The notion of locality in (Quantum) Physics should be clearly defined

[Demystifier]

Einstein didn't intepret Lorentz ether theory but introduced an entirely new concept, i.e., introducing a new description of space and time and a new realization of inertial frames.

So did Bohm, he didn't interpret QM but introduced an entirely new concept, i.e., introducing a new description of microscopic phenomena and a new realization of uncertainty relations.

 

[vanhees71]

Yes, but it doesn't lead to any other prediction of observable facts than standard QM. I've nothing against Bohmian mechanics within non-relativistic 1st quantized QM. I've not seen a convincing realization for relativistic QFT. I know that you disagree with the latter.

 

[Demystifier]

Something that's no observable nor necessary to formulate the theory isn't needed and complicates the issues.

You mean, something like Feynman path integrals in non-relativistic QM?

 

[vanhees71]

Path integrals are just another mathematical tool to formulate QM, and it's applicable in relativsitic QFT too.

 

[Demystifier]

I've nothing against Bohmian mechanics within non-relativistic 1st quantized QM.

Really? Isn't it an unnecessary complication?

I've not seen a convincing realization for relativistic QFT. I know that you disagree with the latter.

We already discussed it. You didn't claim that it doesn't make the same predictions. You objected that it is not based on the same foundations. In particular, you couldn't accept that gauge potentials can be "real", even though there is no experiment that can't be interpreted by a "real" gauge potential. Is an interpretation with a "real" gauge potential more complicated? Probably is, but Bohmian interpretation of 1st quantized QM is also more complicated, and yet you have nothing against it.

 

[Demystifier]

Path integrals are just another mathematical tool to formulate QM

Do you know any problem in non-relativistic QM that is easier to solve with path integrals, then with other methods? (I know that path integrals simplify a lot calculations in gauge field theories, but that's not what I'm asking about.)

 

[vanhees71]

No, path integrals are usually pretty nasty. The solution of the most simple non-relativistic model for the energy eigenstates of the hydrogen atom took a long time to achieve at all, and that's why Feynman abandoned the path-integral for use in introductory QM lectures. Even in the cases, where you can calculate the propagator exactly using the path-integral description the operator approach (using the Heisenberg picture) or wave-mechanics is simpler. That's for cases, where you have to evaluate a Gaussian path integral (free particle, harmonic oscillator, particle in a homogeneous force field). In the path-integral approach you need some regulator scheme. I think the most convenient one is the heat-kernel approach, although for free particles and the harmonic oscillator you can do it by brute-force evaluation as the continuum limit of the time-lattice calculation.

Indeed it's full advantage the path integral shows for many-body calculations and QFT. Indeed the quantization of non-Abelian gauge theories is much simpler than the equivalent operator formalism, which has been developed after the path-integral approach (Faddeev-Popov quantization) had been developed and BRST symmetry discovered. The latter plays the key role in the operator quantization (Kugo et al).

 

[Fra]

Yes, but it doesn't lead to any other prediction of observable facts than standard QM.

Generally, for me at least: the merit of a competing theory, that may be built on different foundations, and have the same predictions in the current domain of corroboration, may offer a different paths forward when we try to generalize the theory to new predictive domains. It's like SR vs GR vs classical mechanics; for everyday physics or standard engineering on earth, classical mechanics is the BETTER effective theory in the sense that it is more efficient to use, to make computations with etc; even though it's "foundations" are questionable. But try to convince a construction engineer that he should use general relativity instead of newtons mechanics to check his construtions. Why would he?

So for me, the main discriminator I use to choose between competing paradigms is; which of them you belive makes extending the theory unification of all forces easier to solve? And here we may have different ideas and noone can tell in advance what is the best one.

I don't know if anyone in the Bohmian program today has any such ambitions though. But not that I am aware of.

/Fredrik

 

[Demystifier]

So for me, the main discriminator I use to choose between competing paradigms is; which of them you belive makes extending the theory unification of all forces easier to solve? And here we may have different ideas and noone can tell in advance what is the best one.

I don't know if anyone in the Bohmian program today has any such ambitions though. But not that I am aware of.

In my case, Bohmian way of thinking taught me that symmetries, especially space-time symmetries, may be emergent than fundamental. Then thinking of symmetries as emergent lead me to new ideas that do not depend on the Bohmian interpretation as such, see e.g. https://arxiv.org/abs/2301.04448

 

[AndreasC]

Einstein didn't intepret Lorentz ether theory but introduced an entirely new concept, i.e., introducing a new description of space and time and a new realization of inertial frames.

But earlier you said that if two theories are observationally equivalent, they are the same theory!

 

[vanhees71]

True, but then usually one chooses the simpler one, i.e., one favors SRT against LET, because the latter makes unobservable additional assumptions (some "ether frame"), which doesn't add anything new, i.e., concerning observable facts about Nature they are otherwise equivalent. From a purely scientific point of view they are the same theory.

 

[Fra]

In my case, Bohmian way of thinking taught me that symmetries, especially space-time symmetries, may be emergent than fundamental. Then thinking of symmetries as emergent lead me to new ideas that do not depend on the Bohmian interpretation as such, see e.g. https://arxiv.org/abs/2301.04448

This is a "insight" I share with you, even if I am not into Bohmian mechanics.

/Fredrik

 

[Demystifier]

True, but then usually one chooses the simpler one

But simplicity is subjective. Personally I find the Bohmian QM simpler than the standard one, for many reasons. One of the reasons is that Bohmian QM does not need the measurement postulate, which standard QM does ("When an observable is measured, then the probability is such and such ...").

 

[vanhees71]

But Bohmian QM doesn't predict anything from that. All there is, also within BM, are the said probabilities.

 

[AndreasC]

True, but then usually one chooses the simpler one, i.e., one favors SRT against LET, because the latter makes unobservable additional assumptions (some "ether frame"), which doesn't add anything new, i.e., concerning observable facts about Nature they are otherwise equivalent. From a purely scientific point of view they are the same theory.

Right, so I think then we can agree that it is a case of two theories, or interpretations of theories, with different foundations, where one eventually came to dominate, despite being observationally equivalent. That is, however, different from the description of shifts only happening because one theory matches the results better. Simplicity is kind of subjective (indeed many physicists at the time probably did not find the counterintuitive aspects of relativity to be "simpler"), what I think is much more indicative that this was the correct idea (which of course may be reappraised in the future) is that this shift in the metaphysics of the theory, despite being observationally equivalent, provided a new way of thought that allowed new breakthroughs, for instance general relativity. So I see this as an example where a change of interpretation led to radical increase in content and understanding, and discovering new properties of nature.

Bottom line is, I definitely agree with what @Fra said. As such, I don't think interpretational (or, to use a "bad" word, "philosophical") investigations are pointless, and honestly I think this is the main problem in this thread.

 

[Demystifier]

But Bohmian QM doesn't predict anything from that. All there is, also within BM, are the said probabilities.

So?

 

[vanhees71]

So, there's no need to consider these fictitious trajectories.

 

[Demystifier]

So, there's no need to consider these fictitious trajectories.

There is no need to make the discussion of this whole thread, and yet we do. Why? Because we want to understand something conceptually. The same is with considering the fictitious trajectories.

 

[Fra]

What is fictious seems to depend on your primary notions. From the perspective of QFT i would say internal observers is fiction because they are ambigous. But OTOH from an agent centered starting point the "observer" that corresponds to QFT is what is fictious because it ia also ambigous in another way.

But seeing both views I think helps see the power of using symmtries as constraints, and the explanatory value of symmetries beeing emergent. I think both mechanisms are at play at once at different levels in the theory.

/Fredrik

 

[vanhees71]

What are "internal observers"?

 

[Fra]

Lets see if Peter Donis will slap my fingers for using the term:

its just a normal observer, but one with finite information capacity, to supposedly be more realistic. Otherwise its a normal observer.

But this limit meas thay cant entertain infinite repeats of experimenta to make perfect ensembles and timeless inferences.

In QFT or qm. Ensembles or arbitrarily large quantum systesm otherwise need to collect, process and represent so much data that its "fictious"

Also an internal observer does not need to be a classical or macro device. Its just tjat its inferencea would be extremely volatile.

Of course QM isnt desgine for internal observers.which is exactly MY problem with it. It is desigen for an ideal "fictive" asymptotic observer. But this fiction is reasonable for small systems.

/Fredrik

 

[PeterDonis]

Lets see if Peter Donis will slap my fingers for using the term

Not for using the term but for making a statement like this. It's off topic and adds nothing to the thread. If you have something to say, just say it. Don't speculate about what, if any, moderator response you will get.

 

[PeterDonis]

its just a normal observer, but one with finite information capacity, to supposedly be more realistic. Otherwise its a normal observer.

But this limit meas thay cant entertain infinite repeats of experimenta to make perfect ensembles and timeless inferences.

In QFT or qm. Ensembles or arbitrarily large quantum systesm otherwise need to collect, process and represent so much data that its "fictious"

Also an internal observer does not need to be a classical or macro device. Its just tjat its inferencea would be extremely volatile.

Of course QM isnt desgine for internal observers.which is exactly MY problem with it. It is desigen for an ideal "fictive" asymptotic observer. But this fiction is reasonable for small systems.

What does any of this (even assuming it's all valid) have to do with the topic of this thread?

 

[Fra]

What does any of this (even assuming it's all valid) have to do with the topic of this thread?

It was an example of that different perspectives/interpretation yields different elements that are fictious. An example that i consider interesting especially for the view of spacetime and its symmetries.

/Fredrik

 

[PeterDonis]

It was an example of that different perspectives/interpretation yields different elements that are fictious.

Is it an example you just made up, or is it taken from something in the literature. Remember that personal speculation is off limits here.

Also, I'm still not clear how this relates to the topic of this thread. This thread is about the specific notion of locality. It is not about general properties of different interpretations.

 

[LittleSchwinger]

My view would be that "non-local" in particle physics either means literal action at a distance, i.e. dynamical coupling between spacelike separated degrees of freedom, or that the fields in the Lagrangian are not point functions of spacetime.

In quantum foundations and quantum information it's a synonym for nonclassical correlations, although in the latter field "nonclassical correlations" seems to me to be more common.