Can anyone explain why empty space has energy?
blind leading the blind, here, but my non-specialist-physics background might be a bit easier to understand than someone who mentions "QCD" at you. the way i understand it is that when you have particles around, they have E.M and gravitational fields, and they have binding forces and so on at the very-close (atomic) level which kiiinda mean that if you get close to them with another particle you either get sucked in, or banged away (like billiard balls) - actually _very_ much like billiar balls, in that you have to get *really* close in order for a deflection to occur [at all] but when you do you really know about it.
and, what we also know is that in non-vacuum there are *lots* of these particles. so, relatively speaking, even in a gas like any one particle really doesn't have to go that far to get banged-up by any other particle.
in other words, your average particle or your average photon (cosmic ray equals a photon with a very high energy content) has a huge amount of "resistance" applied to it, in *all* directions pretty much. this "resistance" means we end up with solid matter (ok gases too) that *stays* solid. stable. follows newton's laws and so on.
in empty space, there is *no such resistance*. there's nothing to get in the way, nothing to interfere with particles or rays. so even the smallest disturbance when two photons (cosmic rays) happen to cross paths, or one hits an atom, can result in "smaaashhh, wheeee!" any by-products of such collisions, which would normally be instantly destroyed by neighbouring particles, preciselybecause there *aren't* any neighbouring particles, the by-products get to stay alive for much longer [possibly forever].
so my take on this is that it's not so much that "empty space has energy", it's that empty space - by *being* empty - doesn't "resist" (so to speak) the creation process of particles. *scratches head*. ... a bit like how if you have one extrovert in a party that's only just started, has huuge rooms, and nobody knows anyone else, the extrovert will stand in the middle of the room happily dancing and the very few other guests else will hug the walls, but if you have *lots* of extroverts in the room, then, well... it's just an another awesome party :)