Reading: The Theory of Special Relativity

The speed of light, represented by the symbol c, is 300 000 000 m/s (meters per second) in a vacuum. If light could travel around Earth’s equator, it would make over 7 trips each second! The very great speed of light makes it dificult to measure changes in the speed of light caused by motion of frames of reference that are familiar to you on Earth.

In the early part of the 20th century, Einstein showed that light does not obey the laws of speed addition that we have seen in objects on Earth’s surface. His theory predicted that light traveled at the same speed in all frames of reference, no matter how fast the frames were moving relative to one another.

As a young clerk in the Swiss patent office, Albert Einstein postulated that the speed of light in a vacuum is the same for all observers. Einstein recognized that light and other forms of electromagnetic radiation (including x – rays, microwaves, and ultraviolet waves) could not be made to agree with the laws of relative motion seen on Earth. Einstein modified the ideas of relativity to agree with the theory of electromagnetic radiation. When he did, he uncovered consequences that have changed the outlook of not only physics but the world.

The ideas of Einstein’s Theory of Special Relativity are stated in two postulates:

  • The laws of physics are the same in all inertial frames of reference. (Remember that inertial frames of reference are those in which Newton’s First Law of Motion holds. This automatically eliminates frames of reference that are accelerating.)
  • The speed of light is a constant in all inertial frames of reference.

The first postulate adds electromagnetism to the frames of reference discussed. Its implications become clear when you begin to ask questions. Is the classroom moving or standing still? How do you know? Remember that an observer in an inertial frame of reference is sure that he or she is standing still. An observer in an airplane would be convinced that he or she is standing still and that your classroom is moving. The meaning of the first postulate is that there is no experiment you can do that will tell you who is really moving.

The second postulate, however, produces results that seem to defy common sense. You can add speeds of objects in inertial frames of reference. But you cannot add the speed of light in motion of an inertial frame of reference.

Physicists say that two events are simultaneous if a light signal from each event reaches an observer standing halfway between them at the same instant. These postulates lead to the idea that simultaneity depends on the observer. You cannot say whether two events in different places occurred at the same time unless you know the position of the observer.

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