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#43 Time and Relativity Theory

February 11, 2008

My name is Thomas. I am currently [reading] your book Time and the Metaphysics of Relativity and have a few questions about it. First of all thank you for writing this excellent study. I am finding it quite enjoyable and fascinating. My question centers around the difference between Einstein's original interpretation of SR and a realistic approach to Minkowski's spacetime formalism.

1) You mention that on a realistic construal of Minkowski's formalism all events exist tenselessly in a four dimensional spacetime. How do we deduce that the spacetime manifold is tenseless? Is it because, as you say on pages 74-77, for one observer S who draws a line of simultaneity with a given event P some event A and another event A' will be past and future respectively yet for another observer moving relative to S who draws a line of simultaneity with P, A and A' will be future and past respectively? If this is how we deduce that the spacetime manifold is tenseless, then would we not also have to regard time as tenseless in Einstein's original interpretation of SR because for a given reference frame two events A and A' will have different temporal locations in another reference frame?

2) In your discussion on pages 30-38 on the conventionality of simultaneity you argue that the light synchronization procedure is ad hoc within Einstein's original interpretation but is perfectly at home in Minkowski's formalism. I understand why the light synchronization procedure is ad hoc in Einstein's original interpretation (we have to assume that the one-way velocity of light is constant but can only measure the round trip velocity) but could not follow your argument as to why it is perfectly natural in Minkowski's formalism. could you explain your reasoning here?



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Dr. craig’s response


I’m glad to find you interested in questions concerning the philosophy of time, Thomas. As you know, I find these issues relevant to how we understand divine eternity and God’s relationship to time.

For readers without your background, let me explain that there are three radically different physical interpretations of the mathematical equations of Einstein’s Special Theory of Relativity which are often run together, even in textbook discussions of the theory. Einstein’s original 1905 interpretation was a theory of familiar 3-dimensional physical objects enduring through time. Einstein held that associated with each uniformly moving observer’s reference frame is a unique time and space, each with its own simultaneity relations, so that there is no over-arching way the world is. Lorentz’s interpretation also involved a world of 3-dimensional spatial objects enduring through time. But he held that there is an absolute, all-embracing time and space with privileged simultaneity relations. But motion affects our clocks and measuring rods so that absolute time and space are veiled to us. Minkowski proposed a different ontology: his world is a world of 4-dimensional objects extended not only in space but also in time. On his view there is an over-arching way the world is, namely, the 4-dimensional description giving a thing’s co-ordinates in spacetime.

So with respect to question (1) Minkowski’s interpretation is tenseless just because it is described in terms of a 4-dimensional geometrical object called spacetime. All points in spacetime exist. Things are extended in time just as they are in space. Hence, there is no temporal becoming. To try to reintroduce temporal becoming just is to abandon the geometrical approach and revert to Einstein’s space and time approach. So it’s not the relativity of simultaneity that implies Minkowski’s interpretation, for, as you note, on Einstein’s interpretation simultaneity is also relative. Rather the difference concerns the fundamental objects of the theory.

As for (2), you somewhat misunderstand me. The point is rather that given that spacetime is Minkowskian, simultaneity relations relative to an observer’s co-ordinate system are not conventional or arbitrary. Since Einstein’s interpretation is equivalent to Minkowski’s when expressed in 4-dimensional language, the same goes for Einstein’s interpretation. But that doesn’t settle whether spacetime is Minkowskian. If Lorentz is right, then spacetime merely appears to be Minkowskian. In fact, however, light does not travel at a uniform velocity relative to every uniformly moving observer. Since Lorentz’s interpretation was observationally indistinguishable from Einstein’s interpretation, Einstein’s claim that the one-way velocity of light is constant was gratuitous.

So the real quarrel here is between Lorentz on the one hand and Einstein and Minkowski on the other. I argue that Einstein’s interpretation, which Einstein himself abandoned as soon as he became aware of Minkowski’s 4-dimensional formulation, is really kooky, so that the choice is really between Lorentz and Minkowski. How can we decide between the two? Well, if you believe, as I do, that time is tensed rather than tenseless, then you should go with Lorentz.

So it’s a choice between a tenseless and a tensed theory of time. Do you think that the difference between past and future is real (the tensed view) or just an illusion of human consciousness (the tenseless view)? Are past and future events on an ontological par (the tenseless view) or is temporal becoming an objective feature of the world (the tensed view)? These are profound questions with far-reaching consequences, not only for science, but even for theology.

For I claim that God’s timeless existence, given that there is a temporal world, is possible only if a tenseless view of time is correct; whereas if a tensed view is right, God exists temporally in absolute time. Since I am firmly convinced that a tensed view of time is correct, I think that Lorentz was, in fact, right, and that God accordingly exists in time.

- William Lane Craig