# Three Men Discuss Relativity (1926)

The other week I read (via Hacker News) the 1926 treatise Three Men Discuss Relativity, by the brilliant science educator J.W.N. Sullivan. Wikipedia calls it “one of the earliest non-technical accounts of Albert Einstein’s general theory of relativity,” which Einstein had published about ten years earlier, circa 1915. Although Sullivan’s book seems daunting at 200 pages, it’s really set in quite large type and can be read in one sitting. It’s organized as a Galilean dialogue among three archetypes: the Mathematical Physicist, the Ordinary Intelligent Person, and the Philosopher.

The most interesting parts for the modern reader, I think, aren’t so much the surprising physical consequences of relativity, as the surprising metaphysical observations about the system that preceded Einstein’s! Recall that Newton’s laws of motion are as follows:

1. A body remains at rest, or in motion at a constant speed in a straight line, unless acted upon by a force.

2. When a body is acted upon by a force, the time rate of change of its momentum equals the force.

3. If two bodies exert forces on each other, these forces have the same magnitude but opposite directions.

Sullivan, pages 33–37:

MATH PHYS:   Newton’s philosophy, by making appeal to unobservable factors, is, as Einstein has said, partly metaphysical. […] We are now in a position to discuss critically the formal foundations of Newtonian theory, his laws of motion. His first law states, you remember, that a body persists in its state of rest or of uniform motion in a straight line, except insofar as it is acted upon by external forces. […] Of course, no body on the surface of the earth is ever entirely free from the action of external forces, and therefore no experimentally controlled body can ever exactly fulfill the law. […]

ORD INT PER:   But is it not a fact that the Newtonian law is found to be obeyed?

MATH PHYS:   Where?

ORD INT PER:   In the solar system, for example.

MATH PHYS:   But the planets do not move in straight lines with uniform velocities.

ORD INT PER:   No, but that is because of the force of gravitation.

MATH PHYS:   Exactly. The force of gravitation is introduced because the unconstrained motion of a body in a straight line with uniform velocity can nowhere be observed. Newton’s law really amounts to a statement that in certain unobservable conditions, namely, at an infinite distance from all gravitating masses, a system of reference could be found […] measured from which the motion of a body thrown into space would be a straight line described with uniform velocity. The statement cannot be put to the test.

[…] That the Newtonian scheme, taken as a whole, describes the observed phenomena extremely well goes without saying. Otherwise it would not have been accepted by scientific men for so long, and we should not now be discussing it. But I am discussing, at the moment, Newton’s first law of motion, and pointing out that it refers to a condition of things which cannot be observed. Newton’s scheme as a whole gives a very good picture of experience, but the manner in which the scheme is built up seems to us, nowadays, very odd. In order to describe the observed happenings in the actual universe, Newton first tells us what would go on outside the universe. In order to make this statement relevant, he then has to introduce an entity, the force of gravitation, to turn the motions natural outside the universe into the motions which take place in the universe.

ORD INT PER:   You do not mean to deny that there is a force of gravitation?

MATH PHYS:   I mean to say that the notion of a force of gravitation is made inevitable by accepting Newton’s first law of motion.

Or again, pages 62–65 (after a discussion of the Michaelson–Morley experiment):

MATH PHYS:   We must remember two principles. One is that phenomena take place in the same way whether we suppose ourselves to be in a system in uniform translatory motion or at rest. The other is that the velocity of light in vacuo is constant.

PHIL:   I should like to point out that the two principles you have just enunciated are of very different kinds. The assumption that the velocity of light in vacuo is constant is, I take it, the result of experiment. Further experiments could conceivably show that it is not always constant. You do not regard it as a necessary preliminary, in your attempt to give coherence and order to natural phenomena, to assume that the velocity of light must be constant.

[…] Your other principle, however, which amounts to saying that rest and uniform motion are indistinguishable, is, although confirmed by experiment as far as the aether is concerned, primarily the enunciation of a certain form, a kind of framework, within which natural phenomena must be arranged if they are to be intelligible to you. […] When you say that natural phenomena do not enable us to distinguish between systems which are in uniform translatory motion with respect to one another, you do not mean that they might do so but as an ascertained fact do not; you mean that there can be nothing in nature corresponding to a distinction that you regard as meaningless.

MATH PHYS:   Yes, that is more or less what I mean. I cannot imagine, for instance, that the laws of a natural process depend on whether it is taking place on my right or on my left, in front of me or behind me. Laws into whose enunciation such considerations entered would obviously not be true laws. It is inconceivable that fundamental laws of nature depend upon the observer to that extent. And I put the difference between uniformly moving systems in the same category. You may call this, if you like, a form imposed upon the laws of nature by my own mind. I admit that you cannot make a science without principles, and those principles must express the constitution of our own minds.

What the Mathematical Physicist says here reminds me very much of what Albert Camus describes (less eloquently and vastly less concisely) in An Absurd Reasoning (1940–1942) — a yearning which Camus terms nostalgia.

To understand is, above all, to unify. The mind’s deepest desire […] is an insistence upon familiarity, an appetite for clarity. To understand the world is to reduce it to the human.

Previously on this blog:

Posted 2022-06-17