Have you ever wondered what the speed of shadow is? Well it can travel faster than light! In a second we will explain how it is possible without breaking the fundamental laws of physics.
Let us begin with a hypothetical experiment:
Imagine you have a light source that is powerful enough to reach the planet Jupiter. A very peculiar characteristic of that light source is that it casts a beam in the shape of a cone that’s broad enough to cover the entire diameter of the planet.
When you pass your finger over the lens, the shadow of your finger will cover the entire diameter of the planet Jupiter. The diameter of the planet is about 142,984 km (142,984,000 m) at its equator. Light requires 0.477 seconds to cover that distance. But if you are quick enough to pass your finger across the lens then the shadow casted by your finger will travel that distance well under 0.477 seconds. That means that it have BROKEN the speed of light.
Well you remember from our previous blog on Cherenkov Radiation we said nothing can travel faster than the speed of light ? Well, that’s the key! Shadows aren’t anything. Shadows are just absence of something especially photons. So if a shadow is moving then the truth is that nothing is moving. There is no sort of information that is moving in shadow.
In the beginning, God said, “Let the four dimensional divergence of an anti-symmetric second rank tensor equal zero.” And there was light.
– Michio Kaku