The experimental evidence in favor of special relativity would seem to be overwhelming. Who other than cranks and idiots could doubt it? At the risk of locating myself in one of those two categories I attempt to explain on this page why to me the evidence is not convincing.
Most people would be surprised to learn that Einstein's special relativity is not the only theory which can explain the litany of experimental evidence which is usually cited in favor of SR. There is a long list of such evidence, including but not limited to Michelson-Morley, Kennedy-Thorndike, Ives-Stilwell, various experiments with muons, and Hafele-Keating. More recently, there is the stunning success of the Global Positioning Satellite (GPS) navigation system, in which satellite-borne atomic clocks are used to send timing signals to receivers on the ground. The whole GPS system depends for its correct operation on our being reasonably correct in our theories about how clocks run and how fast microwave radiation propagates.
There have been many alternative explanations put forward to explain these results, some of which have not stood the test of time, and others which are still unrefuted. I will focus here on just one of them, not because I believe it will ultimately prevail (although it may), but because its fate is intimately tied with that of Einstein's theory.
There exists a theory which makes exactly the same predictions as Einstein's, using exactly the same Lorentz transformations, in which lengths contract and clocks slow down in exactly the same proportion as in Einstein's SR. This theory is known by various combinations of the names of the four men who contributed to it: Larmor, Lorentz, Poincar? and Ives. Here I will call it Larmor-Lorentz.
The Larmor-Lorentz theory says that there is an ether rest frame, even though we have no way of measuring our velocity with respect to it. Physical bodies contract and physical processes slow down when in motion with respect to the ether, in exactly the proportion specified by the Lorentz equations (and by SR, when the observer happens to be in the ether rest frame). But time is absolute and space is Euclidean, and length does not equal time.
Some authors have claimed that if these two theories, Einstein's and Larmor-Lorentz, always make the same prediction regarding the result of a given experiment, then they are really the same theory. This was the view taken by the historian E. T. Whittaker , who reasoned that since Poincar? had it first (in 1904), the theory should be called the relativity theory of Poincar? and Lorentz. For this slight to Einstein, he earned the permanent scorn of Einstein's supporters (see, for example, Pais ).
But Whittaker was wrong. There is a difference between the two theories, and the difference is this: In Einstein's version all inertial reference frames are on equal ground. The reference frame of the muon hurtling toward earth at 0.99c is just as valid as that of earth itself, or that of the cosmic background radiation. Each reference frame defines its own simultaneity. It is this equality of all reference frames which mandates the prime relativistic law that nothing can travel faster than light. A signal travelling faster than light in one reference frame could be viewed in another reference frame as arriving at its destination before it left its source, thus violating causality.
In Larmor-Lorentz, on the other hand, only the ether rest frame defines true simultaneity. Although light travels at velocity c with respect to the ether, and matter must travel at velocity less than c, there is no metaphysical reason why other kinds of information, for example quantum effects, cannot travel faster than c.
If we admit that Einstein's SR and Larmor-Lorentz are indeed two different theories, then we should be able to specify an experiment which will discrimate between them. Einstein's supporters are immediately faced with a dilemma: any experimental result which invalidates Larmor-Lorentz must also invalidate Einstein's SR, because SR claims to be valid in all inertial reference frames, of which the ether rest frame chosen by Larmor-Lorentz is one.
The converse is not true, however. Any result which indicates faster than light signalling is death to special relativity, but leaves Larmor-Lorentz still standing.
Incredibly, there is such a result, though its implications are still controversial. In 1982 Alain Aspect  performed an experiment in which the equations of quantum field theory predicted a faster than light effect. Special relativity was thought to prohibit such an effect. It was a straight up and down test of a crucial disagreement between two highly successful theories.
The experiment was performed and found in favor of quantum field theory. Measurements taken at one event apparently affected measurements taken at another event located outside the light cone of the first event. Was relativity theory immediately jettisoned because it failed a crucial test? Not at all. The nature of the effect is such that it cannot be used to transmit information faster than light, and this allows relativists to claim that causality has not been violated.
Perhaps so, but it seems to me that relativists are way out on a limb here. They have started from the fact that we are currently unable to detect any motion with respect to a preferred frame and from there extrapolated to an inviolable "principle of relativity" which says that we will never be able to do so, no matter how advanced our technology becomes.
As circumstantial evidence piles up against relativity (e.g., what physicists call the "vacuum" looks a lot like what used to be called "ether", and there's that embarrassing cosmic background radiation), they rely on the fact that there have been no technical violations of relativity to pretend that all is well. I, for one, prefer to cast my lot with the cranks and idiots.
- Whittaker, Edmund Taylor, A History of the Theories of Aether and Electricity (American Institute of Physics, 1987).
- Pais, Abraham, "Subtle is the Lord ...": The Science and the Life of Albert Einstein (Oxford University Press, 1982) p. 168.
- Aspect, A., J. Dalibard and G. Rogers, Physical Review Letters 49, 1804 (1982). See Nick Herbert, Quantum Reality: Beyond the New Physics (Anchor Books, 1987) for a layman's introduction to the issues involved.
- Larson, D. J., "An absolute theory for the electrodynamics of moving bodies," Physics Essays 7(4), 476 (1994).