The Michelson-Morley experiment (A-level only)

What you'll learn:

• What the "luminiferous ether" was and why physicists believed it existed.
• How the Michelson-Morley interferometer splits and recombines light.
• Why the experiment's famous "null result" proved that absolute motion cannot be detected.
• How this led directly to the revolutionary idea that the speed of light is invariant.

The Search for the Ether

In the late 19th century, physicists had a problem. They knew that light was a wave, and all the waves they had studied so far required a physical medium to travel through. Sound travels through air, water waves travel through water, and seismic waves travel through rock.

Because light can travel from the Sun to the Earth through the vacuum of space, scientists proposed the existence of the luminiferous ether (or simply, the ether). They believed the ether was a massless, rigid substance that filled the entire universe and acted as the medium for light waves.

If the ether filled all of space, it was considered an "absolute" frame of reference. Because the Earth orbits the Sun, it must be hurtling through this stationary ether. This motion should create an "ether wind", much like how you feel a wind against your face when driving an open-top car on a still day.

The Michelson-Morley experiment was designed to detect this ether wind, thereby proving the existence of the ether and demonstrating the Earth's absolute motion through space.

The Principle of the Interferometer

To detect the ether wind, Albert Michelson and Edward Morley built a highly sensitive device called an interferometer.

Here is how the apparatus works:

1. A beam of monochromatic light is directed at a half-silvered mirror (acting as a beam splitter), angled at 45 degrees.
2. Half of the light passes straight through to a plane mirror ($M_1$), while the other half is reflected at 90 degrees to a second plane mirror ($M_2$).
3. Both mirrors are set at exactly the same distance, $L$, from the beam splitter.
4. The mirrors reflect the light back to the beam splitter, where the two beams recombine.
5. The recombined beam is directed into a viewing telescope.

Because the two beams have travelled different paths, they will undergo superposition when they recombine, creating an interference pattern of alternating light and dark fringes in the telescope.

According to the ether theory, the beam of light travelling parallel to the ether wind should take a slightly different amount of time to return than the beam travelling perpendicular to it.

Rotating the Apparatus

Michelson and Morley floated their heavy stone apparatus on a pool of liquid mercury so it could be smoothly rotated.

The plan was simple:

1. Observe the interference fringes in the telescope.
2. Slowly rotate the entire apparatus through 90 degrees.
3. As the apparatus rotates, the path that was parallel to the ether wind becomes perpendicular, and vice versa.
4. This swap in travel times should cause the beams to arrive slightly more or less out of phase than before, resulting in a fringe shift in the telescope.

Based on the known speed of the Earth in its orbit, they calculated that they should see a shift of about $0.4$ of a fringe width.

The Null Result

When Michelson and Morley performed the experiment, they observed no significant fringe shift whatsoever.

No matter what time of day they ran it, or what season of the year (as the Earth changed direction in its orbit), the interference pattern never shifted. This failure to observe the expected outcome is known in physics as a null result.

Significance of the Failure to Detect Absolute Motion

The null result was a shock to the physics community. It had profound implications:

1. The ether does not exist. Light does not need a physical medium to travel through space.
2. Absolute motion cannot be detected. There is no "stationary background" to the universe against which you can measure your speed.

The Invariance of the Speed of Light

If the speed of light were affected by the Earth's motion, the travel times in the interferometer would have been different. Since the travel times were identical regardless of orientation, this meant the light was travelling at the exact same speed along both paths.

This led directly to a radical new idea, which Albert Einstein later formalized as the second postulate of Special Relativity:

If you shine a torch from a stationary car, the light leaves you at $c$ (approximately $3\times 10^8{m\; s}^{-1}$). If you shine the torch forwards from a rocket travelling at $0.9c$, the light still leaves you at $c$, and an observer on the ground will also measure that light travelling at exactly $c$!