Between 1887 and 1905 there were several attempts, most notably by the Dutch physicist Hendrik Lorentz, to explain the result of the Michelson-Morley experiment in terms of objects contracting and clocks slowing down when they moved through the ether. However, in a famous paper in 1905, a hither to unknown clerk in the Swiss patent office, Albert Einstein, pointed out that the whole idea of an ether was unnecessary, providing one was willing to abandon the idea of absolute time.
A similar point was made a few weeks later by a leading French mathematician, Henri Poincare. Einstein’s arguments were closer to physics than those of Poincare, who regarded this problem as mathematical. Einstein is usually given the credit for the new theory, but Poincare is remembered by having his name attached to an important part of it.
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, no 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 cis 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, any normal object is forever confined by relativity to move 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.
Notably 顯著地
在1887年到1905年之間，人們曾經好幾次企圖去解釋麥克爾遜——莫雷實驗。最著名者為荷蘭物理學家亨得利克·羅洛茲，他是依據相對於以太運動的物體的收縮和鍾變慢的機制。然而，一位迄至當時還不知名的瑞士專利局的職員阿爾貝特·愛因斯坦，在1905年的一篇著名的論文中指出，只要人們願意拋棄絕對時間的觀念的話，整個以太的觀念則是多餘的。
幾個星期之後，一位法國最重要的數學家亨利·彭加勒也提出類似的觀點。愛因斯坦的論證比彭加勒的論證更接近物理，因為後者將此考慮為數學問題。通常這個新理論是歸功於愛因斯坦，但彭加勒的名字在其中起了重要的作用。
這個被稱之為相對論的基本假設是，不管觀察者以任何速度作自由運動，相對於他們而言，科學定律都應該是一樣的。
這對牛頓的運動定律當然是對的，但是現在這個觀念被擴展到包括馬克斯韋理論和光速：不管觀察者運動多快，他們應測量到一樣的光速。這簡單的觀念有一些非凡的結論。可能最著名者莫過於質量和能量的等價，這可用愛因斯坦著名的方程E＝mc2來表達（這兒E是能量，m是質量，c是光速），以及沒有任何東西能運動得比光還快的定律。由於能量和質量的等價，物體由於它的運動所具的能量應該加到它的質量上面去。換言之，要加速它將變得更為困難。這個效應只有當物體以接近於光速的速度運動時才有實際的意義。例如，以10％光速運動的物體的質量只比原先增加了0.5％，而以90％光速運動的物體，其質量變得比正常質量的兩倍還多。當一個物體接近光速時，它的質量上升得越來越快，它需要越來越多的能量才能進一步加速上去。實際上它永遠不可能達到光速，因為那時質量會變成無限大，而由質量能量等價原理，這就需要無限大的能量才能做到。由於這個原因，相對論限制任何正常的物體永遠以低於光速的速度運動。只有光或其他沒有內稟質量的波才能以光速運動。
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