TITLE: Boltzmann brain vs Actual universe probability actual calculations
QUESTION [0 upvotes]: I have been reading lots of physics lately about how universe came into being. One idea is that universe is long gone (thermal death) and it might not exist the way I feel it and I am a sequence of Boltzmann brains fluctuating into existence over incredibly long amounts of time which makes me experience my past and present, and the brains that will be quantum fluctuated will be my future.
The likelihood of universe that we know fluctuating into existence is much less likely due to the size of fluctuation needed. But then I think of calculating probability of universe fluctuating once (and then evolving the way we all know) against many Boltsmann brain fluctuating to represent my lifetime I have to think of the size of a brain.
What sort of Boltsmann brain am I? I don't necessarily have to be human brain I could be software that fluctuates on a microchip and every-time I fluctuate I can be a different kind of brain. Has anyone done any sort of calculations that I could look at what size of fluctuation did they use to compare probabilities.
I have read quite a few articles that claim that we are likely to be Boltzmann brain but couldn't find any actual math done, I would be surprised if there is no article with calculations on the web.

REPLY [1 votes]: The problem with the calculation is that it depends a lot on the underlying assumptions. Once you assume that Boltzmann brains are plausible, restricting the calculation to our brain structure and our current model of the universe seems absurd. What should be computed is the probability over all possible universes and all possible structures that give rise to consciousness. The problem is, that we cannot currently ever compute something more simple than that, which is the likelihood of our current universe, even is we restrict ourselves to computable universes (for general non-computable universes there is not even a meaningful measure of probability). For a more detailed discussion you can start with https://en.wikipedia.org/wiki/Mathematical_universe_hypothesis

REPLY [1 votes]: The fundamental problem here is well described in this article. You can step aside a bit from the assumptions made, the problem is a generic one. In any generic cosmological model, you'll get Poincaré recurrences and then the most likely ones will be where you exist as a Boltzmann brain like state. But this then causes predictions of your observations that violate our observations, because you'll be far more likely experience a far more random universe than we actually observe.
{"set_name": "stack_exchange", "score": 0, "question_id": 205826}