- #1
- 8
- 2
What is the difference between the GTR vacuum and the vacuum of quantum theory? What is the speed of light in the vacuums, specifically what can be predicted and what must be measured?
Um, that they are two different theories?Pikkugnome said:What is the difference between the GTR vacuum and the vacuum of quantum theory?
Our physical theories based on Lorentz invariance predict that there should be a finite invariant speed. Our measurements bear this out.Pikkugnome said:What is the speed of light in the vacuums, specifically what can be predicted and what must be measured?
You seem to be confusing theoretical models with reality. GR and quantum theory are theoretical models. As I have already said, both of them agree that there should be a finite invariant speed. And as far as the numerical value of that speed, as I have already said, that depends on your choice of units; in natural units it is ##1##. It doesn't even make sense to ask whether GR or quantum theory predicts a particular value for the speed, since humans' choice of units is not something theories of physics can predict.Pikkugnome said:I am wondering about the speed of light in GTR vacuum, since it predates quantum theory. It is surprising that the speeds should be the same, as the vacuums could be different.
You've labelled this thread as B-level, but it's really an A-level question. Here's my attempt at an answer that's somewhere in between...Pikkugnome said:What is the difference between the GTR vacuum and the vacuum of quantum theory?
Note, though, that in the algebraic approach to QFT, all of these unitarily inequivalent constructions can be handled in a unified manner. So I think the jury is still out on exactly what the physical meaning is of the mathematical fact that unitarily inequivalent constructions exist.strangerep said:the short (probably still puzzling) answer to your question is that, in curved spacetime, there is an infinity of inequivalent versions of the simplistic vacuum of ordinary (non-interacting) QFT.
The "physical meaning" is that the choice of mathematical framework (Fock space) is not quite right. Find the right structure and collect your Clay Institute prize.PeterDonis said:Note, though, that in the algebraic approach to QFT, all of these unitarily inequivalent constructions can be handled in a unified manner. So I think the jury is still out on exactly what the physical meaning is of the mathematical fact that unitarily inequivalent constructions exist.
Pikkugnome said:What is the difference between the GTR vacuum and the vacuum of quantum theory? What is the speed of light in the vacuums, specifically what can be predicted and what must be measured?
Ahem.pervect said:Informally, though, the quantum vacuum is full of virtual particles, while GR, being a classical theory, is not.
Which can be reached here:strangerep said:Arnold Neumaier's insight articles on virtual particles.
Those theories complement each other to predict the very accurate experimental evidence.Pikkugnome said:I am wondering about the speed of light in GTR vacuum, since it predates quantum theory. It is surprising that the speeds should be the same, as the vacuums could be different. Are the speeds different, should they be, I have no idea.