No, not 20 m / s. For them, the ball moves at 30 m / s (that is to say 10 + 20). So much for common sense. The different comes from the fact that they measure from different “reference frames”, one moving, the other stationary.

Everything is fine, however; Everyone agrees on the result. If the ball strikes the person, the underworld would calculate the same impact time. Yes, the people of the car see the ball moving at a slower speed, but they see the passer -by moving to them (from their point of view), so it works the same thing at the end.

This is the other main postulate of special relativity: physics is the same for all reference frames – or specific frameworks, for all “inertial” or non -acceleration frameworks. Observers can move at different speeds, but these speeds must be constant.

Anyway, now, you may be able to see why it is actually bizarre that the speed of light is the same for all observers, whatever their movement.

Waves in an empty sea

How did Einstein have this crazy idea? I will show you two resons. The first is that light is an electromagnetic wave. Physicists have long known that light behaved like a wave. But the waves need a AVERAGE “Wave” in. The waves of the ocean require water; Sound waves require air. Remove the middle and there is no wave.

But then, what medium was the sun spent when it was crossing space? In the 1800s, many physicists thought there must be a means in space, and they called him Aether Lumiferous Because it’s fun to say.

In 1887, Albert Michelson and Edward Morley designed an intelligent experience to detect this ether. They built a device called interferometer, which divided a bundle of light in half and sent the halves along two paths of equal length, bouncing on the mirrors and merging again in a detector, like this: