Concepts of Simultaneity: From Antiquity to Einstein and Beyond , Max Jammer , Johns Hopkins U. Press, Baltimore, MD, 2006. $49.95 (308 pp.). ISBN 978-0-8018-8422-1
All physicists know that something important happened to the concept of simultaneity in 1905. But most are unaware that in the past 100 years a feud has developed over whether the simultaneity of distant events in a given reference frame is fact or convention. Max Jammer, a professor emeritus of physics and former president at Bar-Ilan University in Israel, has written various books, including three classics published by Harvard University Press: Concepts of Space: The History of Theories of Space in Physics (1954), Concepts of Force: A Study in Foundations of Dynamics (1957), and Concepts of Mass in Classical and Modern Physics (1961). Since the last of the three was first published more than 40 years ago, one might be curious as to whether his new book, Concepts of Simultaneity: From Antiquity to Einstein and Beyond, is as good as the rest.
Jammer discusses notions of simultaneity in various contexts, including Egyptian hieroglyphs, Aristotle's works, and the tenseless Hopi language. Early philosophers criticized astrology by addressing the simultaneity of distant events. For example, Sextus Empiricus challenged the Chaldean method of casting horoscopes at birth: An astrologer on high ground waited to hear a gong when a woman gave birth, but because of the sound delay, the astrologer did not note the position of the stars at exactly the moment of birth. St. Augustine of Hippo denounced astrology by arguing that even though two women at distant places were known to have given birth at the same time because messengers dispatched from each birthplace met midway between the two, the children still had distinct lives.
Coincidentally, Albert Einstein's method of determining simultaneity resembled that of St. Augustine. Thus Jammer highlights various precursors to the modern conception of simultaneity. For example, the scholastics discussed whether there exists only one time. Since 1676, Ole Rømer's demonstration that the speed of light is finite revealed that the constellations as we see them are illusions—not the real, simultaneous positions of stars. Later, in the early 18th century, Gottfried Leibniz ascribed logical priority to simultaneity over other notions of time. Then in 1905 Einstein showed that the simultaneity of distant events varies among inertial systems in relative motion. That relativity of time entailed a new physics.
Yet even in a single system, Einstein established simultaneity as a matter of convention by freely stipulating that the speed of light is equal in opposite directions. Most physicists think instead that the equal speed of light in all directions is a matter of fact. Some seem repulsed by the idea that the foundations of physics involve key elements that are not set empirically. Simultaneity is central to special relativity. So, some people might think that simultaneity as essentially a convention would lead to, as Jammer puts it, “disastrous consequences for the philosophical understanding and epistemological status of physics and with it of the whole of modern science.”
Thus physicists and philosophers have contrived various proposals to prove the isotropy of the speed of light. Those include clocks coupled mechanically by rigid shafts, infinitely slow clock transport, observations of Jupiter's moons, and the use of galvanometers, cavity microwave resonance, and optical interference as sources of measurement. Such methods, however, have suffered refutations and retractions as critics have shown that they involve tacit assumptions equivalent to the standard simultaneity convention. Hence, John Norton once compared this persistent quest to the search for a perpetual motion machine.
In 1970 John Winnie derived a relativistic kinematics independent of the assumption that light speed is isotropic and showed that the theory shares identical experimental consequences with special relativity. Jammer expertly clarifies questions of symmetry and transitivity of nonstandard synchronisms. Meanwhile, some writers have advanced various nonempirical arguments based on simplicity and on causal theories of time to attempt to refute the claim that simultaneity is a convention. Various writers, even Jammer, have criticized such arguments, though Jammer cites a couple of arguments that he does not refute. He actually achieves the tone of an impartial scholar, which is much preferable to that of an impatient and biased judge. He treats conventionality as an unresolved, fundamentally important issue in physics.
The author also analyzes simultaneity in the context of the general theory of relativity. He points out that in a general system of reference it is impossible to globally define simultaneity between any two events. Thus a key concept that sprouted special relativity lost its general validity in the later theory.
Concepts of Simultaneity excels at clearly explaining subtle but important issues. The book is incisive and valuable; it will appeal not only to historians and philosophers of physics but also to physicists drawn to the elements of special relativity. It is indeed as good as Jammer's earlier classics. Physicists used to have no comprehensive resource for catching up on the thought-provoking literature on simultaneity. Now they do.
