Responding to Philip Gibbs, George Ellis wrote (Dec. 31, 2008 @ 05:50 GMT):
[1] "By the way I did not emphasize it, but the choice of world lines I make is that of the Landau reference frame, representing the velocity given by the local average of all energy and mass fields in a small neighbourhood."
Responding to Vesselin Petkov, George Ellis wrote (Dec. 31, 2008 @ 05:52 GMT):
[2] "Causality operates as you state it, but only in the EBU region that already exists.
This domain is coming into being at the present instant; and what is the present instant now will in another fraction DT of time be in the past.
[3] "As I believe spacetime is quantised, I am happy to assume DT cannot be taken infinitesimally small but rather has a finite lower limit (associated with the Planck time); so there are no paradoxes associated with the present having zero time extent. I am happy if it has a very small but finite duration.
[4] "As regards the quantum case, say the double-slit experiment, I have not attempted a description of how particles exist or not in quantum theory. Whatever works for you in the BU words for me in the EBU according to principle (A), except for cases involving entanglement, which I come to in a minute. I hold by the statement that quantum theory only predicts probabilities, not specific outcomes.
[5] "Finally, as regards the issue of the EPR-type *experiments*: you state "those experiments are perfectly explained by the BU because the future exists there. But they cannot be explained by the EBU since, on that view, it follows that the non-existing future can determine the outcome of an experiment."
[6] "But as already stated in a previous post, the EPR analysis is not a relativistic analysis: it is based in the Schroedinger equation.
[7] "As long as any experiment whatever gives a future outcome that is not at present determined even in principle, the EBU description trumps the BU. And we have plenty of such experiments in quantum theory. The relativistic interpretation of the EPR experiments will have to adjust to this fact."
Comment #1: Given the statement [1], about "the local average of all energy and mass fields in a small neighbourhood", and the statement [3], I am under the impression that George Ellis should define the gravitational energy in some 'elementary increment of time' which has "a very small but finite duration". This will be a daunting task, to say the least, because it will require a rigorous explanation of how a "fraction DT of time" [2] is "associated with the Planck time" [3].
Comment #2: By assuming that causality operates "only in the EBU region that already exists" [2], he presupposes the possibility that some physical stuff, call it B , fills in "the present instant now" in a "fraction DT of time", which in turn *implies* that there was (past perfect) some physical stuff A , whose (potential?) future was somehow related, or perhaps influenced, by B . Thus, in order to distinguish EBU from BU, George Ellis has to somehow eliminate totally A in the "fraction DT of time" in which B is present. But the very meaning of B is relational, that is, 'with respect to A and C'. The latter, C , is needed to define B , just as much as A is needed to define B .
I am unable to understand the difference between EBU and BU hypotheses. I'm sure George Ellis can recognize St. Augustine's comments on time in the puzzle above. In modern language, the puzzle was explained by David Bohm as follows (Wholeness and the Implicate Order, Ark edition 1983, p. 204):
"So, if we say that the velocity of a particular *now* (at t_2) is (x_2 - x_1) / (t_2 - t_1) we are trying to relate *what is* (i.e., x_2 and t_2) to *what is not* (i.e., x_1 and t_1). We can of course do this *abstractly and symbolically* (as is, indeed, the common practice in science and mathematics), but the further fact, not comprehended in this abstract symbolism, is that the velocity *now* is active *now* (e.g., it determines how a particle will act from now on, in itself, and in relation to other particles). How are we to understand the *present activity* of a position (x_1) now non-existent and gone for ever?"
Comment #3: Regarding statements [4] - [7], they refer to the clash of STR and QM, which is still unresolved. One of the reasons is that EPR involves counterfactual reasoning, namely, what could have been the outcomes of measurements of "two" (in fact, one) entangled observable(s), had there been some 'now-at-a-distance' reference frame in which one can verify the alleged "instantaneous" correlation. I don't think counterfactual reasoning can be used to support EBU or BU. It is just one big mess.
Happy New Year.
