There is a hypothesis that universes with reversible time can&#x27;t exist. Or time travel of any kind. Such universes, the hypothesis goes, can&#x27;t have any meaningful cause and effect, like we know it. Causality is an extremely important property of our universe. And it&#x27;s really weird that our universe has it, if the hypothesis isn&#x27;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: <a href="http:&#x2F;&#x2F;lesswrong.com&#x2F;lw&#x2F;fok&#x2F;causal_universes&#x2F;" rel="nofollow">http:&#x2F;&#x2F;lesswrong.com&#x2F;lw&#x2F;fok&#x2F;causal_universes&#x2F;</a>