Jurassic Quark

Einstein's theory of special relativity proposed that, since nothing travels faster than the speed of light, not even information, that if two objects are traveling faster than the speed of light away from each other, they cannot "see" each other moving at 2 times the speed of light.

This would mean that the photons that moved between them would have to travel at 2 times the speed of light, and this can't happen, light always travels at the speed of light to a local observer, no matter which direction the observer is moving (which was known from Michelson and Morley's experiments).

 
This means that when traveling near the speed of light, in order to receive photons that reach you at the speed of light relative to you, your velocity has to change in a strange way.

The change in your position is the same, but the time for you while you are moving that distance slows down relative to your closeness to the speed of light. 

(alternatively, it can be said that, for a constant time, space stretches in the direction you're moving such that you cover less of it for a particular time frame.)

 
You can't increase your velocity to the speed of light because you keep losing time as you get faster, which plateaus off to your travel in time being 0 at the speed of light. 

an interesting side effect of this is that particles that do travel at the speed of light, like photons, experience no time. 

If a photon had a watch, the hands wouldn't move between when it was created and when it was destroyed or absorbed!)

 
This implies that, to truly know where an object is, you have to have a fourth dimension that is allowed to change: time. 

This is when time was identified as the fourth dimension: in physics, you can locate something as L = f(x, y, z, t). 

You need to know how time is changing for an object, as well as its position, to locate it in a new concept: spacetime, that is, the set of all possible locations in space or time.

https://www.reddit.com/r/askscience/comments/32cvoo/could_someone_please_explain_the_fourth_dimension/?st=jbm40wa5&sh=67973bca