Time Dilation and Length Contraction

Until the beginning of the 20th century time was thought to have a constant unvarying flow. A minute in North America was the same as a minute in Africa and anywhere else in the universe. But Einstein's theory of relativity changed that notion. He had discovered a bizarre aspect of the universe that is still not fully understood. A second of time here is much different than a second of time on a fast-moving object. Clocks on fast-moving objects actually slow down as compared to clocks on stationary objects. This is known as time dilation.

Understanding time dilation takes us back to the Michelson and Morley experiments of 1887. At that time it was thought light waves behaved like cars on a freeway (to use a more contemporary analogy). If one used a radar gun to measure the speed of an approaching car one might find that it was traveling 50 miles per hour. But if you are driving toward the car at 50 miles per hour the measurement would be different. The measured speed would be 100 miles per hour, the sum of your motion and the motion of the approaching car. Likewise, following the car at the same speed would produce a measurement of zero miles per hour. It is obvious that placement of the radar gun will produce different measurement of the car's speed. And it is equally obvious that light should behave in the same way. That is what made the Michelson and Morley experiments so revolutionary. The speed of light remained constant regardless of the speed of the measuring equipment.

Einstein delved deeply into this apparent contradiction and established the speed of light as a universal constant. To formalize this idea he used mathematics developed by Hendrik Lorentz, who had made the following observation: The speed of light in a moving frame of reference would remain constant if lengths contract in the direction of motion. For example, from the point of view of a stationary observer, a spaceship traveling at 86.6% the speed of light would measure one half its length. This idea was developed from the field of electrodynamics, which showed that the distribution of charged particles put into high-speed motion would similarly contract.

Along with length contraction, Einstein discovered that time was not equal between moving frames of reference. with each increment of speed, moving clocks slow down in relation to stationary clocks. This aspect of time has been firmly established by experiment. Though the time intervals are extremely small at our speeds, atomic clocks have become so accurate that time dilation has been measured. The observed slowing of clocks with speed conforms to the predictions of special relativity.

However, length contraction and the slowing of the clocks is what the stationary observer sees. within moving vehicles, time and lengths appear to remain the same as they always were. Changes in length and time can only be inferred from knowledge about one's speed with respect to light. Further, it is a premise of special relativity that the view from within a moving vehicle is equivalent to all other views.

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