In the seventeenth century, Isaac Newton (1642–1727) proposed three laws of motion and one of gravitation to describe and predict the motions of objects on the Earth and in the heavens. Newton's laws worked very well, and became the centerpiece of the system of laws known as Newtonian physics. In the late nineteenth century, however, physicists began to be troubled by certain experiments that did not obey the predictions of the physics they knew. The need to account for these anomalies led to the development of both relativity and quantum mechanics in the early twentieth century.
One such anomaly was the Michelson–Morley experiment, which disproved the hypothesis that light waves propagate through an intangible, universe-filling substance called the ether as ripples propagate in water. The puzzle for nineteenth-century physicists was this: light was guaranteed by Maxwell's well-tested electro-magnetic theory to have a specific velocity (186,282 miles per second [299, 972 km/sec], usually designated c). The Michelson–Morley experiment showed that light does not travel at c with respect to the ether, while certain astronomical tests had also ruled out the emitter theory of light, according to which every beam of light travels at c with respect to its source, like a bullet fired from a gun. Yet if a beam of light does not move at c with respect either to ether or to its source, what does it move at c with respect to? Einstein's answer was simple yet radical: light moves at c with respect to everything, all the time. From this single bold hypothesis Einstein unfolded the entire theory of special relativity.
Special relativity is called "special" because its equations are valid only for one special set of cases, namely, systems of phenomena moving in straight lines at constant velocities (intertial reference systems). Einstein at once sought to extend his equations to describe reference systems undergoing acceleration, not just inertial reference systems. This required him to account for gravity, which accelerates all objects. Since Newton, physicists had conceived of gravity as a "force," a mysterious attraction exerted instantaneously by every bit of matter on every other. Newton himself had been uncomfortable with this notion, but could not think of an alternative; Einstein did. As part of his general theory of relativity, he proposed that gravity is a manifestation of the geometry of space itself, and that this geometry is imposed on space by the matter in space. This implies that space is a thing having specific, changeable properties, not a featureless void—"absolute space." The proposal that space is not absolute was one of Einstein's most daring ideas.
On the basis of his general theory Einstein made a number of interesting claims, such as that gravity propagates at the speed of light, that light itself must be diverted by gravity, that time passes more slowly in a stronger gravitational field than in a weaker, and that the Universe is finite in size. Scientists at once began looking for ways to test some of these claims. In 1919 a total eclipse of the Sun allowed British astronomers to photograph stars whose light had, at just that time, to graze the Sun on its way to Earth. Ordinarily, the Sun's glare prevents such observations; during an eclipse, they are possible because the Sun is blocked by the Moon. Einstein predicted that the stars' light would be bent a certain, measurable amount by the Sun's gravity, changing the stars' apparent position. The observations were made, and Einstein's prediction was confirmed precisely.
Another early check on general relativity was its ability to solve the long-standing puzzle of the orbit of the planet Mercury, which has peculiarities that cannot by explained by Newton's laws. Each time Mercury orbits the Sun the point at which it comes closest to the Sun, its perihelion, shifts by a small amount. This small but measurable motion—termed precession—had first been measured by the French astronomer Urbain Leverrier (1811–1877) in 1859. The rate of precession of Mercury's orbit is 43 seconds of arc per century, meaning that about three million years are required for a complete cycle of Mercury's perihelion around the Sun. All planets, including the Earth, precess, but the effect was only measurable for Mercury because Mercury is closest to the Sun, where the gravitational field is strongest. Newton's theory of gravity could not account for Mercury's precession, but general relativity could—a strong argument in its favor. In later years, many experiments have confirmed the predictions of general relativity to high precision.
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