Not long after the special theory of relativity was published in 1905, the French physicist Paul Langevin first formulated one of the best-known implications of the theory, the twin paradox. If one twin could be sent to a nearby star at a speed approaching the speed of light while the other twin remained on earth, a strange thing would happen: the twin who traveled to the star would, on his return to earth, find that his brother was either very aged or even dead. Special relativity explains that this occurs because time does not move at the same rate for both twins. If two people or things do not share a frame of reference, time does not progress at the same rate for them. Instead, the rate of passage of time is relative to frame of reference. It was this implication of relativity that drew the strongest criticism. Its opponents feared what might follow from the idea that time is relative to frame of reference. After all, if time—one of the fundamental irreducible quantities of physics—varied with frame of reference, less fundamental aspects of reality must also be subject to variation, and the objective basis of science would be lost. One critic, for example, suggested that special relativity rejected science established on objective experiment “in favor of psychological speculations and fantastic dreams about the universe.” Another was opposed to special relativity because its proponents “deny that any concrete experience underlies these [mathematical] symbols, thus replacing an objective by a subjective universe.” In short, what they feared was that relativity, if established, would lead to epistemological relativism.
Relativism threatens to undermine the basis of science and of objective knowledge in general. The different kinds of relativism (ethical, epistemological, cultural, aesthetic) focus on different aspects of a thing, but they share in common the belief that some aspect of a thing is as it is only with respect to frame of reference. There are things to recommend this idea. We know, for example, that frame of reference affects perception. We also know that the reality of a thing is rarely as simple as it appears to be from any one frame of reference. These two things taken together suggest that the truth of a thing is never exhausted by what is revealed in a single frame of reference. Instead, different—perhaps even contrary—truths are revealed from different perspectives. The flexibility of such relativism is appealing because it both allows for and explains the perspectival nature of truth. But it also has a serious weakness since this view suggests that there is no objective basis of knowledge. If what is seen from each frame of reference is true in it and no one frame of reference is privileged to call itself the right one, knowledge is always conditioned by one’s viewpoint, that is, it becomes subjective, as the second critic quoted above feared. Perhaps even more importantly, its extreme forms suggest that not only is there no objective basis but that we are all trapped in our subjective viewpoints, unable to leave our own frame of reference to see what anyone else sees. We can never understand reality as understood from other perspectives because we quite literally cannot see from another point of view. As a result, there is no way of resolving conflicts between the truths of different perspectives or of ascertaining any more about reality than what one’s own frame of reference reveals.
But Einstein’s relativity is not relativism. He acknowledges the significance of frame of reference; indeed, frame of reference is so critical that it even determines the rate of passage of time. It genuinely determines at least part of the truth. However, had his critics considered his work more carefully, they would have realized that acknowledging the role played by frame of reference is not the same as advocating some form of relativism. Quite the contrary: relativity is founded on a principle which is not compatible with relativism. According to Einstein, the principle of relativity the restricted sense states that, for two systems K and K’ in uniform motion relative to one another, “Relative to K’ the mechanical laws of Galilei-Newton hold good exactly as they do with respect to K.” In other words, the laws of nature that apply in one system (or frame of reference) apply and apply in the same way in all others. This is the opposite of relativism at least insofar as it insists that some things hold in all frames of reference. Light, for example, travels at the same speed in all frames of reference. Einstein later reinforces this point in his discussion of general relativity when he insists “the laws [of nature] themselves must be quite independent of the choice” of frame of reference. Einstein’s adherence to this principle does lead to the conclusion that the passage of time differs with frame of reference, but this difference is attributable to the fact that all frames of reference obey the same laws, and this fact in turn makes it possible to determine how the passage of time differs between frames of reference. In other words, Einstein’s theory recognizes real differences between systems, but it also believes that those differences can be understood and accounted for from any given frame of reference. It provides truth with the flexibility that makes relativism so appealing but also allows us to bridge frames of reference in a way that extreme relativism denies is possible.
The philosophical relevance of relativity lies in its implications about truth in general. Knowledge of reality, Einstein says, is not simple. It is neither something universal nor something that is entirely determined by frame of reference. Instead, truth is the result of the operation of laws within a particular context, and one cannot know truth without knowledge of both of the laws and the context. This notion suggests that truth may be harder to ascertain since it is determined by two variables rather than one, but it also carries with it the advantage of acknowledging the role of perspective while allowing us to overcome its limits.
 I. Bernard Cohen, Revolution in Science (Cambridge, Massachusetts: The Belknap Press of Harvard University Press, 1985) 411-412.