Re: How is energy stored in vacuum?

Stijn,

Energy can take many forms, and nearly all of them require matter (but not
necessarily molecules). Energy can exist as electromagnetic radiation even
if there is no matter present, but in a perfect vacuum without radiation,
there truly is no energy. Even if there is radiation present the
temperature is undefined, since by definition temperature is a measure of
the average kinetic energy of the particles in a system. Every once in
awhile you hear someone say that the background radiation in the vacuum of
space makes the temperature at least 3-4 Kelvin, but this is not correct.
What they mean is that if a certain object (say for instance, a black body
- one with a reflectivity of zero) were placed in deep space away from
other heat sources, it would reach equilibrium at 3-4 Kelvin. I'm
nit-picking, I know, but I think this will alleviate some of your confusion.

If you initially had a system which contained no mass and no radiation (and
therefore no energy), and you added some mass which also contained no
energy (don't ask me how you could do this because technically this
situation can never exist anyway), you'd still have a system with no
energy, even though it would now contain some mass. If you added some mass
that did contain energy, you would be adding energy to the system and since
you now have some particles you can define a temperature for the system. 

David Coit