After a century, Einstein's special theory of relativity, which describes the motion of particles moving at close to the speed of light, has held up remarkably well. But as scientists probe the edges of the current knowledge of physics with new tests, they may find effects that require modifications on the venerable theory.

Several current theories, designed to encompass the behavior of black holes, the big bang and the fabric of the universe itself, could lead to violations of special relativity. So far, recent, updated versions of century-old experiments show no signs that Einstein's vision is reaching its limits. Various tests are ongoing, however, and a new generation of ultraprecise, space-based experiments is set to launch in the next few years, offering some chance, however slim, of observing signs of the laws that will eventually supersede relativity.

"The thing that is very interesting is that our technology has reached the point where we can probe for effects of this type," says V. Alan Kostelecky of Indiana University. "I think there's a very good chance we could see an effect."

The crux of special relativity is that light moves at the same speed for everything, regardless of which way it is pointed or how fast it is moving relative to anything else. This has some well-known consequences: space and time are linked, so that distances shrink and time slows down at high speeds. Einstein saw that if electromagnetic laws, which dictate the speed of light, truly hold throughout space and time, these counterintuitive effects are inescapable. (The technical name for this attribute of nature is Lorentz invariance.)

Special relativity assumes that spacetime has no structure of its own that might pick out a preferred orientation. But physicists think that a theory combining quantum mechanics and gravity will show that spacetime is made up of pieces, like light is made of photons. The structure of these pieces, as well as thus far unnoticed forces predicted by some of these theories, could mean that space has a slight grain to it.

Looking for Deviations

Investigators don't necessarily need to know anything about this new physics to look for deviations. A test theory from the 1940s proposed that special relativity rested on three pillars and so three different tests were necessary to confirm it. Two of them would look for changes in the speed of light using laboratories pointing in different directions or moving at different velocities. The other would make sure that time was slowing down or speeding up appropriately at high speeds. (See box for more about the third test.)

A group from the universities of Konstanz and Duesseldorf in Germany has recently reported results from the first two kinds of experiment. In an updated version of the Michelson-Morley test, which dates back to the 1880s, the researchers send laser light into two optical cavities, set at right angles to each other. The light forms a standing wave in each cavity, with a frequency that depends on the cavity length and the speed of light in that direction. If light can go faster in one direction of space than another, rotating the apparatus should reveal this effect as a change in the relative frequencies between cavities. The team's preliminary results, reported in May at the annual Conference on Lasers and Electro-Optics, showed no deviation from special relativity.

Early this year the researchers also found that Einstein's theory passed the most accurate version yet of the Kennedy-Thorndike test. First performed in the 1930s, Kennedy-Thorndike is the least accurate of the three tests that the original test theory requires, so improvements here are key, says group member Holger M. Mueller of the University of Konstanz. They compared the resonance of a standing light wave with an atomic clock over a period of 190 days, during which time Earth's orbital speed changes by 60 kilometers a second. The result was three times as accurate as previous Kennedy-Thorndike measurements, and the group expects 10 times tighter bounds from future tests with more accurate clocks.

"Twanging" Particles

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 Et-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.

Clocking Relativity

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.