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Starting from the assumption of a cosmology in which Boltzmann brains dominate over evolved ones, it is not immediately obvious to me that there is a real problem, since only Boltzmann brains indistinguishable from evolved brains will be posing the question in the first place.

In other words, the vast majority of Boltzmann brains will not be troubled by the paradox because their memories will not be consistent with a stable/large physical universe, and so they would never ask such a question in the first place.

So we must only consider the subset of Boltzmann brains whose experiences would be so consistent with a universe like ours that they would pose the question "why are we not Boltzmann brains?" And in such cases, even though we may be Boltzmann brains, our experiences would be indistinguishable from evolved brains.

Is this a criticism of the Boltzmann brain paradox that has been dispatched with, or is it a legitimate cause to be suspicious of whether it is really a paradox at all?

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    $\begingroup$ In Boltzmann's scenario, you would merely have to imagine that the universe is stable and whatnot. I.e. your whole memory itself would be a figment of imagination, having accidentally come together a nanosecond earlier, and soon to burst apart and rejoin equilibrium. $\endgroup$
    – Ruben Verresen
    Commented Jul 26, 2017 at 10:29
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    $\begingroup$ @MitchellPorter The room for the extended discussion has been frozen. But I have to say that the conversation there is very strange: It seems OP keeps making up more and more ad hoc conjectures about the possible behaviour of conciousness "flowing" between different Boltzmann brains (the existence of which, to be noted, is also complete conjecture.) This continues with unjustified conclusions for epistemology. Not to mention the beyond hand wavy references to probability in a setting in which, in my opinion, the use of probability is at least unjustified and probably (pun intended) non-sense. $\endgroup$
    – ArtOfProblemSolving
    Commented May 29, 2020 at 22:37
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    $\begingroup$ To make this point clearer: Anybody can put arbitrary probabilities on things which are constructed such that there is no way to rigorously quantify them. I mean you can make arbitrary twists to your "prior probabilities". Maybe us having this discussion is a prior probability, too? I think the arguments of OP lead into this list. Additionally, if you change what exactly constitutes a Boltzmann brain, you can easily change your probabilites by a factor of much more than a Googolplex. $\endgroup$
    – ArtOfProblemSolving
    Commented May 29, 2020 at 22:46
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    $\begingroup$ These self-references are used to sidestep any sort of falsifiability, along the lines of the creationist argument that "well it might look like the universe is more than 10'000 years old, but that is only because god created it that way". (This is nothing compared to "The next time you ask the question the previous logic is exactly the same and your memories are statistically more likely to be 'reset' to be consistent with stability".) $\endgroup$
    – ArtOfProblemSolving
    Commented May 29, 2020 at 22:57
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    $\begingroup$ The cream topping is the unbelievably baseless (and - from a utilitarian viewpoint - also useless) conjecture that "quantum mechanical randomness is 'things [in the conciousness of a Boltzmann brain] going wild' in benign small-enough ways." I am sorry for being a bit petulant, but I really fail to see how this has something to do with physics -- or science at all for that matter... $\endgroup$
    – ArtOfProblemSolving
    Commented May 29, 2020 at 23:02

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There is some basic condition that must be satisfied by any scientific theory explaining what the world really is: the proposed structure of the world must allow for its scientific cognizability.

This is a practice requirement: if a scientific theory suggests that we cannot do science, it is useless in the very broadest sense, even if such theory is actually true.

And when we try to put forward a scientific theory that the existence of the universe is explained by a random process like Boltzmann's brain, we immediately get a contradiction with the above basic condition.

Namely, if the Universe appeared as a result of an absolutely random process, then it is much more likely that it has just appeared in its current state and, therefore, all cosmology is nonsense, all cosmogony is nonsense, and even the very reasons that prompted the hypothesis of the Boltzmann brain are nonsense.

Thus, on the one hand, the question makes sense. On the other hand, the proposing answer has as its consequence the meaninglessness of the question. This is a problem.

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  • $\begingroup$ You are essentially re-stating the Boltzmann brain problem. If it is not a problem (such as for a reason posed in this question) then this obviates such a concern. $\endgroup$
    – user1247
    Commented Dec 22, 2020 at 3:07
  • $\begingroup$ We cannot do science in the Boltzmann brain-like world, because every world property reason would be "because our world was just created with such property". - why apples always fall down? - they don't, but our world was by chance just created in such state, that we think, that apples always fall down $\endgroup$
    – warlock
    Commented Dec 22, 2020 at 8:57
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    $\begingroup$ You say "And in such cases, even though we may be Boltzmann brains, our experiences would be indistinguishable from evolved brains." Yes, our experience would be indistinguishable, but in one case science is nonsense, and in other case science works. $\endgroup$
    – warlock
    Commented Dec 23, 2020 at 13:09
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    $\begingroup$ If our world is a Bolzmann brain and we want to do science, we should decide which questions are allowed to answer “just because our world was created as it is” and which ones are not. Why is this answer considered acceptable for the question "why was the entropy of the Universe so low in the past?", but not for the question "why elephants do not fly?" $\endgroup$
    – warlock
    Commented Dec 24, 2020 at 16:59
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    $\begingroup$ @user1247 The experiences can be made indistinguishable by conditioning on BBs that only observe the same macrostate as we do; However, under the alternative hypothesis that we come from a low entropy past, this observed macrostate is indeed evidence that our memories etc. are correct and that we can make inferences based on our observations (i.e. do science), while the latter is not true under the „Boltzmann observer hypothesis“ (for more details see preposterousuniverse.com/blog/2008/01/14/boltzmanns-universe). $\endgroup$
    – Maximilian Janisch
    Commented May 24, 2023 at 7:00

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