The Second Coming as a Solution to the Boltzmann Brain Problem

Defeating entropy, not just death

Epistemic status: kind of amusing speculative theology—don’t take it too seriously, and speak with your personal care provider local physicist to see if this solution is right for you. Though I did ask Aron about it once, and he didn’t see an obvious problem with it.

The Boltzmann brain problem is one of the bigger problems in physics. The core issue, as I understand it, is as follows: entropy goes up over time. This means that things get less and less orderly over time. The universe will only have a finite period of order, before entering an infinitely long period of disorder.¹

During this period of disorder, sometimes brains will randomly fizz into existence. These brains will sometimes have thoughts—sometimes even memories—but will disappear quickly. Brief note of terminology: these brains that randomly fluctuate into existence in outer space are called Boltzmann brains. Bad news: if there’s only a finite period of initial stability, before an infinite period of disorder, then there will be vastly more Boltzmann brains than normal brains.

This is a problem. If there are vastly more Boltzmann brains than normal brains, then you should think you are probably a Boltzmann brain. You should think, in other words, that your brain randomly fizzed into existence and you are about to die. You should expect your vision to begin blacking out as you freeze in some dark celestial void. But that’s not what you observe (I assume). And if you are a Boltzmann brain randomly appearing from a chemical fluke, you have no reason to trust your reasoning—for this reason, reason can never tell you that you’re a Boltzmann brain.

The core puzzle is that each of the following seems plausible:

1. You’re probably not a Boltzmann brain.

2. There are vastly more Boltzmann brains than non-Boltzmann brains.

3. If there are vastly more Boltzmann brains than non-Boltzmann brains, then you probably are a Boltzmann brain.

1 is self-evident and 2 is a pretty straightforward point of probability. So the thing to give up is 3. But 3 seems implied by our best theory of physics, on which entropy continually goes up.

Now, I won’t claim that there are no other good solutions to the problem. I don’t know any physics. My sense is that there isn’t an agreed upon solution and this is a pretty serious problem, but due to my ignorance, I cannot definitively comment on the plausibility of a physics-based solution.

Instead, let me propose a speculative theological solution open to Christians: perhaps 3 is false because the second coming will occur before Boltzmann brains appear.

The second coming is depicted in the New Testament. It’s when Jesus returns, judges the living and the dead, and all that jazz. But presumably after the second coming, the universe will be very different. It won’t just keep spitting out Boltzmann brains. Matthew 24 says:

Immediately after the tribulation of those days the sun will be darkened, and the moon will not give its light, and the stars will fall from heaven, and the powers of the heavens will be shaken. Then will appear in heaven the sign of the Son of Man, and then all the tribes of the earth will mourn, and they will see the Son of Man coming on the clouds of heaven with power and great glory. And he will send out his angels with a loud trumpet call, and they will gather his elect from the four winds, from one end of heaven to the other.

Revelation 6:12–14:

“The sun became black as sackcloth… the whole moon became like blood… the sky vanished like a scroll that is being rolled up, and every mountain and island was removed from its place.”

Revelation 21:1:

“Then I saw a new heaven and a new earth; for the first heaven and the first earth had passed away, and the sea was no more.”

That doesn’t sound like a post heat death world where particles randomly bop around, occasionally producing brains. So if the second coming happens, then there won’t be Boltzmann brains after it. This allows us to resist the conclusion that Boltzmann brains will outnumber real brains. If the universe is radically transformed before Boltzmann brains appear, then there won’t be loads of Boltzmann brains.

It would be one thing if this was some extra theological claim that Christians had to believe in. If so, then this would be a problem because you pay a cost when you add extra bits to your theory to explain data. But this is already part of the Christians theory. They already think the second coming is going to happen. This gives them a nice free solution to the Boltzmann brain problem.

This solution isn’t just for Christians. Muslims and Jews believe in a day of judgment also. So maybe that will stop Boltzmann brains from proliferating too!

Admittedly, this won’t solve the whole problem. Some multiverse models imply that there are more Boltzmann brains than normal brains because most of the multiverse is very high entropy. These will still face problems even if they think the second coming happens before low entropy universes devolve to high entropy universes—their problems come mostly from all the universes that start in a high entropy state, rather than from the low entropy universes devolving into a high entropy state. But still, a second coming makes it a lot easier to solve the problem.

It’s rare that theology gives good solutions to scientific problems. But if the scientific problem arises from forecasting the future history of the universe, and it’s already part of your theology that the future won’t be as the scientists pressing the problem guess, then that’s a nice upshot of your theory. It means your theory avoids the problem. The major religions, therefore, have a nice way to avoid the Boltzmann brain problem.

1. Occasionally, during this period of disorder, order will randomly form, but the basic point remains: order is rare, disorder is common.