There is a hypothesis that universes with reversible time can't exist. Or time travel of any kind. Such universes, the hypothesis goes, can't have any meaningfu

There is a hypothesis that universes with reversible time can't exist. Or time travel of any kind. Such universes, the hypothesis goes, can't have any meaningful cause and effect, like we know it. Causality is an extremely important property of our universe. And it's really weird that our universe has it, if the hypothesis isn't true. The space of possible universes without causality is much larger than restricted subset that obeys that rule. This is difficult to explain, so see here: http://lesswrong.com/lw/fok/causal_universes/ http://lesswrong.com/lw/fok/causal_universes/