Nope, NOT 20 m/s. To them the ball is moving at 30 m/s (i.e., 10 + 20). So much for common sense. The difference arises from the fact that they are measuring from different “reference frames,” one moving, the other stationary.
It’s all good, though; everyone agrees on the outcome. If the ball hits the person, the miscreants and the bystander would calculate the same time of impact. Yes, the people in the car see the ball moving at a slower speed, but they also see the bystander moving toward them (from their perspective), so it works out the same in the end.
This is the other main postulate of special relativity: The physics are the same for all reference frames—or to be specific, for all “inertial,” or non-accelerating, frames. Observers can be moving at different velocities, but those velocities have to be constant.
Anyway, now maybe you can see why it’s actually quite bizarre that the speed of light is the same for all observers, regardless of their motion.
Waves in an Empty Sea
How did Einstein get this crazy idea ? I’m going to show you two reasons. The first is that light is an electromagnetic wave. Physicists had long known that light behaved like a wave. But waves need a medium to “wave” in. Ocean waves require water; sound waves require air. Remove the medium and there is no wave.
But then, what medium was sunlight passing through as it traveled through space? In the 1800s, many physicists believed there must be a medium in space, and they called it the luminiferous aether because that’s fun to say.
In 1887, Albert Michelson and Edward Morley devised a clever experiment to detect this aether. They built a device called an interferometer, which split a beam of light in half and sent the halves along two paths of equal length, bouncing off mirrors, and merging again at a detector, like this:
Illustration: Rhett Allain