In our last bit of World Science Festival coverage, we discussed how inflation has produced an expanding, inhabitable universe. That still leaves the question of why that universe seems to be filled with strange stuff like virtual particles and dark energy. A different panel tackled that question and, in true quantum fashion, the session that described the contents of the universe came before the one that described its creation.

The session, which was moderated by radio host John Hockenberry, started out with a historical perspective on the fabric of the universe from Cambridge's John Barrow. Barrow described various views of whether it might be possible for a space to exist that was devoid of the sorts of matter we're familiar with. Reactions to the prospects, from the time of Aristotle onwards, were mixed, but they were primarily based on philosophical grounds. Things really didn't get close to our modern conception of a vacuum—one with states that could change over time—until the time of James Clerk Maxwell. The advent of quantum mechanics finally made the description and study of vacuum states a quantitative science.

The panel discussion featured Paul Davies, George Ellis, and Frank Wilczek. The latter described how quantum mechanics changed our view of what constitutes a vacuum by analogy: imagine you're an intelligent fish. You'd probably develop physics that are appropriate for your environment, which is water. What quantum mechanics has done, Wilczek suggested, is show us what sort of physics operates once you do the equivalent of taking the water away. What's left, in the case of our universe, is space that isn't really empty—instead, it's filled with quantum fields (notably the Higgs field) and virtual particles that pop briefly into existence before being annihilated by collisions with their antiparticle counterparts.

The striking thing about this, in contrast to the multiverse, is that we actually have evidence for the existence of these vacuum fields and virtual particles. The clearest the authors described comes in the form of the Casimir effect, in which two plates are brought close to each other in a vacuum. Their proximity excludes the existence of some of the quantum fields in the space in between them, leaving that area, in effect, more empty than the vacuum outside the plates. The net result is a pressure that drives the plates together, and that force has been experimentally verified.

That led to another key point: the vacuum that persists in empty areas of the universe is actually more energetically favorable than space that's devoid of Higgs fields and virtual particles. In short, if you somehow devised an experiment that could clear these things out, they'd spontaneously re-form. So, empty space isn't really empty as we might understand it, but it's a lot easier to have that stuff there than having space that was closer to our traditional conception of empty.

As an aside, someone pointed out that the Casimir effect can apparently be used to describe the thermal decay of black holes through the emission of Hawking radiation. I'd always understood that to happen through the formation of virtual particle-antiparticle pairs on either side of the event horizon and, indeed, one of the panelists brought that up. But it turns out that Paul Davies hates that way of conceptualizing things, as he indicated that the area inside the event horizon is too small for this to be a reasonable probability event (although I may have gotten that wrong—things were moving rather quickly there).

The focus on nothing also took a familiar experiment and turned it on its head. Brookhaven's Relativistic Heavy Ion Collider had always been presented as a way of generating a quark-gluon plasma in order to examine its behavior. But Wilczek pointed out that as the plasma evaporates, researchers might be able to glimpse a different form of nothing that's been left behind, if only briefly (before our mundane bit of nothing returns). As Davies put it, "finding nothing is not the same as not finding anything."

At the same time, it's clear that what we know about nothing doesn't explain everything. When asked about the apparent directionality of time, Wilczek responded by saying, "it's remarkable how well we've done by ignoring all that stuff." It's also had nothing to say about String Theory which, "hasn't made significant contact with empirical reality yet." Still, it made for quite an entertaining time, and the panelists were happy to stay and field questions from the audience afterwards.