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?

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – David Z
    Apr 10, 2015 at 16:59
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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. $\endgroup$ Jul 26, 2017 at 10:29
  • $\begingroup$ @RubenVerresen, I realize this, but the question still stands. The point is that those BB's that could stand to reflect on whether or not they are in the process of rejoining equilibrium are going to be BB's from outside that reference class of observers. $\endgroup$
    – user1247
    Jul 26, 2017 at 16:51
  • $\begingroup$ Can you please absolutely clearly state what version of the paradox you are referring to? Many people refer to the "paradox" in "Boltzmann paradox" more in the sense of a "suprising fact" rather than "contradictive statement". $\endgroup$
    – Void
    Sep 28, 2017 at 10:02
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    $\begingroup$ @ArtOfProblemSolving, the concept of Boltzmann Brains are a fairly standard consideration in cosmology. Don't shoot the messenger, or act as though OP is making the concept up, or making up the fact that it is a consideration in the field of physics (in addition to philosophy of science). OP (me) is asking about a mainstream concept in physics. Please take your criticisms up with physicists like Andrei Linde, not me. $\endgroup$
    – user1247
    May 30, 2020 at 23:15

1 Answer 1


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.

  • $\begingroup$ 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
    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
    Dec 22, 2020 at 8:57
  • $\begingroup$ Yes, you are essentially just re-stating the classic Boltzmann brain problem. $\endgroup$
    – user1247
    Dec 23, 2020 at 11:56
  • $\begingroup$ 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
    Dec 23, 2020 at 13:09
  • $\begingroup$ Again, you are just repeating the classic Boltzmann brain problem. It is not a problem if you establish that our experience is indistinguishable from us not being Boltzmann Brains, because in that case by definition of "indistinguishable" science works just fine. $\endgroup$
    – user1247
    Dec 24, 2020 at 15:17

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