In recent years, physicists have found that they have many more ways to look for breakdowns in special relativity. In particular, Kostelecky and collaborators have formulated an "extension" to the Standard Model of particle physics based on string theory. The theory supposes that all elementary particles are actually extended, one-dimensional objects that vibrate in higher dimensions. All but the four familiar dimensions of space and time would be imperceptible to the naked eye but might lead to measurable effects. The different particles, including those responsible for the forces in nature, would correspond to strings twanging in different modes.
Some of these vibrations might show up as additional fields of force, weaker than gravity and therefore harder to detect. But if the fields were to interact with certain particles, they might reveal themselves as deviations from special relativity for those particles. The standard model extension catalogues the forms these violations could take for protons, neutrons, electrons and a slew of others. String theory is not advanced enough to say at what point, exactly, special relativity would break down or by how much, however. And for the most part, the tests are independent of one another, so physicists have to cover all their bases. "It's not like you would see this and not that and say, 'Bingo,'" says Ronald L. Walsworth of the Harvard-Smithsonian Center for Astrophysics, in Cambridge, Mass.
A number of groups have already begun casting the experimental net, at least on a part-time basis. Blayne R. Heckel and his colleagues in the University of Washington's E¿t-Wash group use a sensitive torsion test, for instance, to reckon whether electrons prefer a certain direction in space. They keep tabs on a test mass of magnetic material whose electrons all have the same spin, or angular momentum, to see if it wobbles more in one orientation than another.
Others, such as Walsworth, monitor highly stable atomic clocks. These are collections of atoms that radiate at a certain frequency. Deviations from special relativity would show up as changes in their frequencies, depending on which way Earth is pointing. Walsworth's group has put the tightest bounds on relativity violations in protons and neutrons, using xenon and hydrogen isotopes that emit coherent microwave radiation. Vernon Hughes of Yale University and his colleagues have done similar experiments with muonium, which consists of an electron orbiting a positively charged muon.
Earth-bound atomic clocks start to become unstable after just a few hours. Gravity, daily temperature changes and mechanical degradation are all sources of error. So the next generation of atomic clock measurements is scheduled to run on satellites or the International Space Station, where microgravity and a shorter rotation time should allow higher accuracy. (See box for list of space tests.)
These tests constitute the scientific equivalent of buying a lottery ticket. "It's got a low probability of success but a tremendously high payoff," Walsworth says. It would be the first step in a whole new understanding of space and time. And that's a lure that few can resist.
JR Minkel is based in New York City.