I’ve tried and tried to understand it but there so much to follow, I can’t grasp it in its entirety.
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It’s just a brain produced by something random; usually quantum fluctuations or “nucleation,” depending on what “kind” of universe we really live in (i.e., the true fundamental properties of the universe).
But other than that, it’s literally just a brain. The problem is: how do we know we’re “real” humans versus just a brain that spontaneously assembled?
If the universe exists for infinite time and under certain circumstances, there would be infinite Boltzmann brains. So the probability you’re an actual human is basically 0: you’re probably a Boltzmann brain with spontaneous memories of being a human.
It’s an indication that something is wrong with your theory of physics.
There are several different boltzmann brains, but probably the easiest to explain is the original. The other versions are broadly similar, but pop up in relation to different theories of physics.
So, anyway: the original Boltzmann brain starts with a pretty basic and important question, namely “why is the universe as we know it here?” At the time, nobody knew about the Big Bang or the expansion of the universe, and the going scientific theory was that the universe was eternal and unchanging. But if that was the case, it should have long since reached as state of maximum entropy…a “heat death” where everything was just a haze of evenly distributed atoms with no stars or planets or anything.
That’s kind of a problem since there are stars and planets and everything.
However, it’s also known that entropy is _probabilistic_. If you take puzzle pieces and shake them in a box, they are going to just get more and more jumbled…get closer and closer to that “heat death” of perfect randomness. But if you shake long enough hypothetically all the pieces _could_ bounce back just right to form the puzzle again.
So the original idea for how we could have the universe is that, over long enough time, the universe just sort of happens to bounce its way out of heat death and back to a low entropy state, and that’s the universe that we see. It’s _staggeringly_ unlikely, but if you have an infinite universe and infinite time, why not? This is the “Boltzmann universe”.
…But..then someone pointed out that, while still staggeringly unlikely, it’s much easier to just spontaneously generate a brain that thinks a momentary thought than an entire visible universe, in the same way that if you are bouncing around a puzzle box it’s much easier to bounce together a couple pieces to make a guy’s head than it is to bounce together the whole puzzle… and in fact you should bounce together random heads a huge number of times compared to bouncing together entire puzzles.
So any theory which relies on random entropy reduction to generate an entire universe has the problem that anyone in the universe is far more likely to be one of those two puzzle-piece heads than a part of a fully built puzzle. This is contradictory and absurd and rather implies that there’s something fundamentally wrong with the theory.
….and indeed, later observations showed there _was_ something fundamentally wrong with the theory, namely that the universe is not eternal and unchanging but instead is expanding and only a few billion years old. As a result, no highly unlikely entropy fluctuations are needed and thus there’s no implication of zillions of boltzmann brains for every actual universe.
Now, even current theories sometimes spit out boltzmann brains under different situations but if you ask me, that just means we don’t know all there is to know about the universe yet.
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