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This assumption was the heart of Relativity. When properly developed, mathematically, it led to astonishing conclusions, some of them (like many scientific facts) "contrary to common sense." Suppose, for instance, that the earth is moving at many feet per second toward a star. This approaching motion does not increase the arrival speed of the star's light, which strikes the earth at exactly the same speed (186,000 miles per second) as if the earth were at rest. Expressed in an equation, it looks like this:
186,000 m p s &velocity of earth = 186,000 m p s
Even if the earth speeds toward the star at 100,000 m p s, it makes no difference :
186,000 m p s & 100,000 m p s = 186.000 m p s
Slow Clocks, Heavy Matter. Obviously, something is wrong, for even Relativity does not abolish simple arithmetic. Einstein's daring conclusion was that only the speed of light is invariable. When the speed of a body changes, its dimensions and its mass and its time also change. As it speeds up, it shrinks (in the direction of the motion); its clocks slow down; its matter grows heavier. If the earth were to reach a speed of 161,000 m p s, every pound of matter in it would double in weight.
Observers on the speeded-up earth would not know that anything had changed.
But with their slowed-down clocks and their shrunken yardsticks, they would measure the arriving starlight in such a way that its speed would come out 186,000 m. p s. Under Relativity, the "absurd" equations above are not absurd.
Shrunken yardsticks are hard to measure, but the increase of mass which Einstein predicted in 1905 has been observed ac curately. Certain material particles shot out by radium move at 185,000 m p s, almost the speed of light. When they are weighed in flight (by a magnetic device), their mass is shown to have increased ac cording to his prediction.
W'hat makes the mass increase? A fast-moving body, Einstein proved mathe matically, has more energy, and energy has mass. Thus the mass of a moving body is its "rest-mass" plus the mass of the energy it contains.
This was a revolutionary concept. If energy can turn into mass by speeding up a moving body, then mass, perhaps, can turn into energy. "Certainly," said Einstein. "Mass, including the mass of all matter, is merely another form of energy." In his famous equation, he gave their equivalent values: E = mc2.*This meant that every pound of any kind of matter contained as much energy as is given off by the explosion of 14 million tons of TNT. It took the world 40 years (until Hiroshima) to appreciate this shocker.
Photons and Quanta. In that same year, 1905, Einstein advanced another theory which many historians of science consider even more important than Relativity. The ether was gone, and although Relativity established the velocity of light as the firmest figure in the universe, it did not supply any medium to carry the waves of light.
At that time nearly all physicists agreed that light consisted of waves whose properties had been observed in great detail. The old theory (favored by Newton) that light was speeding corpuscles had been abandoned. But the theory had one great advantage: corpuscles can move through space by themselves. Unlike waves, they need no medium to carry them.