从以太论到狭义相对论
From aether theory to Special Relativity
摘要：
在19世纪末，光被认为是一种在称为以太的物质介质中传播的电磁波。麦克斯韦波动方程中出现的速度c是光相对于以太的速度。因此，根据伽利略的速度相加，实验室中的光速将不同于c。这种差异的测量将揭示实验室（地球）相对于以太的运动（一种绝对运动）。然而，地球的绝对运动从未被证明。伽利略的速度加法是基于长度和时间间隔不变（与运动状态无关）的假设。这种思考时空的方式源自我们的日常经验，也是牛顿经典力学的核心。然而，在1905年，爱因斯坦通过假设光在任何惯性系中以相同的速度c传播来挑战伽利略的速度相加。在此过程中，爱因斯坦将相对性原理扩展到麦克斯韦定律所描述的电磁现象。在爱因斯坦的狭义相对论中，以太不存在，绝对运动也没有意义。光速的不变性迫使伽利略变换被洛伦兹变换取代。因此，相对论性的长度收缩和时间膨胀进入了我们对时空的理解。牛顿力学必须重新表述，这导致了质能等价的发现。
英文摘要：
At the end of the 19th century light was regarded as an electromagnetic wave propagating in a material medium called ether. The speed c appearing in Maxwell's wave equations was the speed of light with respect to the ether. Therefore, according to the Galilean addition of velocities, the speed of light in the laboratory would differ from c. The measure of such difference would reveal the motion of the laboratory (the Earth) relative to the ether (a sort of absolute motion). However the Earth's absolute motion was never evidenced. Galileo addition of velocities is based on the assumption that lengths and time intervals are invariant (independent of the state of motion). This way of thinking the spacetime emanates from our daily experience and lies at the heart of Newton's Classical Mechanics. Nevertheless, in 1905 Einstein defied Galileo addition of velocities by postulating that light travels at the same speed c in any inertial frame. In doing so, Einstein extended the principle of relativity to the electromagnetic phenomena described by Maxwell's laws. In Einstein's Special Relativity the ether does not exist and the absolute motion is devoid of meaning. The invariance of the speed of light forced the replacement of Galileo transformations with Lorentz transformations. Thus, relativistic length contractions and time dilations entered our understanding of the spacetime. Newtonian mechanics had to be reformulated, which led to the discovery of the mass-energy equivalence.
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