The Michelson-Morley Experiment

Introduction

  • In 1887 two physicists named A.A.Michelson and E.W.Morley created an experiment in search of what was named the "Aether Wind."
  • Assuming aether permeated all matter, the Earth's rotation would cause a relative movement between the Aether and the surface of the Earth. This relative moment is known as the Aether Wind.
  • The experiment works on a concept that can be explained with the analogy of two boats travelling the same distance at the same speed on a river with a current.
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  • Assume that the boats are going at 5km/h, the distance they travel is 2km and the current is 3km/h
    • For Boat A: It has to battle the current directly, and its speed is reduced to 2km/h, and it takes 1 hour to complete the trip.
    • For Boat B: While it has to battle current, it does not have to as much as Boat A. If Pythagoras' theorem is used, it is found that its speed vertically is 4km/h. Thus it takes 30 minutes to complete its trip.
    • For return trips, Boat A is helped by the current with a speed of 8km/h, and Boat B maintains the speed of 4km/h. Ultimately, they take different amounts of time to go forward and back.
  • From this, it can be seen that depending on the direction one travels with a current, its speed will change.
  • When applied to light, the existence of the Aether Wind implied that light travelling one direction will be at a different speed in the other direction, similar to the current of a river.

The experiment

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  • Michelson and Morley set up an experiment to find this relative speed using this concept in an "interferometer"
    • A beam of light is shot from S into M, a half-silver mirror that splits up the ray into 2 perpendicular directions.
    • Assuming the Aether wind is travelling in v's direction:
    • For Beam 1: this is similar to Boat A, where it battles wind on the way there, and when bounced off full-mirror M1, flows with wind.
    • For Beam 2: this is Boat B's case, moving diagonally with the wind.
    • When the two beams are reflected into telescope T, due to their different speeds, they create an interference pattern.
    • This apparatus was then rotated 90 degrees, which would create a different interference pattern due to Aether Wind's constant direction.
  • However, they discovered no difference in the interference patterns, despite the extreme sensitivity and many, many attempts.
    • There was one attempt where they put the apparatus on a cement block floated in goddamned Mercury.

Results and Implications

  • Michelson and Morley received a null result as they could not determine the speed of the Aether Wind.
  • Scientists of the time were initiall baffled by this null result, but it only created doubt in the aether model.
  • Some suggestions were made to adapt this lack of a result, but all were un-experimentable.
  • Until Einstein and Maxwell's theories in 1905, the Aether model remained used, but was waning in popularity.
    • Einstein used the MM experiment as proof for his competing theory.
  • Eventually the theory was phased out since no one used it any more.
  • Thus it can be said that the MM experiment casted doubt onto the Aether model and allowed competing and possible correct models to be recognised.