Reference frame of the vacuum -- which forum?

[danb]

TL;DR

What is the best forum for questions related to QFT?

I have a question about interactions between matter and the vacuum, but I don't see a forum for that. Quantum field theory seems to be the most closely related subject. What would be the best forum for questions related to QFT?

 

[PeterDonis]

Summary:: What is the best forum for questions related to QFT?

This one, since the question is about quantum field theory.

 

[danb]

Thanks.

 

[PeterDonis]

I have a question about interactions between matter and the vacuum

If your question about the twin paradox is the one you are referring to, it has nothing whatever to do with this. It has been moved to the relativity forum since it's a straightforward question about relativity and has nothing to do with QM or QFT.

 

[danb]

it's a straightforward question about relativity and has nothing to do with QM or QFT.

Just for the record, I completely disagree with this characterization. I'm not questioning the accuracy of relativistic formulas, and I don't expect experts on relativity to be very objective about the legitimacy of their favorite subject. My question is about the time evolution of matter, which is a quantum-mechanical process.

 

[Dale]

My question is about the time evolution of matter, which is a quantum-mechanical process.

Then you really wrote it poorly. What you wrote belongs in the relativity forum.

 

[phinds]

... I don't expect experts on relativity to be very objective about the legitimacy of their favorite subject.

Yes, but in your other thread you were not questioning the objectivity of scientists, you were disagreeing with well-established scientific theory, the validity has been demonstrated over and over again. Now, it seems, you not only want to question science, you want to impugn the integrity of scientists.

The very title of the thread "Reference frame of the vacuum" is based on confusion. There is no such thing, so it appears that you intend to continue your insistence that there is an absolute frame of reference somewhere. Defying well established science isn't going to lead you anywhere but to further confusion.

 

[phinds]

@danb, humans evolved in an extremely narrow band of physical existence and there was just naturally never any reason to understand the very small (quantum mechanics) or the very large (cosmology). Because of this lots of stuff in both QM and GR are not just counter intuitive, they make it clear that "common sense" and "intuition" are often counterproductive in studying them.

 

[danb]

you were disagreeing with well-established scientific theory, the validity has been demonstrated over and over again.

I was questioning the philosophical basis of the theory, not its correspondence to experimental data. Gauge theory gravity has established that the paradigm of curved spacetime may not be as objectively real as people think.

Now, it seems, you not only want to question science, you want to impugn the integrity of scientists.

Nobody's perfect. I'm not saying physicists are any worse than anybody else, only that we're all human. Do you disagree that physicists are human?

The very title of the thread "Reference frame of the vacuum" is based on confusion. There is no such thing

You don't know that. Nobody has proved that the vacuum can't have a reference frame.

Defying well established science isn't going to lead you anywhere but to further confusion.

Questioning well-established science is how discoveries are made. Adhering to unproven dogma only continues the existing confusion.

 

[PeterDonis]

My question is about the time evolution of matter, which is a quantum-mechanical process.

Please give a reference (textbook or peer-reviewed paper) for this claim.

Gauge theory gravity

Please give a reference.

Nobody has proved that the vacuum can't have a reference frame.

You can't prove a negative. If you think the vacuum does have a preferred reference frame, it's up to you to provide evidence for that claim. Nobody has done so yet.

Adhering to unproven dogma

Any further comments along these lines will receive a warning. Theories of physics with decades worth of experimental data confirming their predictions to many decimal places are not "unproven dogma".

If you want to claim that there are alternative theories that can match the same predictive track record with different philosophical foundations, you need to give references to the scientific literature that expounds those theories and demonstrates that predictive track record.

And, btw, Lorentz Ether theory is not one of those "alternative theories", at least not for purposes of discussion here. See under "non-mainstream theories" in the PF global guidelines:

https://www.physicsforums.com/threads/physics-forums-global-guidelines.414380/

 

[danb]

lots of stuff in both QM and GR are not just counter intuitive, they make it clear that "common sense" and "intuition" are often counterproductive in studying them.

And this is why I have my doubts about the idea that the vacuum doesn't have a reference frame. I know the comments I've made aren't popular around here. I'm not trying to push anyone's buttons. But it seems to me that relativity makes exactly the mistake that you're warning me about. The whole concept of the vacuum not having a reference frame derives from the classical, everyday experience that it seems "empty". People often describe it as an "empty void", and how can nothingness have a reference frame? The principle of relativity dates all the way back to Galileo, and that's pretty classical.

But quantum field theory suggests something very different. It suggests that the vacuum is a very real medium of some kind and matter is simply a quantized vibration of that medium. In that context, the principle of relativity seems quaint and old-fashioned. Gauge theory gravity makes accurate predictions without assuming that space is curved, and ether theories do likewise without the idea of spacetime. I'm not resisting modern ideas in favor of intuitive traditions, I'm suggesting something even more radical. I'm suggesting that a gauge theory of gravity may be a closer approximation to reality and may be easier to quantize.

 

[PeterDonis]

I know the comments I've made aren't popular around here.

It's not a matter of popularity. We don't discuss personal opinions. We discuss physics that has appeared in acceptable sources--that usually means textbooks or peer-reviewed papers. If you cannot give specific references to support the positions you are taking, that should be a big, big red flag to you that the position you are taking is just your personal opinion and is not a valid basis for discussion here. If nothing else, you can't expect to have any opinion worth discussing if you aren't familiar with the scientific literature on the topic--and if you are, you should have no trouble providing references.

So you have a simple choice: either give references, or this thread will be closed.

 

[PeterDonis]

The whole concept of the vacuum not having a reference frame derives from the classical, everyday experience that it seems "empty".

Nonsense. The concept of the vacuum not having a preferred reference frame is a simple consequence of Lorentz invariance of physical laws, including the laws of quantum field theory. Which, btw, makes your use of quantum field theory to try to justify your position an obvious case of your opinion not being worth discussing because you obviously are unfamiliar with the scientific literature.

 

[PeterDonis]

a gauge theory of gravity

For which you have given no references to even tell us what you mean by it. Either give references, or this thread will be closed.

 

[danb]

a gauge theory of gravity

For which you have given no references to even tell us what you mean by it. Either give references, or this thread will be closed.

Wikipedia: Gauge theory gravity
article by David Hestenes, Arizona State University (pdf): Gauge Theory Gravity with Geometric Calculus
arxiv from Phil. Trans. Roy. Soc. Lond. A356 (1998) 487-582: Gravity, Gauge Theories and Geometric Algebra
Proceedings of the Third Conference held at Deinze, Belgium, 1993: Gravity as a Gauge Theory in the Spacetime Algebra

 

[atyy]

In quantum field theory, the term"vacuum" does require a choice of reference frame.
The vacuum is simply the ground state of the system.
The ground state is a quantum state.
Like all quantum states, a choice of reference frame is required.
However, probabilities of measurement outcomes calculated using the quantum state do not depend on the reference frame chosen.

 

[Dale]

But quantum field theory suggests something very different.

Quantum field theory is fully relativistic. The vacuum in QFT is not described as having a unique reference frame.

Questioning well-established science is how discoveries are made.

Yes, but in the usual professional scientific venues, not here.

 

[danb]

Well, thank you for responding, everyone. I appreciate your answers to my questions, and I hope I didn't cause too much trouble.

 

[PeterDonis]

@danb, thank you for the references. IIRC we have had several previous PF threads on Hestenes' Geometric Algebra, which is much more general in application than just gauge theory gravity.

Gauge theory gravity itself, as Hestenes notes in his introduction, is one member of the general class of theories of gravity in flat spacetime, as opposed to interpreting gravity as spacetime curvature. Locally (meaning not just "locally" in the sense of the equivalence principle but in the sense of "on a continuous open neighborhood of arbitrary size), one can always reinterpret a curved spacetime theory as a field theory on flat spacetime (although any such reinterpretation will not, in general, be unique--there will be many possible ways of doing it in any particular case). The difference arises globally, in the kinds of global configurations that are considered to be "allowed"--flat spacetime theories are more restrictive in this respect than standard GR with the curved spacetime interpretation. So that is one potential way of distinguishing between these general types of theories experimentally, although not one we are likely to be able to make use of any time in the near future.

 

[PeterDonis]

In quantum field theory, the term vacuum does require a choice of reference frame.

No, it doesn't. The vacuum state of any QFT is Lorentz invariant, so it is the same in all frames.

Like all quantum states, a choice of reference frame is required.

A choice of frame for a particular representation of the state might be required. But all such representations will be physically equivalent.

 

[atyy]

No, it doesn't. The vacuum state of any QFT is Lorentz invariant, so it is the same in all frames.

Yes, I was confused. I meant in general, when one has time evolution in quantum field theory, a choice of frame is required. However, one can discuss the vacuum without time evolution.

 

[PeterDonis]

article by David Hestenes

I have to say, after reading through this article, that IMO Hestenes is not giving a fair presentation of "standard" GR. One example that jumped out at me:

He claims that the line element in his equation (203) does not appear in any GR textbook. But as far as I can tell, this line element is just the Schwarzschild geometry in Painleve coordinates (he calls it "Newtonian gauge"), which AFAIK appears in multiple GR textbooks and is certainly not any novelty (it was first discovered in 1922).

So while the Geometric Algebra/Geometric Calculus formalism is certainly an interesting way to mathematically model physics in general, and gravity in particular, I would be highly skeptical of claims on Hestenes' part that mainstream GR is simply ignoring things that he pays attention to. I think what we have here, at least at present, is a simple matter of personal preference for one formalism over another. Perhaps some day there will be an actual experimental test that can distinguish the two, but I don't see one coming any time soon.