Lesson 5: Theory of Relativity

The theory of relativity is a very complex and difficult subject to understand. We will just discuss the very basics of the theory here. The theory of relativity is actually two theories that Albert Einstein came up with in the early 1900s. One is called “special” relativity and the other is called “general” relativity. Special Relativity states that the speed of light is always constant.  This means that no matter what you do to light it will always go the same speed. The Theory of General Relativity is the one which redefined the laws of gravity.  It says that it is impossible to tell the difference between gravity and the force of inertia from a moving object. We will talk mostly about special relativity here. 

Imagine you are in a rain storm, and the wind is blowing against your back. If you started running, the rain wouldn’t hit your back as hard. It would be travelling slower compared to you. Scientists would say that the rain was travelling slower relative to you.And, of course, if you turned around and ran toward the rain, it would hit you even harder than if you stood still. Scientists would say that the rain was moving faster relative to you.

Back then, scientists thought that light acted like rain drops in a rain storm. They thought that if the earth was moving around the sun, and the sun was moving around the galaxy, they should be able to measure how fast they were moving through space. All they would need to do was measure how the speed of light “changed.” And that’s exactly what they did. But they discovered something very strange. 

The scientists had discovered that light didn’t act like raindrops or anything else in the universe. No matter how fast you were moving, and no matter what direction you were heading, the speed of light was always the same. This was very confusing, and it took a man named Albert Einstein to figure out how this could be.

Read the next pages of this lesson to find out the main effects of special relativity.