Jerry E Bishop. Asian Wall Street Journal. Dec 10, 1991.

Working alone in Berlin in 1915, Albert Einstein completed a meticulous set of equations that yielded a new concept of gravity. To test the equations, he applied them to a decades-old puzzle about why the planet Mercury annually changes its orbit about the sun.

When the equations predicted those changes exactly, the 36-year-old physicist immediately realized that he had made a startling discovery.

“I was beside myself with ecstasy for days,” he wrote to a friend a few months later. But it would be four years before the rest of the scientific world would grasp his enormous achievement: He had ensured that his new general theory of relativity would supersede Isaac Newton’s laws of motion and gravity, which had dominated science for more than two centuries.

So far, Einstein’s work has stood up better than Karl Marx’s or Sigmund Freud’s. It has turned back repeated challenges and remains, even now, the very basis of modern physics. His general theory is widely regarded, says physicist-author Jeremy Bernstein, as “the most perfect and aesthetically beautiful creation in the history of physics, perhaps in all of science.”

In addition, the theory has spawned surprisingly practical applications: It enables engineers to guide spacecraft through the solar system. It has helped astronomers find black holes, pulsars, quasars and other mysterious celestial objects. It plays a central role in cosmologists’ debates over the origins of the universe.

First, Einstein discarded the notion that acceleration is absolute by pointing out a principle that scientists had used for two centuries without realizing it. He recalled that Galileo had disproved the ancients’ assumption that a heavy object falls faster than a lighter one. Galileo showed that, if there isn’t any atmosphere, two objects of different masses—say, a feather and a lead ball—dropped off the Tower of Pisa will accelerate at the same rate and hit the ground at the same time.

Newtonians had explained that although gravity may tug at the more massive object with greater force, that object’s resistance to acceleration—its inertia—is also greater and offsets the stronger tug. So, all falling objects on Earth, regardless of their mass, accelerate at the same rate. Einstein found the equivalence of gravitational mass and inertial mass intriguing, and he said he “guessed that in it must lie the key to a deeper understanding of inertia and gravitation.”

With this realization, Einstein reached his most momentous conclusion, his “principle of equivalence.” This principle says, in essence, that if two phenomena produce equivalent effects, they must be manifestations of the same fundamental law.

Einstein then applied the principle of equivalence to acceleration. If people floating freely about inside a spaceship suddenly plunge to the floor, they can’t tell whether their craft has been accelerated by its rockets or had been falling and stopped. For the travelers, the effects of acceleration and gravitation are the same. They can tell whether they have accelerated or stopped falling only in relation to something else, such as a planet. Acceleration is relative, not absolute.

The implications of acceleration and gravity being equivalent were far-reaching. If a spaceship is accelerating and a beam of light is coming in through a window on one side, the beam, as it crosses the ship’s interior, won’t hit a spot on the wall precisely opposite the window. To its astronauts, the beam would be curved. And if acceleration makes light curve, the principle of equivalence indicates that gravity also makes light curve.

In 1911, to test the concept that gravity and acceleration are equivalent, Einstein stuck his neck out: He predicted that starlight grazing the sun’s surface would be curved by 0.83 seconds of arc by solar gravity. (“It should have been 0.87, but arithmetic was never one of Einstein’s strong points,” writes Banesh Hoffman, a scientific collaborator of Einstein’s, in his 1972 biography of him.) Einstein, however, still thought in Newtonian terms of gravity being a force, and his calculations, if astronomers had been able to check them by observation, would have been found wrong, even aside from his arithmetical mistake. It would be four years before he developed his general theory of relativity and discovered why his 1911 calculations were wrong.

But Einstein first had to grapple with a far different problem. Newtonians said the influence of gravity was instantaneous everywhere; if a star suddenly disappeared, the disappearance of its gravity would be felt instantly throughout the universe.

However, Einstein’s special theory said nothing, even gravity, could travel faster than light. At first, the inconsistency frustrated him. In 1912, however, a mathematician friend, Marcel Grossman, introduced him to tensor calculus. After the two men sweated through this complex mathematics, Einstein found the right equations in 1915, and the general theory of relativity fell into place. Gravity, he concluded, wasn’t a force of nature but a geometrical distortion in the fabric of space-time.

Although most people perceive the universe in three dimensions—height, width and depth—the mathematics that Einstein used described a four-dimension universe, with time the fourth dimension. In a four-dimension universe, any mass caused the fabric of the universe, of space-time, to curve. The curves bent into space-time affected the trajectories of moving matter.

Matter moving through the universe always follows the shortest distance between two points. In a flat universe, this is a straight line, but in a universe of curves, the shortest distance between two points is a curved line. Hence, the Earth and other planets circle the sun, not because the sun holds them on strings of gravity but because the sun’s mass pulls a dimple into space-time and the planets follow the curved walls of the dimple.

Right off the bat, Einstein noted a phenomenon where the physics of general relativity worked better than Newton’s did. Ever since the mid-19th century, physicists and astronomers had been puzzled by Mercury’s movement around the sun. Newtonian physics predicted that the point of Mercury’s closest approach to the sun, its perihelion, would change every Mercurial year. Observations found that the gravitational pull from other planets was moving Mercury’s perihelion—but a little bit more than Newtonian physics predicted.

On that momentous day in Berlin in 1915, Einstein used his new general theory to calculate how much Mercury’s perihelion would change if space-time were curved. The answer exactly matched the observations.

However, the real test of the general theory, as it emerged in 1915, came four years later. Einstein’s theory predicted that starlight grazing the sun would be bent by the sun’s gravity more than Newtonian physics forecast. During a 1919 eclipse, British astronomers found that the degree of bending was exactly what Einstein predicted, a discovery that made headlines world-wide.

Since 1960, a dozen experiments have tested relativity. Radio transmissions from spacecraft sent to Mars, Venus and Mercury have refined measurements of the bending of light by the sun to within 0.1% from a possible 20% error in 1919.

Relativity got major support in 1974, Dr. Will says in his book. Astronomers discovered what appears to be two extremely dense stars rotating rapidly around each other and pulsing out a radio signal every 59 thousandths of a second. This rapidly rotating “pulsar” has the equivalent of a rapidly changing perihelion. In less than three months, the pulsar gave a far more accurate measure than Mercury does of how much this point of closest approach differed from that foreseen in Newton’s “flat” universe. And the difference was what Einstein’s theory predicted.

Moreover, using the pulsar as a celestial clock, scientists confirmed the warping of time, predicted by the special theory of relativity, and the gravitational stretching of light waves towards the color red, predicted by the general theory.

Now, the pulsars are suggesting one more confirmation of general relativity. In 1916, Einstein described gravitational waves comparable to light waves and predicted that any object that emits gravitational waves would lose energy. In 1979, a century after Einstein’s birth, Joseph Taylor, the University of Massachusetts astronomer who, with a student, Russell Hulse, discovered the pulsar, announced that the pulsar was indeed slowing down at exactly the rate predicted by Einstein.

This report’s hint that the pulsar was emitting gravity waves set off a continuing search to detect such waves rippling past the Earth from vibrations in distant space. Other experiments, Dr. Will says, also are in the wings. One is Stanford University’s plan to put four super-precise gyroscopes into polar orbit in 1996. If general relativity is correct, the gyroscopes should change their angle relative to distant stars by a tiny fraction of a degree every year.

And more work remains to be done, of course. The general theory of relativity describes one force of nature—gravity—but doesn’t incorporate the other natural forces such as electromagnetism and the forces that hold atoms together. From the 1920s until his death in Princeton, New Jersey, in 1955, Einstein worked on theories that would incorporate the other natural forces into his concept of gravity as a geometrical warping of space-time. He never succeeded—and no one else has, so far.

But that hardly detracts from Einstein’s enormous achievements. “What I find to be truly amazing,” Dr. Will writes, “is that this theory of general relativity, invented almost out of pure thought, guided only by the principle of equivalence and by Einstein’s imagination … turned out in the end to be so right.”