FlatEarth":1fnm8e78 said:

ramparts":1fnm8e78 said:

A couple of things: you're thinking more broadly of quantum field theory (QFT). Quantum electrodynamics (QED) is, as you might tell from the name, a particular branch of QFT dealing with electrodynamics - that is, the electromagnetic force, explaining how it behaves on small scales where quantum effects take over.

The reason I referred to QED is because L. Krauss does in his lecture, but thanks for the clarification. Are you now studying the subject?

On my own, but I'll understand a lot more when I get into graduate school and take some classes

ramparts":1fnm8e78 said:

Yes, space *is* filled with a vacuum energy, with virtual particles popping in and out of existence, as a result of QFT. But it's hard to argue (and I've never seen a physicist do so) that that is a description of what spacetime is "made" of; rather, just another of many things occupying spacetime.

Yes, and I find this puzzling! It

*could be *that space is comprised of virtual particles, and without them there would be no space. This would help explain how gravity warps space, and why there is a cosmic speed limit (because, if true, space is a medium).

Erm, it could be. It's really a matter of how you interpret it, and there's not really much of a reason to interpret these virtual particles as

*defining* spacetime rather than being something that happens

*in* spacetime. Think of spacetime as the background, these virtual particles are really more of object

*on* that background. It's not helpful to think of them as comprising the background.

ramparts":1fnm8e78 said:

It is almost certainly true, however, that we will know more about spacetime - possibly having a final description of it - when we have a quantum gravity theory. The only well-tested theory we currently have that deals explicitly with the nature of spacetime is general relativity, in which gravity is the result of spacetime's curvature. **(A thought: if QFT's vacuum energy really were describing what spacetime is "made of", as you seem to be suggesting, then on large scales it would look like general relativity. It doesn't.)** Quantum gravity describes what happens to gravity on small scales, where the universe is quantum - just as QED describes electromagnetism on quantum scales. However, gravity has this additional interpretation of being the background itself, rather than a force on the background. This means that our quantum description of gravity is going to have to explain how spacetime behaves when exposed to quantum weirdness, and that's the biggest remaining mystery about spacetime: we just don't know how spacetime gets quantized. The quantum gravity theory will, presumably, solve this problem, and we'll know a lot more about the fundamental nature of spacetime as a result.

You lost me in the bolded text! Why doesn't vacuum energy look like/fit GR on a large scale?

I'm glad you picked that out!

When we have two different descriptions of the same thing on different scales, they should "look like" each other when we change the scales. Take gravity, for example. Newton described gravity as a force that scales as 1/r^2. Einstein conceived gravity in a completely different way - as the curvature of spacetime - with a completely different set of equations. The mathematical language, differential geometry, is completely different. But when you look at general relativity in the so-called weak field limit - when gravity isn't all that strong, distances aren't big, speeds aren't high - then its equations actually reduce down to Newton's equations. It's quite lovely.

If QFT's virtual particles described what spacetime

*was* and how it acted, then you would well expect that when you considered how their governing equations acted on large scales, you would recover the equations of general relativity. No such thing happens. So clearly there is more to spacetime than the virtual particles, because they don't give you the right answer on large scales

When you say “we just don't know how spacetime gets quantized”, are you referring to the mechanism that causes virtual particles to appear in space?

Not entirely. Forget about the virtual particles for a sec - those are somewhat tangential. What I'm talking about is how spacetime jives with quantum mechanics - how is spacetime subject to the uncertainty principle? Does it have to come in discreet chunks, like particles do? These are questions to which we don't know the answer.

I tend to think the discovery of gravity at the quantum level would change GR to a degree. Rather than gravity being the result of curved space, gravity would be considered a force like the other forces, with its own carrier particle. Do you see it that way?

Good post, ramparts. I'm hoping to learn more about QFT from you. Hope you don't charge a lot.

haha I charge not a thing, mostly because then I would be overcharging

Yes, GR might be changed in its interpretation. However, GR describes space at scales much larger than the quantum, so knowing what happens on quantum scales would leave GR unchanged

*except* in the regions (namely black holes and the big bang) where both theories are important.

However, you're definitely right in noting the tension between the two intepretations of gravity: as a force (with a carrier particle) and as the curvature of spacetime. Both are almost certainly true in some way, much like how particles can also be rightly seen as waves. It depends which view is more convenient for you. However, one of the big questions going into quantum gravity is how far down the spacetime picture holds - is it an effect of smaller processes that is only viable on large scales, or does this background actually exist at the most fundamental levels? The spacetime intepretation and its governing equations are unimaginably elegant, so it is difficult for me to conceive of it being just an illusion created by something entirely non-geometric on smaller scales. However, it's a possibility.

String theory, the most viable theory of quantum gravity we have, probably has an answer to these questions, but I don't know enough about the subject to say what that is. I will wait to learn the subject rather than peek at Wikipedia for the answer

SpeedFreek":1fnm8e78 said:

If QED or QFT says that space is made up of virtual particles, how does that define spacetime as a thing? What does it say time is made from?

That's a good question. I think I was in error for saying it defines spacetime. I should have said space. The time question is another problem, but it is said that virtual particles are borrowed from the future, so what does that say about time on the quantum level?

In fact, I should have learned by now to qualify all unproven statements with “I believe...”! As in “I believe QFT/QED describes what space is.”

I will say this: don't separate time and space so easily. If there's one thing we learn from GR, it's that the two are fundamentally the same!