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Then what is gravity, this mysterious force that Newton believed exerted its influence instantaneously over the greatest distances? According to Einstein, it really is not a force at all, but a property of what came to be called spacetime. In this world picture, the universe is shaped by the three spatial dimensions of ordinary experience, plus the added dimension of time—one that cannot be described by the sacred Euclidean geometry of Einstein's youth. In his search for a new "metric" to describe spacetime, Einstein again turned to his old friend Grossmann, now a distinguished mathematician. Grossmann provided the necessary mathematical tool: an obscure non-Euclidean geometry (developed by the 19th century German mathematician Bernhard Riemann) that could accommodate Einstein's new four-dimensional world.
Tying everything neatly together in ten complex "field" equations, Einstein in 1916 published his general relativity theory. Unlike the special thepry, it had almost no immediate intellectual predecessors. Even today, scientists marvel at the mental processes Einstein used to develop it. Says Nobel Laureate Physicist Richard Feynman of Caltech: "I still can't see how he thought of it."
' Hard as it is to visualize, Einstein's curved four-dimensional space-time "continuum" is often likened to a suspended rubber sheet stretched taut but deformed wherever heavy objects—stars, galaxies or any other matter—are placed on it. Thus, according to Einstein, a massive body like the sun curves the space-time around it. The planets, instead of being held in their elliptical orbits around the sun by the force of gravity, move along the curved pathways of spacetime.
To prove his dumbfounding theories, Einstein first used the field equations to clear up a puzzling anomaly in the orbital motion of the planet Mercury. Over a century, the point closest to the sun in Mercury's elliptical orbit moves 43 seconds of arc more than Newtonian mechanics dictated that it should. Scientists had been unable to explain this difference. But when the Einstein equations were applied to Mercury's orbit, they precisely accounted for the extra 43 seconds of arc.
In another thought experiment, Einstein imagined that his hypothetical elevator, accelerating at a tremendous rate, was traveling at close to the speed of light. In that case, a beam of light entering through a hole in the wall would appear to a scientist inside the elevator to bend down in an arc and exit at a lower point on the opposite wall. Reason: even as the light moves across the elevator, the elevator is moving "up." But the scientist inside, aware only that his feet are pressing on the floor (because of the acceleration), assumes that gravity is bending the beam.
The experiment suggested—and Einstein's equations showed—that gravity would indeed curve light.
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