How is realism understood in QM?

[PeterDonis]

Thread closed for moderation.

 

[PeterDonis]

Some off topic, overly speculative posts and their responses have been deleted. Thread reopened.

 

[physika]

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Do they have position and momentum prior to being measured?

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but what is Position ?

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[Morbert]

Sorry, only saw this now

If the result of an interaction depends on the state of both interacting parts, it makes no sense to describe it as a real property of one part.

Ok I see what you mean now. We cannot divorce 'that which is measured' from the experimental context in BM (which I am not very familiar with) since the 'that which is measured' is under-determined by the initial configuration and wavefunction of the measured system.

I have never understood why Griffiths' (in)consistent histories interpretation is named realist. IMHO it is simply adding some more details and denotations to Copenhagen. But that would be a different discussion.

Griffiths presents measured properties as noncontextual. E.g. According to him, we really can talk about a spin measurement outcome as revealing a pre-existing property without reference to an experimental context, provided we accept that no single family of histories will be able to describe all properties resolvable by experiment.

 

[physika]

If the result of an interaction depends on the state of both interacting parts, it makes no sense to describe it as a real property of one part.

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and in any case, if the particle have no spin at the moment (simply at rest, not spinning, static..), does not exist ?
make no sense...

and related, position only makes sense, only if there are some things to refer to such a "position".
(positions and other properties of objects are only meaningful relative to other objects).

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[Elias1960]

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and in any case, if the particle have no spin at the moment (simply at rest, not spinning, static..), does not exist ?
make no sense...
and related, position only makes sense, only if there are some things to refer to such a "position".
(positions and other properties of objects are only meaningful relative to other objects).

In the usual realistic interpretations, the state of a system is described by its configuration space trajectory $q(t) \in Q$. So, the same reality as in a classical Lagrange formalism. The wave function is in some of them also part of reality (dBB) in others it is epistemic (Caticha's entropic dynamics). But variables which already in the classical theory depend on the Lagrangian (and that means, possibly depend on external forces), like the momentum $p = \frac{\delta L}{\delta \dot{q}}$, are contextual, thus, depend in the quantum variant on the configuration of the "measurement device" too.

Position is only a very special case of a configuration. Relativism is also irrelevant, QT is not a relativistic theory.

 

[EPR]

How does the Bohmian view explain the stability of matter? Why isn't the particle(a point charge) radiating as it orbits the nucleus?

 

[Elias1960]

How does the Bohmian view explain the stability of matter? Why isn't the particle(a point charge) radiating as it orbits the nucleus?

Hm. You think the explanation given by standard QM is not sufficient? Why?

If the QM explanation is fine, then the BM explanation is simple. One derives the QM rules in quantum equilibrium, and then applies the QM explanation.

 

[EPR]

Hm. You think the explanation given by standard QM is not sufficient? Why?

If the QM explanation is fine, then the BM explanation is simple. One derives the QM rules in quantum equilibrium, and then applies the QM explanation.

Standard QM does not have a point charge continuously orbiting a nucleus.

 

[Elias1960]

No, it is a point charge in QM too. The configuration is defined by a single point position.

And from the Schroedinger equation follows a continuity equation for the probability density in the configuration space, so to claim that there is no continuity is unjustified too.

 

[EPR]

Show me a peer-reviewed paper that says that electrons have definite positions in the atom at all times. Configuration space is not spacetime. This claim is of the same character as particles trajectories.

 

[Demystifier]

Show me a peer-reviewed paper that says that electrons have definite positions in the atom at all times.

https://journals.aps.org/pr/abstract/10.1103/PhysRev.85.166

 

[Demystifier]

How does the Bohmian view explain the stability of matter?

By postulating that velocity of the electron at a given position is determined only by gradient of the wave function at that position.

Why isn't the particle(a point charge) radiating as it orbits the nucleus?

Because the electron and the EM field obey the Bohmin equations of motion, which differ from classical equations of motion.

 

[Demystifier]

I have never understood why Griffiths' (in)consistent histories interpretation is named realist. IMHO it is simply adding some more details and denotations to Copenhagen. But that would be a different discussion.

The existence of history does not depend on its measurement, that's why some call it "realist". But instead of depending on measurement it depends on the framework, which conceptually is the most difficult part of the interpretation. What determines the right framework in the absence of measurement? It's hard to tell clearly. It seems that the interpretation says that any framework can be used, but one just should not use two different frameworks at once. So this interpretation is more about how we should think about phenomena, rather than about what the phenomena really are. In this sense it is not a realist interpretation.

 

[kith]

Because the electron and the EM field obey the Bohmin equations of motion, which differ from classical equations of motion.

This answer should be enough but I don't find it completely satisfying. There is the pitfall of clinging too much to classical concepts when learning QM but dismissing intuitions based on classical mechanics too quickly may also lead to missed opportunities for understanding.

For the topic at hand, an answer which takes into account classical intuitions seems to be possible: The electron simply doesn't move and the part of its energy which could be considered kinetic is actually (quantum) potential energy in dBB. Do you find this point of view sensible?

 

[EPR]

Because the electron and the EM field obey the Bohmin equations of motion, which differ from classical equations of motion.

So is the electron, an electrically charged particle, moving around the atom or not? How is Bohmian motion different from classical motion? If the electron has definite positions along a trajectory, it must be radiating energy.

 

[PeterDonis]

is the electron, an electrically charged particle, moving around the atom or not?

It depends on the interpretation. In the Bohmian interpretation, it is.

How is Bohmian motion different from classical motion?

Because there is a different equation of motion, as @Demystifier said. The Bohmian equation of motion includes the quantum potential, which is not present in the classical equation of motion.

If the electron has definite positions along a trajectory, it must be radiating energy.

In classical physics, it would be. But in the Bohmian interpretation of QM, no, it does not.

 

[Demystifier]

The electron simply doesn't move and the part of its energy which could be considered kinetic is actually (quantum) potential energy in dBB. Do you find this point of view sensible?

That's wrong. In dBB electron moves, except in the ground state.

 

[Demystifier]

So is the electron, an electrically charged particle, moving around the atom or not?

It moves, except in the ground state.

How is Bohmian motion different from classical motion?

The equations of motion are different, which should answer your question.

If the electron has definite positions along a trajectory, it must be radiating energy.

Why it must radiate energy? If you have an answer at all, it must be based on classical physics. But Bohmian mechanics is not classical physics.

 

[kith]

That's wrong. In dBB electron moves, except in the ground state.

I see. The electron standing still corresponds to the wave function being real-valued (or having a global phase which can be divided out), right?

So basically there is a way to picture things in dBB which somehow complies with the classical intuition: the electron loses energy by interacting with the electromagnetic field until it comes to rest when it reaches the ground state. The corresponding story in ordinary QM is that the electron cannot be localized better than in the ground state because of the HUP. Of course, both stories are quite handwavy and shouldn't be used as a substitute for solving the equations of motions.

 

[Demystifier]

I see. The electron standing still corresponds to the wave function being real-valued (or having a global phase which can be divided out), right?

Right.

 

[A. Neumaier]

Because the electron and the EM field obey the Bohmian equations of motion, which differ from classical equations of motion.

Please point to a paper that specifies Bohmian equations of motion for an electron and the EM field. I must have overlooked the existence of these - in Bohmian treatments I have seen, the EM field seems to be emerging rather than have its own equation of motion.

 

[Demystifier]

Please point to a paper that specifies Bohmian equations of motion for an electron and the EM field. I must have overlooked the existence of these - in Bohmian treatments I have seen, the EM field seems to be emerging rather than have its own equation of motion.

See e.g. D. Bohm, Phys. Rev. 85 (1952) 180, Appendix A.

 

[A. Neumaier]

See e.g. D. Bohm, Phys. Rev. 85 (1952) 180, Appendix A.

Thanks. I hadn"t noticed this.

But the treatment is nonrelativistic, and would lead to the well-known infinities in a relativistic treatment to one loop order. Did anyone develop a renormalized version of this, or is Bohm"s treatment still the state of the art?

 

[EPR]

How would this work in practice? "Motion" is defined as a continuous process, not 'motion' done in quantum jumps. How can motion in DeBB for an electron be defined, if it goes from A to B without traveling the distance from A to B?
I understand the electron is also its associated wave in configuration space but the particle aspect is supposed to be real valued along the distance from A to B.

 

[A. Neumaier]

"Motion" is defined as a continuous process, not 'motion' done in quantum jumps. How can motion in DeBB for an electron be defined, if it goes from A to B without traveling the distance from A to B?

The solution of any differential equation results in continuous trajectories.

 

[Demystifier]

Thanks. I hadn"t noticed this.

But the treatment is nonrelativistic, and would lead to the well-known infinities in a relativistic treatment to one loop order. Did anyone develop a renormalized version of this, or is Bohm"s treatment still the state of the art?

I think nobody renormalized it in a way you would like to. People usually regularize it by some lattice-type regularization and assume that a sufficiently fine lattice gives sufficiently fine results. I know you disagree, but we already discussed it several times so let us not go into it once again.

 

[EPR]

The solution of any differential equation results in continuous trajectories.

What about tunneling? Which DeBB equation of continuous motion describes it?

 

[A. Neumaier]

What about tunneling? Which DeBB equation of continuous motion describes it?

The equations are always the same, except that the potential changes.

The electron moves continuously either through the barrier or along a reflected path, depending on the initial conditions. I don"t know whether anyone has looked at this problem numerically so that one could see how the motion proceeds.

 

[A. Neumaier]

People usually regularize it by some lattice-type regularization

Bohm's version??

I think they abandon Bohm's version and introduce other beable degrees of freedom, and do not keep Bohm's beables.

 

[EPR]

The equations are always the same, except that the potential changes.

The electron moves continuously either through the barrier or along a reflected path, depending on the initial conditions. I don"t know whether anyone has looked at this problem numerically so that one could see how the motion proceeds.

I am struggling to come to grips with "electron goes through barrier without like charges repelling and pushing the charges apart".

 

[PeterDonis]

I am struggling to come to grips with "electron goes through barrier without like charges repelling and pushing the charges apart".

Remember that the quantum potential is also present in the equations of motion, so you cannot use your classical intuitions about what will happen.

 

[EPR]

Remember that the quantum potential is also present in the equations of motion, so you cannot use your classical intuitions about what will happen.

Fine, but then how is it continuous motion? How is it continuous Bohmian motion if a particle goes from A to B without traversing the distance between A and B?

 

[PeterDonis]

how is it continuous motion?

Because, as has already been noted, the motion is the solution of a differential equation.

How is it continuous Bohmian motion if a particle goes from A to B without traversing the distance between A and B?

Nobody said the particle did not traverse the distance between A and B. In the Bohmian interpretation, it does. It just doesn't obey the classical equations of motion when doing so, because the Bohmian equations of motion include the quantum potential. The quantum potential is not something magical that converts continuous motion to discrete jumps. It's just a potential that is present in the quantum differential equation that is not present in the classical differential equation.

 

[EPR]

Nobody said the particle did not traverse the distance between A and B. In the Bohmian interpretation, it does. It just doesn't obey the classical equations of motion when doing so, because the Bohmian equations of motion include the quantum potential.

Assuming that the above is true and the electron travels from A to B continuously, how is it able to do quantum tunneling?
You are saying it goes from A to B continuously but also does so in discreet steps due to the quantum potential(e.g. during tunneling). Both can't be true.