Chapter 13: Star Birth and Death

Einstein was sitting in his patent office in Bern when he had what he later called the happiest thought of his life: he realized that a person falling freely will not feel their own weight. He realized that motion or acceleration caused by gravity and acceleration caused by any other force were equivalent. Gravity is no different from any other force.

Imagine that you are in a sealed elevator feeling your normal weight. Could you devise any experiment that would tell you whether you were accelerating through deep space at 9.8 meters per second squared — the Earth's gravitational acceleration — or just sitting stationary on the Earth's surface? No! In either case, you would feel your normal weight. If you dropped an apple it would fall to the floor normally. In one situation gravity accelerates the apple toward the floor and in the other case, an outside force causes the floor to accelerate upward to meet the apple. Now let's look at Einstein's happy thought. Imagine you are in a sealed elevator with a sense of weightlessness. Could you devise any experiment that would tell you whether you were floating in deep space or plunging in free-fall toward the Earth's surface? No again! In either case, an apple would float freely in the space around you.

Another prediction of general relativity is that clocks slow down in strong gravity the same way it slows down in fast-moving spacecraft. In tests, atomic clocks do run slower on the Earth's surface than at the top of a tall building, where the gravity is slightly weaker! In space, we have found that the gravitational distortion of space slows a light signal that passes near the Sun by 250 millionths of a second.

General relativity has passed all its observational tests with flying colors. Most of the tests so far have been carried out in the relatively weak gravity of the solar system, where the effects of curved space are subtle. The phenomenon that first led to a conflict with Newton's theory is the slight advance or precession in the point of nearest approach of Mercury to the Sun. The effect is very small, only 0.5 seconds of arc per year. Since Einstein showed that radiation has an equivalent mass, it follows that light must respond to gravity just as particles do. Photons leaving a massive object suffer a gravitational redshift, caused by the photons losing energy as they climb out of the gravitational well. The gravitational redshift of the Sun is only 0.0002%, but it has been detected.

Starlight is deflected slightly when it passes close to a massive object like the Sun. Einstein's theory first gained wide acceptance after the solar eclipse expedition of 1919 showed that starlight was deflected by the predicted amount. (Most physicists were on the edge of their seats waiting for the announcement of the results of the experiment. Einstein went to bed; he was supremely confident that general relativity was the correct theory of gravity.) More recently, pulsars have been used as accurate clocks to confirm predictions of general relativity, resulting in the award of the 1993 Nobel Prize in Physics to Joseph Taylor and Russell Hulse.