Silly Questions About Space; not due to being high, I swear.

[Yoshi525]

I didn't really understand what you were trying to say.
E=mc2, so it doesn't matter whether there is energy or matter.
The two things are interchangeable.

From relativistic view point that is true. From a practical/Newtonian view point it is not. The concept of a vacuum was contrived under (pre)Newtonian physics, when most people think of a vacuum, and when a vacuum is considered for any practical purpose invariant mass
is used. So whilst it is, sort of true - on a practical basis it is not. Hence why i said 'Not really' (because i knew someone would pwn me ). You can theoretically have a region of space devoid of any invariant mass, this is in keeping in what the word 'vacuum' essentially means.

 

[Paladin Solo]

You forgot space drag.

Most people do, until it's too late.

 

[syllogz]

Yoshi:

Yes, what it boils down to is definition and when to use it.

Your classical physics definition is fine, vacuum is space without matter. And it is perfectly usable in laboratory or in industry.
However, once you are trying to use it in the confines of General Relativity, or Quantum Physics, or Quantum Gravity, or String Theory, then you have to stick with the "vacuum energy" definition.
(Mind that classical physics, which includes special relativity, is an asymptotic solution to the other theory I've just listed. Or at least supposed to be.)
And since we were discussing something that falls into after-general-relativity-cosmology, I think we should use the non-classical definition.

Yeah, I know, someone should have come up with a different name for this "new kind" of the vacuum. Too bad you still would want to have that classical solution...