2017-04-08 12:54:25 UTC
Image lightning striking miles away. In a literal instant it reaches me. Moments later I hear a thunderclap. The sound is a wave, and the light is a particle, but both are the same.
Now imagine timing the flash of light, and the time between it and the thunder.
Here is relativity.
If you travel towards the lightning fast enough, the difference between the speed of light and the thunder will change because light is a constant.
The speed of sound is 340.29 m / s
So if you travel towards the lightning at 340.29 m/s, the sound of the thunder will get to you much quicker than it would if you were standing still and the lightning would reach you at the same time.
And we have no other way of measuring time without this sort of relativity!