Secrets of the Old One

Was Einstein Right? Putting General Relativity to the Test

by Clifford M. Will
Basic Books, 274 pp., $18.95

“Newton, forgive me,” Einstein wrote in an autobiographical essay. “You found the only way which, in your age, was just about possible for a man of highest thought and creative power.” What was Einstein asking forgiveness for? That is the subject of this splendid book by Clifford Will, a physicist at Washington University, in St. Louis.

The subject is general relativity, or Einstein’s theory of gravity, and how it has repeatedly been confirmed since 1960 by major experiments. But first some background.

The simplest kind of relative motion was fully understood by the ancients. If you are on a large ship that moves at a steady rate through calm waters, you can toss a ball back and forth as easily as on shore, even though the ball follows complicated paths relative to the stationary land. Of course the land is not really stationary. The earth rotates and goes around the sun. The sun moves relative to the stars of our Milky Way galaxy. The galaxy in turn rotates and moves relative to other galaxies. Is there some sort of fixed reference frame against which a final, absolute motion can be defined?

Yes, said Newton. Motion is absolute with respect to space. Before Einstein, physicists trying to explain how light can go through a vacuum—waves seem to require a medium to transmit them—postulated a fixed substance called the ether. Experiments had shown that the speed of light through this imagined ether was independent of the speed of its source. It should be possible, therefore, to determine the absolute motion of the earth with respect to a “stagnant” ether by measuring the speed of light in different directions on the earth’s surface. The famous Michelson-Morley experiment of 1881 proved this could not be done. There was no trace of an “ether wind” generated by the earth’s motion.

In 1905, apparently unaware of the Michelson-Morley results, Einstein published his special theory of relativity. Essentially, it discarded the notion of an ether, and asserted that light (or any other portion of the electromagnetic spectrum) has a constant relative velocity regardless of the motion of an observer. If you travel alongside a light beam at half the speed of light, or even go the opposite way, the beam will always go past you at about 186,000 miles per second. Granting this absolute value for the speed of light relative to “the observer”—an observer moving in any direction at any speed—all sorts of strange effects involving space, time, mass, and energy, including the famous formula E = mcu2, inexorably follow.

The special theory concerned only motions in one direction at a constant speed. What about accelerated motions, such as the violent inertial effects astronauts undergo when their ship blasts off, or the inertia that caused a young earth to bulge at its equator? Inertia is the tendency of bodies to stay at rest or continue moving in a straight line unless an external force acts on them. It is hard to walk on a merry-go-round…

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