Background

In the 19th century, physicists developed various theories regarding the nature of light. One physicist, James Clerk Maxwell, developed a set of four equations which became the foundation of electromagnetism. His equations predicted that light is an electromagnetic wave that propagates at a speed of 3.0 × 10⁸ m s^-1 in a vacuum. However, the strange part which baffled physicists at the time was that the equations also implied that the speed of light was constant and independent of the motion of the source or the observer.

This was contrary to classical physics and Galilean relativity, which suggested that the speed of light should depend on the relative motion of the source and the observer. For example, if a star is emitting light at a speed of 3.0 × 10⁸ m s^-1 and a spaceship is moving towards the star at 1.0 × 10⁸ m s^-1, then according to classical physics, the speed of the light emitted by the star relative to an observer in the spaceship should be 4.0 × 10⁸ m s^-1. However, according to Maxwell’s equations, the speed of light when measured from the spaceship would still be 3.0 × 10⁸ m s^-1.

To reconcile this, most physicists believed that the constant speed of light predicted by Maxwell's equations was the speed of light relative to a medium. They called this hypothetical medium of light the “aether”. This idea arose from the fact that mechanical waves such as sound, earthquake and water waves require a medium for propagation, so they believed that light waves must also require a medium. Due to light's ability to travel through a vacuum, they believed that space must be filled with this so-called aether. However, the aether was purely hypothetical and no evidence had been found to support it.

The Michelson-Morley Experiment

If the aether really did exist, Earth must be moving through it. As the Earth moves through the aether, which was assumed to be stationary in space, an “aether wind” would be passing in the opposite direction. It was presumed that light travelling in the direction of Earth’s velocity would be slowed down as it is travelling against the aether wind, so it would have a slower apparent speed (c - v, where v is the velocity of the Earth relative to the aether). Conversely, light travelling opposite to Earth’s motion would be travelling with the aether wind and appear faster (c + v).

In 1887, physicists Albert Michelson and Edward Morley developed an experiment with the aim of measuring the velocity of Earth relative to the aether. Their experiment was based on the idea that light travelling parallel to Earth’s velocity through the aether would be slowed down compared to light travelling perpendicular to Earth’s velocity.

They set up a device called an interferometer, which splits a light beam into two perpendicular paths before recombining them to form an interference pattern.

Here’s how it works: a light source projects a beam of light towards a 45° half-silvered mirror. This mirror splits the light into two beams:

• Beam A travels in the direction of the supposed aether velocity, reflects, and returns to the interferometer.
• Beam B travels in a plane perpendicular to the aether velocity, reflects off another mirror, and returns to the interferometer.

The two beams recombine, creating an interference pattern of bright and dark regions. To test for the presence of the aether, the apparatus is rotated 90 degrees. If the aether existed, the velocities of Beams A and B would change, resulting in a sideways shift in the observed interference pattern, which was the expected result.

However, no shift in the interference pattern was observed when the apparatus was rotated 90 degrees; a null result (i.e. a negative result). This meant the speeds of beams A and B were unchanged when the apparatus was rotated, suggesting that the speed of light was the same in both directions.

Significance

Although the experiment did not directly disprove the aether model, it failed to provide evidence to support the aether’s existence. It suggested that light does not require a medium for propagation, and that the speed of light is constant in all directions. Therefore, it encouraged physicists to explore other theories, notably Einstein’s special theory of relativity, which proposed that:

• The aether does not exist and is not necessary for light to propagate.
• The speed of light is constant, regardless of the observer's motion or reference frame.

Therefore, despite being labelled as the “most famous failed experiment”, the null result of the Michelson-Morley experiment is ultimately significant as it provides evidence in support of Einstein’s theory of special relativity, which greatly influenced modern physics.

VCE Sample Question and Explanation:

Question:

Michelson and Morley devised an experiment with the aim of measuring the velocity of Earth relative to the aether. They observed the fringe pattern at a detector resulting from interference between two light beams, one travelling parallel to Earth’s velocity and the other travelling perpendicular to it. The measurement was then repeated with the apparatus turned through an angle of 90°, and no change was seen in the interference-fringe pattern.

Explain the significance of this null observation. (3 marks)

Answer: found here

Full Animated Explanation of The Experiment: