I read a post on here about boltzmann brains a few days ago, and got curious on the subject. From the articles I read, it seems that they are saying that we are almost 100 percent likely to be BBs instead of humans. These ideas have worried me tremendously.

I also read this post (Is the Boltzmann brain problem really clearly established as a problem?) Which only amplified my anxiety because that post seemingly says that yes, we probably are BBs.

Are those article's statements that we are almost certainly BBs true? And is that other post also correct?

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    $\begingroup$ would it matter, I think therefore I am. don't worry $\endgroup$ Sep 27, 2019 at 18:42
  • $\begingroup$ Some people think that our universe is a simulation in an alien computer. Some people think that you have doppelgangers in other universes. Some people think that a vacuum instability could destroy our universe at any moment. Physics, like modern life, is full of things that can provoke anxiety, and it is good to learn coping mechanisms so that you enjoy the life that you have. $\endgroup$
    – G. Smith
    Sep 27, 2019 at 19:27
  • $\begingroup$ Hi Curdoz. Welcome to Phys.SE. This is your 3rd question about Boltzmann brains. Please don't repost a closed question in a new entry. Instead, you are supposed to edit the original question within the original entry. $\endgroup$
    – Qmechanic
    Sep 27, 2019 at 20:26
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    $\begingroup$ @Qmechanic The 'this is a duplicate question' on my last post said I could ask a new question, so I reformatted it and I did ask a new question. The third question you speak of was around 1 and a half years ago. Also, the post you linked as being a duplicate is not my question at all. $\endgroup$
    – Curdoz
    Sep 27, 2019 at 20:51
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    $\begingroup$ @Curdoz Why would it give you anxiety? Before your Boltzmann brain existence would (or does) end, you'd know that it (or something very much like it) would begin (and, maybe, has begun) again, after a simple, old-fashioned, relatively uncontroversial Poincare recurrence time. Myself, I kind of like the possibility, although I try not to act on it except for occasional oversleeping. $\endgroup$
    – Edouard
    Sep 28, 2019 at 3:32

3 Answers 3


You aren't a Boltzmann brain.

Sean Carroll wrote a blog post about this that you should read. It's funny, and gets its point across well.

The key point is that it's a premise of the Boltzmann brain argument that we aren't Boltzmann brains, so it makes no sense to conclude from the argument that we are.

How do we know that we aren't? I think the argument for that goes something like this.

Complex systems can arise by random chance, but the probability is low. The more complex the system, the lower the probability.

The probability of a brain randomly appearing in some location is not zero, but it's ludicrously smaller than the probability of that location containing, say, hydrogen gas.

The probability of a brain and a surrounding body and ecosystem and fake fossils suggesting that the brain actually evolved through natural selection appearing randomly is ludicrously smaller than the probability of just a brain appearing randomly, never mind the hydrogen gas.

Nonetheless, you can construct cosmological models so ludicrously large that the chance of the brain + fake evidence appearing somewhere is pretty high.

The thing is, though, that if the model is large enough for even one random brain + fake evidence to exist, it should also contain a huge number, an absolutely ludicrously huge number, of random brains without the evidence, because that's so much more likely. So, if you're a random brain, then with ludicrously high probability you should not see evidence of evolution. But you do. This is a good reason to be ludicrously confident that you aren't a random brain.

  • $\begingroup$ The post you link to does not make the argument you say it makes. Moreover, your conclusions don't follow from your premises. For example: it is extremely unlikely that my exact combination of genes would come together in any particular meeting of a sperm with an egg. Yet they did. Is this a good reason to be ludicrously confident that I am not the result of such a random process? Of course not! $\endgroup$
    – N. Virgo
    Sep 27, 2019 at 18:39
  • $\begingroup$ @benrg "The probability of a brain randomly appearing in some location is not zero, but it's ludicrously smaller..." How does one decide whether something has a ludicrously small probability, or is just plain impossible? Assigning any probability at all (other than exactly zero) means that you are claiming the thing exists. $\endgroup$
    – D. Halsey
    Sep 27, 2019 at 18:42
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    $\begingroup$ (contd. from my prev. comment) The actual argument in Carroll's post is that we only have a high probability of being Boltzmann brains if we assume the universe is vastly, vastly bigger than modern cosmology thinks it is. Boltzmann proposed that the universe was essentially a gas in equilibrium, existing over sufficiently long spatial and temporal scales that the entire observable universe could be a random fluctuation. The argument Carroll refers to is an argument against that theory. If you take that context away, it doesn't really make sense. $\endgroup$
    – N. Virgo
    Sep 27, 2019 at 18:44
  • $\begingroup$ (Though to be honest I've always thought that argument was a bit sketchy. If there are fluctuations at all scales then there will be some at the scale of the observable universe, and the existence of lots of smaller ones doesn't change that. In addition, I'm not entirely convinced that it is more likely for a brain to just fluctuate into existence on its own, versus a whole universe fluctuating into existence, in which brains evolve. But this is all an aside, and probably not best suited to a comment thread.) $\endgroup$
    – N. Virgo
    Sep 27, 2019 at 18:48
  • $\begingroup$ @Nathaniel The argument I made in my answer isn't meant to be the same as argument in the blog post. Carroll does make arguments like mine elsewhere, e.g. in this post which is linked from the other other one. $\endgroup$
    – benrg
    Sep 27, 2019 at 18:53

The simple answer is that the argument that Boltzmann brains are more probable than our own brains is faulty. Sure the likelihood that a naked brain will appear spontaneously is greater than the probability that a brain with a body will appear spontaneously. But that's not how nature works.

An example: there are an infinite number of non-spherical shapes, so a naive guess might be that a vanishingly small number of planets are spherical assuming that their shapes result from random processes. But all planets are spherical. Why? Because they are not formed by random processes.

The same is true of human brains. We, and our brains were formed by evolutionary processes as non-random as the physical processes that form planets. Yes, randomness plays a part, but the end result is largely determined by factors that smooth out the randomness and keep the process moving toward an end state determined by a tendency to change toward the minimum energy configuration in the case of planetary shapes, and by Darwinian competition in the case of human brains.

Bottom line: those who accept arguments asserting that Boltzmann brains exist with more likelihood than biological brains simply don't understand evolution.


The way these things work, is that some intellectual comes up with a bizarre new theory of reality and is delighted with their own cleverness; and then some sensitive member of the public takes the words of this authority seriously, because they are an authority, and their own mental health suffers.

This happens with an idea as old and simple as determinism (am I not actually the author of my own actions, even though it feels like I am?); it happens with the many worlds interpretation of quantum mechanics (am I committing terrible acts in other possible worlds at every moment?); it happens with the idea that we are living in a computer simulation.

There is no cure-all for this, except perhaps actually achieving a knowledge of the nature of reality which the human race has not yet attained; and it is always possible that something counterintuitive will in fact be true, and yet maddeningly intolerable to the human mind. But in general, one should not believe ideas that are in radical contradiction with the bedrock facts of experience.


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