Relativity and the Cosmos

by Alan Lightman

In November of 1919, at the age of 40, Albert Einstein became an overnight celebrity, thanks to a solar eclipse. An experiment had confirmed that light rays from distant stars were deflected by the gravity of the sun in just the amount he had predicted in his theory of gravity, General Relativity. General Relativity was the first major new theory of gravity since Isaac Newton's, more than two hundred and fifty years earlier.

Einstein became a hero, and the myth building began. Headlines appeared in newspapers all over the world. On November 8, 1919, for example, the London Times had an article headlined: "The Revolution In Science/Einstein Versus Newton." Two days later, The New York Times' headlines read: "Lights All Askew In The Heavens/Men Of Science More Or Less Agog Over Results Of Eclipse Observations/Einstein Theory Triumphs." The planet was exhausted with World War I, eager for some sign of humankind's nobility, and suddenly here was a modest scientific genius, seemingly interested only in pure intellectual pursuits.

What was General Relativity? Einstein's earlier theory of time and space, Special Relativity, proposed that distance and time are not absolute. The ticking rate of a clock depends on the motion of the observer of that clock; likewise for the length of a "yard stick." Published in 1915, General Relativity proposed that gravity, as well as motion, can affect the intervals of time and of space. The key idea of General Relativity, called the Equivalence Principle, is that gravity pulling in one direction is completely equivalent to an acceleration in the opposite direction. (A car accelerating forwards feels just like sideways gravity pushing you back against your seat. An elevator accelerating upwards feels just like gravity pushing you into the floor.

If gravity is equivalent to acceleration, and if motion affects measurements of time and space (as shown in Special Relativity), then it follows that gravity does so as well.In particular, the gravity of any mass, such as our sun, has the effect of warping the space and time around it. For example, the angles of a triangle no longer add up to 180 degrees and clocks tick more slowly the closer they are to a gravitational mass like the sun.

Many of the predictions of General Relativity, such as the bending of starlight by gravity and a tiny shift in the orbit of the planet Mercury, have been quantitatively confirmed by experiment. Two of the strangest predictions, impossible ever to completely confirm, are the existence of black holes and the effect of gravity on the universe as a whole (cosmology).

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