《时间简史》:为什么物体的速度不能超过光速?

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Mia2014
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《时间简史》:为什么物体的速度不能超过光速?

帖子 Mia2014 » 周六 4月 03, 2021 4:14 pm

简单的说,因为物体的能量和质量是相互转化的,速度加快,质量也相应增加,质量越大的物体,提高速度需要更多能量。物体速度越接近光速,质量越接近无限大,达到光速需要的能量就是无限大,因此也是不可能的。

摘自《时间简史》:

相对论的基本假设是,相对于所有观察者的速度,科学定律对于所有自由移动的观察者应该相同。牛顿的运动定律确实如此,但现在这个想法已扩展到包括麦克斯韦的理论和光速:无论观察者移动多快,所有观察者都应测量出相同的光速。这个简单的想法会产生一些显着的后果。也许最广为人知的是质量和能量的相同性,它用爱因斯坦著名的方程式E = mc2进行总结(其中E是能量,m是质量,c是光速),以及定律是没有比光速更快的运动了。由于能量和质量相等,物体由于其运动而具有的能量将增加其质量。换句话说,它将使增加速度变得更加困难。该效果仅对于以接近光速的速度移动的对象才真正重要。例如,以光速的10%计,物体的质量仅比正常质量多0.5%,而以光速的90%时,物体的质量将是其正常质量的两倍以上。当物体接近光速时,其质量会以更快的速度上升,因此需要越来越多的能量来进一步加快其速度。实际上,它永远不可能达到光速,因为届时它的质量将变得无限大,并且通过质量和能量的等效,它将需要无限量的能量才能到达那里。出于这个原因,通常情况下,相对论将它永远限制在比光速慢的速度上。只有光或其他没有固有质量的波才能以光速运动。

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》

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