A Brief History of Relativity

What is it? How does it work? Why does it change everything? An easy primer by the world's most famous living physicist

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This required abandoning the idea that there is a universal quantity called time that all clocks measure. Instead, everyone would have his own personal time. The clocks of two people would agree if they were at rest with respect to each other but not if they were moving. This has been confirmed by a number of experiments, including one in which an extremely accurate timepiece was flown around the world and then compared with one that had stayed in place. If you wanted to live longer, you could keep flying to the east so the speed of the plane added to the earth's rotation. However, the tiny fraction of a second you gained would be more than offset by eating airline meals.

Einstein's postulate that the laws of nature should appear the same to all freely moving observers was the foundation of the theory of relativity, so called because it implies that only relative motion is important. Its beauty and simplicity were convincing to many scientists and philosophers. But there remained a lot of opposition. Einstein had overthrown two of the Absolutes (with a capital A) of 19th century science: Absolute Rest as represented by the ether, and Absolute or Universal Time that all clocks would measure. Did this imply, people asked, that there were no absolute moral standards, that everything was relative?

This unease continued through the 1920s and '30s. When Einstein was awarded the Nobel Prize in 1921, the citation was for important--but by Einstein's standards comparatively minor--work also carried out in 1905. There was no mention of relativity, which was considered too controversial. I still get two or three letters a week telling me Einstein was wrong. Nevertheless, the theory of relativity is now completely accepted by the scientific community, and its predictions have been verified in countless applications.

A very important consequence of relativity is the relation between mass and energy. Einstein's postulate that the speed of light should appear the same to everyone implied that nothing could be moving faster than light. What happens is that as energy is used to accelerate a particle or a spaceship, the object's mass increases, making it harder to accelerate any more. To accelerate the particle to the speed of light is impossible because it would take an infinite amount of energy. The equivalence of mass and energy is summed up in Einstein's famous equation E=mc2, probably the only physics equation to have recognition on the street.

Among the consequences of this law is that if the nucleus of a uranium atom fissions (splits) into two nuclei with slightly less total mass, a tremendous amount of energy is released. In 1939, with World War II looming, a group of scientists who realized the implications of this persuaded Einstein to overcome his pacifist scruples and write a letter to President Roosevelt urging the U.S. to start a program of nuclear research. This led to the Manhattan Project and the atom bomb that exploded over Hiroshima in 1945. Some people blame the atom bomb on Einstein because he discovered the relation between mass and energy. But that's like blaming Newton for the gravity that causes airplanes to crash. Einstein took no part in the Manhattan Project and was horrified by the explosion.

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