Light, Matter, and Energy

Abstract

All straight, steady speeds are relative. This is known to us: When we overtake a car on the highway, it moves relative to us slower than for the oncoming drivers. However, we know also that the speed of light is absolute. Hence no matter if a light beam overtakes us, or we travel towards it: It always passes us at the same speed! It means that the speed of light is one of the fundamental constants of nature. That this makes sense, without any ado, was the great insight of Einstein. We show in this chapter that therefore the mass of any object must increase with increasing speed, eventually preventing matter from overtaking light. However, at the same time, the energy of the object also increases. This leads directly to the famous equation E=mc ², showing that mass and energy are like two sides of the same coin. We also show that light is pure energy which cannot rest, but carries mass and can convert into mass.