Besides the fluctuating activity of quantum fields, space is filled with several layers of more permanent, substantial stuff. These are plenums, or ethers, in something closer to the original spirit of Aristotle and Descartes—they are materials that fill space. In some cases, we can identify what they’re made of and even produce little samples of it.
Physicists usually call these material ethers condensates. One could say that they (the ethers, not the physicists) condense spontaneously out of empty space as the morning dew or an all-enveloping mist might condense out of moist, invisible air.
The best understood of these condensates consists of bound quark-antiquark pairs. Here we are talking about real particles (σ mesons, to be precise), not those ephemeral, virtual particles that come and go spontaneously. The usual name for this space-filling mist of quark-antiquark pairs is “chiral symmetry breaking condensate,” but let me just call it what it is: the Qbar Q (pronounced Q-Q bar, for quark-antiquark) background.
The QQ background forms because perfectly empty space is unstable. Suppose that we clean out space by removing the condensate of quark-antiquark pairs. This is something we can do more easily in our minds, with the help of equations and computers, than in laboratory experiments, although as we’ll discuss the experiments aren’t altogether impossible. Having done this (we compute), quark-antiquark pairs have negative total energy. The mc² energy cost of making those particles is more than made up by the energy you liberate by unleashing the attractive forces between them, as they bind into little molecules. So perfectly empty space is an explosive environment, ready to burst forth with real quark-antiquark molecules.
Chemical reactions usually start with some ingredients A, B and produce some products C, D; then we write
A+Bxrightarrow C+D
and if energy is liberated
A + B → C + D + energy
In that notation, our reaction is
→ Q+ Q +energy
No reactants (other than empty space) required! Fortunately, the explosion is self-limiting. The pairs repel each other, so as their density increases it gets harder to fit new ones in. The total cost for producing a new pair includes an extra fee, for interacting with the pairs that are already there. When there’s no longer a net profit, the production stops. We wind up with the space-filling condensate, QQ, as the stable endpoint.
An interesting story, I hope you'll agree. How do we know it's right?
One answer is that it’s a mathematical consequence of equations—the equations of QCD—that we have many other ways of checking. But while that may be an adequate logical answer, since the checks are very detailed and convincing, it’s not
35