Nothing vs. empty space
is empty space really nothing?
The physicist's 'nothing' is an example of something to the philosopher for which 'nothing' is well, let this philosopher explain in a review of "A Universe from Nothing" by Lawrence Krauss:
empty space governed by quantum mechanics (or any other laws of physics, or even just the laws of physics by themselves) is not nothing, and not even an “example” of nothing (whatever an “example of nothing” means), but something. And it remains something rather than nothing even if it is a “good first approximation” to nothing (which is what Krauss presumably meant by “good first example”). When people ask how something could arise from nothing, they don’t mean “How could something arise from almost nothing?” They mean “How could something arise from nothing?” That is to say, from the absence of anything whatsoever -- including the absence of space (empty or otherwise), laws of physics, or anything else.
I'd like to add to Alfred Centauri's Answer.
First, forget about space for a moment.
Let's take, for example, the second quantised electromagnetic field, since this is the field most wonted to me.
The only things that are believed to be real in modern physics are this field and other quantum fields like it. There are only a handful of them. When we witness physical phenomena we are seeing interactions between these quantum fields.
The second quantised electromagnetic field can be thought of as a infinite gathering of quantum simple harmonic oscillators, one for each classical plane wave mode of Maxwell's equations. The eigenstates of quantum simple harmonic oscillators are discrete and they are evenly spaced by an amount of energy $h\,\nu$, where $\nu$ is the frequency of the oscillator in question. So each oscillator can change its state discontinuously, by taking up or shedding a whole number multiple of this basic energy "chunk" $h\,\nu$ from or to another quantum field. So the interactions of the electromagnetic field with the other quantum fields in the world is by way of these discrete packets. I like to think of these packets not so much as billiard balls but more like discrete data packets that are swapped between networks on the Internet, thus giving being to "stuff that happens" on the Internet. Often it doesn't even make much sense to ask "where" these communications are happenning. The quantum fields of the World talk to each other in discrete, chunky, communications, thus giving being to everything that we see happenning around us. When these chunky communications involve the electromagnetic field, we call them photons.
Where are these quantum oscillators? Remember we haven't even talked about space, I ask you to forget about it! The answer is that they are nowhere in particular and everywhere all at once! For the quantum fields I spoke of are the space around us. We don't need to deal with the mysterious concept of a "void" any more in physics (an idea that actually used to give me nightmares as a child): empty space is nothing more than what we see when the quantum oscillators of the quantum fields of the World are all in their ground states!
"Empty space" is quite different from nothing, as the former is made out of quantum fields.