The Real Crisis That Led to Relativity

Electromagnetism & Relativity

Course 2 · Chapter 7 · The Real Crisis That Led to Relativity

Chapter 6 established that light is an electromagnetic wave. But every other wave 19th-century physicists knew of needed a medium to travel through — water waves need water, sound needs air. What, then, was light waving through? This chapter covers the real experiment that broke the answer physicists had settled on, and the real, more complicated story of what came after.

The Luminiferous Ether

Physicists proposed that space itself was filled with an invisible medium — the "luminiferous ether" — whose vibrations light waves consisted of. The real, honest problem was that the ether needed genuinely contradictory properties to work at all: it had to be thin and frictionless enough that planets and everyday objects moved through it with no detectable drag whatsoever, yet simultaneously rigid enough to support waves propagating at Maxwell's own real 3×10&sup8; m/s — a combination no known real substance could plausibly satisfy.

The Michelson-Morley Experiment (1887)

American physicists Albert A. Michelson and Edward W. Morley set out, between April and July 1887 at what is now Case Western Reserve University in Cleveland, Ohio, to directly detect Earth's own motion through this proposed ether. Their reasoning: as Earth orbits the Sun, it should move through the stationary ether, creating a real, detectable "ether wind" — light travelling with or against this wind should take a measurably different time than light travelling across it.

Their real apparatus, a Michelson interferometer, split a single light beam into two perpendicular paths, each 11 m long, then recombined them to produce an interference pattern — sensitive enough to detect the tiny predicted timing difference. To eliminate vibration, the entire apparatus floated on a pool of mercury.

⚠ A Real, Genuinely Famous Failure Michelson and Morley expected a fringe shift of roughly 0.4 fringes as the apparatus rotated relative to the predicted ether wind — easily large enough to measure with their real instrument. What they actually measured was a shift of 0.02 fringes or less — by their own account, roughly one-fortieth of the expected displacement, and well within the range of pure measurement noise. Michelson himself reported to Lord Rayleigh that any real ether motion, if it existed at all, had to be less than one-sixth of Earth's own orbital speed. This is genuinely regarded as one of the most famous "failed" experiments in the history of physics — not because anything went wrong with the equipment, but because a confident, specific prediction came back with a real, honest, and completely unambiguous null result.

Attempted Rescues Before Einstein

The null result did not immediately overturn the ether concept — physicists first tried to save it. George FitzGerald in 1889, and Hendrik Lorentz independently in 1892, each proposed that objects moving through the ether physically contract, ever so slightly, along their own direction of motion — by precisely the amount needed to cancel out the timing difference Michelson and Morley had expected to see. At the time, this "Lorentz–FitzGerald contraction" was a genuinely ad hoc fix, invented specifically to explain away one troublesome result rather than derived from any deeper principle.

Einstein's Real Relationship to the Experiment

The popular version of this story treats Michelson and Morley's 1887 null result as the direct spark that led Einstein to invent special relativity in 1905. The real, documented history is honestly more complicated.

🔗 A Correction Worth Making Directly Einstein's own 1905 paper cites no prior work at all — a genuinely unusual choice for a paper of its significance. When later historians investigated what had actually shaped his thinking, Einstein himself explicitly denied that the Michelson-Morley experiment was a significant direct influence on his own reasoning. What he did credit with real, considerable influence was Lorentz's own 1895 electromagnetic theory, the separate Fizeau experiment on the speed of light in moving water, and philosophical works — notably Henri Poincaré's Science and Hypothesis, which Einstein and his friends are documented to have closely studied and discussed over several years. The best current historical picture is that Einstein arrived at special relativity primarily through general theoretical reasoning about Maxwell's own equations (this course's own Chapter 6) and the principle of relativity itself, rather than as a direct response to any single experimental result — even one as famous as Michelson-Morley's.

Expectation vs. Reality

What Was Believed	What Was Found
Space is filled with a detectable luminiferous ether	No ether wind was detected, to a precision roughly 20× smaller than expected
Earth's motion through the ether should shift the interference pattern by ~0.4 fringes	Measured shift: 0.02 fringes or less
The M-M null result directly inspired Einstein's 1905 relativity paper	Einstein himself denied this was a significant direct influence

Hands-On Exercises

Exercise 1

Explain, in your own words, why the luminiferous ether needed to have genuinely contradictory physical properties - name both properties directly, and explain why each was necessary for the ether theory to work at all.

→ Solution

Exercise 2

Michelson and Morley expected a fringe shift of about 0.4 fringes, but measured a shift of only about 0.02 fringes. Calculate what fraction (as a ratio, e.g. "1 in X") the measured shift represents of the expected shift, and explain in one sentence why this made the null result so difficult to dismiss as ordinary measurement error.

→ Solution

Exercise 3

Explain, in your own words, why the popular story that "Michelson-Morley's null result directly inspired Einstein's theory of relativity" is a real oversimplification, and name what Einstein himself actually credited as significant influences on his own 1905 paper.

→ Solution

Quick Reference

The luminiferous ether was proposed as light's own necessary medium, but needed impossible, contradictory properties (frictionless yet rigid)
Michelson & Morley (1887, Cleveland): expected a ~0.4 fringe shift from Earth's motion through the ether; measured ~0.02 or less — a genuine null result
FitzGerald (1889) and Lorentz (1892) independently proposed length contraction as an ad hoc rescue of the ether concept
Einstein's 1905 paper cites no prior work; he himself denied Michelson-Morley was a significant direct influence, crediting Lorentz's 1895 theory, the Fizeau experiment, and Poincaré instead

Next chapter: Special Relativity: Real Time Dilation & Length Contraction — where the Lorentz-FitzGerald contraction this chapter introduced as an ad hoc fix gets its real, derived justification, and Einstein's own actual reasoning finally takes centre stage.