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. Time Dilation: The name given to the prediction of Special Relativity that an observer measuring the rate of a clock that is in relative motion with respect to himself will find the rate of the clock to be slow when compared with an identical clock that is not moving. It does not matter whether the moving clock is approaching or receding.

A clock is a device for creating events...things that happen. The ticking of a clock is an event and each tick has a "where" and a "when." When we create two events, they are separated by an amount of time. So, anything that creates a regular series of events is a clock. The beating of your heart is a clock and every time it beats, a certain amount of time has elapsed.

The clock in the animation creates its events by ticking whenever a light signal moves from one end to the other of the clock. In the animation, the ticking of the clock is indicated by a light that turns from green to red. You could imagine the light being reflected back to its origin to complete a "tock," then doing it again and again. But the beating of someone's heart as the person moves by could just as well be the "ticks" of a clock. Any moving clock, be it the light clock we demonstrate here, or the beating of a human heart, will be judged by a stationary bystander to be running slow for the reason demonstrated in this animation.

When you begin the animation by clicking below, you will see a "Stationary Car" button and a "Moving Car" button. The animation begins with the "Moving Car" but you can show how the clock works from the viewpoint of someone who carries the clock along with himself in a car by clicking on the "Stationary Car" button. From this viewpoint (riding with the car), of course, the clock and the car are at rest.

When the car is moving, see how the time measured by two identical clocks would be measured if one clock is at rest (the "bystander's clock") and the other moves (the "car's clock"). You will see that they don't tick simultaneously. The bystander sees the light move a shorter distance in his clock than the light must move to make the moving clock tick. But actual experiment (which forms the Second Postulate) tells us that in both clocks the light moves with the same speed. Since the ticking of the moving clock lags the ticking of the bystander's clock, the bystander judges that the moving clock is running slow. This is the effect called "time dilation." It is summarized by saying that "moving clocks run slow." If the 'clock' is a human heartbeat, the bystander will judge that the moving heart is running slow.

Click here to begin the animation.


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