A correspondent writes:
British (Catholic) historian Paul Johnson in his wonderful Modern Times attributes relativism's rise to Einstein! So does Einstein's latest biographer.
There are two questions that must be distinguished. The first is whether Einstein's Theory of Relativity entails either moral or cognitive (alethic) relativism. The second question is whether Einstein's revolutionary contributions to physics, via their misinterpretation by journalists and other shallow people (am I being unfair?), contributed to an atmosphere in which people would be more likely to embrace moral and cognitive relativism. The first question belongs to the philosophy of science, the second to the sociology of belief. The questions are plainly distinct.
The answer to the first question is a resounding No. Since physics has nothing to do with moral questions — which is not to say that moral questions do not arise in the technological application of physical knowledge or in its dissemination or in the construction of experiments, etc. — it is quite clear that neither STR nor GTR nor any physical theory has any logical consequences in respect of metaethical doctrines such as moral relativism. And as for cognitive or alethic relativism, far from its being entailed by the Theory of Relativity, I should think that the latter presupposes the absoluteness of truth.
Take the Galilean principle of the additivity of velocities. Suppose I'm on a train moving with velocity v1. I fire my gun in the direction of the train's travel. The projectile's muzzle velocity is v2. The projectile's total velocity is v1 + v2. But STR implies that the additivity of velocities breaks down at relativistic speeds, speeds approaching the speed of light. Now the proposition that the principle of the additivity of velocities fails at relativistic speeds is not merely true relative to STR, but true absolutely. And the same goes for any number of other propositions of STR and GTR such as the one bearing upon the conversion of mass and energy, E=mc^2, or that the speed of light remains a constant 186, 282 mi/sec. Or consider the proposition that motion and rest are relative to reference-frames. That proposition's truth is not relative to any reference-frame or to any conceptual framework either.
In short, Einstein's Theory of Relativity, far from entailing relativism about truth, presupposes, and thus entails the absoluteness of truth. But I don't need to make that strong a claim to refute the thesis that the Theory of Relativity entails the relativity of truth. It suffices to point out that the theory is logically consistent with the absoluteness of truth.
As for the sociological question, I suppose one would have to grant that misinterpretations and shallow expositions of the Theory of Relativity did contribute to the spread of moral and cognitive relativism. But of course that is not the responsibility of Einstein or modern physics but the responsibility of those shallow-pates we call journalists. (Am I being unfair a second time?)