Friday, June 24, 2022

Boltzmann brain blackouts

Some cosmological theories lead to the worrisome conclusion that most people with present brain states like ours are Boltzmann brains—random aggregations of molecules in space that came together to form a brain in a little bubble of oxygen. Usually when people talk about Boltzmann brains, they talk of how this induces a sceptical problem for the theory that generates them. Thinking about Boltzmann brain issues that way leads to messy epistemological questions such as whether we get to simply assume that we have hands, and the like. Moreover, if there is evidence for the cosmological theory, then that becomes evidence for Boltzmann brains, which then undermines the evidence for the cosmological theory, and that’s all a mess.

Here is how I suggest we think about what happens when a cosmological theory T leads to a Boltzmann brain issue. The vast majority of Boltzmann brains—even ones with brain states like ours—are short-lived. Their bubble of oxygen dissipates in the absence of gravity, and after a brief moment of hypoxia they die. So think of the point this way. If a cosmological theory predicts a large ratio of Boltzmann brains to ordinary evolved brains, then the theory makes an empirical prediction: in a moment you are extremely likely to start blacking out. So just do the experiment: wait a moment and see if you’re blacking out. If you’re not, then you’ve got very strong disconfirmation of the cosmological theory, and you’re done with it. You don’t have to worry about self-defeat, Moorean questions about whether you have two hands, or anything deep like that. (And if you are blacking out, then if it’s a Boltzmann brain related blockout, you’ll be dead in a moment. If you do come back to, and not in the afterlife, that’s massive evidence against the theory again, but now you should see a doctor about your blackout problem.)

In fact, you don’t even have to wait: on cosmological theories that generate too many Boltzmann brains, you should expect to already be starting to black out—because most of the Boltzmann brains will be extremely short-lived.

Objection: There will be long-lived Boltzmann brains, too.

Response: Sure. But for entropic reasons they will be much less common than the short-lived ones. You might, of course, worry that in many of these cosmological scenarios there are infinitely many Boltzmann brains, and infinitely many are short-lived and infinitely many are long-lived, and you can’t say that the short-lived ones are more common. The short-lived ones will be more common in a “typical” large finite region, but overall we just have infinity. Now, if you are worried about this—and I think you should be—then that worry already applied at the beginning of the story when you looked at the ratio of ordinary to Boltzmann brains, because there will be infinitely many of each on such a cosmological theory, and the formulation of the problem that I gave at the beginning, namely that Boltzmann brains greatly outnumber ordinary brains, is inaccurate. (I think if you do have this worry, then the theory has another problem, namely that probabilistic reasoning makes no sense in a world described by the theory. That is a kind of sceptical and self-defeat problem, but of a different nature.)

My point in this post is modest: if you want to say that Boltzmann brains greatly outnumber ordinary brains, then instead of thinking deep stuff about self-defeat of theories and scepticism, you should just think of the theory that generates this prediction as falsified by future observation.


Alexander R Pruss said...

Suppose that for antisceptical reasons you think that you should dismiss any hypothesis on which you didn't live as long as common sense days you did. Then the falsified prediction of the cosmological theory changes: it predicts we're in a bubble of "normal" reality whose radius is equal to your age times the speed of light. And that's disconfirmed by looking whether stars beyond that distance disappear. And they don't.

Sam Harper said...

I don't think this response to Boltzmann brains works. The reason you don't find yourself blacking out is because you JUST NOW came into existence completely with false memories of a past that didn't actually happen. At any moment, it only SEEMS like you've been around a while because those memories were JUST NOW implanted in you. If you are a Boltzmann brain, then you will surely die in the very next moment. The only reason you haven't died yet is because you JUST NOW came into existence. I don't know if I'm explaining that well or not.

Alexander R Pruss said...

Boltzmann brains aren't instantaneous. They come into existence and then die soon after that once they run out of oxygen. Most of them just barely have enough oxygen to be conscious for a very short period. But that short period isn't a moment, and their perishing isn't instantaneous.

Sam Harper said...

I don't see why it would need to be instantaneous for my point to go through. Regardless of how short or long Boltzmann brains last, it would still be the case that the reason you don't experience yourself going out of existence is because you just now came into existence. After all, you only ever experience the immediate present. You don't experience the past even a fraction of a second ago, nor the future a fraction of section from now. So you could've come into existence a fraction of a second ago, and that's why you haven't begun to deteriorate yet.

Alexander R Pruss said...

Response 1: You could have. But the theory predicts that most of your life would be a deterioration. You would probably only have a small fraction of a second of mental clarity, and then have started going down into darkness, and the going down into darkness time would probably be significantly longer than the brief moment of clarity. In other words, the theory predicts that you would already be experiencing the deterioration.

Response 2: Don't worry about any of the fancy sceptical stuff. Just use the ordinary scientific method. You're conscious now and aren't blacking out. The theory predicts you will soon be blacking out. So wait and see. You waited and saw and there was no blackout. All well and good. Theory disproved.

Zsolt Nagy said...

It is so funny. In the other post it is stated, that it is supposedly "notoriously hard" difficult to come up with an account of matter and here it is stated or implied the matter oxygen to be an essential part or necessary matter for a functional brain.
I myself cannot come up with such a great irony.