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.