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Newton's Simple World. For 200 years before Einstein, physicists had faithfully followed a set of basic laws published by the great Sir Isaac Newton in 1687. Their faithfulness had paid off. Sir Isaac led them to many triumphs and promised them many more.
Newton's laws were high-school simple. He assumed the existence of two independent entities—mass and force, which interacted as follows:
1) Every body (mass) continues in its state of rest, or of uniform motion in a straight line, except so far as it may be compelled by force to change that state.
2) Any two bodies attract one another with a force (gravitation) which is proportional to the product of their masses divided by the square of the distance between them.
Upon these basic rules (and others closely related), physicists built an imposing structure of knowledge. They predicted the motions of the earth, the moon, the planets. They derived a maze of useful mechanical sub-laws. They explained the behavior of gases, and discovered the nature of heat. Newton's laws did not account for everything, but the physicists felt that this was due to their own ignorance. Eventually, they were sure, all phenomena could be explained in Newton's terms.
When conflicting facts were discovered by increasingly sensitive instruments, physicists tended to ignore them, or to explain them away by highly artificial creations. Most famous of these was the ether—a tenuous material supposed to fill all space. Ether was necessary (in Newtonian physics) for carrying light waves.
End of the Ether. The ether had another valuable property: it was at rest—"the calm ether-sea"—while everything else in the universe was in motion. Thus it provided the only stable "frame of reference." The earth, for instance, was thought to have "absolute motion" through the motionless ether.
In 1887 came that dreadful day when the ether was done to death. Two U.S. physicists, Albert A. Michelson and E. W. Morley, measured the speed of light simultaneously in two directions at right angles to one another. The speeds were expected to differ slightly because of "ether drift" past the earth. They turned out to be exactly the same, proving conclusively that ether did not exist.
Loss of the ether left the physicists inconsolable. Without it, light waves had no medium to carry them. The vital "frame of reference" was gone. No motion was "absolute" now. The motion of every moving body could be measured only "relative" to some other moving body.
For nearly 20 years, the physicists worked hard to "save" the ether. But the ether could not be saved, and with it went the authority of Newton's scientific decalogue, which depended upon it. Science, the guiding mind of technological civilization, was in crisis.
Albert Einstein, then an unknown clerk in a Swiss patent office, rescued science. In his Theory of Special Relativity (1905) he abandoned Newton's assumption of independent mass and force. In its place he put the assumption, well supported by observation, that the speed of light in a vacuum is constant, no matter what the speed of its source.