National Geographic : 2001 Oct
Shoar, the Waves of the Sea, the faces of men, and all other naturall things." Newton believed that light was particulate-"multitudes of unimag inable small and swift Corpuscles of various Sizes, Springing from Shin ing bodies at great distances one after another." Newton was such a giant on the scientific landscape that his rivals had little luck pushing the the ory that light is a wave. The wave theory did not begin to rebound until the titans of 19th-century science joined the battle to understand light and overwhelmingly came down on the side of waves. It was James Clerk Maxwell, a Scot, who in the 1860s made one of the most essential break throughs. He had been studying electricity and magnetism and realized that they propagated through space at-coincidence?-the speed of light. Light, he concluded, is an "electromagnetic" wave. The particle versus wave debate wound up with a kind of truce, gov erned by quantum mechanics: Light is produced by changes in the energy level of electrons. Light moves through space as a wave, but when it encounters matter it behaves like a particle. It simply doesn't fit into one of our neat little categories. "Light, indeed, is different from anything else we know," writes Sidney Perkowitz, a physicist at Emory University and the author of Empire of Light. The era of permanent uncertainty began in 1900, when Max Planck's experiments with heat radiation implied that light pounded against matter in discrete chunks-quanta, he called them-like bullets from a machine gun. This seemed contrary to Maxwell's equations, and Planck was reluctant to believe it. Enter Albert Einstein. It's common knowledge that Einstein, in pro mulgating the special theory of relativity, destroyed the mechanical, deterministic Newtonian universe. He achieved this theoretical break through by thinking about ... yes, light. Einstein did "thought exper iments," and in one he asked what would happen if you could ride a beam of light and look at an adjacent beam. Wouldn't the adjacent beam of light appear motionless? Maxwell's equations didn't seem to allow Slight to slow down or stop when moving through space. Einstein's answer-that light's speed is constant for all observers regardless of their own velocity-obliterated the classical conception of space and time. The groundwork was laid for Einstein by a famous experiment in 1887 by American scientists Albert Michelson and Edward Morley. The Light passe Earth, according to the orthodoxy of the time, moved through a fixed S ether that filled space. No one had ever detected this ether, but common th ugh t sense required its existence. Michelson and Morley set out to detect it by laboratory of measuring the speed of light when light beams traveled with, and per pendicular to, the motion of the Earth. They expected light to show the Isaac Ne t effects of the "current" of this ether as Earth hurtled along. It didn't. The speed of light was the same no matter its direction. Scientists, including and n e Michelson and Morley, were aghast and hoped that the results were looked th simply wrong. Einstein accepted them. There is no ether, he said. There's no absolute location in space. There isn't even any absolute time. Sa eg Iwill confess that relativity makes my head spin. A beam of light from the headlamp of a speeding locomotive ought to move faster-says common sense-than the beam from a stationary flashlight. It doesn't. And there's nothing anyone can do about it. Einstein's relativity presents all manner of head-scratching implica tions. It reveals that as objects approach the speed of light, time slows down. At the speed of light itself, time stops. This fact can help us think about the journeys made by starlight and THE POWER OF LIGH'