Einstein, Relativity and Absolute Simultaneity

William Lane Craig, Quentin Smith

• Publisher: Routledge
• Year: 2007
• ISBN:
  9781134003884
• Edition: 1
• Topic: Philosophy
• Subtopic: Philosophy History & Theory

Einstein, Relativity and Absolute Simultaneity is an anthology of original essays by an international team of leading philosophers and physicists who have come together to reassess the contemporary paradigm of the relativistic concept of time. A great deal has changed since 1905 when Einstein proposed his Special Theory of Relativity, and this book offers a fresh reassessment of Special Relativity's relativistic concept of time in terms of epistemology, metaphysics, and physics.

1 The metaphysics of special relativity:
three views

William Lane Craig

Introduction

A physical theory is comprised of two components: a mathematical formalism and a physical interpretation of that formalism. Competing theories which differ only in virtue of their divergent physical interpretations can be extremely difficult to assess if they are empirically equivalent in their testable predictions. Considerations which are metaphysical in nature may then become paramount.

The Special Theory of Relativity (hereafter SR) provides a case in point. Herman Bondi has remarked that ‘‘there is perhaps no other part of physics that has been checked and tested and cross-checked quite as much as the Theory of Relativity’’ (Bondi 1964: 168). Indeed, muses J. G. Taylor, ‘‘as far as special relativity is concerned all has been worked out and tested;’’ the theory has enjoyed ‘‘remarkable successes, and absolutely no failures’’ (Taylor 1975, preface). The empirical success of SR’s testable predictions can, however, be misleading, dulling us to the truly controversial nature of the correct physical interpretation of the theory’s formalism. The fact is that the only version of SR which is experimentally verifiable, as Geoffrey Builder points out, ‘‘is the theory that the spatial and temporal coordinates of events, measured in any one inertial reference system, are related to the spatial and temporal coordinates of the same events, as measured in any other inertial reference system, by the Lorentz transformations’’ (Builder 1971: 422). But this verifiable statement is underdeterminative with regard to the radically different physical interpretations of the Lorentz transformations given, respectively, by Einstein, Minkowski, and Lorentz.

During the decades in which positivism dominated the philosophy of science these differences tended to be glossed over, since empirically equivalent physical interpretations of the same mathematical formalism were regarded as but different linguistic expressions of the same theory. But with the collapse of positivism – arguably the most important event in philosophy in the second half of the twentieth century (Burge 1992: 49) – such indifference toward the fundamentally different ontological structures of space, time, and space-time which appear in these three interpretations can no longer be ignored. Unfortunately the articulation of a post-positivist philosophy of space and time has only scarcely begun. Minkowskians have issued critiques of Einsteinians in the effort to justify the former’s space-time realism, and the largely marginalized neo-Lorentzians have criticized what we might call the received interpretation of SR (an Einsteinian-Minkowskian amalgam which fails to differentiate these viewpoints) for its denial of relations of absolute simultaneity; but I know of no critical appraisal in the literature which lays these three interpretations side by side and attempts to come to some adjudication of them. In this paper I propose to do just that.

Three relativistic interpretations

The Einsteinian interpretation

SR, as Einstein originally formulated it, postulates a 3+1-dimensional ontology, not a 4-dimensional ontology (Einstein 1981).¹ That is to say, it is a theory of familiar physical objects enduring through time. Space and time are relativised to reference frames, which serve to define distant simultaneity and along with it notions like rest, motion, speed, and velocity. Light is postulated to have the constant velocity c in every reference frame. Because physics is relativised to reference frames, clocks run at different rates and measuring rods have different lengths relative to different frames. Such an interpretation of SR implies an anti-realist or instrumentalist understanding of Minkowski space-time.² There is no tenselessly subsisting manifold of events; space-time is a theoretical construct only, a geometrical representation of a theory which is really about physical objects enduring through time. A Minkowski diagram will prove to be a helpful tool, but it neither depicts reality nor implies an ontology. A good representative of this original Einsteinian perspective is the French physicist Henri Arzeliés. In his Relativistic Kinematics, Arzeliés asserts, ‘‘The Minkowski continuum is an abstract space of four dimensions, the sole role of which is to interpret in geometrical language statements made in algebraic or tensor form. . . . The four-dimensional continuum should therefore be regarded as a useful tool, and not as a physical ‘reality’’’ (Arzeliés 1966: 258). While it is true that relativity theory banishes the notions of absolute spatial and temporal intervals from physics, nonetheless ‘‘It is perfectly clear that in relativity, the ordinary three-dimensional space (which is Euclidian in special relativity) and the time of pre-relativistic physics is employed’’ (Ibid).

The Minkowskian interpretation

There is no gainsaying Arzeliés insofar as Einstein’s original formulation of SR is concerned. But it is also indisputable that once having encountered Minkowski’s geometrical formulation of the theory, Einstein became an outspoken realist concerning space-time (Einstein and Infeld 1938: 219; Einstein 1961: 150; Einstein and Besso 1979: 276–77).³ Minkowski took his space-time ontologically: it was not merely a geometrical representation of the world of space and time as described by Einstein’s SR; rather it was the world. When he said, ‘‘A point of space at a point of time, that is, a system of values x, y, z, t, I will call a world-point. The Multiplicity of all thinkable x, y, z, t systems of values we will christen the world,’’ (Minkowski 1952: 76) he was making self-consciously a metaphysical statement, proposing a new ontology. Heralding ‘‘a metamorphosis of our concept of nature,’’ Minkowski declared, ‘‘Henceforth, space by itself, and time by itself, are doomed to fade away into mere shadows, and only a kind of union of the two will preserve an independent reality’’ (Ibid., 75, 76). On this second interpretation of SR, the notions of reference frames, invariant velocity of light, distant simultaneity, relative motion or rest, and so forth, so central to the Einsteinian interpretation, play no role.⁴ Rather the central feature of this interpretation is the light cone structure at any space-time point, which determines the geometrical properties of space-time. In 1911 A. A. Robb was able to recover all the geometric structure of Minkowski space-time on the basis of the single relation after among its points, conjoined with several conditions of that relation (Robb 1913). Taking Robb’s relation to be extensionally equivalent to some sort of causal relation, recent theorists have defined causally the Lorentz group of transformation equations (Zeeman 1964: 490–93), orthogonality to a time-line in Minkowski space-time (Malament 1977: 293–300), and the metrical congruence of intervals in that space-time (Winnie 1977: 134–205). Space-time realists debate intramurally whether causality is truly constitutive of, rather than merely (at best) coextensive with, Robb’s fundamental relation,⁵ but the point remains that the familiar physical entities of the Einsteinian interpretation make no appearance in the space-time interpretation. These two interpretations of relativity theory thus present strikingly different metaphysical visions of reality; they are as radically divergent in their ontologies as is relativity theory itself in comparison with the Newtonian physics of absolute time and space. Minkowski’s space-time approach to relativity theory, especially with the development of the General Theory of Relativity (GR), has come to be the dominant mode of presentation and discussion of relativity.

The Lorentzian interpretation

It is an interesting historical fact that neither of the giants of late nineteenth century physics to whom Einstein looked for inspiration in his work on SR, H. A. Lorentz and Henri Poincaré, was ever convinced, despite being fully apprised of the empirical facts, of the truth of the Einsteinian or Minkowskian interpretations of the Lorentz transformations. Well after Einstein had formulated his SR and as he struggled to craft a GR, Lorentz in particular continued to study and lecture on problems of relativity, often in connection with Einstein. By 1908 Lorentz had already realized the incompatibility of his electron theory with Planck’s quantum hypothesis, and by the 1911 Solvay Congress there was a general sense that the electron theory would have to be radically reformed in light of the advent of quantum physics (McCormach 1970: 486–88). Nonetheless, since Lorentz’s attempted explanation of the phenomenon of length contraction in terms of the deformable electron was not essential to his basic physical interpretation of SR, Lorentz continued to adhere to an approach to relativity theory which preserved the classical notions of space and time. A theory may be classified as Lorentzian just in case it affirms that (i) physical objects are n-dimensional spatial entities which endure through time; (ii) the round trip vacuum propagation of light is isotropic in a preferred (absolute) reference frame R_o (with speed c = 1) and independent of the velocity of the source; and (iii) lengths contract and time rates dilate in the customary special relativistic way only for systems in motion with respect to R_o (Maciel and Tiomno 1989: 507–8).

Lorentz always spoke appreciatively of Einstein’s alternate approach and lectured sympathetically on both SR and GR, while remaining finally unconvinced that Einstein had abolished the classical conceptions of time and space. Writing in 1910, he contrasted his view with Einstein’s:

Assume there were an aether; then there would be among all systems x, y, z, t one singled out in that the coordinate axes as well as the clock is at rest in the aether. If one conjoins with this the idea . . . that space and time are something wholly different and that there is a ‘true time’ (simultaneity would then exist independently of location, in accord with the circumstance that it is possible for us to conceive of infinitely great velocities), then one easily sees that this true time would have to be indicated just by clocks which are at rest in the aether. If, then, the principle of relativity were generally valid in nature, then one would not be in a position to determine whether the coordinate system employed is that distinguished one. One thus comes to the same results as when one in agreement with Einstein and Minkowski denies the existence of the aether and the true time and treats all coordinate systems as equivalent. Which of the two modes of thought one may agree with is best left to the individual.

(Lorentz 1934: 211)⁶

Lorentz, realizing that his aether compensatory interpretation is empirically equivalent to the Einstein-Minkowski interpretations, leaves it up to the individual to choose which he shall adopt. But Lorentz preferred the classical conceptions of time and space on metaphysically intuitive grounds, as he made clear in his 1922 lectures at Cal Tech:

All our theories help us form pictures, or images, of the world around us, and we try to do this in such a way that the phenomena may be coordinated as well as possible, and that we may see clearly the way in which they are connected. Now in forming these images we can use the notions of space and time that have always been familiar to us, and which I, for my part, consider as perfectly clear and, moreover, as distinct from one another. My notion of time is so definite that I clearly distinguish in my picture what is simultaneous and what is not.

(Lorentz 1927: 221)

Here Lorentz refuses to jettison what he took to be the intuitively obvious reality of absolute simultaneity among events in the world just because one cannot determine which spatially separated events are simultaneous or because Einstein’s operationally re-defined notion of simultaneity is relative to reference frames. Moreover, he sees no good reason to scrap the intuitive distinctness of space and time in favor of Minkowski’s unified reality, space– time.

A major reason that Lorentz remained unconvinced was that he was not a positivist. In 1913 he wrote,

According to Einstein it has no meaning to speak of motion relative to the aether. He likewise denies the existence of absolute simultaneity.
It is certainly remarkable that these relativity concepts, also those concerning time, have found such a rapid acceptance.
The acceptance of these concepts belongs mainly to epistemology . . . It is certain, however, that it depends to a large extent on the way one is accustomed to think whether one is attracted to one or another interpretation. As far as this lecturer is concerned, he finds a certain satisfaction in the older interpretations, according to which the aether possesses at least some substantiality, space and time can be sharply separated, and simultaneity without further specification can be spoken of. In regard to this last point, one may perhaps appeal to our ability of imagining arbitrarily large velocities. In that way, one comes very close to the concept of absolute simultaneity.
Finally, it should be noted that the daring assertion that one can never observe velocities larger than the velocity of light contains a hypothetical restriction of what is accessible to us, [a restriction] which cannot be accepted without some reservation.

(Lorentz 1920a: 23)

Here Lorentz clearly discerns the foundational role played by Einstein’s verificationist theory of meaning in his formulation of SR and rejects it. In defense of absolute simultaneity, Lorentz appeals to the use of arbitrarily fast signals, even though they are not presently observable. He disregards the assumption that it is meaningless to speak of such unobservables. Elsewhere Lorentz affirms that it makes sense, if there is an aether, to speak of motion relative to it even if observers could not detect such m...

Table of contents

• Cover Page
• Title Page
• Copyright Page
• Illustrations
• Introduction
• 1. The Metaphysics of Special Relativity: Three Views
• 2. Finding ‘‘Real’’ Time In Quantum Mechanics
• 3. A Radical Rethinking of Quantum Gravity: Rejecting Einstein’s Relativity and Unifying Bohmian Quantum Mechanics With a Bell-Neo-Lorentzian Absolute Time, Space and Gravity
• 4. Hidden Variables and the Large-Scale Structure of Space-Time
• 5 Non-Local Correlations In Quantum Theory: How the Trick Might Be Done
• 6. The Zero Acceleration Discontinuity and Absolute Simultaneity
• 7. Global Positioning System and the Twins’ Paradox
• 8. A Defense of Absolute Simultaneity
• 9. Cosmic Simultaneity
• 10. Presentism, Eternalism and Relativity Physics
• 11. The Special Theory and Absolute Simultaneity