The most common answer to this question is: Albert Michelson and Edward Morley, in 1887. It appears in textbooks, on Wikipedia, and in Google's AI Overview. And it is wrong.
What Michelson and Morley demonstrated, with impressive precision, was that the Earth does not move through a rigid, stationary luminiferous ether in the way that a boat moves through water. Their interferometer found no "ether wind" — no directional dependence in the speed of light at the level of Earth's orbital velocity (30 km/s).
This is a genuine and important null result. But it is not a disproof of the ether. It is a disproof of one specific model of the ether: a solid, motionless, mechanical medium at absolute rest.
What the Experiment Actually Ruled Out
The 1887 experiment tested for an ether wind — a measurable difference in the speed of light along the direction of Earth's motion versus perpendicular to it. The expected fringe shift, based on a rigid stationary ether, would have been detectable with their instrument's sensitivity.
They found no shift. Conclusion: light does not propagate through a rigid, stationary medium in which the Earth moves without disturbance.
But this conclusion does not extend to:
- A fully dragged ether (Fresnel's original proposal, later refined by Stokes)
- A flowing ether whose velocity field is determined by nearby masses
- A superfluid ether with zero viscosity and no drag signature
- A Lorentz-covariant ether with no preferred rest frame to detect
- Einstein's own "ether of general relativity" — spacetime endowed with physical properties
Each of these models would produce exactly the same null result in a Michelson-Morley interferometer. The experiment cannot distinguish between "no ether" and "an ether with no detectable wind."
Einstein's Reversal
The narrative usually stops in 1905: Einstein showed the ether was unnecessary, and so it was abandoned. What the narrative omits is everything that came after.
In 1920, Einstein delivered a lecture at the University of Leiden titled "Ether and the Theory of Relativity." In it, he stated:
According to the general theory of relativity space without ether is unthinkable; for in such space there not only would be no propagation of light, but also no possibility of existence for standards of space and time.
And:
More careful reflection teaches us however, that the special theory of relativity does not compel us to deny ether. We may assume the existence of an ether.
In 1924, he wrote in "Concerning the Aether":
We will not be able to do without the aether in theoretical physics, that is, a continuum endowed with physical properties; for general relativity... rules out direct action at a distance.
Einstein spent 30 years after special relativity — from 1916 to his death in 1955 — working within a framework in which space possesses physical properties. He called it an ether. The textbooks usually omit this.
The Question Nobody Asks
If the ether was truly "disproved," why do these facts hold?
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The quantum vacuum has measurable physical properties. Vacuum permittivity ( F/m), vacuum permeability ( H/m), and the impedance of free space () are experimentally measured constants of "empty" space.
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The Casimir effect (1948, experimentally confirmed 1997) demonstrates measurable forces arising from vacuum fluctuations between conducting plates.
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The Lamb shift (1947, Nobel Prize 1955) shows that atomic energy levels are shifted by interaction with the quantum vacuum.
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Analog gravity (Unruh 1981, Visser 1998, Steinhauer 2016) demonstrates that a flowing fluid medium produces emergent spacetime geometry indistinguishable from general relativity. Steinhauer observed spontaneous Hawking radiation in a Bose-Einstein condensate, published in Nature Physics (2016).
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Superfluid dark matter (Berezhiani & Khoury 2015, Physical Review D) models dark matter as a superfluid whose phonon excitations mediate MOND-like forces at galactic scales.
None of these developments use the word "ether." All of them describe a medium pervading space with physical properties that influence the propagation of light, the dynamics of particles, and the geometry of spacetime.
As Nobel laureate Robert Laughlin wrote in 2005:
The modern concept of the vacuum of space, confirmed every day by experiment, is a relativistic ether. But we do not call it this because it is taboo.
What This Project Does Differently
The ether physics monograph at Section 1 takes the step that mainstream physics has been reluctant to take: it names the medium, formalises its dynamics, and derives testable consequences.
The result is a mathematical framework containing 1,253 equations, 28 theorems, and 17 falsifiable predictions — all derived from a single superfluid Bose-Einstein condensate substrate. It reproduces general relativity exactly (Section 3), resolves the vacuum catastrophe (Section 4), and makes parameter-free predictions for galaxy rotation curves (Section 4) that match observed data.
Nobody disproved the ether. What was disproved was a specific, crude model of it. The question is not whether space has physical properties — it manifestly does. The question is what those properties are, and what they predict.