IS THE SPEED OF LIGHT CONSTANT?
Late in life Einstein couldn’t even remember whether he’d been aware, in 1905, of the Michelson-Morley experiment, which showed that the speed of light is constant in all directions. Although we now think of the experiment as having shattered absolute space and time, that result was not at all what Albert Michelson and Edward Morley expected when they did their experiment in 1887, in a basement of the Western Reserve University in Cleveland. They were trying to detect variations in the speed of light to prove the existence of the “luminiferous aether,” a perfectly immobile, transparent, and mysterious substance that was thought to pervade the universe. Physicists had conceived of the ether as a medium that could transmit light waves, the way air or water transmits sound. The ether had to exist, but no one had seen it—it was a bit like dark matter today.
Because Earth orbits the sun at 18 miles per second, Michelson and Morley reasoned that they should be able to detect an ether wind blowing through their Cleveland basement with the help of an experimental setup called an interferometer. They split a light beam in two with a half-silvered mirror, sent the two halves of light off at right angles, and then bounced them off mirrors about five feet away. The two men expected that light waves traveling against the ether wind, in the same direction as Earth’s motion, would be slowed and would arrive back at the starting point slightly later than the light waves traveling across the wind. When the two beams were recombined, the offset waves would interfere with each other to produce a distinctive pattern of light and dark bands. Michelson and Morley made their measurements with extraordinary care but saw no disruption in the pattern—the light beams traveled the same speed in all directions, impervious to any ether wind.
One null result, of course, did not rid the universe of the ether. Michelson went to his grave in 1931 convinced that it had to exist. But by then Einstein had changed most people’s minds and persuaded them to accept the simple, beautiful truth: The velocity of light, c, really is different from any other velocity. Anyone who measures the speed of any light beam—or any other type of electromagnetic radiation—will get the same value for c: 299,792,458 meters per second. There is no absolute space, Einstein decided, ether-filled or otherwise. Seen in that light, the Michelson-Morley result made perfect sense.
And yet today theorists are questioning the absoluteness of c. Some versions of string theory, the most popular candidate for a unified theory, say there could be extremely feeble force fields left over from the Big Bang that point in different directions in different parts of space. An experiment to measure c might produce variations depending on how the setup was oriented with respect to one of those fields, variations far smaller than Michelson and Morley could have detected.
Several groups are looking for such variations with modern versions of the Michelson-Morley experiment. Peter Wolf, Sebastien Bize, and their colleagues at the Paris Observatory measure c with microwaves oscillating at 12 gigahertz inside a small sapphire crystal. Microwaves reflecting back and forth within the crystal line up and reinforce each other, or resonate—as long as they are moving precisely at c. If c were to change because the orientation of the crystal had changed with respect to some “preferred” direction of space, then the resonant frequency of the sapphire oscillator would change as well. The apparatus containing the crystal is bathed in liquid helium, chilling it to a few degrees above absolute zero to make sure that the crystal doesn’t expand or contract by even a femtometer.
Over a period of months, as Earth spins on its axis and revolves around the sun, the Paris researchers monitor their oscillator, comparing it with the microwaves from a hydrogen maser (microwave laser), which shouldn’t be affected by Earth’s motion. “What we measure is that small frequency difference,” says Bize. “We look for modulations that correlate with the motion of Earth.”
Another group, based at Humboldt University in Berlin, uses a slightly different setup, comparing the outputs of a pair of sapphire oscillators. Over the past several years the two groups have achieved broadly comparable null results. “The speed of light in any two directions is the same to about one part in a quadrillion,” says Holger Müller, a former member of the Berlin team who now works at Stanford. That’s equivalent to knowing the U.S. gross national product to within a penny.
Over vast distances, even such slight variations could become meaningful. If two photons differed that much in velocity, and if they left a galaxy a billion light-years away at the same instant, they would arrive at Earth 30 seconds apart. A few years ago physicist Giovanni Amelino-Camelia at La Sapienza University in Rome had an idea for staging just such a race to test the constancy of c in a new way. Some theories of quantum gravity require space-time itself to be grainy—to be made up of discrete quanta, presumably around 10-35 of a meter across because that’s the scale at which Einstein’s field equations generate their bothersome infinities. (A proton is a hundred million trillion times bigger.) Amelino-Camelia calculated that light photons might navigate this cosmic foam at slightly different speeds, depending not on their direction—the possibility the Michelson-Morley experiments test for—but on their energy.
In 2007 or so, NASA plans to launch GLAST, the Gamma Ray Large Area Space Telescope. Its main purpose is to allow astronomers to study such events as gamma-ray bursts, which are mysteriously powerful explosions in distant galaxies. But it could also serve as Amelino-Camelia’s finish line. All the photons in a burst, he reasons, must leave the starting blocks at about the same time. If you compare a lot of high-energy photons with a lot of relatively low-energy ones, you should find that on average, after a billion-year race, the high-energy ones reach GLAST’s detector sooner—by about a millisecond. He and other quantum gravity theorists are pretty excited by that possibility, which just goes to show what they’re up against.