Einstein's Special Relativity
Time Dilation, Lorentz Contraction, and Other Relativity Effects
Mar 15, 2007
Why special relativity?
The Michelson-Morley experiment performed in the late 19th century led to special relativity. Using a device known as a Michelson interferometer, Michelson and Morley were trying to measure the difference in the speed of light for a light beams traveling parallel and perpendicular to the direction of Earth's travel around the Sun. They expected the difference to be Earth's orbital speed. The experiment did not work as expected. They were not able to measure a difference in the speed a light beam traveled in the two directions. Because their negative result led to the Special Theory of Relativity, the Michelson-Morley experiment has been called the most significant negative experiment in the history of science.
Einstein's assumption
Rather than trying to understand why the Michelson-Morley experiment didn't work, Einstein effectively took the result as his starting point. He made the basic assumption that the speed of light is a fundamental constant in the universe and that all observers in any reference frame that is not accelerating will measure the same value for the speed of light. There is however historical controversy as to how much Einstein was actually influenced by the Michelson-Morley experiment. He might have made this assumption, even if the experiment had not been performed. In either case, his assumption that any observer moving at any constant velocity will measure the same value for the speed of light led to special relativity. Basically if the speed of light can't change for different observers moving at different speeds, some other things, such as length and time, must change. Einstein found a number of surprising consequences to this assumption.
Special relativity theory
Einstein published his Special Theory of Relativity in 1905. Some of the major points are:
- Space-time - Space and time are fundamentally interrelated rather than two distinctly different quantities. Time is essentially a fourth dimension complementing the three spatial dimensions.
- Simultaneous events - Whether or not two events are simultaneous depends on the observer. One observer might see two events as occurring simultaneously, another as one of the events occurring first, and a third as the other event occurring first.
- Lorentz contraction - An object moving near the speed of light will appear shorter as seen by an outside observer at rest. The amount of contraction depends on its speed, and its length approaches zero as its speed approaches the speed of light. To an observer moving along with the object its length appears normal.
- Time dilation - As seen by an outside observer at rest, time will move more slowly for an object moving close to the speed of light. At the speed of light, time will stop as seen by an outside observer. To an observer moving along with the object all appears normal
- Mass increase - The mass of an object moving close to the speed of light will increase as seen by an outside observer. The mass will approach infinity as the speed approaches the speed of light. Again to an observer moving along with the object the mass remains the same.
- Speed of light limit - As an objects speed approaches the speed of light its mass approaches infinity. Therefore it would take an infinite external force to accelerate any object with mass to the speed of light. Therefore light, and anything else with no mass, can travel at the speed of light. But an object with mass can not reach the speed of light. The best it can do is come arbitrarily close. Nothing can travel faster than the speed light travels in a vacuum. The speed of light in a vacuum is the ultimate speed limit in the universe.
- E=mc2 - E represents energy, m represents mass, and c2 represents the speed of light squared. According to this famous equation mass and energy are interchangeable. Matter can change to energy and vice versa. The equations is sort of a conversion factor telling us how much matter corresponds to a certain amount of energy. For example, in nuclear reactions some of the mass is converted to energy according to this equation.
These relativistic effects seem strange to us because they only become significant at speeds of at least 10% the speed of light, which is 300,000 km/s. Because we have never traveled anywhere close to these speeds, we have never experienced these effects. However experiments in which subatomic particles are accelerated to these high speeds have so far confirmed all predictions Einstein's from special relativity theory. About a decade later, Einstein published his general theory of relativity.
