"To compute a consistent universe with a low-entropy terminal condition and high-entropy initial condition, you have to simulate lots and lots of universes, then throw away all but a tiny fraction of them that end up with low entropy at the end. With a low-entropy initial condition, you can compute it out locally, without any global checks. So I am not yet ready to throw out the arrowheads on my arrows."
Here's the problem with this argument. Your simulations are occurring as a sub-history of a universe where the second law of thermodynamics alr... (read more)
The symmetry corresponding to conservation of energy is time translation, not time reversal. That said, I have other reasons to be doubtful of Yudkowsky's thesis here. In particular, I think it really does come down to entropy.