Frank Wilczek: What is Space?
The space-filling medium we have the most definite ideas about is called the “Higgs condensate” (after Peter Higgs, a Scots physicist who pioneered some of the ideas). We have a lot of information about how this condensate affects the motion of fundamental particles—especially quarks, electrons and their leptonic brethren, and the W and Z bosons responsible for the so-called weak interactions. We don’t, however, know what it’s made out of, or what waves in it do. One idea is that it’s made out of just one kind of particle (the Higgs particle), and waves in it just break up into Higgs particles. Other ideas, which I favor, suggest that it’s made out of at least two kind of particles, and that waves in it make three additional kinds (similarly to how waves in Q bar Q make π mesons). It’s exciting that the LHC, soon to begin operation at the CERN laboratory near Geneva, should have what it takes, in terms of energy and instrumentation, to produce and detect any of these hypothetical particles.
In the general theory of relativity, Einstein used the concept of curved space-time to construct a theory of gravity. According to Newton's second law of motion, bodies move in a straight line at constant velocity unless a force acts upon them. The general theory of relativity modifies this law to postulate that bodies follow the straightest possible paths through space-time (so-called geodesics). When space-time is curved, even the straightest possible paths acquire bumps and wiggles, as they must adapt to changes in the local geometry. Putting these ideas together: bodies respond to the topography of space-time. The resulting wiggles in a body's space-time trajectory (in more dignified language, changes in its direction and speed), provide, according to general relativity, an alternative and more accurate description of the effects formerly known as gravity.
We can describe general relativity using either of two mathematically equivalent ideas: curved space-time, or metric field. The metric field is like the legend of a map, which allows a flat chart to represent a bumpy terrain. Mathematicians, mystics, and specialists in general relativity tend to like the geometric view because of its elegance. Physicists trained in the more empirical tradition of high-energy physics and quantum field theory tend to prefer the field view, because it corresponds better to how we (or our computers) do concrete calculations.
Once it’s expressed in terms of the metric field, general relativity resembles the field theory of electromagnetism. In electromagnetism, electric and magnetic fields bend the trajectories of electrically charged bodies, or bodies containing electric currents. In general relativity, the metric field bends the trajectories of bodies that have energy and momentum. The other fundamental interactions also resemble electromagnetism. In QCD, the trajectories of bodies carrying color charge are bent by color gluon fields; in the weak interaction, still other types of charge and fields are involved; but in all cases the deep structure of the equations is very similar.
These similarities extend further. Electric charges and currents affect the strength of the electric and magnetic fields nearby—that is, their average strength, ignor-
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