Using my recent interview on To The Best Of Our Knowledge about the Krauss and the "Universe From Nothing" controversy as a pretext, I thought it would be a good idea to write a bit about what physics says of "nothing," and how this tricky notion evolved. (Here is something I wrote for 13.7 on this a few weeks back.)
We may start with Aristotle, who decided that "Nature abhors a vacuum" and thus declared that there was no such thing as nothing, understood as absolute emptiness. Spaced was filled up with aether, the same stuff that made up all celestial objects, from the moon up. Aristotle was reacting to the atomists, who, before him, had declared that matter was made of indivisible atoms moving in the void.
Fast forward to the early 17th century and Descartes also states that there is no such thing as empty space. To explain the motion of planets around the Sun and of the moon around the Earth, Descartes suggested the existence of swirling vortices of an aether-like substance that filled all space. He didn't accept the notion of action-at-a-distance, preferring to justify motions in terms of contact between material objects.
When Newton entered the scene, he would take aim at Descartes' plenum, showing it to be an impossibility: if there were such stuff in space, orbits would be unstable and the Moon would spiral down toward us. So, Newton proposed that gravity acts as a mysterious force at a distance, propagating instantaneously in empty space.
The ping pong continued in the 19th century, when Maxwell figured out that light was made of waving electric and magnetic fields. As with water or sound waves, light waves needed a supporting medium. There came the aether again, now to allow for light waves to propagate through space. Also, with the notion that every source of force has an associated field around it, the notion of empty space goes down the drain: gravitational and electromagnetic fields fill space and carry energy around. So, even in the classical era of physics, space was not empty.
In 1905, a 26-year-old Einstein did away with the "luminiferous aether," explaining that electromagnetic waves don't need any support to propagate: weirdly, and like nothing else, they can move in empty space. (Possibly, gravitational waves can do this too, a topic for another week.)
Together with Einstein's relativity came quantum mechanics, the physics of atoms and subatomic particles. And in the world of the very small, the rules are very different. In particular, nothing ever stands still. There is a residual jittery motion that is inherent to all matter, becoming most noticeable for small objects. Quantum mechanics changed our conception of reality; we are also wavy and jittery, although not to a measurable extent, at least from quantum effects. However, electrons and the like are always on the move. You try to pin one down, and it moves faster away. This is Heisenberg's Uncertainty Principle, which established a fundamental limit as to what we can know about the positions and velocities of particles.
Since motion has energy, the Uncertainty Principle also says that energy is never really zero. In modern physics, we represent every particle as an excitation of a field: an electron is a little lump of energy in the electron field; a quark is a lump of energy in the quark field, and so on. So, we don't think of particles anymore, but of their fields, filling up all of space.
Not quite Aristotle's aether, but certainly not empty space either. In fact, when we apply the Uncertainty Principle to these fields, their jitteriness is translated into energy fluctuations of varying duration: even if the field has zero energy (its sitting at its "vacuum"), it will fluctuate for short times. Very short-lived fluctuations in energy can have substantial values. And since E=mc2, these fluctuations can produce particles of matter for short times, what we call "virtual particles" or "vacuum fluctuations."
So, modern physics populates empty space with incessant vacuum fluctuations, tiny particles that come in an out of existence, like bubbles in a soup.
If we now bring in gravity as one of the fields that fills up space, and remembering that gravity is understood as an effect due to the curvature of space, quantum fluctuations of the gravitational field lead to fluctuations on the curvature of space (and time). Carrying this notion to the extreme, and not yet measured, we can contemplate a vacuum of space, and hence a quantum nothingness, where a fluctuation may trigger the existence of a patch of space that has a chance of growing to become a whole universe. This is how modern cosmologists describe the creation of a universe out of nothing, as the result of a vacuum fluctuation of flat space.
Now, given the number of concepts involved in describing this, clearly it's a major stretch to say that this is a creation of a universe out of "nothing," at least a nothing that means nothing in its colloquial sense. There is no question that modern physics creates the possibility that universes may pop into existence as a result of vacuum fluctuations. But the choice of calling this mechanism a "creation out of nothing" with no baggage attached is, to say the least, an exaggeration.
Explanations of this sort necessitate a huge conceptual structure to support them and cannot exist without it. It should be enough to celebrate human inventiveness, capable as it is to arrive at such a phenomenal mechanism of creation, without having to make it godlike. The human mind cannot create on a vacuum.