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# Relativity of Simultaneity Help

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By — McGraw-Hill Professional
Updated on Sep 18, 2011

## Relativity of Simultaneity

Relativity theory has a reputation as something only geniuses can understand. However, the basics of relativity are no more difficult to grasp than the fundamentals of any other theory. Some of the ideas put forth to explain astronomical observations before Einstein came up with his theory were more esoteric than relativity itself. This chapter contains a little bit of mathematics, but it doesn’t go beyond the middle-school level.

There are two aspects to relativity theory: the special theory and the general theory . The special theory involves relative motion, and the general theory involves acceleration and gravitation.

Before we get into relativity, let’s find out what follows from the hypotheses that the speed of light is absolute, constant, and finite and that it is the highest speed anything can attain.

When he became interested in light, space, and time, Einstein pondered the results of experiments intended to find out how the Earth moves relative to the supposed medium that carries electromagnetic (EM) waves such as visible light. Einstein decided that such a medium doesn’t exist—EM waves can travel through a perfect vacuum.

## The Ether

Before Einstein’s time, physicists determined that light has wavelike properties and in some ways resembles sound. However, light travels much faster than sound. Also, light can propagate through a vacuum, whereas sound cannot. Sound waves require a material medium such as air, water, or metal to get from one place to another. Many scientists suspected that light also must need some sort of medium to travel through space. What could exist everywhere, even between the stars and galaxies, and even in a jar from which all the air was pumped out? This mysterious medium was called luminiferous ether , or simply ether .

If the ether exists, some scientists wondered, how could it pass right through everything, even the entire Earth, and get inside an evacuated chamber? How could the ether be detected? One idea was to see if the ether “blows” against the Earth as our planet orbits around the Sun, as the Solar System orbits around the center of the Milky Way galaxy, and as our galaxy drifts through the Cosmos. If there is an “ether wind,” then the speed of light ought to be different in different directions. This, it was reasoned, should occur for the same reason that a passenger on a fast-moving truck measures the speed of sound waves coming from the front as faster than the speed of sound waves coming from behind.

In 1887, an experiment was done by two physicists named Albert Michelson and Edward Morley in an attempt to find out how fast the “ether wind” is blowing and from what direction. The Michelson-Morley experiment , as it became known, showed that the speed of light is the same in all directions. This cast doubt on the ether theory. If the ether exists, then according to the results obtained by Michelson and Morley, it must be moving right along with the Earth. This is quite a coincidence; it implies that the Earth is stationary relative to absolute space, and no one believed that in 1887. Attempts were made to explain away this result by suggesting that the Earth drags the ether along with itself. Einstein could not believe this. He decided that the results of the Michelson-Morley experiment had to be taken at face value: The speed of light is constant in every direction. Einstein believed that the Michelson-Morley experiment would have the same outcome for observers on the Moon, on any other planet, on a space ship, or anywhere in the Universe.

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