## 《时间简史》：为什么物体的速度不能超过光速？

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### 《时间简史》：为什么物体的速度不能超过光速？

The fundamental postulate of the theory of relativity, as it was called, was that the laws of science should be the same for all freely moving observers, on matter what their speed. This was true for Newton’s laws of motion, but now the idea was extended to include Maxwell’s theory and the speed of light: all observers should measure the same speed of light, no matter how fast they are moving. This simple idea has some remarkable consequences. Perhaps the best known are the equivalence of mass and energy, summed up in Einstein’s famous equation E=mc2 (where E is energy, m is mass, and c is the speed of light), and the law that nothing may travel faster than the speed of light. Because of the equivalence of energy and mass, the energy which an object has due to its motion will add to its mass. In other words, it will make it harder to increase its speed. This effect is only really significant for objects moving at speeds close to the speed of light. For example, at 10 percent of the speed of light an object’s mass is only 0.5 percent more than normal, while at 90 percent of the speed of light it would be more than twice its normal mass. As an object approaches the speed of light, its mass rises ever more quickly, so it takes more and more energy to speed it up further. It can in fact never reach the speed of light, because by then its mass would have become infinite, and by the equivalence of mass and energy, it would have taken an infinite amount of energy to get it there. For this reason, and normal object it forever confined by relativity to more at speeds slower than the speed of light. Only light, or other waves that have no intrinsic mass, can move at the speed of light. --- from 《A Brief History of Time》