What is Theory of Relativity?

The theory of relativity is a very complex and confusing subject. We will only discuss the basics of the theory here.

The theory of relativity is actually made up of two theories developed by Albert Einstein in the early 1900s. One is called “special” relativity and the other is called “general” relativity. Here we will mainly talk about special relativity.

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 all inertial frames of reference.

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 light source.

What does “relative” mean?

The first rule listed above is quite confusing. What does it mean? Well, before Albert Einstein, scientists believed that all motion happens relative to a reference point called “eether”. Einstein stated that ether does not exist. He says that all movements are “relative”. This means that the measure of motion depends on the relative speed and position of the observer.

A Relative Example

An example of relativity is to imagine two people on a train playing table tennis. The train is moving at about 30 m/s northward. When the ball is bounced back and forth between two players, the ball appears to be moving north at about 2 m/s and then south at 2 m/s.

Now imagine someone standing by the train tracks watching a table tennis game. As the ball moves north, it will appear to be traveling at 32 m/s (30 m/s plus 2 m/s). When the ball is hit in the opposite direction, it still seems to be moving north, but with a speed of 28 m/s (30 m/s minus 2 m/s). To the observer beside the train, the bullet appeared to be still flying north.

As a result, the speed of the ball depends on the “relative” position of the observer. The people on the train will be different from those on the side of the track.

E = mc2

One of the results 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 the energy of an object is accompanied by a change in its mass. This concept became important in the development of nuclear power and the nuclear bomb.

Length Contraction

Another interesting result of special relativity is the contraction of length. Length contraction occurs when the shorter the objects appear, the faster they move relative to the viewer. This effect only occurs when the object reaches very high speed.

To give you an example of how very fast moving objects look shorter. If a 100-foot-long spaceship were flying near you at half the speed of light, it would appear to be 87 feet long. If it accelerated to 0.95 the speed of light, it would only be 31 feet long. Of course, these are all relative. To those aboard the spaceship, it still seemed 100 feet long.