Einstein proposed two theories of relativity- Special Relativity in 1905, and General Relativity in 1915. Although general relativity is the theory that we will most closely consider in the following topics, Special Relativity is important to understand as it defines much of what we now consider basics of physics and sets a base for General Relativity.

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• Special Relativity

Special Relativity is a theory on the nature of matter moving through space and time, and it combines these two properties into a continuum called Space-Time. The heart of the theory is that different observers moving at different rates of speed will find that the laws of physics and the speed of light remain the same, even though they will have different perceptions of time and distance. Because of this, simultaneous events can appear to happen at different times, or events happening hours apart to one person could happen at the same time to another. In other words, your reality is determined by your point of view.

Other important conclusions came from special relativity, including the fact that a moving object is shortened in the direction of movement, time runs more slowly for a moving object, no body can travel at or above the speed of light, and that a body has more mass when it is moving than when it is at rest. These effects are usually too small to be observed with our normal velocities.

Einstein's famous equation, E=mc˛, also stems from special relativity. The greater mass of a moving body suggests a connection between kinetic energy and mass. From there, Einstein guessed that all energy and mass would be related. E=mc˛ shows this relationship.

• General Relativity

General Relativity is an extension of special relativity, but it is mainly a theory of gravity. It defines gravity as a curvature of the four-dimensional space-time suggested by special relativity.

The best way to picture this would be to imagine space-time as a rubber sheet. If you were to drop a lead ball onto the sheet, it would curve downwards. This indentation would be the ball's gravitational field. A heavier ball would create a larger 'field'.  If you were to drop a marble or much lighter lead ball (because a very dense marble would create a 'field' of it's own!) Near one of the fields, it would roll in, much as a small celestial object would be pulled in by a larger one.

There are, of course, many other conclusions based on general relativity. General relativity gives a description of how material bodies will move in the presence of gravitational fields, and the mathematical equation gives the precise value for curvature caused by bodies of different masses. It predicts that extremely massive stars can collapse on themselves to form Black Holes. General relativity shows that gravity can bend light, such that some of the stars we see in the night sky are not readily visible to us, but the sun curves their light so we can see them. And finally, general relativity shows that a straight line is not always the shortest way between two points. A geodesic is what the line signifying this distance is known as.  In Euclidean, or plane, geometry, the geodesic is a straight line. But when relativity comes into play, geodesics can become curved. (Think of the circle routes used by planes on long or overseas flights.)

Such are the basics of relativity. By the way, have you ever wondered why it's called relativity? It's because it all depends on the relative motion of the observers.