Therefore, P(Sentient Aliens or Simulation) and P(You are a Boltzmann Brain) move in opposite directions when updated with new evidence.
If those are the only two possibilities then of course they do. A complete set of probabilities must sum to 1 and so P(Aliens) is simply (1 - P(Bolzmann)).
If those are the only two possibilities then of course they do. A complete set of probabilities must sum to 1 and so P(Aliens) is simply (1 - P(Bolzmann)).