恒速度c

One of the postulates of special relativity states that:

The speed of light in free space has the same value in all inertial frames of reference.

This seems strange, and it does not agree with the things we observe in everyday life. For example, if I observe a fast motorcycle while standing still, and then I observe it while driving in my almost-as-fast car, then the motorcycle cannot pull ahead of me as quickly, so it does not appear to be as fast! Thus, the motorcycle has different speeds in the "moving car" reference frame and in the "standing still" reference frame.

This somewhat strange property of light was observed in the famous Michelson-Morley experiment of 1887. The experiment discredited the existence of an "aether"--a medium through which electromagnetic waves travel and which can be used to define an "absolute" or "preferred" reference frame. It is now generally accepted that electromagnetic waves do not need a medium to travel. The experiment showed that light was always perceived to travel at the same speed, regardless of the speed of the observer.

Einstein took the aforementioned experiment results and used them as a postulate on which to ground his theory of special relativity, which utterly redefined the way we understand the world around us.

So what is this special speed, the speed of all electromagnetic waves? It is denoted $c$, and it equals exactly $299, 792, 458$ meters per second. In fact, because of the fact that the speed of light is a universal constant, to the best of our knowledge, the meter was actually redefined in terms of the speed of light! The meter is now defined as the distance that light travels in $\frac{1}{299,792,458}$ of a second (the second is also defined by a universal constant of nature, having to do with electron transitions in a cesium isotope).

The speed of light is also $c=\frac{1}{\sqrt{\epsilon_0\mu_0}}.$ Here, $\epsilon_0 = 8.85\times 10^{-12} \frac{\text{C}^2}{\text{Nm}^2}$ and $\mu_0 = 4\pi\times 10^{-7} \frac{\text{Tm}}{\text{A}}.$ This computation was carried out by Maxwell--he used the four most important equations of electricity and magnetism (now called Maxwell's equations) to show that electric fields and magnetic fields can propagate through space, resulting in electromagnetic waves. Inspection of the wave equation for electromagnetic waves yields the wave speed $c.$