Einstein discovered how our normality is maintained 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 time and space are the same for both, they get a different speed of light and different physics. Einstein’s conclusion was that space had to shrink and time dilate to keep the speed of light constant, otherwise flashlights might not always shine and mirrors might not always reflect! Lorentz saw his transformations as mathematical curiosities but Einstein saw them as what made Poincare’s relativity principle work. Einstein recognized that for the universe to be as Poincare described, space and time had to change as Lorentz described. Time and space had to change to make physics invariant.
The implications of this conclusion are strange indeed. Imagine a rocket flying past a space station in orbit (Figure 5.3). It doesn’t seem possible that people on the rocket and on the space station both get the same speed of light, but in fact they do! If they didn’t, our physics wouldn’t work on Mars. Einstein’s conclusion, that time and space change when one moves, has been verified by experiment.
Yet who is really moving – is the rocket going past the space station or is the orbiting space station passing the rocket? It turns out that it doesn’t matter. If the rocket moves, its space and time contract and dilate, or 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. For any particle model, this extraordinary finding makes no sense at all.
It seems weird that time and space change to maintain normality when we move but as Einstein said, this is why the universe isn’t weird. That the speed of light stays the same no matter how fast we are moving is why we live in a “normal” universe.