I How is realism understood in QM?

[Lynch101]

To put it very briefly, a realist is someone who thinks that scientific theories aim at describing the world as it is (of course, within the limits of human epistemic access to reality), while an anti-realist is someone who takes scientific theories to aim at empirical adequacy, not truth. So, for instance, for a realist there truly are electrons out there, while for an anti-realist “electrons” are a convenient theoretical construct to make sense of certain kinds of data from fundamental physics, but the term need not refer to actual “particles.” It goes without saying that most scientists are realists, but not all. Interestingly, some physicists working on quantum mechanics belong to what is informally known as the “shut up and calculate” school, which eschews “interpretations” of quantum mechanics in favor of a pragmatic deployment of the theory to solve computational problems.

Thanks LJ.

There appear to be 3 categories here, 1) most scientists, 2) "not all", and those who prefer to 3) "shut up and calculate".

With regard to the "anti-realist", as described above, I would have questions as to how their position differs from the realist or those who prefer to "shut up and calculate". As I understand it, [quantum] realists and [quantum] anti-realists differ in their interpretation of the mathematical formalism. Anti-realists, tend to adopt an instrumentalist+ interpretation. That is, where those who prefer to "shut up and calculate" adopt a strict instrumentalist approach, taking the formalism to be nothing more than a tool for prediction and choosing to remain silent on ontological questions, anti-realists take an instrumentalist interpretation + an anti-realist interpretation i.e. making specific statements about ontology.

Is that an accurate representation?

 

[Lord Jestocost]

...instrumentalism is usually categorized as an antirealism, although its mere lack of commitment to scientific theory's realism can be termed nonrealism...

from: https://en.wikipedia.org/wiki/Instrumentalism

Fifteen years ago, I mused in a Reference Frame column on how different generations of physicists differed in the degree to which they thought that the interpretation of quantum mechanics remains a serious problem (Physics Today, Physics Today 0031-9228 42 4 1989 9 April 1989, page 9 ). I declared myself to be among those who feel uncomfortable when asked to articulate what we really think about the quantum theory, adding that “If I were forced to sum up in one sentence what the Copenhagen interpretation says to me, it would be “Shut up and calculate!”

N. David Mermin, Physics Today 57, 5, 10 (2004)

 

[Lynch101]

...instrumentalism is usually categorized as an antirealism, although its mere lack of commitment to scientific theory's realism can be termed nonrealism...

from: https://en.wikipedia.org/wiki/Instrumentalism

Fifteen years ago, I mused in a Reference Frame column on how different generations of physicists differed in the degree to which they thought that the interpretation of quantum mechanics remains a serious problem (Physics Today, Physics Today 0031-9228 42 4 1989 9 April 1989, page 9 ). I declared myself to be among those who feel uncomfortable when asked to articulate what we really think about the quantum theory, adding that “If I were forced to sum up in one sentence what the Copenhagen interpretation says to me, it would be “Shut up and calculate!”

N. David Mermin, Physics Today 57, 5, 10 (2004)

There is a distinction to be made between instrumentalism and anti-realism, in the context of quantum interpretations. "Shut up and calculate" is a strict instrumentalist approach to the mathematical formalism. It remains silent on foundational questions or questions of interpretation [of the mathematical formalism]. Anti-realism doesn't remain silent on such questions however. As mentioned, it adopts an instrumentalist+ interpretation.

Suppose I do measure the position of the particle, and I find it to be at point C. Question: Where was the
particle just before I made the measurement? There are three plausible answers to this question, and they serve to characterize the main schools of thought regarding quantum indeterminacy:

1. The realist position: The particle was at C. This certainly seems reasonable, and it is the response Einstein advocated. Note, however, that if this is true then quantum mechanics is an incomplete theory, since the particle really was at C, and yet quantum mechanics was unable to tell us so. To the realist, indeterminacy is not a fact of nature, but a reflection of our ignorance. As d’Espagnat put it, “the position of the particle was never indeterminate, but was merely unknown to the experimenter.” Evidently is not the whole story—some additional information (known as a hidden variable) is needed to provide a complete description of the particle.

2. The orthodox position: The particle wasn’t really anywhere. It was the act of measurement that forced it to “take a stand” (though how and why it decided on the point C we dare not ask). Jordan said it most starkly: “Observations not only disturb what is to be measured, they produce it …We compel [the particle] to assume a definite position.” This view (the so-called Copenhagen interpretation), is associated with Bohr and his followers. Among physicists it has always been the most widely accepted position. Note, however, that if it is correct there is something very peculiar about the act of measurement—something that almost a century of debate has done precious little to illuminate.

3. The agnostic position: Refuse to answer. This is not quite as silly as it sounds—after all, what sense can there be in making assertions about the status of a particle before a measurement, when the only way of knowing whether you were right is precisely to make a measurement, in which case what you get is no longer “before the measurement”? It is metaphysics (in the pejorative sense of the word) to worry about something that cannot, by its nature, be tested. Pauli said: “One should no more rack one’s brain about the problem of
whether something one cannot know anything about exists all the same, than about the ancient question of how many angels are able to sit on the point of a needle.” For decades this was the “fall-back” position of most physicists: they’d try to sell you the orthodox answer, but if you were persistent they’d retreat to the agnostic response, and terminate the conversation.

Emphasis here is the authors own, I've just emboldened instead of italicised.

#1 above corresponds to the realist position (obviously) while #3 corresponds to the "shut up and calculate" (SUAC) position. #2 corresponds to the anti-realist position. As you can see, there is a difference between the SUAC position and the anti-realist position.

 

[Morbert]

No. Realist interpretations are in no way obliged to treat the spin as real. In fact, they don't do it.

There's an important distinction to be made between real and primitive/fundamental. E.g. From Goldstein's "Quantum Physics Without Quantum Philosophy" regarding Bohmian mechanics

In this regard, it might be objected that while spin may not be primitive, so that the result of our “spin measurement” will not reflect any initial primitive property of the system, nonetheless this result is determined by the initial configuration of the system [...] it is some property of the system and in particular it is surely real.

I.e. We can interpret spin as a real property even if we understand it as entailed by the configuration

There are also realist interpretations that treat spin as fundamentally as position (e.g. Robert Griffiths's consistent histories)

 

[Elias1960]

There's an important distinction to be made between real and primitive/fundamental. E.g. From Goldstein's "Quantum Physics Without Quantum Philosophy" regarding Bohmian mechanics

In this regard, it might be objected that while spin may not be primitive, so that the result of our “spin measurement” will not reflect any initial primitive property of the system, nonetheless this result is determined by the initial configuration of the system [...] it is some property of the system and in particular it is surely real.

This quote suggests that this would be Goldstein's position. If you look at the context, it is not. Spin measurement is contextual. And that means that the measurement result depends not only on the state of the system, but also on the state of the measurement device.

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.

There are also realist interpretations that treat spin as fundamentally as position (e.g. Robert Griffiths's consistent histories)

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.

 

[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.

.
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]

.
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.