What is a Vacuum in General Relativity?

[Brook]

Hi what constitutes a vacuum? I mean it has to be made of something, since it it part of the space time continuum. In school you learn that a vacuum is empty space or area without matter, but it occurs to me that a vacuum must be made of something and there must be something there. What is it?, else the universe will be something inside a huge vacuum.

 

[Simon Bridge]

Classically a vacuum is simply nothing. It is not "made of" anything.

In Quantum Mechanics, the concept of a vacuum is a bit more complex ... but the concept of space is rather complex too. But there is nothing intrinsically wrong with the idea of the materials of the Universe being inside of a huge nothingness.

Wikipedia has a useful overview of the different ideas - have a read of that and get back to us on anything you don't follow. However, I think you'll be better served by getting used to the classical ideas about space before you venture into the advanced concepts.

 

[Brook]

Thanks for your response Simon. I guess you mean the wikipedia article on vacuum.
Ok well let's get comfortable with the classical concept.

So if the materials of the universe are inside a huge nothingness, then does it mean that this nothingness is infinite? This seems strange, because the vacuum is in fact part of the universe. If a scientist create a vacuum in a tube in a lab (just saying) then that vacuum exist in the space around the lab, so it is part of the universe.

I guess what I am thinking is that in the same way that time doesn't exist without space, so too a vacuum doesn't exist without space. correct?

 

[Naty1]

So if the materials of the universe are inside a huge nothingness, then does it mean that this nothingness is infinite? This seems strange, because the vacuum is in fact part of the universe.

The universe is not 'inside' anything insofar as is known. The Universe is everything like the vacuum of outerspace, galaxies, stars, and the cup of coffee I am drinking.

If a scientist create a vacuum in a tube in a lab (just saying) then that vacuum exist in the space around the lab, so it is part of the universe.

A lab vacuum in everyday discussions just means most of the air has been removed. It is not empty in there because for examples light is passing through it. But if if you block all exterior fields, and we can't block gravity, there will still be some vacuum energy.

I guess what I am thinking is that in the same way that time doesn't exist without space, so too a vacuum doesn't exist without space. correct?

In everyday non scientific dreaming, those are ok, but we don't know either of those things for sure. Time and space may have emerged together from a bang, but may not have in another universe...one that may have failed to evolve as ours has, for example. Or either might have existed without the other; they DO seem to go together in this universe: "Time is nature's way of keeping everything from happening at once." and you can make up one for space, like "Space is nature's way of keeping everything from happening at one place.'

Keep in mind nobody knows EXACTLY what time and space and vacuum are. What we have are some good explanations that underlie our observations, not final definitive understandings.

For a quick read, try the first paragraph in Wikipedia on Casimir effect...that experiment seems to show space is NOT empty!

http://en.wikipedia.org/wiki/Casimir_effect

 

[Simon Bridge]

Thanks for your response Simon. I guess you mean the wikipedia article on vacuum.
Ok well let's get comfortable with the classical concept.

So if the materials of the universe are inside a huge nothingness, then does it mean that this nothingness is infinite?

Of course not: there is nothing about a vacuum that requires infinite nothingness.

This is a common topic - you should google around the concept of the "structure of space time". There are many "bounded Universe" models. The basic concept is that if you keep traveling on one direction for long enough, you'll end up approaching the place you started out but from the opposite side.

 

[USAGeorge]

When we talk of nothing or nothingness we first have to define"nothing". In science nothing as in a vacuum one should say nothing is a state when everything that can be removed has been removes. I would venture to say that absolute nothingness can never exist,even before the big bang..Think of it this way...Absolute nothingness can have no boundaries and must therefore run out into infinity...
When we say the room is empty,that it has nothing in it,we speak in general terms. There is a whole world of things there,for example there is air,radio waves and let's not forget the walls ceiling and floors ect. Just because our senses can not see/feel what is there does not make it so. I say all this in general terms because "nothing" has to be defined properly.

 

[Simon Bridge]

What wrong with absolute nothing also having zero extent? (Isn;t "extent" as in "runs out into infinity" a "something"?) Also - what is the need to define an absolute so globally? Could it not make sense to have a local absolute nothingness inside some container?

Whatever - the question is not about "nothingness" but about "vacuum"... later posts refined this to speak about the classical vacuum.

The classical vacuum has an established definition already.

 

[Joseph King]

When you say "infinite", be careful. Infinite is like a forbidden word in physics. A more exact way to describe it is "infinitely finite".

 

[ikjyotsingh]

The correct definition of a vacuum in terms of general relativity, is a system with an isolated mass. You can have an isolated mass, with an asymptotically flat spacetime, which implies that the stress-energy tensor in the Einstein field equations is zero. Just for your own information, there is only one spherically symmetric, static, and vacuum solution in general relativity, it is the Schwarzschild solution.

You will notice from the Schwarzschild metric that although it is a vacuum solution, there is a "Mass" term. The most unfortunate thing about how people describe vacuums is they say "space is empty", this is not correct, because a vacuum solution has the existence of a mass. What they mean to say is that a vacuum physically constitutes in this case an isolated mass in an asymptotically flat spacetime.

The second possible case of a vacuum that is non-empty is if one considers a non-zero cosmological constant. Looking at the field equations, with a vacuum (Tab = 0), we see that:
Rab - 1/2Rgab + Lambda g_ab = 0
Which implies that upon taking the trace:
R = -4Lambda.

Clearly, this is a vacuum, but the Ricci scalar is non-zero, so the spacetime is clearly not empty. The Lambda has been thought to correspond to dark energy.