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. We will talk mostly about special relativity here.
You can learn more about two very important aspects of the theory of relativity on this page about the speed of light and time dilation.
Special Relativity
There are two main ideas that make up Einstein's theory of special relativity.
1. The principle of relativity: The laws of physics are the same for any inertial reference frame.
2. The principle of the speed of light: The speed of light in a vacuum is the same for all observers, regardless of their relative motion or the motion of the source of the light.
What does "relative" mean?
The first principle listed above is pretty confusing. What does this mean? Well, before Albert Einstein, scientists thought that all motion occurred against a reference point called the "ether". Einstein claimed that the ether did not exist. He said that all motion was "relative". This meant that the measurement of motion depended on the relative velocity and position of the observer.
A Relative Example
One example of relativity is to imagine two people on a train playing ping-pong. The train is traveling at around 30 m/s north. When the ball is hit back and forth between the two players, the ball appears to the players to move north at a speed of around 2 m/s and then south at the speed of 2 m/s.
Now imagine someone standing beside the railroad tracks watching the ping-pong game. When the ball is traveling north it will appear to travel at 32 m/s (30 m/s plus 2 m/s). When the ball is hit in the other direction, it still appears to travel north, but at a speed of 28 m/s (30 m/s minus the 2 m/s). To the observer by the side of the train, the ball always appears to be traveling north.
The result is that the speed of the ball depends on the "relative" position of the observer. It will be different for the people on the train than for the person on the side of the railroad tracks.
E = mc2
One of the results of the theory of special relativity is Einstein's famous equation E = mc2. In this formula E is energy, m is mass, and c is the constant speed of light.
An interesting result of this equation is that energy and mass are related. Any change in an object's energy is also accompanied by a change in mass. This concept became important in developing nuclear energy and the nuclear bomb.
Length Contraction
Another interesting result of special relativity is length contraction. Length contraction is when objects appear shorter the faster they are moving in relation to the observer. This effect only occurs as objects reach very high speeds.
To give you an example of how objects moving very fast appear shorter. If a spaceship 100 feet long was flying by you at 1/2 the speed of light, it would appear to be 87 feet long. If it sped up to .95 the speed of light, it would only appear to be 31 feet long. Of course, this is all relative. To people on board the space ship, it would always appear to be 100 feet long.
