The Michelson-Morley Interferometer (AQA A Level Physics)

What was the Luminiferous Aether?

• The Dutch physicist Christiaan Huygens had developed the wave theory of light
  • All other known waves at the time (sound and water) travelled through a medium, so physicists assumed light did too
  • Huygens called this medium for light waves the "luminiferous aether" (or just aether) and physicists wanted to study its properties

What was the Aim of the Michelson-Morley Experiment?

• In 1849, Hippolyte Fizeau measured the speed of light in moving water
  • One beam of light travelled with the current and the other travelled against the current of the water
  • Through the interference between the two beams, he found that light moving in the same direction as the medium travelled faster than light in a direction opposing the motion of the medium
  • Interestingly, the speed decrease when travelling against the medium was greater than the speed increase when travelling with the medium
• In the 1880s, Michelson and Morley wished to use a similar method to prove the existence of the aether
  • If the aether existed, then the Earth was travelling through it
• Light travelling in the direction of the Earth's motion would be travelling against the aether
  • The motion of the aether against the light was called the aether wind 
  • Like with water, if light was travelling into the aether wind, it would be travelling more slowly than light travelling perpendicular to the aether wind

Theory of motion of light relative to the "aether wind" as a result of the Earth's motion through it

Light travelling into the aether wind was predicted to travel a small fraction slower than light travelling perpendicular to the aether wind

• The aim of the Michelson-Morley experiment was to use this difference in speeds of light to determine the absolute motion of the Earth relative to the aether
  • Absolute motion refers to the idea that the motion of all bodies in the Universe could be measured relative to the aether
  • The phase difference in the two beams of light can be used to determine the Earth's motion relative to the aether

What was the Michelson-Morley Interferometer?

• Michelson and Morley created a device called an interferometer
  • This consisted of two arms of identical lengths at right angles to each other, with mirrors at each end and a beam splitter (a semi-silvered mirror which allows some light to pass through and reflects some light) at their intersection

A diagram showing the arrangement of the Michelson-Morley Interferometer

Upon reaching the beam splitter, some light was reflected and some was transmitted, forming two beams that where initially coherent. The plane of glass ensured both beams travelled through the same distances of glass and air.

• White light travelled from a source to the beam splitter, where some travelled along path AB and some travelled along path AC
  • For the reflected beam, a plane of glass was placed in its path to ensure both beams travel through the same amount of air and the same amount of glass
  • Both beams are reflected by mirrors at B and C and meet at an eyepiece
• Both beams are from the same source and are therefore coherent
  • This means they will form an interference pattern
• The Michelson-Morley interferometer was set up to float on a bath of mercury
  • This allowed it to be rotated with minimal friction
• An interference pattern would be observed with one beam of light being slowed by the aether wind
  • Rotating the interferometer would then affect the phase difference of the beams differently, causing a phase shift in the interference pattern
  • The predicted shift was 0.4x the width of one fringe in the interference pattern, so the equipment was designed to detect changes of 0.01 fringe widths

The Results

• Michelson and Morley performed the experiment at different angles and at different times of the day (so the Earth had also rotated relative to the aether)
  • However, their results only ever showed displacements of the interference project around 0.02 fringe widths and not even in the expected orientations
  • These values were far too small to be significant and most likely experimental noise
• This null result led scientists to the following conclusions:
  • The aether does not exist and therefore light is a wave able to travel without a medium
  • The speed of light is unchanged by the Earth's motion - it is invariant
• If there is no medium from which to measure the motion of the Earth, there is no absolute motion - everything is moving with respect to everything else