Boltzmann Brains, Evil Demons, and Why It’s Occasionally a Good Idea to Listen to Philosophers

There’s been a bit of a buzz recently about a paper Sean Carroll posted to the arXiv, “Why Boltzmann Brains Are Bad”. The argument in the paper isn’t new, it’s something Carroll has been arguing for a long time, and the arXiv post was just because he had been invited to contribute a piece to a book on Current Controversies in Philosophy of Science.

(By the way: in our field, invited papers and conference proceedings are almost always reviews of old work, not new results. If you see something on arXiv and want to know whether it’s actually new work, the “Comments:” section will almost always mention this.)

While the argument isn’t new, it is getting new attention. And since I don’t think I’ve said much about my objections to it, now seems like a good time to do so.

Carroll’s argument is based on theoretical beings called Boltzmann brains. The idea is that if you wait a very very long time in a sufficiently random (“high-entropy”) universe, the matter in that universe will arrange itself in pretty much every imaginable way, if only for a moment. In particular, it will eventually form a brain, or enough of a brain to have a conscious experience. Wait long enough, and you can find a momentary brain having any experience you want, with any (fake) memories you want. Long enough, and you can find a brain having the same experience you are having right now.

So, Carroll asks, how do you know you aren’t a Boltzmann brain? If the universe exists for long enough, most of the beings having your current experiences would be Boltzmann brains, not real humans. But if you really are a Boltzmann brain, then you can’t know anything about the universe at all: everything you think are your memories are just random fluctuations with no connection to the real world.

Carroll calls this sort of situation “cognitively unstable”. If you reason scientifically that the universe must be full of Boltzmann brains, then you can’t rule out that you could be a Boltzmann brain, and thus you shouldn’t accept your original reasoning.

The only way out, according to Carroll, is if we live in a universe that will never contain Boltzmann brains, for example one that won’t exist in its current form long enough to create them. So from a general concern about cognitive instability, Carroll argues for specific physics. And if that seems odd…well, it is.

For the purpose of this post, I’m going to take for granted the physics case: that a sufficiently old and random universe would indeed produce Boltzmann brains. That’s far from uncontroversial, and if you’re interested in that side of the argument (and have plenty of patience for tangents and Czech poop jokes) Lubos Motl posted about it recently.

Instead, I’d like to focus on the philosophical side of the argument.

Let’s start with intro philosophy, and talk about Descartes.

Descartes wanted to start philosophy from scratch by questioning everything he thought he knew. In one of his arguments, he asks the reader to imagine an evil demon.


Probably Graz’zt. It’s usually Graz’zt.

Descartes imagines this evil demon exercising all its power to deceive. Perhaps it could confound your senses with illusions, or modify your memories. If such a demon existed, there would be no way to know if anything you believed or reasoned about the world was correct. So, Descartes asked, how do you know you’re not being deceived by an evil demon right now?

Amusingly, like Carroll, Descartes went on to use this uncertainty to argue for specific proposals in physics: in Descartes’ case, everything from the existence of a benevolent god to the idea that gravity was caused by a vortex of fluid around the sun.

Descartes wasn’t the last to propose this kind of uncertainty, and philosophers have asked more sophisticated questions over the years challenging the idea that it makes sense to reason from the past about the future at all.

Carroll is certainly aware of all of this. But I suspect he doesn’t quite appreciate the current opinion philosophers have on these sorts of puzzles.

The impression I’ve gotten from philosophers is that they don’t take this kind of “cognitive instability” very seriously anymore. There are specialists who still work on it, and it’s still of historical interest. But the majority of philosophers have moved on.

How did they move on? How have they dismissed these kinds of arguments?

That varies. Philosophers don’t tend to have the kind of consensus that physicists usually do.

Some reject them on pragmatic grounds: science works, even if we can’t “justify” it. Some use a similar argument to Carroll’s, but take it one step back, arguing that we shouldn’t worry that we could be deceived by an evil demon or be a Boltzmann brain because those worries by themselves are cognitively unstable. Some bite the bullet, that reasoning is impossible, then just ignore it and go on with their lives.

The common trait of all of these rejections, though? They don’t rely on physics.

Philosophers don’t argue “evil demons are impossible, therefore we can be sure we’re not deceived by evil demons”. They don’t argue “dreams are never completely realistic, so we can’t just be dreaming right now”.

And they certainly don’t try to argue the reverse: that consistency means there can never be evil demons, or never be realistic dreams.

I was on the debate team in high school. One popular tactic was called the “non-unique”. If your opponent argued that your plan had some negative consequences, you could argue that those consequences would happen regardless of whether you got to enact your plan or not: that the consequences were non-unique.

At this point, philosophers understand that cognitive instability and doubt are “non-unique”. No matter the physics, no matter how the world looks, it’s still possible to argue that reasoning isn’t justified, that even the logic we used to doubt the world in the first place could be flawed.

Carroll’s claim to me seems non-unique. Yes, in a universe that exists for a long time you could be a Boltzmann brain. But even if you don’t live in such a universe, you could still be a brain in a jar or a simulation. You could still be deceived by an “evil demon”.

And so regardless, you need the philosophers. Regardless, you need some argument that reasoning works, that you can ignore doubt. And once you’re happy with that argument, you don’t have to worry about Boltzmann brains.

14 thoughts on “Boltzmann Brains, Evil Demons, and Why It’s Occasionally a Good Idea to Listen to Philosophers

  1. Wyrd Smythe

    You’re describing what I see as the first of two leaps of faith we must take to escape collapsing into the black hole of solipsism. That first leap takes us to cogito ergo sum and allows us to believe our consciousness is what it appears to be and persists as the core of our identity. Then we have to make the leap of accepting other minds as equally real and persistent, plus we accept the “empirical realism” of the physical world.

    And, sure, we can (and occasionally should) cast doubt on those two assumptions, but until we do make those leaps of faith, we’re stuck in doubt and solipsism and can’t progress further. (Perhaps less so in the latter case. One could invent mathematics and some forms of philosophy as an isolated solipsistic mind.)

    Accepting reality as (more or less) what it appears to be seems necessary to navigate through it.

    That said, I’m not sure I grant the existence of universes possessing laws that would allow and drive the spontaneous formation of a working brain — complete with coherent memories. Further, our consciousness is smeared out at least over several milliseconds, so it’s not just the spontaneous formation of a brain in an instant, it’s the spontaneous formation of a process in motion. I tend to find the possibility incoherent.


  2. subgraph

    All this talk about Boltzman brains and simulations seems like a complete waste of time to me. The pragmatic approach is obviously the correct one here.
    How would knowing I am a BB change anything about my life? My goal as a physicist would still be understanding the rules of the reality I am inhabiting.
    And what makes the reality BBs exist in any more ‘real’ than mine anyway?

    People who think about nonsense like this too much, just become easy prey for snake-oil vendors:


    1. 4gravitonsandagradstudent Post author

      This seems like a good objection against Boltzmann’s original formulation, but I’m not sure it works in Carroll’s setup, mostly because I’m a little unclear on what sort of properties these late-time universes are supposed to have. Basically, the crucial bit here is that you need the simple states that eventually evolve into brains, and enough time for them to evolve, and I don’t know if the latter is supposed to be a problem for some reason. (This is also why I’m not sure about Wyrd Smythe’s objection above: I don’t know how the universe Carroll is envisioning is supposed to evolve in time.)

      Liked by 1 person

      1. JollyJoker

        Humm, maybe a high cosmological constant would make evolution impossible but Boltzmann brains still ok? Seems odd if Carroll’s universe would be completely incompatible with the observations we have on ours.


        1. 4gravitonsandagradstudent Post author

          Upon reflection/re-reading some of Carroll’s argument, it looks like I was just confusing myself. The reason that brains that evolved aren’t more common in this kind of universe than brains that spontaneously form is because the universe is in thermal equilibrium, so the sort of large temperature gradients (like stars) that fuel evolution are uncommon, and more to the point uncommon than smaller temperature gradients (like some brief squishy brain).

          This also should answer Wyrd’s objection: you can indeed get something that lasts some unspecified amount of time, if needed.

          Liked by 1 person

  3. Will

    I think Carroll’s argument and the Cartesian argument are totally different.

    Carroll’s argument (as I understand it, I haven’t read the current paper, but I’ve read other stuff he wrote) goes like this:
    1) There are certain classes of cosmologies in which Boltzmann brains are vastly more numerous than real universes.
    2) There is no point in “doing” “science” if I am a Boltzmann brain.
    3) Therefore, working cosmologists should start from the assumption that the laws of physics aren’t consistent with the copious production of Boltzmann brains, otherwise there is no point to their work.

    As you point out, this argument also applies to the evil demon scenario. This is a good thing! It means that scientists can go about their work just assuming that everything they experience is not the whim of an evil demon. What a relief!

    (This argument, though, doesn’t apply to the simulation scenario. Someone living inside in the simulation might consider it a worthwhile task to attempt to discover the rules that govern the simulation.)

    But the crucial thing is that Carroll’s argument doesn’t attempt to actually disprove the hypothesis that I am Boltzmann brain or that evil demons don’t exist. On the other hand, Descartes attempts to affirmatively prove his entire philosophy.

    Carroll: Assume no Boltzmann brains -> Laws of universe not consistent with Boltzman brains
    which translates into:
    Carroll: Assume no evil demons -> Real world exists

    While in Descartes arguments the causation is reversed.
    Descartes: (long chain of arguments starting with cogito ergo sum) -> Real world exists/No evil demon

    For Descartes, no evil demons is a conclusion, not a premise. For Descartes, there is no true “cognitive instability”. The skeptical part of the Meditations is just a prelude to the constructive part. By the end, he thinks he can prove everything starting from the one certain truth, cogito ergo sum, which is not an “uncertainty” or a “cognitive instability”.

    Liked by 1 person

    1. 4gravitonsandagradstudent Post author

      Yeah, I agree that the analogy with Descartes was a bit of a stretch. There have been philosophers who actually tried to build something on the “no cognitive instability” platform, but they’re typically more recent (and don’t go off on a tangent and make falsifiable physics predictions). I was mostly just amused by the precedent of someone trying to use very non-empirical criteria to ground a physics argument.


  4. Lubos Motl

    Dear 4gravitons, thanks for highlighting this interesting alternative perspective. Yup, the Boltzmann Brain must be classified just as another reincarnation of an Evil Daemon whose possible existence questions whether we may trust even the most basic reasoning we use. Nice to hear that philosophers are no longer obsessed with Evil Daemons, although they don’t exactly agree why they lost their interest. I would guess that they are not enthusiastically endorsing such daemons because those began to look increasingly ludicrous in the wake of the progress in natural science and technology, something that the philosophers didn’t really contributed to. Evil Daemons – and maybe even God, as Feynman has said – are hiding and thriving in the ignorance and this habitat for them has simply shrunk as many corners have been illuminated.

    But I don’t see how the philosophers may really help to reasonably lead one through the jungle of possible Evil Daemons if you assumed that such a jungle still exists. They’re changing their views without good reasons or arguments.

    Despite the sympathies, I would probably choose to disagree with your thesis that things like the existence of Evil Daemons or a vortex around the Sun emulating gravity cannot be settled by the scientific method. As I understand the scientific method, it surely can settle such things at a huge confidence level – and it may settle many things that look even deeper or more metaphysical than yours. It seems to me that this possible disagreement between us could be a repetition of our disagreement on whether “Is physics of the sixth place of decimals important?” (Google search). Decimals may be important but physics is here primarily to solve bigger and deeper questions – and it can do so and has done so many times.

    The alternative theories with assorted Evil Daemons do make various predictions. Even if you consider a very diluted, highly generalized version of an Evil Daemon theory, it typically makes some predictions that are quickly disfavored by the empirical data, often very simple-to-collect empirical data. A vortex of fluid just ultimately behaves differently than the gravitational field and one would need to adjust many vortices to agree with the observations and it’s very unlikely that they have all the right parameters – while gravity allows to explain all these observations with one (G) or several (mass) parameters.

    The reasons to think that there is any good reason why “science could imply within a realistic particle+cosmological model that we are Boltzmann brains” are just wrong. Whether it’s likely that you are a Boltzmann brain – or “whether any Yes/No proposition is right” – is given by the probability that is an intensive quantity and the probability of the birth of a (particular) Boltzmann Brain is simply expo-exponentially tiny, so we’re almost certainly not ones. Carroll and others use an alternative, “extensive” notion of probability that can be inflated indefinitely by simply noticing that one can have many copies of something over spacetime. But the probability simply isn’t an extensive quantity and Carroll’s reasoning is wrong for this reason (and many others).

    But even if we were controlled by Evil Daemons or programmers of the giant simulation we inhabit, or anything like that, science wouldn’t become meaningless. Natural science is about the explanation of our observations and relationships between them, and predictions of those. Even if an Evil Daemon is hiding or obfuscating some “deep underlying image of the reality”, the goal of science is to predict what we will actually perceive. In other words, in the presence of an Evil Daemon, the purpose of science is to figure out the methods how the Evil Daemon wants to deceive us and predict what he wants us to believe next time etc. If He is obfuscating the deep reality very well, it really means – according to the scientific, i.e. empirically based understanding of the truth – that He is successfully changing the truth itself. As far as science goes, the truth is what He wants us to believe that the truth is because the truth in the scientific sense is the result of most most enlightened, accurate, and careful evaluation of the data we perceive. And if the data we perceive are basically fabricated by the Evil Daemon, then the truth is what the Evil Daemon wants us to believe that the truth is.

    And after all, the very suggestion that the daemon is evil is just a biased insult. The daemon does whatever He does and if we love science, we must love Him, too! In this sense, I think that already the stigmatization of these forces that could be behind something in the Universe as “dark” or “evil” is an irrational, antiscientific, unjustifiable prejudice.

    We could use the name Evil Mother Nature for Nature that respects relativity, too. It’s some shrew that is doing Her dirty job and hiding the luminiferous aether from us. Is she? Well, the relativistic theory without the aether is simply more elegant and especially more compatible with the data than the theories with the aether. So we choose to trust relativity over aether. Could it be a totally fabricated illusion? Yes but the fabtication would have to fake almost everything. And if I take the viewpoint or assumption that this fabrication is taking place, is Nature evil because She is doing so? I don’t think so. If Nature is faking relativity, she is really saving us from experiencing the non-relativistic Hell and gives us the pleasant feeling of living in a proper relativistic world. It’s the negative emotion that is being added that is most wrong about this whole way of thinking. We should embrace the data that may be naturally extracted from the observations. When a theory can be made compatible with the data, it’s a successful theory, not an evil one. And when a theory cannot be made compatible with the data, it’s not because the theory is evil, it’s because it’s probably wrong.

    It may be exciting to scare ourselves with emotions, references to morality, evil of the deamons, and creepiness of brains floating in some hypothetical environment. But at the end, science really dictates us to throw away the emotions. The questions are whether one theory is right or wrong, not whether concepts in the theories are evil. And the comparison of predictions with the observations – and the comparisons of scores of different theories that they get in these empirical tests – may resolve lots of questions, including those that have been thought of as a domain of philosophers.


  5. John M.

    Actually, there’s a reason adherents of multiverse theory should be much more concerned about Boltzmann Brains than philosophers are with with Descartes’ evil demon scenario. The multiverse theory (so the argument goes) entails that the probability of your being a thoroughly delusional Boltzmann Brain is MUCH higher than the probability of your being a genuine homo sapien with reliable perceptions, memories, etc. There’s nothing in philosophy, by contrast, to suggest that the existence of an evil demon controlling your thoughts is actually more probable than not. (Even Descartes was perfectly aware of that, by the way.)

    Put another way, the threat of skepticism in philosophy is just the problem of explaining how it can be rational to believe one’s perceptions, memories, etc. are reliable given the POSSIBILITY they are not. The threat of Boltzmann Brains in multiverse theory is the problem of explaining how it can be rational to believe one’s perceptions, memories, etc. are reliable given the HIGH PROBABILITY they are not.

    Big, big difference.


    1. 4gravitonsandagradstudent Post author

      Yeah, as (I think) I replied to earlier comments, Descartes evil demon is not really the best example of this kind of philosophical skepticism, just the most recognizable one. The problem of induction ought to be a better analogy: there simply isn’t any way to argue that induction is more probable than not to work that isn’t circular. If you think you’re likely to be a Boltzmann Brain, you don’t have fully rigorous recourse to induction, because your memories don’t reflect persistent laws. But in general, you don’t fully rigorous access to induction, because there isn’t an independent justification for induction. Your likelihood of being a Boltzmann Brain doesn’t actually make the situation any worse.


      1. John M.

        Thank you for your response, but I still disagree. Lacking a non-circular defense to support a belief and actually having compelling evidence that the belief is very likely false are two very different things. Both Descartes’ evil demon example and your example about inductive reasoning are cases of the first sort (along with all other skeptical worries in philosophy), whereas the Boltzmann Brain problem is a case of the latter–and much more serious–sort.

        Let me try to make the point a different way. Most philosophers–and I’m one for the record–are not skeptics in the strong sense. That is, we think people are justified in holding at least a few assumptions needed to get inquiry off the ground that cannot be supported with argument (e.g., the assumption that induction is reliable). So you’re exactly right to suggest that philosophers don’t typically stress out over lacking a compelling defense of such beliefs. But it’d be a rare philosopher indeed who thinks you’d still be justified in holding a belief once you’re presented with compelling evidence (that even you yourself readily recognize) renders the belief very likely false.

        And that’s precisely what the Boltzmann Brain problem is supposed to do for the multiverse theorist. If you think universes randomly fluctuate into existence ad infinitum, then you should (says the argument) readily recognize that the probability of your being a Boltzmann Brain is extremely probable. Thus you don’t merely lack a non-circular argument to support your assumption that your cognitive faculties are reliable (we all lack that); rather, you are tied to a theory that actually entails that your faculties are almost certainly delusory.

        Solution? Either (a) find some way to suggest (like Carroll is trying to do) that Boltzmann Brains wouldn’t vastly outnumber reliable observes in a multiverse, or (b) give up the multiverse theory.


        1. wrf3

          Mathematical induction is reliable. Experiential induction less, and sometimes much, less so. I suspect the sun will rise tomorrow. One day it won’t.

          To make matters worse, evidence is only compelling under a shared axiomatic system.

          The third solution, (c) is to not worry about it. As long as the sun appears to rise, my dog appears to love me, and the software I write appears to work, just enjoy it. If it’s all an illusion, I can’t do anything about it, anyway. Boltzmann brains aren’t, in practice, any different from the “God as liar” problem.


        2. 4gravitonsandagradstudent Post author

          That’s a fair point. I think the crux of the issue might be the question of how strongly you hold your initial assumptions. That is, if you expect people to assume that the world is predictable in order to get their reasoning off the ground, the question is whether they’re allowed to revise that assumption if they find evidence to the contrary, and if so, how strong that evidence needs to be.

          If you reason that you’re most likely a Boltzmann Brain you need two elements: some cosmological picture that predicts an abundance of Boltzmann Brains, and some version of the Copernican principle (you have to assume you’re a typical observer in this respect). You can reject the cosmological picture (Carroll’s argument, or rejecting the multiverse), you can accept that you are a Boltzmann Brain (rejecting induction, and thus any assumption you used to justify induction), or you can reject the application of the Copernican principle here.

          Leaving the strength of the scientific argument aside for the moment (and thus whether you can reject the multiverse or use Carroll’s approach), you have two principles that can trade off, the assumption you use to justify induction and the Copernican principle. And while it probably depends on how you justify either, I would think that the assumptions you used to underpin induction would be held stronger than the Copernican principle, which if anything seems like a less necessary sub-case. So even if you reason yourself into a universe filled with Boltzmann Brains, it seems like you ought to conclude that you are a very atypical observer, not that you yourself are a Boltzmann Brain.

          Now of course, if you’re a very atypical observer then any attempt to get predictions out of the multiverse via anthropic reasoning fails. It’s still not a good situation for multiverse proponents by any means. But it does seem like a qualitatively different situation than the multiverse being directly logically self-defeating, and worth distinguishing.



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