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I frequently hear that Albert Einstein’s Special Relativity forbids the Aether. This opinion is voiced by people who believe that Einstein rejected the aether. Instead, Einstein abandoned the aether because it was controversial, and he didn’t need it for his relativity theories. But see for yourself what the Master wrote in “Ether and the Theory of Relativity,” an address given on 5 May 1920 at the University of Leiden. Please click onto
http://turnbull.mcs.st-and.ac.uk/~history/Extras/Einstein_ether.html.
It is a remarkable document.
Special Relativity is an almost obvious, simple model of how light travels from one object to another in the Universe. The conjecture in this essay is that this requires an aether that is carried along as the Universe expands; more exactly, expansion of the aether defines the expansion of the Universe.
Physics Nobel prizewinner Frank Wilczek (in 2004, the Gross-Politzer-Wilczek award for “asymptotic freedom”) has resuscitated the aether in his new book, The Lightness of Being: Mass, Ether, and the Unification of Forces. Wilczek’s aether is quite different from the classical Maxwell-type aether: The new aether is a space-filling medium that is teeming with extremely small virtual particles that come and go, as revealed by quantum mathematics. In the present essay, however, we only consider the “space-filling” aspect of the aether.
Special Relativity requires celestial objects that are not accelerating, that are part of an inertial frame or inertial platform. Although this is never exactly true, it is approximately correct because the accelerations are small compared to the velocity of light. Let’s assume, therefore, that two planets are acceleration-free: The first planet is US, the Earth. The second planet, THEM, is receding from US at a speed, relative to US, of 100 million meters/second. This is one-third the velocity of light, which is 300 million m/s as measured on Earth (these are approximate values, of course).
According to Special Relativity – and this is a very sensible and reasonable assumption – the velocity of light is, also, 300 million m/s as measured by the people on planet THEM, relative to THEM. The empirical evidence is that the spectra of celestial objects  – electromagnetic field output versus frequency – are the same everywhere we look in the Universe, except for a Doppler frequency shift toward red if the object is retreating from US (and blue if it is approaching).
Consider this simple scenario: An unimaginably powerful beam of light is sent from US toward THEM. It starts out at a speed of 300 million m/s, but gradually increases in speed to 400 million m/s as the Universe expands. The beam finally arrives at THEM with a speed of 300 million m/s relative to THEM, but 400 million m/s relative to US. The gradual increase in speed is accompanied by a stretching of the wave that constitutes the beam. It stretches by a factor of 4/3, a ratio of 1.33, which corresponds, by definition, to a red shift of z = 0.33.
The controversial argument presented here is that there is no way that a light beam, by itself, would stretch by a factor of 1.33. This requires that the Universe should be filled with the aether, whose density decreases as the Universe expands. The aether, of course, carries the light beam along, imposing the stretches needed to keep the local velocity of light constant at 300 million m/s. But the aether raises several questions: Does it consist of tiny particles? Does the aether cause the expansion of the Universe? How does the aether originate? At the present time, the answer to these questions is that nefarious word, “Somehow,” or we don’t know.
Two of the famous conclusions that accompany special relativity are time dilation and length contraction:
1) That unbelievably powerful beam of light reveals that, as seen by US, the clocks on planet THEM run slow; in the present example, which is receding from US at 100 million m/s, their clocks run slow by a factor of 0.943. In fact, this time dilation applies to the entire planet THEM; the inhabitants age more slowly than we do (but, relative to themselves, aging is normal).
2) Another revelation, as seen by US, is that all objects on planet THEM are shortened in the direction away and towards US.  In the above example receding at 100 million m/s, length is contracted by a factor of 0.943.
Time dilation and length contraction have nothing to do with an aether, and that is why Einstein could ignore the aether. They depend only on the 300 million m/s local velocity of light. A few simple geometrical constructions shows why, on a receding (or approaching) object, relative to US, time dilation and length contraction would be observed (see Figs. 8-1 and 8-4 in my 2006 book, Einstein’s Greatest Mistake; Abandonment of the Aether).
Please don’t visualize the above light beam as suddenly speeding up from 300 to 400 million m/s. Nothing could be further from reality. The expansion is the gentlest of breezes, barely perceptible to local inhabitants. Consider the following oversimplified model (which hopefully contains a grain of truth): The expansion begins at the Center of the Universe point (COTU) point, with the outer edge shaped like a sphere, expanding at the speed of light, the radius increasing at a rate of 300 million m/s. A light-year is 9.460 × 1015 m so that, at the present age of the Universe, 13.7 billion years, the radius of the Universe as a sphere is
9.460 × 1015 × 13.7 × 109 = 1.296 × 1026 meters.
Evidence that the Universe is finite comes from H.W.M. Olbers (1758-1840). He pointed out that the night sky of an infinite, homogeneous universe should be bright, no different from the daylight sky. This notion is called Olbers’s paradox. Wherever we look, the integrated effect of distant, and yet more distant, stars and galaxies should yield nothing less then a bright, sunlit sky everywhere. According to this simple test, it appears as if the Universe is finite, with the Earth at its center, because the night sky is uniformly dark.
To get a more palatable scenario, imagine that we have a stick 300 million meters long (the distance light travels in one second) reaching out from the COTU point. How much does the end of the stick stretch in one second? Here we have a simple proportion: The radius of the Universe, 1.296 × 1026 m, increases by 300 × 106 m in one second. Then the stretch of the stick, x, is to its length, 300 × 106, as the increase in radius, 300 × 106,  is to the radius, 1.296 × 1026. We get that x = 7 × 10–10 m, or x = 7 angstroms. That is an easily visualized and familiar distance because the diameter of an atom is one angstrom, and the diameter of a water molecule is 3 angstroms.
In 100 years, the stretch gets longer by a very respectable 2 meters! During a human lifetime, then, we may be able to measure the local expansion of the Universe.
What about the people living near the edge of the Universe, where things are hopping at nearly the speed of light? Well, they are ignorant of all that commotion. Relative to them, the nearby celestial objects are practically stationary. Looking back toward US on Earth, they conclude, from the red shift, that the Earth is moving away from them as the Universe expands. It is all a matter of relativity.
As seen from the Earth, the sky is reasonably uniform (except for the Milky Way), but finite, so perhaps we happen to be at or near the COTU point after all. Perhaps people who live near the edge of the Universe, on the other hand, looking at the sky, see half of it blacked out; they “see” the void of outer space, which is devoid of the aether, and  filled with a true vacuum.

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