No, not 20 m/s. For them, the ball moves at a speed of 30 m/s (i.e. 10 + 20). For common sense, so much. The difference is due to the fact that they are measured from different “reference frames”, one moves, the other moves.
However, everything is fine. Everyone agrees with the result. If the ball hits a person, then the wrongdoing and bystanders will calculate the same impact time. Yes, the people in the car see the ball moving at a slower speed, but they also see the bystanders moving towards them (from their perspective), so in the end it works the same.
This is another major assumption of the special theory of relativity: the physics of all reference frames is the same, or specific, for all “inertial” or non-acceleration frames. The observer can move at different speeds, but these speeds must be constant.
Anyway, now maybe you will understand why the speed of light for all observers is the same regardless of their movement, which is actually strange.
Waves in the sky and sea
How did Einstein get this crazy idea? I’ll show you two reasons. First of all, light is electromagnetic waves. Physicists have long known that light behaves like waves. But the waves need one Medium “Wave” in the waves. Sound waves require air. Remove media without waves.
But what medium does the sun pass through as it passes through space? In the 1800s, many physicists believed that space had to have a medium, which they called Glowing ether Because it’s very interesting.
In 1887, Albert Michelson and Edward Morley designed a clever experiment to detect ether. They built a device called an interferometer that splits a beam of light in half and sends half along two paths of equal length, bounces from the mirror, and merges again on the detector:
Illustration: Rhett Allain
