We have to bear in mind that all our propositions involving time are always propositions about simultaneous events.
-- Albert Einstein, 1905
Introduction
Modern physics, as is well known, led to a radical revision of the fundamental concepts of classical physics, such as the concepts of space, time, matter, energy, and causality.
On the 14 December 1900, the “birthday” of the quantum theory, Max Planck announced what he then called “the natural constant h” and what later, under the name of “Planck’s constant,” became the “trademark” of quantum mechanics. For a similar reason Friday, 30 June 1905, may be called the “birth-day” of the theory of relativity, for on that day Albert Einstein’s seminal paper on the special theory of relativity was received by the editorial board of the Annalen der Physik.
Priority questions are generally, and rightly, regarded as nugatory and not worth mentioning. In the present case, however, the claim that, despite the chronological precedence of the birth of the quantum theory, it was Einstein’s 1905 relativity paper that initiated the conceptual revolution of modern physics was made most eloquently not, as may be expected, by a relativist but by one of the fathers of quantum mechanics, Werner Heisenberg. In his Gifford Lecture, delivered at the University of St Andrews in the winter semester 1955/56,
Heisenberg declared:
“Within the field of modern physics the theory of relativity has played a very important role. It was in this theory that the necessity for a change in the fundamental principles of physics was recognized for the first time.” [1]
A similar statement had already been made by Heisenberg in 1934 when he said:
“The fundamental presuppositions of classical physics, which led to the scientific picture of the 19th century, had been challenged for the first time by Einstein’s special relativity.”
“It was the assumption that it is meaningful without further consideration to call two events simultaneous in the case they do not occur at the same place.”
Heisenberg’s statement, that Einstein’s 1905 analysis of the concept of distant simultaneity (i.e., of spatially separated events) inaugurated the modern physical world picture, can be confirmed by the fact that Einstein himself once admitted:
“By means of a revision of the concept of simultaneity in a shapable form I arrived at the special relativity theory.”[2]
A magniloquent formulation of Heisenberg’s claim was given more recently by the cosmologist Julian B. Barbour:
“Einstein’s definition of simultaneity opened the door into a world as unexpected as the one inadvertently discovered by Copernicus when trying to save uniformity in the heavens.”[3]
Heisenberg, who knew, of course, his teacher’s statement concerning the birth of the quantum theory, could nevertheless make this priority claim because he realized that the philosophical implications of a new physical theory did not necessarily need to be recognized at the birth of that theory. Indeed, as we know from documentary evidence, prior to 1906 Planck and his colleagues thought it was possible to “fit” the constant h into the conceptual framework of classical physics.[4] However, Einstein’s 1905 relativity page immediately left no doubt that the classical notions of space and time could no longer be maintained.
It was precisely because of its revolutionary innovations that the more conservative members of the editorial board of the Annalen considered the paper, with its seemingly bizarre notions of time dilation, length contraction, and relativity of simultaneity, written by a clerk of a patent office, more as a piece of “science fiction” than as a serious scientific work. Thus, what later was called “possibly the most important paper that has been written in the twentieth century”[5] might have been returned to its author as unfit for publication had it not been for Planck, who as the representative of the German Physical Society was the chairman of the editorial board and who immediately understood the paper’s importance.
It would be wrong to assume that this notion became the subject of critical attention only with the advent of the theory of relativity. The fact that many philosophers, including such prominent thinkers as Aristotle, Leibniz, and Kant, thought that this notion required closer analysis seemed not to be well known, certainly not among physicists. Einstein’s treatment of this concept has a noteworthy prehistory that can be traced back to antiquity. It also has an equally important posthistory. As Graham Nerlich, a philosopher of the University of Adelaide, rightly remarked in 1982: “It is hard to overestimate the impact of Einstein’s definition of distant simultaneity on philosophy in this century, set, as the words were, in the context of a highly successful theory of physics.”[6]
To show that such a statement is justified, at least as far as the philosophy of physics is concerned, it suffices to recall that, in accordance with the theory of relativity, even such an elementary concept as the length of a line segment or of a rod, moving relative to an inertial system, involves the concept of distant simultaneity. That not only temporal but also spatial measurements depend on the notion of simultaneity follows from the simple fact that “the length of a moving line-segment is the distance between simultaneous positions of its endpoints,” as Hans Reichenbach, in the chapter entitled “The Dependence of Spatial Measurements on the Definition of Simultaneity” in his influential book The Philosophy of Space and Time[7] convincingly demonstrated. Having shown that “space measurements are reducible to time measurements” he concluded that “time is therefore logically prior to space.” Since, in turn, the notion of time, as Einstein demonstrated in 1905, presupposes a definition of simultaneity, it is clear that, indeed, the importance of the concept of simultaneity for kinematics, and therefore for physics in general, can hardly be exaggerated.
This holds especially for the theory of relativity. For instance, P. F. Browne rightly pointed out that all relativistic effects are ultimately “direct consequences of the relativity of simultaneity.”[8] Or as Ernan McMullin wrote: “The ‘relativity’ of the new theory—one of the most solidly verified theories in the entire range of physics—is chiefly, therefore, a relativity of simultaneity.”[9]
One of the major problems debated by philosophers of science in our time is the controversial question of whether the concept of distant simultaneity denotes something factual, empirically testable, or at least unambiguously definable, or whether it refers to merely an object of a convention, that is to an arbitrary stipulation without any factual content, as to which events are to be called simultaneous. This “problem of the conventionality thesis concerning the concept of distant simultaneity” has far-reaching implications. If, as mentioned above, the concept of distant simultaneity is a fundamental ingredient in the logical structure of the theory of relativity but is in reality nothing but a convention, the question naturally arises of whether this does not imply that the whole theory of relativity and with it a major part of modern physics are merely fictions devoid of any actual content. A positive answer to this question would have disastrous consequences for the philosophical understanding and epistemological status of physics and with it of the whole of modern science.
The concept of simultaneity also plays a significant role in other branches of physics, in particular, in quantum mechanics. A well-known example is the quantum-mechanical entanglement {such as the one exhibited in the Einstein-Podolsky-Rosen type experiments,} where the outcome of spatially separated measurements are instantaneously correlated. Indeed, “the uneasiness of fit between relativity and quantum mechanics regarding the treatment of measurements hinges on the concept of simultaneity.”[10]
As we will see, the notion of simultaneity also plays an important role in classical physics. Unfortunately, textbooks in the physical sciences, unless dealing exclusively with the theory of relativity in general, completely ignore the notion of simultaneity. A noteworthy exception is Herbert Goldstein’s excellent text on classical mechanics which states explicitly at the beginning of its first chapter that “basic to any presentation of mechanics are a number of fundamental physical concepts, such as space, time, simultaneity, mass and force.”[11]
One of the major objectives of this treatise is to show that this concept has occupied the attention of philosophers and scientists throughout the whole history of human thought and played an important role in the writings of such intellectual giants as Aristotle, St. Augustine, Leibniz, and Kant.
It would be a serious mistake to associate the concept of simultaneity exclusively with philosophical or scientific reasoning. In fact, it was at the level of prescientific apprehension, a fundamental ingredient in the process of human apperception and conception of time. As Gerald James Whitrow pointed out, “our conscious appreciation of the fact that one event follows another is of a different kind from our awareness of either event separately.
If two events are to be represented as occurring in succession, then—paradoxically—they must also be thought of simultaneously.” [12]
Footnotes
W. Heisenberg, Physics and Philosophy (New York: Harper & Row, 1958), p. 110 ↩︎
Quoted by A. Fölsing, Albert Einstein: a Biography (New York: Viking, 1997), p. 176; ↩︎
J. B. Barbour, Absolute or Relative Motion? (Cambridge: Cambridge University Press, 1989), vol. 1, p. 676. ↩︎
Cf. M. Jammer, The Conceptual Development of Quantum Mechanics (New York: McGraw-Hill, 1966), p. 22; enlarged edition published as vol. 12 in the series The History of Modern Physics (New York: The American Institute of Physics, 1989), p. 17. See also T. S. Kuhn, Black-body Theory and the Quantum Discontinuities (Oxford: Clarendon Press, 1978), pp. 125–126. ↩︎
R. W. Clark, Einstein—The Life and Time (New York: Avon, 1972), p. 116. ↩︎
G. Nerlich, “Simultaneity and convention in special relativity,” in R. McLaughlin (ed.), What? Where? When? Why? (Dordrecht: Reidel, 1982), p. 130. ↩︎
H. Reichenbach, The Philosophy of Space and Time (New York: Dover Publications, 1958), chapter 3. ↩︎
P. F. Browne, “Relativity of rotation,” Journal of Physics A10, 727–744 (1977). Quotation on p. 731. ↩︎
E. McMullin, “Simultaneity,” in the New Catholic Encyclopedia (New York: McGraw-Hill, 1967), vol. 13, p. 234. ↩︎
H. Chang and N. Cartwright, “Causality and realism in the EPR experiment,” Erkenntnis 38, 169–190 (1993). ↩︎
H. Goldstein, Classical Mechanics (Cambridge, MA: Addison-Wesley, 1951), p. 1. ↩︎
G. J. Whitrow, The Natural Philosophy of Time (London: Thomas Nelson, 1961), p. 75. ↩︎