by Daniel Ranard
First I will focus on Einstein's relativity, before touching on relativism in philosophy. To me, the story begins with two opposing accounts of what physics is. According to one account, physics provides an objective description of the world itself, like an encyclopedia entry on "The Universe." The encyclopedia tells you what stuff the world is made of and how that stuff behaves. This approach might be called the realist approach: physics presents objective facts about the real world.
Others prefer an "operationalist" account that focuses on the individual. By this account, physics is simply a collection of rules telling the individual what to expect in various circumstances. It's like a personal guidebook for experience: it predicts what you will observe when you follow various experimental procedures, like following a recipe in a laboratory. Unlike the realist's encyclopedia entry, the operationalist's guidebook does not attempt to describe the real world objectively. Instead, it prescribes how your experience should lead you to predict future experiences, using your own observations. Operationalists avoid referring to fundamental aspects of nature, like mass or length. To the operationalist, the length of an object is not some fundamental property – it's just a number you observe when you measure the object with a ruler, and any notion of length must be accompanied by well-specified procedure for how to measure it.
Realism and operationalism are not necessarily opposed, but they suggest different ways of approaching any physics question: should you consider the realist's encyclopedia, or the operationalist's guidebook? Einstein would defend realism later in life, especially as he and struggled with the meaning of quantum mechanics. However, I suspect it was operationalist thinking that led to his first creative breakthroughs.
The trouble with the realist approach is that sometimes it's hard to reconcile a radically new observation with what's already written in the encyclopedia. By attempting to describe the fundamental nature of the world, the realist may harbor implicit assumptions that are difficult to escape. For instance, when Einstein was still a student, the realist's encyclopedia entry might have said: "The universe is built of absolute space and time, which provide the arena for motion." Moreover, the encyclopedia would have listed lengths and durations of time as fundamental, universal quantities.
Before Einstein, physicists were confused by the results of certain experiments, like the Michelson-Morley experiment. Many physicists wanted to edit the encyclopedia to account for the surprising results, guessing incorrectly that the universe was permeated by a new substance they called aether. But Einstein asked how to fill in missing sections of the guidebook instead, building a theory of what any single observer would measure and experience.
In Einstein's relativity, different observers tell different stories about what they see. A woman on a train may see her suitcase as a foot wide, but the man watching from the station sees it as 11.999 inches. That makes it difficult to write the encyclopedia: what should it say about the woman's suitcase? But it's easy to write the guidebook. The text of the guidebook is the same for all readers, but each reader fills in the blanks with her own observations, crunches her own numbers, and creates her own set of "facts" with which to make new predictions. The woman writes down "Suitcase: 1 foot" in the margin of her book, while the man writes "11.999 inches" in his own copy, and each creates their own, internally consistent perspective. Each understands that the other will see the suitcase differently, and neither claims to have measured the "true" length.
The philosophical relativists of the 20th developed ostensibly similar ideas, occasionally inspired directly by Einstein's work. For instance, moral relativists claim there are no objective facts about what is right or wrong, only facts relative to the perspective of some society. Cultural relativists argue that there is no objective interpretation of individual experience, but rather people interpret their experiences using the apparatus of their particular culture. Most strikingly, conceptual or ontological relativists claim that objects can only be said to exist relative to some conceptual framework.
On the face of it, philosophical relativism may resemble an extension of Einstein's relativity. The relativists don't want to describe morality or personal experiences using a universal encyclopedia. Rather, they want to individuals and groups to frame their own stories in their own words. In this sense, the philosophical relativists are more radical than the physicists. The physicists all use the same guidebook, written in the same language, espousing the same general principles by which each observer is guided to craft a different narrative. The guidelines are universal, even though individual accounts differ. In contrast, ethicists and anthropologists who are relativists may follow different principles when constructing narratives for different groups.
Philosophical relativism and Einstein's relativity also differ more drastically, though. Einstein may have liked thinking about the guidebook, but ultimately he also formulated his theory using the encyclopedia. He asked how to stitch together the perspectives of different observers in a consistent way, describing an objective world that gives rise to different observers and their experiences. Einstein's revision of the encyclopedia requires new language and concepts – for instance, he writes of a unified spacetime, with spacetime events witnessed by multiple reference frames – and he discards old concepts that were not properly universal, like the "length" of the suitcase. Or, if you prefer thinking about a guidebook, you might say that Einstein ultimately edited the guidebook so that the guide not only tells you how to write down your own facts, but also tells you how to reconcile these facts with the facts reported by others.
Einstein used a form of philosophical relativism to great advantage in physics, focusing on individual experience in order to break down old assumptions about which concepts are universal—breaking down, for example, the assumption that we all observe the length of the suitcase in the same way. But he also explained how to stitch these perspectives together, and he strived for realism until the end of his life.
I can't say what lesson should be drawn for philosophical relativists, if any. But perhaps the lesson is this: after claiming some concept is relative, don't stop there. Try, at least, to develop new language and concepts that are universal, acknowledging both individual observations and shared facts, sewing together each perspective in a new framework without denying any single one.