Michelson-Morley experiment

Albert Abraham Michelson was an American physicist known for his work on the measurement of the speed of light and especially for the Michelson–Morley experiment. In 1907 he received the Nobel Prize in Physics. Born: December 19, 1852, Strzelno, PolandBornStrzelno, Poland Died: May 9, 1931, Pasadena, CADiedPasadena, CA

Edward Williams Morley was an American scientist famous for his path-breaking measurements of the atomic weight of oxygen, and for the Michelson–Morley experiment. Born: January 29, 1838, Newark, NJBornNewark, NJ Died: February 24, 1923, West Hartford, CTDiedWest Hartford, CT

Michelson’s 1878 Rotating Mirror Experiment Picture credit German American physicist A.A. Michelson improved a device previously used by Focault to measure the speed of light Instead of Foucault's 60 feet to the far mirror, Michelson used 2,000 feet.. Using this method, Michelson was able to calculate c = 299,792 km/s This was accepted as the most accurate measurement of c for the next 40 years.

A step back Since Newton, the scientists thought that the light needed a medium to propagate. Such medium filled up the interstellar space and was called Aether.

The waves need a medium to propagate and after Newton it was commonly accepted that light is a wave … Right?

So the Aether was the medium where light propagates. But what is exactly the aether? Naturally, something that allows solid bodies to pass through it freely is a little hard to get a grip on. The structure of the matter was being understood, but still the scientists had no clue about what stuff the aether was made of. Although hypotheses of the aether vary somewhat in detail they all have certain characteristics in common. In 19th century theories, aether is considered to be a physical medium composed of particles with a very small mass occupying every point in space, including within material bodies.

The movement of the planets through the aether had to create a disturbance in this medium; such disturbance was called “aether wind”. The earth rotates around the sun at a speed of 67,000 miles per hour (about 100,000 km/hr, so it was supposed to create a noticeable disturbance in the aether, … right?

Back to Michelson (1887) Detecting the aether wind was the next challenge Michelson set himself after his triumph in measuring the speed of light so accurately. But Michelson realized that, just as the speed of sound is relative to the air, so the speed of light must be relative to the aether. This must mean, if you could measure the speed of light accurately enough, you could measure the speed of light travelling upwind, and compare it with the speed of light travelling downwind, and the difference of the two measurements should be twice the speed of the wind.

Unfortunately, it wasn’t that easy. All the recent accurate measurements had used light travelling to a distant mirror and coming back, so if there was an aether wind along the direction between the mirrors, it would have opposite effects on the two parts of the measurement, leaving a very small overall effect. A good measure of the speed of the light and the effect of the aether wind had do be done with a small instrument in the lab.

At this point, Michelson had a very clever idea for detecting the aether wind. As he explained to his children (according to his daughter), it was based on the following puzzle: Suppose we have a river of width 100 feet, and two swimmers who both swim at the same speed of 5 feet per second. The river is flowing at a steady rate of 3 feet per second. The swimmers race in the following way: they both start at the same point on one bank. One swims directly across the river to the closest point on the opposite bank, then turns around and swims back. The other stays on one side of the river, swimming upstream a distance (measured along the bank) exactly equal to the width of the river, then swims back to the start. Who wins?

The swimmer who swims along the bank of the river is easy to understand: When he swims upstream he has to fight the current so he advances with a speed of 2 ft/sec. When he swims downstream he is helped by the current so he advances with a speed of 8 ft/sec. He takes a total time of 100/2+ 100/8 =62.5 sec. To complete his task. The swimmer going across the flow is trickier. It won’t do simply to aim directly for the opposite bank- the current will carry the swimmer downstream.

To succeed in going directly across, the swimmer must actually aim upstream at the correct angle (of course, a real swimmer would do this automatically). Thus, the swimmer is going at 5 feet per second, at an angle, relative to the river, and being carried downstream at a rate of 3 feet per second. If the angle is correctly chosen so that the net movement is directly across, in one second the swimmer must have moved four feet across: the distances covered in one second will form a 3,4,5 triangle. So, at a crossing rate of 4 feet per second, the swimmer gets across in 25 seconds, and back in the same time, for a total time of 50 seconds. The cross-stream swimmer wins. This turns out to true whatever their swimming speed. (Of course, the race is only possible if they can swim faster than the current!)

Michelson’s great idea was to construct an exactly similar race for pulses of light, with the aether wind playing the part of the river. The scheme of the experiment is as follows: a pulse of light is directed at an angle of 45 degrees at a half-silvered, half transparent mirror, so that half the pulse goes on through the glass, half is reflected. These two half-pulses are the two swimmers. They both go on to distant mirrors which reflect them back to the half-silvered mirror.

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At this point, they are again half reflected and half transmitted, but a telescope is placed behind the half- silvered mirror as shown in the figure so that half of each half-pulse will arrive in this telescope. Now, if there is an aether wind blowing, someone looking through the telescope should see the halves of the two half-pulses to arrive at slightly different times, since one would have gone more upstream and back, one more across stream in general. To maximize the effect, the whole apparatus, including the distant mirrors, was placed on a large turntable so it could be swung around.

But … (after the Mechanical universe documentary….) The experiment showed clearly that the aether wind does not exist. Because of that, Michelson and Morley believed that their experiment was a failure! More importantly, the experiment shows that the speed of the light is not influenced by the motion of earth. This is a very important conclusion that opened the doors to the theory of relativity.