eli5 What’s Boltzmann Paradox?
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The first bit comes from advanced physics – we know that sometimes ‘random things’ happen in the universe. For example, there a measurable, non-zero possibility that a dinosaur wearing a toupee and singing New York, NY will appear on my front lawn when I hit ‘comment’ and then vomit up the spider from Home Alone. Is this *likely?* Of course it isn’t. But it’s *possible* with some insanely, infinitesimally small – but calculable – likelihood.
So by extension – let’s assume it’s also possible that a goopy flesh thing, let’s call it a ‘brain’, containing all of my thoughts, memories, and experiences just *appears* floating in deep space somewhere. Like the dinosaur this is *possible*, just really really really really unlikely, but again – measurably possible.
Now, let’s consider the universe as it really is. All the eons and eons of things that occurred in exactly the way they did, forming the cosmos in exactly the way they are, with every decision and moment and interaction going the way it did that leads up to this very moment of my writing this explanation for you. How likely is it that *that* happened exactly that way? Pretty fucking unlikely, considering all the possible other options.
What if it’s 1,000x *more likely* that a memory-flesh-brain appears in space than a universe lead to this moment? What if that’s what *you are*. A flesh-brain floating in space for a fraction of a second pre-programmed with all the memories of your existence.
Absurd, yeah it is. 100,000,000x more like than the universe actually existing as we perceive it? Maybe.
So then what?
The Boltzmann paradox is a reference to a thought experiment named after a scientist who was writing on thermodynamics and cosmology a century ago. Basically, it predicts that if the universe is made up of random structures coming together and falling apart, at any given point in time there should be more spontaneously created brains floating in deep space than there are intelligent observers like humans looking up at space from planets.
If even very large structures like galaxies ultimately emerge basically at random points in the universe out of gas and dust, it must be true that very small structures also emerge basically at random. In fact, the number of very small structures (little gas clouds, say) must be a lot higher than the number of very big structures (galaxies, say), for the same reason that you wouldn’t be surprised at rolling a six but you might be surprised at rolling six sixes in a row.
But the implication of this is that every once in a while, the random little object that spontaneously comes together won’t be a little gas cloud: it’ll be something with more random structure. Even something that looks exactly like a body, say. Or a brain. In the thought experiment, we call this a “Boltzmann brain”: a brain that has come together through random fluctuations of matter in the universe, out in deep space, rather than as part of a person on a planet.
This might seem incredibly unlikely, but in a big enough universe where structures are randomly coming together and falling apart all the time, even the incredibly unlikely must happen once in a while.
And there comes the paradox part. In a sufficiently huge universe, there must be a very large number of Boltzmann brains at any given moment in time. In fact, at any given point in time, there must be more “Boltzmann brains” (randomly created brains floating in space) than there are brains in the heads of actual intelligent observers of the universe, like ourselves.
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