Ontology is to quantum theory what hardware is to computation theory

[Demystifier]

Q(F)T is correct, and local realistic theories are wrong.

Does it mean that locality is wrong? Or that realism is wrong? Or that both are wrong? What's your hunch?

 

[vanhees71]

Since relativistic QFT in its standard form is based on locality (for causality reasons) it's "realism" that's wrong. Under "realism" I understand that all observables take determined values, and this is obviously wrong in all types of QT.

 

[Fra]

R: But the world is made of something, irrespective of whether we measure it or not. The measurement disturbs its behavior, but the stuff it is made of is the same, whether we measure it or not. Would you agree?
TP: Yes, that's probably true.
R: So you don't care about the stuff the world is made of.
TP: True, because it's a philosophical question.

Here I think TP is confused, because, if we would ask...

Would we expect the "response" to a disturbing measurement, to be influenced by what the stuff "is" or is "made of"? I would guess TP would say yes, and then it's not a philosophical question because TPs expectations and thus actions would depend on the answer. Now consider "TPs" friend, who is also a TP that studies him an his experiments, now this "philosophical equation" enters the hamiltonian of the composite system!

/Fredrik

 

[martinbn]

Only to the point. To demonstrate this, let me present a typical dialogue between a "realist" (R) and a "typical physicist" (TP) how I see them.

This dialog only confirms what I said earlier. You mix notions. Already the first answer is meaningless.

TP: The world is made of atoms, which are made of finer objects such as protons, neutrons and electrons.
R: Sure, but what these objects are? For example the electron, is it the same thing as its wave function?
TP: To the best of our knowledge, yes, the electron is nothing else but a wave function.

No, it is not. It doesn't even make sense to say that the electron is the same as its wave function. You are identifying the territory and the map, and they are not the same things.

The rest of the conversation builds on this and introduces further confusions. Now, I don't know if you do this intentionally or not, or you are just being imprecise, but my problem with the foundations of QM is that most of it is just that, introducing confusions like these and then writing papers on "solving" these "problems".

I am not a physicist, let alone a typical one, but I am willing to have the conversation as the TP, and we can see where it leads.

So my answer to the first question would be: "No, the electron is not the same as its wave function."

 

[Structure seeker]

Since relativistic QFT in its standard form is based on locality (for causality reasons) it's "realism" that's wrong. Under "realism" I understand that all observables take determined values, and this is obviously wrong in all types of QT.

Let me quote myself:

In the paper "A system's wave function is uniquely determined by its underlying physical state" [1] it is concluded based on free choice that interpreting the wavefunction as an objective reality is possible

So I disagree. You talk about realism as if it must be about the classical point-like properties. But if we accept that reality consists of wavefunctions, that is totally unharmed by the Bell tests.

 

[martinbn]

Let me quote myself:

Th link gives a paper on integration of tensor fields! And it doesn't seem related!

 

[Structure seeker]

My fault, sorry. I mean this link: https://iopscience.iop.org/article/10.1088/1367-2630/aa515c

And also wavefunctions evolve locally, only for entangled particles their quantum properties can only be described by their density matrix (a wavefunction describing their combined properties) so measuring that wavefunction here affects also the wavefunction there.

 

[Demystifier]

No, it is not. It doesn't even make sense to say that the electron is the same as its wave function. You are identifying the territory and the map, and they are not the same things.

Not me, it's the TP who wrongly identifies the territory and the map.

 

[martinbn]

Not me, it's the TP who wrongly identifies the territory and the map.

But you put the words in his mouth!

 

[Demystifier]

I am not a physicist, let alone a typical one, but I am willing to have the conversation as the TP, and we can see where it leads.

So my answer to the first question would be: "No, the electron is not the same as its wave function."

OK, let's try!
R: But wave function represents some properties of the electron. Is there another mathematical object that represents electron properties more directly than the wave function?

 

[Demystifier]

But you put the words in his mouth!

The words I put in his mouth represent his actual words. The words I put are the map, while his true words are the territory.

 

[martinbn]

OK, let's try!
R: But wave function represents some properties of the electron. Is there another mathematical object that represents electron properties more directly than the wave function?

The wave function represents the state of the electron. Other mathematical objects, for example some operators, represent the observables.

 

[martinbn]

The words I put in his mouth represent his actual words. The words I put are the map, while his true words are the territory.

Can you quote the exact words and say whe they belong to?

 

[Demystifier]

The wave function represents the state of the electron. Other mathematical objects, for example some operators, represent the observables.

Which of those, the state or the observables, changes with time? Is that change described by a deterministic equation?

 

[martinbn]

Which of those, the state or the observables, changes with time? Is that change described by a deterministic equation?

Are you changing the subject? You know the answers. There is no disagreement between R and TP on this. Most importantly how is this related to ontology?

 

[Demystifier]

Are you changing the subject? You know the answers. There is no disagreement between R and TP on this. Most importantly how is this related to ontology?

Please answer the question, you will see soon how this is related to ontology. I just need that YOU explicitly say that (even if I know it), so that you cannot later say that you didn't say it.

 

[martinbn]

Please answer the question, you will see soon how this is related to ontology. I just need that YOU explicitly say that (even if I know it), so that you cannot later say that you didn't say it.

Ok, fine. In the Heisenberg picture the observables depend on time and satisfy the Heisenberg equations.

 

[vanhees71]

Let me quote myself:

So I disagree. You talk about realism as if it must be about the classical point-like properties. But if we accept that reality consists of wavefunctions, that is totally unharmed by the Bell tests.

In relativistic QFT there are no wave functions but quantum fields.

Even in non-relativistic QFT, where a consistent 1st-quantization formulation exists, the wave function has a probabilistic meaning. The idea that it represents, e.g., an electron as Schrödinger thought originally, contradicts basic observations about electrons, and that's how the probabilistic interpretation by Born became unavoidable. Even nearly 100 years later, there's no other interpretation that is consistent with all observations.

 

[vanhees71]

Ok, fine. In the Heisenberg picture the observables depend on time and satisfy the Heisenberg equations.

In the Heisenberg picture the operators that represent observables in the QT formalism are time dependent and their time evolution is, by definition, governed by the full Hamiltonian. This is of little physical significance though since the physically meaningful quantities derived from the observable operators and the statistical operator are independent under arbitrary unitary transformations, which can be time dependent. In other words the physics is independent of the choice of the picture of time evolution.

 

[Demystifier]

Ok, fine. In the Heisenberg picture the observables depend on time and satisfy the Heisenberg equations.

So where does quantum randomness come from?

 

[martinbn]

So where does quantum randomness come from?

How does this relate to ontology! Now i have the feeling that you are just shooting in the dark hoping to hit something.

 

[vanhees71]

Where does the determinism in classical physics come from?

 

[kith]

No, it is not. It doesn't even make sense to say that the electron is the same as its wave function. You are identifying the territory and the map, and they are not the same things.

I agree that a typical physicist would answer this question differently but I think the map-territory distinction becomes murkier as the concepts become more abstract.

For example, I think many physicists don't distinguish between field quantities and the underlying physical system. I had a discussion with @Demystifier and @A. Neumaier about this 10 years ago.

 

[PeterDonis]

let me present a typical dialogue between a "realist" (R) and a "typical physicist" (TP)

You are misrepresenting the TP position. The difference between TP and R is not that R "cares" about ontology while TP does not, but that TP understands that at our current state of knowledge, we do not have good answers to ontological questions, and therefore does not see much point in continuing to beat one's head against the wall about them, while R refuses to admit this and keeps on asking the questions even though we do not have good answers to any of them. Understanding that we currently do not have good answers to certain questions is not at all the same as not "caring" about those questions at all.

 

[Demystifier]

You are misrepresenting the TP position. ... TP understands that at our current state of knowledge, we do not have good answers to ontological questions, and therefore does not see much point in continuing to beat one's head against the wall about them,

If you read the whole dialogue, you will notice that I said that. So I don't think that I misrepresent the TP.

 

[Demystifier]

How does this relate to ontology! Now i have the feeling that you are just shooting in the dark hoping to hit something.

I'm not, just be patient and we will come to that.

 

[Demystifier]

Where does the determinism in classical physics come from?

From the Newton equation.

 

[PeterDonis]

If you read the whole dialogue, you will notice that I said that.

You had TP say there are no good answers, yes. But that's by no means all you had TP say. You had TP say lots of other things that I don't think were appropriate for the TP viewpoint.

What you should have had TP say was something like: "I know you feel compelled to ask these ontological questions. But we currently have no good answers to any of them. So what's the point of discussing them?"

How would you have R answer that?

 

[PeterDonis]

Please answer the question, you will see soon how this is related to ontology. I just need that YOU explicitly say that (even if I know it), so that you cannot later say that you didn't say it.

I'm not, just be patient and we will come to that.

I don't think these sorts of word games are appropriate here. If you have an argument to make, then make it.

 

[martinbn]

I'm not, just be patient and we will come to that.

Assume i give the textbook asnwers to all your questions and get to thw point.

 

[Demystifier]

You had TP say there are no good answers, yes. But that's by no means all you had TP say. You had TP say lots of other things that I don't think were appropriate for the TP viewpoint.

What you should have had TP say was something like: "I know you feel compelled to ask these ontological questions. But we currently have no good answers to any of them. So what's the point of discussing them?"

How would you have R answer that?

R: So you do not care much about ontological questions, because there is no point in discussing them. Am I right?

 

[PeterDonis]

R: So you do not care much about ontological questions, because there is no point in discussing them. Am I right?

TP: No. I have already said that admitting that we have no good answers to certain questions is not the same as not caring about them.

 

[Demystifier]

Assume i give the textbook asnwers to all your questions and get to thw point.

TP: The randomness is in the wave function collapse, which happens during the measurement.
R: But collapse contradicts locality.
TP: No, because collapse is just an update of our subjective knowledge. The collapse is not a change of real physical stuff.
R: But do you assume that real physical stuff exists?
TP: Of course.
R: But Bell proved that if real physical stuff exists, then, during some types of measurements, this real physical stuff necessarily changes in a way which contradicts locality.
TP: Yes, by which he proved that "real physical stuff" does not exist.
R: But you just said that you assume that real physical stuff exists.
TP: The notion of "real physical stuff" is a vague concept without a true relevance to physics.
R: So you don't care about ontology?
TP: Exactly, I'm a serious scientist so I don't care much about the vague notion of "ontology".
R: I rest my case.

 

[vanhees71]

From the Newton equation.

The Schrödinger equation then also implies that QM is deterministic.

 

[Demystifier]

The Schrödinger equation then also implies that QM is deterministic.

So why then QM is probabilistic in a way in which classical mechanics isn't? Where does the difference come from?