Simultaneity Help

By — McGraw-Hill Professional
Updated on Sep 12, 2011


There are two aspects to Albert Einstein’s relativity theory: the special theory and the general theory . The special theory involves relative motion, and the general theory involves acceleration and gravitation. However, before we get into relativity, let’s find out what follows from the hypothesis 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 came to believe that such a medium doesn’t exist and that EM waves can travel through a perfect vacuum.

The Luminiferous Ether

In the 1800s, physicists determined that light has wavelike properties and in some ways resembles sound. But light travels much faster than sound. However, light can travel through a vacuum, whereas sound cannot. Sound waves require a material medium such as air, water, or metal to propagate. Most scientists thought light also must require some sort of medium, but what? What could exist everywhere, even in a jar from which all the air was pumped out? This mysterious medium was called luminiferous ether , or simply ether . It turned out to be nothing but a figment of the imagination.

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, and 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 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 seemed to be too great a coincidence. Attempts were made to explain away this result by suggesting that the Earth drags the ether along with itself. Einstein could not accept that. He took the results of the Michelson-Morley experiment at face value. 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.

The Speed Of Light Is Constant

Einstein rejected the notion of luminiferous ether. Instead, he proposed an axiom: In a vacuum, the speed at which light, or any other EM field, travels is an absolute constant. This is the case regardless of the motion of the observer with respect to the source. (In media other than a vacuum, such as glass, this axiom does not apply.) Armed with this axiom, Einstein set out to deduce what logically follows.

Einstein did all his work by using a combination of mathematics and daydreaming that he called “mind journeys.” He wasn’t an experimentalist but a theorist. There is a saying in physics: “One experimentalist can keep a dozen theorists busy.” Einstein turned that inside out. His theories have kept thousands of experimentalists occupied.

Practice problems of these concepts can be found at: Relativity Theory Practice Test

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