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# Special Theory of Relativity

## Interpret motion between different objects that are moving at constant speeds relative to each other.

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Looking for the Aether

### Looking for the Aether

Credit: Benjamin D. Esham
Source: http://commons.wikimedia.org/wiki/File:Michelson-Morley_experiment_(en).svg

Thought to be the medium for electromagnetic and gravitational forces, the aether theory was disproven by the well known Michelson – Morley experiment.

#### Amazing But True

• For many years, physicists proposed the existence of a medium call the aether. This medium was initially thought to be necessary as a medium for the propagation of electromagnetic and gravitational forces. It wasn't until the late 1800s when the Michelson-Morley experiment was performed was the idea for the aether finally rejected.
• The Michelson-Morley experiment was done through the use of a beam separator and an interferometer. By taking into account that if an aether frame existed, motion relative to the frame would be easily to detect since the Earth is moving relative to this frame.
• The experiment was performed by splitting a beam of light such that their paths were perpendicular to one another. The beams were then reunited after reflecting them off of another mirror. When the beams reunited, an interference pattern was formed. Since the pattern is sensitive to any differences in the speed of light, it would be relatively easy to detect any differences in the velocity of the two beams. When Michelson and Morley viewed the results, they were able to see a very small shift in the phases of the two light beams, but the shift was so small that it fell within the error of the experiment. Their experiment showed that light travels at same speed in an Earth based reference frame, disproving the aether theory.

#### Show What You Know

Using the information provided above, answer the following questions.

1. If the aether did exist, what would have been the change in the experiment described?
2. How is this in agreement with Einstein’s postulate of special relativity?

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