Brian's Guide to Einstein and Gravity
         
                Due to the nature of this project, this will not be a full explanation of Einstein's general relativity theory but rather a quick introduction.  The first part of general relativity involves gravity and I will try to explain this as best I can.
                Einstein theorized that the existence of gravity was a result of a plane that runs through the whole universe called space time.  This plane would be curved by massive objects but not because of its gravitation but because of some other phenomena.  One way to visualize the curving of space time is with a bowling ball.  The bowling ball, represents the massive object.  When it is placed on a sheet of flexible metal, it causes the metal to bend downwards as shown in the diagram below.

            The sheet under the bowling ball represents space time and if a smaller ball were placed on this sheet, it would roll towards the larger one because of the curvature in space time.  This rolling, or attraction, is what we call gravity.  Therefore, gravity is not a force in itself, but rather the result of a curved space time.
                Black holes then do not pull space time because of their infinite gravity, but rather the mass of the singularity causes space time to curve into it and as a result, a strong gravitational pull is produced.  Also, you might notice that light sort of 'falls' into a black hole the way water might run into a hole.  This is because the path of light follows along the lines of space time.
             
        Special Relativity
         
                Another theory from Einstein that is actually more common in our lives than general relativity is special relativity.  Special relativity is all about the speed of an object in relation to the speed of a larger object that supports or contains the moving object mentioned first.  For instance, suppose the speed of a bullet were 800 m/s.  If a man riding a train that is travelling at 30 m/s fires the bullet in the direction the train is travelling the speed of the bullet relative to the man who shot it and anyone on the train at the time, would be 800 m/s.  However, if someone were standing beside the train, the speed of the bullet would be equal to 800 m/s plus the speed of the train.  As a result, the bullet's velocity become 830 m/s if you were to watch it from off the train and 800 m/s if you were on the train.  If the man were to turn around and fire the bullet in the opposite direction, the speed relative to the train would still stay the same but the speed relative to someone off the train would be the speed of the bullet MINUS the speed of the train which would equal 770 m/s.
                Another part of this theory involves the principle of equivalence.  It states that the effect of gravity can be produced anywhere even away from a planet.  Suppose a man drops a ball on earth.  The same result could be produced in space if a man were to drop the ball from inside a spaceship that was moving upwards at a constant speed.  The same principle applies for free-falling objects.  In space, we feel no gravitational pull and as a result it seems as though we were floating.  The same result would be produced if an elevator were falling to the ground at a uniform speed.
             

                That ends my short guide to relativity.  Click on the Black Hole banner to return to the index to read more on Black Holes.