalues of the operator associated with that observable.

Yes, eigenvalues, plural. But the experiment outcome is not a set of eigenvalues, but only one single eigenvalue. Nature in someway has chosen that one and not any other of the set. The process by which nature makes that choice is not addressed by QM. Right?

Thanks!
 
  • #15
By your definitions, if nature happens to be nondeterministic, then I think every scientific theory must be incomplete by your definition.


Have you tried your hand at understanding some of the non-collapse interpretations, like MWI or Bohm?
 
  • #16
computerphys said:
Yes, eigenvalues, plural. But the experiment outcome is not a set of eigenvalues, but only one single eigenvalue. Nature in someway has chosen that one and not any other of the set. The process by which nature makes that choice is not addressed by QM. Right?

Thanks!

So, you object that the position or momentum can have any possible value?
 
  • #17
GeorgCantor said:
If you push it that far, there isn't ever going to be a complete theory of anything.

When a theory predicts an experiment outcome under its scope, we should say it is complete, as for example, Newtonian Mechanics. Of course, NM gets out of its scope when relativistic corrections are needed. So, at least we have a complete theory of something here.

In contrast, my point is that QM seems to me to be a theory that cannot predict an experiment outcome under its own scope, as for example the position of a free electron (after measuring its momentum).

Do you agree?

Thanks!
 
  • #18
Hurkyl said:
By your definitions, if nature happens to be nondeterministic, then I think every scientific theory must be incomplete by your definition.

Not exactly. If you consider the proper scope/range of a given theory, NM would be complete but QM wouldn't be.



Hurkyl said:
Have you tried your hand at understanding some of the non-collapse interpretations, like MWI or Bohm?

Do you think that a non-collapse interpretation could "fix" the QM incompleteness?

Thanks!
 
  • #19
What principle of Nature requires that we should simultaneously know the position and momentum of a particle?

QM says that the state of a system is described by a wave function. It also provides an equation that gives the time-evolution of the wave function. It is true that we cannot solve this equation for all but the simplest cases, but that does not mean that the theory is incomplete. A similar situation arises in classical mechanics, where the three-body problem is unsolvable, but that does not mean the theory is incomplete.
 
  • #20
Dickfore said:
So, you object that the position or momentum can have any possible value?

I am not sure if I understand what you are asking me ... but according to the experiment of measuring the position of a free electron you get one and only one eigenvalue. Do you agree with it?

Of course, according to QM, you get a set of eigenvalues, not just one. So, I must conclude that QM cannot predict a fact, an event, a measurement (under proper QM scope/range).

Einstein called it an incomplete theory. I just would like to know whether he was wrong or not, and where is the flaw in this way of reasoning.

Thanks!
 
  • #21
computerphys said:
Not exactly. If you consider the proper scope/range of a given theory, NM would be complete but QM wouldn't be.
Explain how Newtonian Mechanics could be complete if reality is nondeterminstic.

Do you think that a non-collapse interpretation could "fix" the QM incompleteness?
It seems you are already predisposed to reject the idea, no matter whether or not it has merit. :tongue:

MWI, for example, is complete and deterministic. Bohm as well. (whether it's accurate at a macroscopic scale is another question)


Nature-is-wavefunctions-collapse-is-nondeterministic-and-there's-nothing-else is also complete. Nondeterminsitic, though.
 
  • #22
Dickfore said:
What principle of Nature requires that we should simultaneously know the position and momentum of a particle?

None, of course. As I said, I accept the uncertainty of QM and of the very nature.

My point is that what we measure in the laboratory cannot be predicted by QM completely. Nature can render it, but QM cannot. So, QM is not as complete as nature. Where is my flaw?


Dickfore said:
It is true that we cannot solve this equation for all but the simplest cases, but that does not mean that the theory is incomplete. A similar situation arises in classical mechanics, where the three-body problem is unsolvable, but that does not mean the theory is incomplete.

I agree with you about this point. The reason why Einstein told Bohr that QM is incomplete wasn't related to the integrability of its equations.

Thanks!
 
  • #23
Computerphys, please respond to Hurkyl.

Thanks!
 
  • #24
Evo said:
Computerphys, please respond to Hurkyl.

Thanks!

Why should he respond to someone?
 
  • #25
Hurkyl said:
Explain how Newtonian Mechanics could be complete if reality is nondeterminstic.

The non-determinisc correction is out of NM scope. So, NM is effectively complete.


Hurkyl said:
It seems you are already predisposed to reject the idea, no matter whether or not it has merit. :tongue:

Sorry, then. No intention to offend or ignore merit. In fact I strongly believe QM interpretations are essential to the understanding of the world.


Hurkyl said:
Nature-is-wavefunctions-collapse-is-nondeterministic-and-there's-nothing-else is also complete. Nondeterminsitic, though.

If you tell me that nature is complete, of course I agree with it.

If you tell me that nature is non-deterministic at this level, I also agree with you.

The problem is not about nature. The problem is about QM when saying QM is complete.

Einstein's point is:

Nature gives us results that QM cannot predict (inside the quantum scope, of course), so QM is incomplete.

Thanks!
 
  • #26
Dickfore said:
Why should he respond to someone?
Because Hurkyl knows what he is talking about and is trying to keep this thread in line so it doesn't get closed.

That's why. :smile:
 
  • #27
Evo said:
Because Hurkyl knows what he is talking about and is trying to keep this thread in line so it doesn't get closed.

That's why. :smile:
Why would it get closed?
 
  • #28
computerphys said:
When a theory predicts an experiment outcome under its scope, we should say it is complete, as for example, Newtonian Mechanics. Of course, NM gets out of its scope when relativistic corrections are needed. So, at least we have a complete theory of something here.

In contrast, my point is that QM seems to me to be a theory that cannot predict an experiment outcome under its own scope, as for example the position of a free electron (after measuring its momentum).

Do you agree?

Thanks!


If nature is that way(electrons not having a definite position and momentum at the same time), then qm is complete.

If you want to posit some hidden variables theory about an underlying hidden reality, make it a non-local one(i.e. a pilot wave that instructs/directs the particle).
 
  • #29
computerphys said:
The non-determinisc correction is out of NM scope. So, NM is effectively complete.

"non-deterministic correction" is a bit of an oxymoron isn't it? I mean, if you're making a correction to a model of a reality, aren't you implying determinism, given that the whole applying of a correction is, on some level, validation of the model? And models don't really have any purpose outside of determinism.
 
  • #30
It is possible, computerphys, that you're confusing chaotic, nonlinear, or stochastic processes with non-determinism.
 
  • #31
GeorgCantor said:
If nature is that way(electrons not having a definite position and momentum at the same time), then qm is complete.

If you want to posit some hidden variables theory about an underlying hidden reality, make it a non-local one(i.e. a pilot wave that instructs/directs the particle).

No, I don't need any hidden variables to get the conclusion that QM is incomplete. As I said before, I do believe in the uncertainty as a fundamental feature of the nature, and I know they are debunked due to Bell and Aspect, so forget hidden variables.

I still don't see why you say QM is complete while you agree with the empirical fact that the outcome of a measurement is a single eigenvalue, and at the same time knowing that QM is not capable of render/predict such a single eigenvalue, but a bunch of them.

Einstein's point in short words:

How can we say that QM is complete when it cannot render accurately experimental outcomes?

Thanks!
 
  • #32
Pythagorean said:
It is possible, computerphys, that you're confusing chaotic, nonlinear, or stochastic processes with non-determinism.

Thanks for the hints, but no. Independently of these uncertainty sources, there is a fact we cannot avoid: the nature renders a single eigenvalue but QM renders a lot of them. So, nature is not completely described by QM. QM is incomplete.

By the way, chaotic or nonlinear cases belong to the Hidden Variables scheme, if I am not wrong. So, we should avoid them.

Pythagorean said:
And models don't really have any purpose outside of determinism

I agree with you, but it does not change Einstein's point:

QM is incomplete because it cannot renders accurately eigenvalues as nature does.
 
  • #33
computerphys said:
No, I don't need any hidden variables to get the conclusion that QM is incomplete. As I said before, I do believe in the uncertainty as a fundamental feature of the nature, and I know they are debunked due to Bell and Aspect, so forget hidden variables.

I still don't see why you say QM is complete while you agree with the empirical fact that the outcome of a measurement is a single eigenvalue, and at the same time knowing that QM is not capable of render/predict such a single eigenvalue, but a bunch of them.


I didn't say qm was complete, look at my first post. In order for me to know this, i would have to know if there exist at the fundamental level such a thing as an electron with a defnite momentum and definite position at the same time. I don't know if this is true, hence i am not taking a side.


Einstein's point in short words:

How can we say that QM is complete when it cannot render accurately experimental outcomes?

Thanks!


Einstein didn't know that lhv would be refuted so any underlying model has to either be non-realistic(counterfactual), non-local or the observers have no free will.

You seem to insist that fundamental particles must have a definite position and momentum at the same time, but what is the justification for this, except a classical mindset?
 
  • #34
Just for the sake of clarity, when I say that QM is incomplete, I am not implying that it should exist a deeper theory that would "fix" QM. In fact, I believe that deeper theory cannot exist.

What I mean by incomplete is fairly simple: nature is given us more information in the experimental outcomes than QM can render/predict. Just that.

The key concept in this thread should be the completeness, its meaning and its applicability to QM. I hope that clarifies my point which I think it is the same that Einstein's.

Thanks again!
 
  • #35
GeorgCantor said:
I didn't say qm was complete, look at my first post. In order for me to know this, i would have to know if there exist at the fundamental level such a thing as an electron with a defnite momentum and definite position at the same time. I don't know if this is true, hence i am not taking a side.

I apologize for my misunderstanding. I respect you taking no side.



GeorgCantor said:
Einstein didn't know that lhv would be refuted so any underlying model has to either be non-realistic(counterfactual), non-local or the observers have no free will.

Yes, Einstein believed in hidden variables as a solution to the incompleteness of QM. But I don't share that point with him. Nevertheless it seems to me that Einstein was right when telling Bohr that QM is not complete due to the lack of accuracy in QM predictions as opposed to the accuracy nature give us in the measurement process.



GeorgCantor said:
You seem to insist that fundamental particles must have a definite position and momentum at the same time, but what is the justification for this, except a classical mindset?

I don't know where I have said that, but I don't think that fundamental particles must have a definite position and momentum at the same time.

What I say is just that what you measure in the laboratory cannot be rendered by QM.

I think that is equivalent to say that QM is not complete, but not meaning there should exist another alternative, deeper and more complete theory.

Just would like to know if this point could be a sound argument under a scientific and philosophical basis.

Thanks!
 

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