exactly science at its best. We’d like our equations to have consequences we can see reflected in the physical world.
A second answer is that we can calculate the consequences having a QQ background, and check whether they match things we see in the physical world. To be more specific, we can calculate whether Q Q, considered as a material, can vibrate, and what the vibrations should look like. This is very close to what “luminiferous ether” fans once wanted to have for light—a good old-fashioned material, more substantial than electromagnetic fields, whose vibrations describe light. Vibrations of Q Q aren’t visible light, but they do describe something quite definite and observable, namely π mesons. Among the many strongly interacting particles, the three π mesons have unique properties. They are by far the lightest, for example, and they never fit comfortably within the quark model, which works well for all other strongly interacting particles. So it’s very satisfying—and if you study the details, it’s very convincing—that they arise in quite a different way, as vibrations of Q Q.
A third answer is the most direct and dramatic of all, at least in principle. We started by considering the thought experiment of cleaning out space. How about doing it for real? Scientists at the relativistic heavy ion collider (RHIC) at Brookhaven National Laboratory, on Long Island, have been working on it, and more such work will be going on at the Large Hadron Collider (LHC). What they do is accelerate two big collections of quarks and gluons moving in opposite directions—in the form of heavy atomic nuclei, like gold or lead nuclei—to very high energy, and make them collide. This is not a good way to study the basic, elementary interactions of quarks and gluons or to look for subtle signs of new physics, because many, many such collisions will be going on at once. What you get, in fact, is a small but extremely hot fireball. Temperatures over 1012 degrees (Kelvin, Celsius, or Fahrenheit; at this level, you can take your pick) have been measured. This is a billion times hotter than the surface of the Sun; temperatures this high last occurred only well within the first second of the Big Bang. At such temperatures, the Q bar Q condensate vaporizes—the quark-antiquark molecules from which it’s made break apart. So a little volume of space, for a short time, gets cleaned out. Then, as the fireball expands and cools, our pair-forming, energy-liberating reaction kicks in, until the stable Q bar Q background is restored.
All this almost certainly happens. "Almost" comes in, though, because what we actually get to observe is the flotsam and jetsam thrown off as the fireball cools. Figure 2 is a photograph of what it looks like. Obviously, the photograph doesn't come labeled with circles and arrows telling you what's responsible for what aspect of this spectacularly complicated mess. You have to interpret it. Today, the most accurate and complete interpretations build in the process of Q Q melting and re-formation we've been discussing, but they're not yet as clear and convincing as we might hope for. People continue to work at it—both the experiments and the interpretation.
The Q Q condensate can be considered an established fact. We vehemently suspect that there are other condensates, too. To explain why, let me start with a parable.
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