Nothing vs. empty space This question quotes Hawking saying: 

[...] you enter a world where conjuring something out of nothing is possible (at least, for a short while). That's because at this scale particles, such as protons, behave according to the laws of nature we call "quantum mechanics", and they really can appear at random, stick around for a while, and then vanish again to reappear somewhere else.

Nowever, is empty space really nothing? is there a distinction between non-existence and the "nothingness" of space?
Perhaps space is something, we just cannot grasp exactly what it is. Anyone can shed light on whether space is something and what exactly that "something" is.
 A: 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.

A: 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.
A: A simple analogy.
Consider a still pool of water. If it is still then there are no vortices and no ripples. If we reserve the word "thing" to mean "vortex, ripple, stuff like that" then when the pool is still, entirely unmoving, then there is "no-thing" there. When the pool is disturbed there will be ripples and vortices, and now there is "some-thing" there. This is a good analogy for the sense in which Hawking and others have suggested that "conjuring something out of nothing is possible" (to quote from the question). I think that in this example Hawking showed a regrettable lapse into language that is liable to mislead, because the "pool" that is involved in quantum field theory is a highly structured pool; Hawking himself studied its properties (quantum fields in a spacetime described by general relativity) and he needed all his intellectual abilities to do so. The very fact that it has properties and equations describing it shows that it is not nothing in the ordinary sense of the word.
A: Start with some shielded vacuum in free fall.  That's empty!  Take a a pair of rigid, planar, in-register, parallel, superconducting, grounded mirror plates and bring them in real close, smaller than a micron separation.  Look up the Casimir effect.
For each allowed photon wavelength in that vacuum there is half-photon of Heisenberg uncertainty.  You have created an etalon in which no half-wavelength larger than plate separation can exist (and also omit periodic destructive interference for smaller wavelengths).  You have squeezed out some extra nothing from the vacuum.  Excess radiation pressure outside the plates pushes them together.  Force varies as the inverse cube of their separation.  It has been measured.
But wait!  There is more! EVIL!  Speed of light normal to the etalon exceeds c = 1 only when the Casimir energy is negative.  The finite temperature Casimir energy is positive in published cases.
http://www.npl.washington.edu/AV/altvw43.html 
http://arXiv.org/abs/gr-qc/0107091 
http://arXiv.org/abs/quant-ph/0010055 
http://arxiv.org/abs/1110.1919 
   Huge Casimir effect at finite temperature in electromagnetic Rindler space 
Phys. Lett. B236 354 (1990) 
Phys. Lett. B250 133 (1990) 
J Phys A26 2037 (1993)
A: one cannot interface "being and nothingness" without fundamental problems and paradoxes associated with infinitesimals and thus the question of "does nothing exist along with something?" is meaningless. Nothing exists only as a relationship as in an absence of something. 
I wanted to mention that the vacuum which is definitely composed of physical things is the only mechanism for supporting quantum entanglement.
The vacuum is everywhere; dark energy is where there was gravity.
