3. Einstein's principle of Special Relativity

This is a text+animation version of the video chapter 3 from Einsteinlight.

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?

Einstein's intuition was that Maxwell's equations were so beautiful that they had to be true, and true in all frames, even if this meant replacing the relativity of Galileo and Newton..

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

Now this is weird and counter-intuitive. Can it really be that the light from the headlights has the same speed with respect to the driver and to a pedestrian?

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.

Go to the text+animation version or video version of chapter 4 from Einsteinlight.