Relativity in brief... or in detail..
3. Einstein's principle of Special Relativity
So, how to reconcile Galilean relativity and electromagnetism? We might say that Maxwell's equations of electromagnetism only work in one special frame of reference. But what would be the preferred frame?
So Einstein started from the principle that:
the laws of electromagnetism, as well as mechanics, are the same for all observers.
IBut, as we've just seen, different observers will have to make more complicated conversions when they compare measurements. Let's see the consequences.
Light is an electromagnetic wave so, if it obeys the same laws in all frames, then
all observers will see light travel at the same speed.
Now, fortunately for Einstein, two American physicists had recently tried to measure the effect of the Earth's rotation and orbit on the behaviour of light. Their results showed no effect.
Read more about Michelson and Morley
But our common sense is developed for everyday speeds, so there's no reason why our intuition should work near the speed of light. So either follow this link, or else just accept Einstein's principle, and see where it leads.