Fundamental Principle The two postulates of the Special Theory of Relativity are: The laws of nature are the same for all observers who are in uniform motion (Motion Symmetry). The speed of light in empty space is the same for all observers in uniform motion regardless of their motion or the motion of the source of light. Definition An "event" is something which happens at a particular place in space and at a particular point in time. It is a "point" in four-dimensional spacetime..

Each of these postulates has been tested to high precision by experiment so that we know that these describe the way that nature actually behaves. The second postulate is where all the "mischief" in Special Relativity lies. The accompanying animation will illustrate some of the strange consequences of this behavior of nature.

"Events" are things that happen in the world. Each event has a "where" and a "when" in space and time. Our limited everyday experience tells us that if two events happen simultaneously ("at the same time"), then everyone will agree that the two events happened simultaneously. Because nature behaves according to the two above postulates, this turns out to be NOT necessarily true!

When you begin the animation by clicking below, you will see a "Stationary Car" button and a "Moving Car" button. Choose the "Stationary Car" first and you will see a light pulse moving outward like an expanding sphere. Because of the Second Postulate, an observer on the car sees the light move away from himself with the same speed in all directions, thus expanding like a growing sphere around him. When it reaches the green lights at the end of the car, an observer in this frame (in which the car is stationary), sees two simultaneous events that change the lights to red.

Now choose the "Moving Car" to see how these SAME two events appear to a second observer (bystander) in a frame in which the car is moving at 80% of the speed of light. Because of the Second Postulate, the bystander also finds herself at the center of the expanding sphere of light. To the second observer the same two events seen by the first observer to be simultaneous are NOT simultaneous. Because of the First Postulate, you can't say which observer is REALLY right since there is no way to say which observer is really at rest or really in uniform motion. That is why we say that the simultaneity of two events is relative and not absolute. It is here that all of the strange consequences of Special Relativity can be traced. (Observe the "length contraction" of the moving car along its line of motion (that's why the wheels aren't round); note the "time dilation" recorded at bottom left (in arbitrary time units). These are also consequences of the postulates of Special Relativity.)

Click here to begin the animation.


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