this post was submitted on 01 Mar 2026
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surely if the universe has an infinite lifespan there could be an infinite number of humans? for whatever passes as a huban at any given time. the two concepts may even overlap.
not that it matters for the day-to-day, anyway.
The logic is that the universe of big bang matter has a limited lifespan. This sets a hard limit on the number of humans via "normal" means.
Boltzmann brains are due to a quirk of quantum mechanics. Matter can come into existence spontaneously. The rate is proportional to the amount (technically the energy content). Given enough time and space, something that would fit the definition of human could spontaneously appear. The odds of this are unbelievably long, but, so long as it's finitely large, in a true infinite universe it will happen an infinite number of times. It's a bit of infinity Vs very large number weirdness.
End result is that there will be a large but finite number of "normal" humans, but an infinite number of Boltzmann brain humans. Therefore, the chances of being an actual "normal" human is effectively infinitesimal.
Agreed about it not mattering, day to day. It's one of those things that is of interest to theoretical physicists, since it might tell us something interesting about the nature of our universe.
it is very interesting, but it's also one of those topics that makes anything else in the conversation not matter.
also do note that i said nothing about thinking sensory inputs are illusory, just that belief is not required for things to exist.
It seems more likely in a universe that is infinitely large that brains would come into existence through simpler deterministic processes like they did on earth than random fluctuations no?
Our best ideas on the big bang put the universe as huge, but finite in space. (Way bigger than the observable universe) The question is time. If time is infinite then Boltzmann brains win.
Matter has a finite life, energy differentials run out. Stars run out of fuel. Black holes evaporate. Even protons eventually fall apart to energy. Then there is endless emptiness.
That emptiness would be finite in space, but infinite in time. Without that last boundary, weird things happen to maths.
If you appeal to heat death then you cannot say brains pop back into existence either because "matter has a finite life," and so it is self-defeating. If brains can pop back into existence due to random fluctuations then surely planets and stars could as well given enough time.