Dear Prof. Ellis,
1. I think that the EBU is a good alternative solution to the problem of time. On the other hand, I do not think that the other versions of BU can be rejected that easy, as it seems to result from your post "on Dec. 29, 2008 @ 17:53 GMT". The standard BU attempts to express the temporal structures in terms of timeless structures. We can consider it, in a way, as a research program of explaining the time itself in terms of timeless structures. But, by adhering to a presentist view, and by reducing the BU functionality to a purely archiving role, there is the danger of explaining the time by appealing to time in a circular way: the EBU includes the passed time in the archived BU, but the evolution happens in a metatime. Another interesting feature the BU has is that it contains all the physical fields in its description. By giving a special role to the present, we introduce a feature which has no correspondent in the matter fields. The BU accounts for the physical fields, but it cannot include an intrinsic present, and maybe doesn't even need. Yet, if it would need to mark the present, a "BU with a bookmark" would solve the problem.
2. You propose as experimental support for deciding between the EBU and the BU the double slit experiment, in the post "on Dec. 29, 2008 @ 17:53 GMT". A. The double slit experiment, as well as other wavefunction collapse experiments, cannot decide between determinism and indeterminism, since there are deterministic approaches to QM which provide the same probabilistic behavior at observer's level. B. Assuming anyway that this experiment proves that the world is fundamentally indeterministic, the BU is not necessarily deterministic. We can have a BU in which knowing the fields and derivatives on a spacelike hypersurface does not determine uniquely the fields on other hypersurface, which has a greater time coordinate. Even more, we can have a branching/forking/splitting BU, which contains the indeterminism and all possible subsequent development, without appealing to presentism.
3. Consider Wheeler's delayed choice experiment, with the photon emitted by a distant star. The observer watching the star will decide whether to measure the "both ways", or the "which way". His decision affects the past history of the observed photon, hence of the observed star. Of course, it is unlikely that he affected the star's state in a significant way, but he affected it at least in a small way. Until the observation, the photon, hence the star (by entanglement), was in an undefined state. Assume now that the photon is never observed, and escapes far from any planet and any possible observer. The Universe will remain in an undetermined state. So, we cannot say that the past block will be ever created. On the other hand, my proposal of a "registry" [1, 2, 3] of incomplete initial data which increases with each observation allows the possibility that the state of the Universe remains undetermined.
4. About the "preferred spacelike surfaces", I think that it is not necessary to violate/break the Lorentz symmetry. One can use for time in EBU, instead of a totally ordered set, a more general relation, which will allow the relativity of simultaneity (in the worst case, the causal structure in General Relativity can be such a relation). But I am afraid that the entanglement can be complicated enough. The measurements of the spins of the two electrons in the EPR-B experiment can be in any spacetime relation. We cannot consider that the wavefunction collapse takes place necessarily along such preferred spacelike surfaces, which are compatible with a spacetime foliation. It is easy to see that, if we associate spacelike surfaces to the collapse, it is possible that these intersect in complicated ways. Moreover, collapse can take place also between events that cannot belong to the same spacelike surface, being for example one in the other's future. This is why I believe that it would be preferable to avoid such "absolute space" solutions.
5. It is hard for me to see that there is a time asymmetry occurring at a fundamental level in Physics. I think that Boltzmann provided a good explanation, at least as a starting point, for the thermodynamic time arrow. The decoherence and the measurement may be based on dissipation or other thermodynamically irreversible processes. Maybe we are not sure, but this is not necessarily a sign of a fundamental time asymmetry. But I agree that the EBU needs such a time asymmetry. On the other hand, the "registry" approach does not rely on an absolute time arrow, nor on an absolute spacetime foliation: we can collect the data in any order, in principle, but it is more likely that the order correlates with the increase of entropy.
You conclude the post by "I could go on, but that should suffice to show there is excellent experimental and observational support for the EBU as against the BU." I think that, at the present moment, we don't have enough support for one side or the other, but, as the discussion around your joke "on Jan. 2, 2009 @ 08:13 GMT" ("The running of this competition gives a nice example of why the EBU model trumps the BU.") seems to suggest, we are free to interpret the same data as sustaining BU or EBU, according to our preferences.
Therefore, I think that, although I like your essay and I consider a good candidate the EBU, the BU still remains a good alternative. On the other hand, I agree that the EBU (as well as my "registry" proposal) reflects well our feelings of flow of time, free-will, and open future.
Warm regards,
Cristi
