Ok, so you’ve heard that nothing can travel faster than the speed of light. (That’s not quite true. The expansion of the universe allows for faster than light travel but that’s another post.) You’re also aware that time slows down the closer you get to the speed of light. You know, the ‘One twin goes off to Alpha Centauri at the speed of light and comes back after 80,000 years but he’s only aged 3 months’ story.
Ever wonder why? Here’s the crib notes.
Everything in the universe always travels exactly at Light Speed.
Time dilation: Special relativity declares a law for all motion: The combined speed of any object’s motion through space and it’s motion though time is always precisely equal to the speed of light.
That’s right, everything. You, me, the computer screen you’re looking at, your grandma’s French toast, Santa Clause… everything.
Everything is traveling through Spacetime: space (the three dimensions we experience and the nine others that m-theory predicts) and time.
Adding the total movement through both space and time always equals light speed. Always. Always. Always.
Since you must travel constantly at exactly the speed of light, when you increase your speed through space, you decrease your speed through time.
Your head (and the rest of you) is traveling through spacetime at the speed of light. But, when you’re at rest (not accelerating) all of your head’s movement is through time, none of it is traveling (accelerating) through space. Every time your head moves (accelerates) through space; in a car, in a plane, in a spaceship… even nodding up and down, some of it’s movement in time is lost since it is now moving through space. Cool huh.
What about light?
Since light waves use all of their motion to travel through space at Light Speed, they have absolutely no motion through Time. Every photon that has ever been produced exists in an ageless state. (To us, the light seems to move through time but to the photon, time is standing still. This is one of the seemingly odd realizations fo Ensteins Theory of Relativity.)
The universe ages, light does not.