That's right! Contrary to what common sense tells us, time and distance are not
fixed. This, too, is the assumption Einstein made.
In our second and third train examples, the speed of light turns out to be
exactly the same for both you and the observer standing along the tracks
because time, as measured by your watch, ticked along at a slower pace than
time measured by the observer. Not only that, distance changed, too. For the
observer, a one-foot ruler whizzing by on the train would have measured less than
The weird thing is that, for you on the train, time wouldn't seem to be moving
slower and your ruler wouldn't be shorter—all would appear normal. However,
time on the rest of the Earth would appear to be ticking along slower and its
rulers would be shorter.
Now let's say you want to do some time travelling. You board a spaceship and
take off for deep space.
The ship approaches the speed of light. Time for you seems to pass as it always
has. It takes you about five seconds to tie your shoe. But to an observer on
Earth (assuming he or she could watch you), you are moving at a snail's pace.
It takes hours to tie your shoe.
Anyway, you continue on your journey. You slow down, stop, and accelerate back
to Earth. You arrive home. You have aged two years during your flight. Two
hundred years have passed on Earth. You have successfully travelled forward
Now you want to go back? Sorry. According to relativity, you can only move
through time in one direction.