In November 1887, American physicists Albert Abraham Michelson (1852–1931) and Edward Williams Morley (1838–1923) published their historic paper in the American Journal of Science (Vol. 34) titled “On the Relative Motion of the Earth and the Luminiferous Ether”.

The Michelson-Morley Experiment delivered the most famous null result in physical science, proving that the speed of light is constant in all inertial reference frames and disproving the existence of a stationary luminiferous ether.

High-Precision Floating Interferometer Setup

Michelson and Morley built an ultra-sensitive optical interferometer mounted on a heavy 1.5-meter square sandstone slab floating in a annular pool of liquid mercury:

Experimental Sensitivity

  • Optical Path Length: Used multiple reflection mirrors to extend the optical arm length to $L = 11\,\text{meters}$.
  • Expected Ether Shift: Moving through stationary ether at Earth’s orbital speed ($v = 30\,\text{km/s}$) predicted an optical fringe shift of:

$$\Delta N = 2 \frac{L}{\lambda} \frac{v^2}{c^2} \approx 0.4\,\text{fringes}$$

The Null Result

Michelson and Morley rotated the floating slab through $360^\circ$ at all hours of the day and seasons of the year. The observed fringe shift was less than $0.005$ fringes—far below the predicted $0.4$ shift!

Catalyst for Special Relativity (1905)

The Michelson-Morley null result baffled classical physics, inspiring George FitzGerald (1889) and Hendrik Lorentz (1892) to propose length contraction ($\gamma = 1/\sqrt{1 - v^2/c^2}$).

In 1905, Albert Einstein resolved the mystery by formulating Special Relativity, postulating that the speed of light in vacuum ($c$) is an invariant universal constant for all observers regardless of motion.

Key Takeaways

  • Year: 1887
  • Key Figures: Albert A. Michelson & Edward W. Morley
  • Core Discovery: Executed the Michelson-Morley Experiment, obtaining a null result for ether drift ($v < 5\,\text{km/s}$).
  • Constancy of $c$: Proved light speed is invariant across all inertial frames.
  • Quantum Relevance: Established the relativistic spacetime background for Special Relativity and quantum field theory.