Chapter 2.8 Interpreting relativity


Everyone now agrees that special relativity is well confirmed by experiment. But there remains stark disagreement about why length contraction and time dilation occur – about what is going on behind the scenes to produce such startling effects. This may come as a surprise. Einstein’s theory is 100 years old. Surely scientists and philosophers would have clearly understood it by now?

But the popular image of science is often different from the way it really works. Consciously or unconsciously, scientists are propagand- ists. To the outside world, they present science as a series of great discoveries, as smooth upwards progress towards truth. But inside science, fierce debates and controversies rage constantly. The public is shielded from these in several ways. First, scientific language is often technical and difficult for non-scientists to penetrate. Secondly, science textbooks used everywhere from elementary school to university tend to conceal disagreement. This helps students by simplifying the material, but it also serves to reinforce the image of science as “objective truth” above all questioning, and thereby reinforces the enormous social and political authority of science.

Disagreement about the interpretation of scientific theories is normal. No major theory of science is free of debate about its truth, meaning and implications.

One task that philosophers perform is the conceptual interpreta- tion of theories in physics. That is, they exploit their talents for clear reasoning and careful definitions to explore what the formulas in physics mean, to unveil what the symbols say about our world.

Physicists today are trained to calculate numbers rather than analyse conceptual arguments, and their verbal interpretations of their own theories are often unreliable. Despite their technical skills, as soon as physicists stop calculating they are sadly quite mortal.

The purpose of interpreting scientific theories is twofold. Science is partly an intellectual quest to understand the world around us, but as science became more successful at making predictions it also became more obscure, technical and mathematical. Thus progress in understanding the world now often depends on first interpreting and thereby understanding the scientific theories we already possess. The second purpose of interpretation is more practical. Advances in science come in many ways. Some are the result of blind trial and error, and some arise when patterns in data are first discerned.

Historically, interpretation and conceptual analysis have been one important route forwards towards new theories and better science.

Many of the important concepts that lie at the foundations of contemporary science were first created by philosophers. Thus today’s philosophers can hope to contribute to our intellectual understanding of the world, as well as to the advance towards better and deeper theories.

Relativity theory so shocked everyone that many different interpretations of the above effects have been advanced and defend- ed. During the 1920s and 1930s, most physicists accepted relativity theory and it became a routine part of their work. Controversy, however, raged loudly and ceaselessly. A number of physicists flatly rejected the theory and concocted paradoxes to show that it could not be true and must be self-contradictory and incoherent. Outside science, quacks and disgruntled cranks barraged scientists with “proofs” of Einstein’s errors. In Nazi Germany, the Nobel-prize winner Johannes Stark bizarrely condemned Einstein’s theories as “Jewish physics”, and used his political power to push research in other directions. In retrospect, those turbulent times were a learning period. Mainstream physicists rebutted the paradoxes, and deepened our understanding of relativity.

During the Cold War, from the 1950s through the 1980s, special relativity was gospel. It was considered the best confirmed of theories, and provided foundations for all advanced work in theoretical physics. Controversies over the interpretation of the theory subsided, and textbook presentations of the theory were standardized. Then came the surprise. Beginning in the 1980s, philosophers and some physicists began to realize that certain experiments (discussed below) were a new and unexpected challenge to our understanding of relativity. That is, while still accepting that the theory worked at a practical level, increasing numbers began to doubt that the standard interpretation was correct.


Similar sorts of interpretational problems arise with ordinary maps. A map of the world may be useful for navigation even though it grossly distorts the shape of the continents and portrays the spherical Earth as if it were a two-dimensional plane. Special relativity makes predictions that turn out to be true, but we can still ask how well it pictures reality.

There are many examples in history of theories that made good predictions but fundamentally misdescribed reality. A simple one is the theory that the Sun will rise every morning. This theory leads to the prediction of a general brightening in the sky at about 6 am, which will be well confirmed. But the theory is false because the Sun does not rise: Earth rotates.

Two key distinctions, or pairs of words, are at the centre of debates over special relativity: “relative” versus “invariant” and “appearance” versus “reality”. “Relative” means related to or dependent on something else. When used as a noun, “relative” means something involved in a relation, which is why we call our cousins relatives. The word “invariant” is used very often in debates over relativity. In this context, a property is invariant when it is independent of the set of rulers and clocks that is used for measuring it. Suppose that different sets of rulers and clocks are all moving relatively to each other, and are used to measure some one property. If all the sets give the same answer, then the property they are measuring is invariant and independent of how it is measured.

Physicists sometimes use the word “absolute” as a synonym for “invari- ant”, but history has encrusted “absolute” with so many different meanings that we will avoid it in these introductory chapters.
The philosophy called “Relativism” holds that truth and values depend on personal beliefs or cultural conditions. Relativism is not the same as Einstein’s theory of relativity. As will be discussed below, Einstein’s relativity theory does not reject objective truth altogether.

It argues that some properties we thought were invariant are not, and introduces new sorts of invariants. In fact, the name “theory of relativity” was not Einstein’s first choice; it was coined by another physicist (Poincaré).

The second distinction between appearance and reality is familiar.

Hallucinations and mirages are cases where appearances diverge from reality. A straight stick appears bent when half submerged in water, even though it is really straight. This distinction is also central to modern science. Earth appears to be flat and motionless, but science tells us this is not really so. For another example, colours are mere appearance. The atoms that make up the objects around us are colour- less, and appear coloured only because they reflect light of different wavelengths into our eyes.

Note that, as defined above, the question of whether a property is invariant or not is a question about appearances. A physicist can test whether lengths are invariant merely by making observations, and need not speculate about whether those measurements faithfully report what is real. Appearances may have the property of invariance.

In debates over special relativity, most people accept that the theory correctly describes appearances. That is, the predictions it makes have so far, without exception, been confirmed. The question that remains is over the reality behind the appearances. What is happening behind the scenes? Can we describe or build models of a world that would explain our observations of length contraction and time dilation?

A theory may make good predictions even though it wrongly describes reality.

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