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DanielLC comments on Causal Universes - Less Wrong
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I'm not sure we're in a causal universe.
According to the theory of timeless physics, our universe is governed by Schrödinger's time-independent equation. If the universe can expand without limit, and the potential energy eventually decreases below the total energy, kinetic energy will be driven below zero. This means that the amplitude will change exponentially as the universe expands. There are two ways this can work. Either the amplitude will increase exponentially, in which case we'd expect to be in one of those negative-energy configuration states, or the amplitude will exponentially approach zero, in which case the universe would look pretty much like it does now. If you just set boundary conditions at the big bang, you'd almost certainly end up in the former case. In order to get the latter case, you have to have part of your boundary conditions that as the universe expands the amplitude approaches zero. This means that the universe is partially governed by how it ends, rather than just how it begins.
There are other explanations, of course. Perhaps amplitude doesn't matter after a while. Perhaps the multiverse is finite. Perhaps timeless physics was wrong in the first place. Perhaps it's something else I haven't thought of. I'm just not sure that the expanding universe boundary condition should be rejected quite yet.
Also, if you accept SSA, the universe is acausal. The probability of being now is dependent on the number of people in the future.
I guess you could still build a causal graph if the universe is defined by initial and end states - you'd just have two disconnected nodes at the top. But you'd have to give up the link between causality and what we call "time".
"But you'd have to give up the link between causality and what we call 'time'."
You'd just have to make it slightly weaker. Entropy will still by and large increase in the direction we call "forward in time". So long as entropy is increasing, causality works. I don't think the errors would be enough to notice in any feasible experiment.