Einstein analyzed why the speed of light stays constant as follows. He imagined a moving train where a floor light reflects from a mirror up on the ceiling. A train passenger sees the light go straight up and down, but a platform observer sees it travel a longer path in the same time (Figure 5.2). If both observers have the same time and space, they will get a different speed of light, and so different physics.
It followed that space had to shrink and time dilate to keep the speed of light constant. Lorentz saw his transformations as mathematical curiosities, but Einstein saw them as what made Poincare’s relativity work. He saw that for the universe to be as Poincare described, space and time had to change as Lorentz described. If space and time didn’t change to make physics invariant (Note 1), light would have a different speed for every observer!
The implications of this conclusion are strange indeed. For example, imagine a rocket flying past a space station in orbit (Figure 5.3). It seems impossible that people on the rocket and on the space station both get the same speed of light, but they do! If they didn’t, our physics wouldn’t work on Mars.
But who is really moving – is the rocket going past the space station, or is the space station passing the rocket? It turns out that it doesn’t matter. If the rocket moves, its space and time contract and dilate, and if the space station moves, the same applies. Regardless of how the rocket and station move relative to each other, distance and time change just enough to keep the speed of light the same for both.
This defies common sense, but experiments have verified that time and space really do change as matter moves faster or slower. It seems weird, but as Einstein said, this is why our universe isn’t weird. Our universe is normal because the speed of light is the same everywhere in it.
Note 1. Einstein preferred the term invariance for his theory but relativity stuck.