Michelson-Morley Experiment & Spectra of Binary Stars

This is part of the HSC Physics course under the topic Light and Special Relativity.

HSC Physics Syllabus

• analyse and evaluate the evidence confirming or denying Einstein’s two postulates:
– the speed of light in a vacuum is an absolute constant
– all inertial frames of reference are equivalent (ACSPH131)

Michelson-Morley Experiment & Spectra from Binary Stars

This video discusses evidence confirming Einstein's two postulates. The Michelson-Morley experiment and spectra from binary star systems both confirm the constancy of light's speed.

The Aether Model

In the 1800s, since all transverse waves were known to propagate through a solid medium, many experiments were conducted to investigate and verify the existence of a solid medium for light. The most popular model of this medium is the luminiferous aether which was theorised to have the following properties:

• stationary in space
• fills all of space and permeates all of matter, allowing light to travel anywhere
• perfectly transparent and thus invisible to the eye
• very low density, allowing objects to move through it freely
• great elasticity, meaning that instantaneous changes (e.g. light's fast oscillations) made the aether lock up and act as a solid whilst slow changes (e.g. planetary movement) made it act as a fluid. This is analogous to the  properties of oobleck.

At the time, Newtonian mechanics was widely accepted to be universally correct. As such, scientists used Newtonian relative motion to theorise that Earth's movement through the aether would cause the aether to move in the opposite direction with respect to an observer on Earth.

This is analogous to you feeling the wind when you stick your hand out the window in a moving car, hence why it is termed the aether wind. It was thought that light would 'feel' this wind and change in speed (which we now know to be incorrect under Einstein's second postulate).

The Michelson-Morley Experiment

In 1887, the Michelson-Morley experiment set out to measure the velocity of the Earth through the aether by detecting the ‘aether wind’ (assuming the aether existed). The aim was not to verify the existence of the aether.

Equipment

• Source of coherent (in-phase) light allows any phase differences and thus interference pattern to be only due to the aether wind.
• A collimator to thin the light source into a beam.
• Half-silvered mirror, whilst allows 50% of the light to pass through and reflects the other 50%, creating two beams.
• Two mirrors to reflect the two beams.
• A microscope to observe the interference pattern
• Sandstone block to minimise vibrations which affect the sensitive interference pattern.
• A trough of mercury to allow for free rotation of the sandstone block. Also minimises vibrations.

Method:

• A beam of light from a monochromatic light source was directed at the 45° half-silvered mirror (Ms) which splits it into two perpendicular beams.
• One beam travels to mirror M1 and the other to mirror M2 .
• The beams are reflected by M1 and M2 and rejoin after passing through Ms .
• It will produce an interference pattern due to the phase differences in the time taken for the seperate light beams to travel their separate paths. This time difference would be due to the aether wind, slowing down or speeding up one or both of the light beams.
• They rotated it the sandstone block around, expecting the interference fringes to move side to side.

Results

The experiment was repeated many times by Michelson and Morley, at different times of the day and year, no change in the interference pattern was observed, constituting a null result.

Conclusions

The experiment was negative (null result) and it was impossible to measure the absolute velocity of the Earth in the aether wind.

There were a few plausible explanations of the null result including:

• Aether had no effect on light's velocity
• Orbital motion of Earth dragged aether with it, causing there to be little to no aether wind.

This experiment did not prove that aether wind did not exist. It only failed to provide evidence for the existence of aether. This is a very important distinction.

Implications of Michelson-Morley's Experiment

The null result of this experiment shone light onto the flaws of the aether model. Aether model supporters were encouraged to develop new explanations to account for the nulls results. It also questioned whether the aether really did exist.

When Einstein established his theory of special relativity, he was able to use it to derive the results of experiments such as the Michelson-Morley experiment without an ad-hoc hypothesis. The ability to account for more observations than the aether model made also did not disprove the aether, but simply made it unnecessary.

As such, the results of the Michelson-Morley experiment serve as retrospective evidence for Einstein's special relativity, aiding its acceptance whilst helping rejecting the aether theory.

Spectra of Light from Binary Stars

A binary star system consists from two stars in orbital motion about their centre mass.

The emission theory suggested that the velocity of light emitted from each of the two stars depended on their orbital velocity. Light emitted from a star that is moving towards Earth would travel faster (c + v). Light emitted from a star that is receding from Earth would travel slower (c – v).

This meant that light emitted from an approaching star will catch up to light emitted from the receding star from an earlier time.

Since the observation of binary star systems depended on light emitted from these stars, the simultaneous arrival of light rays emitted from different times will result in a distorted image. This distort image would in turn imply that the binary stars are in a non-Keplerian orbit i.e. they do not obey Kepler's laws.

However, no binary star systems have been observed to be non-Keplerian. This suggests the emission theory is incorrect, and that the speed of light is not affected by the orbital velocity of binary stars.

This observation is consistent and supports the postulates of special relativity.

Previous section: Special Relativity – Einstein's Postulates

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