Ask a physics student what Einstein said about the ether and you will hear: He showed it was unnecessary in 1905, and it was abandoned.
This is half a story. The full story is far more interesting, and far more relevant to modern physics.
1905: The Rejection
In his paper "On the Electrodynamics of Moving Bodies," Einstein wrote that the introduction of a "luminiferous ether" would prove superfluous. He needed only two postulates — the principle of relativity and the constancy of the speed of light — to derive the Lorentz transformations. The ether, as a preferred rest frame, was not needed.
This much is accurate. But what Einstein rejected was a very specific thing: a rigid, stationary medium that defined absolute rest. He did not reject the possibility that space might have physical properties.
1916: General Relativity Changes Everything
When Einstein completed general relativity in 1915-16, he discovered something that forced him to reconsider. In GR, spacetime is not a passive background — it is a dynamical entity with physical properties. It curves, it carries energy, it propagates waves. The metric tensor determines distances, times, causal structure, and the motion of matter.
This is, by any reasonable definition, a medium.
1920: The Leiden Address
On May 5, 1920, Einstein delivered a lecture at the University of Leiden titled "Ether and the Theory of Relativity." In it, he made statements that directly contradict the popular narrative.
On the necessity of the ether:
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 (measuring-rods and clocks), nor therefore any space-time intervals in the physical sense.
On special relativity not requiring denial:
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; only we must give up ascribing a definite state of motion to it.
On the new ether:
The ether of the general theory of relativity is a medium which is itself devoid of all mechanical and kinematical qualities, but helps to determine mechanical (and electromagnetic) events.
1924: "Concerning the Aether"
Four years later, Einstein wrote an essay titled "Uber den Ather" (Concerning the Aether), published in the proceedings of the Natural Sciences Society of Basel:
Instead of 'aether', one could equally well speak of 'the physical qualities of space'.
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.
The aether of general relativity differs from those of classical mechanics and special relativity in that it is not 'absolute' but determined, in its locally variable characteristics, by ponderable matter.
What Changed Between 1905 and 1920?
Einstein realised something the popular account misses: the old ether and the new ether are different things.
| Property | 19th-century ether | Einstein's GR ether |
|---|---|---|
| Rigid body | Yes | No |
| Preferred rest frame | Yes | No |
| Mechanical properties | Yes | No |
| Physical properties | Yes | Yes |
| Determines light propagation | Yes | Yes |
| Determines metric (distances, times) | No | Yes |
| Curved by matter | No | Yes |
The 19th-century ether was a solid. Einstein's ether is a field. But both are media that pervade space and determine the behaviour of light and matter. The discontinuity between them has been exaggerated; the continuity has been suppressed.
Einstein Was Not Alone
Other major physicists have said similar things:
Paul Dirac (1951), in Nature:
...with the new theory of electrodynamics we are rather forced to have an aether.
John Stewart Bell advocated a return to Lorentzian ether theory as "the cheapest solution" to the problems raised by EPR and Bell inequalities.
Robert Laughlin, Nobel laureate (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.
The Ether Physics Programme
The monograph at Section 2 takes Einstein's position seriously. If general relativity endows space with physical properties, the next question is: what physical system has those properties?
The answer explored in the monograph is a superfluid Bose-Einstein condensate. Its acoustic metric reproduces the Painleve-Gullstrand form of the Schwarzschild solution exactly (Section 3). Its field equation is the Einstein equation (Section 3). Its zero-point energy matches the observed dark energy density (Section 4). And it makes 17 falsifiable predictions that neither CDM nor MOND can make.
Einstein said space without ether is unthinkable. Perhaps it is time we took him at his word.