Chapter 2: Michelson-Morley Experiment
The 1887 Michelson-Morley experiment is one of the most famous "failed" experiments in physics. Its null resultāfinding no evidence for Earth's motion through the aetherāpaved the way for Einstein's special relativity.
The Luminiferous Aether
In the 19th century, light was known to be a wave. All known waves need a medium to propagate: sound through air, water waves through water. What medium did light waves travel through?
Scientists hypothesized the luminiferous aetherāan invisible substance permeating all space. Light waves were vibrations in this aether, and the speed of light (c) was the wave's speed relative to the aether.
Required Properties of the Aether
- ā¢ Extremely rigid (to support high-frequency oscillations)
- ā¢ Perfectly transparent and massless
- ā¢ Permeating all matter without friction
- ā¢ Stationary in absolute space
If the aether existed, Earth moving through it would create an "aether wind." Just as you feel wind when cycling, the aether wind should affect the measured speed of light.
The Interferometer
Albert Michelson and Edward Morley used an interferometer to detect Earth's motion through the aether. The device split a light beam into two perpendicular paths, then recombined them to create an interference pattern.
How the Experiment Works
- 1. A light source sends a beam to a half-silvered mirror (beam splitter)
- 2. The beam splits: one arm parallel to Earth's motion, one perpendicular
- 3. Each beam reflects off a mirror and returns to the beam splitter
- 4. The recombined beams create an interference pattern
- 5. The apparatus is rotated 90Ā° and the pattern is observed again
If there's an aether wind, the beam traveling parallel to Earth's motion should take longer than the perpendicular beam. Rotating the apparatus should shift the interference fringes.
Expected vs. Actual Result
Earth orbits the Sun at about 30 km/s. Even if the aether were at rest relative to the Sun, this orbital velocity should produce a measurable effect.
Expected
A fringe shift of about 0.4 fringes when rotating the apparatus 90Ā°. The Michelson interferometer was sensitive enough to detect shifts of 0.01 fringes.
Actual Result
Null result! No significant fringe shift was observed. The experiment was repeated at different times of year, orientations, and elevationsāalways null.
The "aether wind" could not be detected. Either there was no aether, or something very strange was happening to rulers and clocks in motion.
Attempts to Save the Aether
Aether Drag (Stokes)
Perhaps Earth drags the aether along with it? But stellar aberration observations contradicted this.
Length Contraction (Lorentz-FitzGerald)
Lorentz and FitzGerald independently proposed that objects contract in their direction of motion by a factor \( \sqrt{1 - v^2/c^2} \). This would exactly cancel the expected time difference!
Local Time (Lorentz)
Lorentz introduced a "local time" that transformed oddly between frames. He didn't realize its physical significanceāthat was Einstein's breakthrough.
These ad hoc fixes explained the null result, but they felt contrived. Why would nature conspire to hide the aether? Einstein would provide the revolutionary answer.
Historical Significance
The Michelson-Morley experiment is often called "the most famous failed experiment in history." Its significance:
- ā¢Demonstrated that the speed of light is the same in all directions
- ā¢Cast serious doubt on the aether hypothesis
- ā¢Provided key evidence that Einstein used to develop special relativity
- ā¢Michelson won the 1907 Nobel Prize for his optical precision instruments
Einstein's radical step: Rather than inventing reasons why the aether was undetectable, Einstein simply abandoned it. The speed of light is constant for all observersānot because of some conspiracy of nature, but because that's how spacetime actually works.