A Could QM Arise From Wilson's Ideas

[WernerQH]

Why is it flawed?

Why should a classical concept retain its usefulness down to the smallest scales of space and time?

 

[Demystifier]

Why should a classical concept retain its usefulness down to the smallest scales of space and time?

Do you know some interpretation of QM that does not retain any classical concept at the smallest scales?

 

[stevendaryl]

But for low energy, gravity is extremely weak, so weak that it safely can be ignored. What remains observable in all those particle colliders are only renormalizable theories.

Okay. I interpreted Wilson's quote as saying that every theory has a low-energy limit that is renormalizable (or looks renormalizable). But in the case of gravity, that low-energy limit is: "no (dynamical) gravity at all". (By "not dynamical", I mean that within QFT, the metric is unaffected by particles.)

 

[vanhees71]

It’s not that you can’t have a background geometry, but that geometry cannot take into account quantum particles.

You can have electrons moving in a background geometry but by definition that background doesn’t include the effect of those electrons. The background geometry would (contrary to the spirit of Newton’s third law) act on the electrons but would not be acted on by them.

The problem indeed is that we don't have a complete theory yet, i.e., the gravitational interaction is not successfully "quantized". Quantum theory describes everything except gravity in a given "background spacetime", i.e., the gravitational interaction is treated classically in the sense that it is reinterpreted as a spacetime which is determined by the Einstein field equations with the classical energy-momentum tensor of the macroscopic matter.

It's a bit like in quantum mechanics, where you describe the electromagnetic field as a classical field. In contradistinction to gravity electromagnetism (and also the weak and strong interaction) have been successfully quantized.

 

[WernerQH]

Even Steven Carlip (post #6) admitted that he cannot prove that gravity needs to be quantized. It's quite a different animal. It differs from the other interactions in that it doesn't couple to a discrete charge. One could even argue that it doesn't interact with elementary particles at all. It just tells them to follow the "most natural" path. I for one can't make sense of a superposition of space-time geometries; only an average background geometry makes sense to me.

 

[stevendaryl]

Even Steven Carlip (post #6) admitted that he cannot prove that gravity needs to be quantized. It's quite a different animal. It differs from the other interactions in that it doesn't couple to a discrete charge. One could even argue that it doesn't interact with elementary particles at all. It just tells them to follow the "most natural" path. I for one can't make sense of a superposition of space-time geometries; only an average background geometry makes sense to me.

But then what is the source (the stress-energy tensor) for the field equations? If it is the energy and momenta of quantum particles, then I don't see how you can get away without needing to quantize gravity. One alternative, possibly, is that the source is the expectation values of the quantum energy/momenta. Expectation values being c-numbers.

 

[Sunil]

Why should a classical concept retain its usefulness down to the smallest scales of space and time?

Why should a useful and successful classical concept lose its usefulness simply because of scales becoming small?

One can reasonably doubt that a classical concept fails if no interpretation exists which supports this concept. Else, there is simply no base for doubt.

 

[WernerQH]

But then what is the source (the stress-energy tensor) for the field equations? If it is the energy and momenta of quantum particles, then I don't see how you can get away without needing to quantize gravity. One alternative, possibly, is that the source is the expectation values of the quantum energy/momenta. Expectation values being c-numbers.

Yes, it's the expectation values. Also pressure is just an average taken over many moving atoms. I see GR as a macroscopic theory, microscopic physics enters only in an averaged form. Today nobody views elastic forces as fundamental, they are reduced to electromagnetic interactions. Gravity may be some kind of residue of the other three interactions, and quantizing gravity similar to, but of course much harder than quantizing elasticity.

 

[WernerQH]

Why should a useful and successful classical concept lose its usefulness simply because of scales becoming small?

Why should classical mechanics and electrodynamics fail to describe atoms?

 

[Sunil]

Why should classical mechanics and electrodynamics fail to describe atoms?

Because the experiment tells us that they fail. Why they fail remains, of course, unknown until the better theory has been found.

If a theory fails, it should be replaced by one which does not fail. Such is life in science.

But the classical philosophical concepts don't fail in such a way. That the founding fathers were unable to find an interpretation in agreement with classical common sense concepts is an irrelevant historical accident, what should matter is only what we know today. And today we have interpretations which are realistic, causal, and have a quite classical ontology, with the wave function interpreted as incomplete knowledge. Why would one reject principles which are viable, compatible with the best theories we have?

 

[Demystifier]

Even Steven Carlip (post #6) admitted that he cannot prove that gravity needs to be quantized.

Perhaps it does not need to be quantized in a sense in which electromagnetism is quantized, but it certainly needs to be quantized in a sense in which Schrodinger cat is quantized.

 

[bhobba]

To me, that’s just mush.

Maybe it is along the lines of Gell-Mann and Hartle:
https://arxiv.org/pdf/1106.0767.pdf

I do not think all the details have been worked out, but it is how I look at the emergence of a classical world from QM. Otherwise, such is a BIG problem, most definitely pointing to QM being incomplete (as Einstein sits laughing on the sidelines).

For those new to the issue, it needs to be said Einstein went through several phases in his attitude towards QM. His final position, contrary to popular myth, was that QM was correct but incomplete. Einstein took Dirac's Principles book wherever he went, and when he could not find it, he would ask: “Where is my Dirac?” Believe it or not, some scholars believe (as I am inclined to) Dirac sided with Einstein.
https://direct.mit.edu/posc/article/16/1/103/15218/Paul-Dirac-and-the-Einstein-Bohr-Debate

Thanks
Bill

 

[Quantumental]

Maybe it is along the lines of Gell-Mann and Hartle:
https://arxiv.org/pdf/1106.0767.pdf

I do not think all the details have been worked out, but it is how I look at the emergence of a classical world from QM. Otherwise, such is a BIG problem, most definitely pointing to QM being incomplete (as Einstein sits laughing on the sidelines).

Isn't Gell-Mann and Hartle just Everett in chronic denial?

 

[bhobba]

Isn't Gell-Mann and Hartle just Everett in chronic denial?

One could equally say that MW is just DH with a confusing sematic waffle added:

In one, we have potentially real outcomes. In the other actually real outcomes. Some may be interested in debating the difference of such semantics - to each their own. I have mentioned it before, and it is just a personal thing; actually real is too weird for me. It's not scientific, just a personal opinion. These days whenever I read about MW, I think of the worlds as potentially real.

It is like solipsism. I can't prove it wrong. However, personally, like most people, I believe it wrong. It simply does not sit well with the world as having an independent objective existence. In probability theory, we think of the outcomes we assign probabilities to as potentially real, and one becomes actually real. We do not think of all possible outcomes as actually real. It is just a convention - but one most people hold to.

Thanks
Bill

 

[*now*]

(Another alternative could be considering just what is relevant…)

The results of this could be interesting- Frontiers | On the Possibility of Experimental Detection of the Discreteness of Time | Physics (frontiersin.org) . I think CH treats time differently, as I think other views can treat it in different ways.

 

[bhobba]

I think CH treats time differently, as I think other views can treat it in different ways.

It treats time the same as the formalism everyone agrees on does. If you want to learn about CH, see:
https://quantum.phys.cmu.edu/CHS/histories.html

Thanks
Bill

 

[*now*]

Yes, it just occurred to me to edit to correct myself, thank you, I was referring to the discussion of the arrow of time from the earlier DH paper linked, and how different views might discuss the formalism, but I won’t edit for continuity.

 

[Quantumental]

It is like solipsism. I can't prove it wrong. However, personally, like most people, I believe it wrong. It simply does not sit well with the world as having an independent objective existence. In probability theory, we think of the outcomes we assign probabilities to as potentially real, and one becomes actually real. We do not think of all possible outcomes as actually real. It is just a convention - but one most people hold to.

Thanks
Bill

Would it not be more fair to say that DH is the solipsist standpoint here? I am conscious and all empirical evidence indicates that it is due to my brain, other people have similar brain structures and behave similarly, hence assuming their consciousness is logical. In this example don't you just choose to believe that somehow, someway, one world is real for no other reason than the fact that the others cannot be observed (like consciousness of others) ?

 

[bhobba]

In this example don't you just choose to believe that somehow, someway, one world is real for no other reason than the fact that the others cannot be observed (like consciousness of others) ?

Sure. But it is what most people do. Other than realising other views are possible, and virtually everyone rejects them, it really doesn't matter. If it worries you, I think a philosophy forum is more suitable than here.

Thanks
Bill

 

[Quantumental]

Sure. But it is what most people do. Other than realising other views are possible, and virtually everyone rejects them, it really doesn't matter. If it worries you, I think a philosophy forum is more suitable than here.

Thanks
Bill

No, I was just curious if there was any logic in the interpretation of DH that lent itself to a one world view beyond denial. Everyone is entitled to their own beliefs of course :)

 

[yjjiang]

Einstein was right. QM is useful, but it is not complete.

I'd say, if it is incomplete, people would have found the missing (global) hidden variables by now. Again, the problem with QM is how to interpret QM to match our intuition.

 

[bhobba]

I'd say, if it is incomplete, people would have found the missing (global) hidden variables by now. Again, the problem with QM is how to interpret QM to match our intuition.

The modern proposed incompleteness is below, say the Plank Scale, another, possibly non-quantum, theory takes over.

Thanks
Bill

 

[stevendaryl]

I'd say, if it is incomplete, people would have found the missing (global) hidden variables by now. Again, the problem with QM is how to interpret QM to match our intuition.

That's not necessarily true. It could be that there is some theory $QM+$ whose differences with ordinary $QM$ are completely negligible when the number of interacting particles is small but become important when there are, say, $10^{10}$ or more interacting particles. It would be very difficult to empirically test the difference, because we can't actually analytically study systems of many particles without making approximations.

 

[*now*]

This is a different paper than the one I posted before. It is about time with operational formulations. It doesn’t mention DH by name but does discuss some views and might be interesting here-
The arrow of time ewline in operational formulations of quantum theory (arxiv.org)

 

[physika]

That's not necessarily true. It could be that there is some theory $QM+$ whose differences with ordinary $QM$ are completely negligible when the number of interacting particles is small but become important when there are, say, $10^{10}$ or more interacting particles. It would be very difficult to empirically test the difference, because we can't actually analytically study systems of many particles without making approximations.

.
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End date 30 April 2027

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[Heidi]

I have a question but i am not sure that it is relevant here.
consider something like the Ads/Cft equivalence but where we have a 3+1 quantum theory equivalent to a holographic 2+1 quantum theory (x,y,t)
suppose that we have a measurement of z in the 3+1 space time.
what about the reality of the z result?