Boltzmann Brains

Published on by Catherine Toulsaly



As for the poet, she only sees the Universe as a symphony of lights and sounds played on the keyboard of time.

Prologue to all the possibles

Along with the Universe, we, too, are gifted with an emotional keyboard, but we don't play it the same way. Concepts, words, and emotions blend together. They, too, have a life of their own. Reading about the genetic diversity among sequoias created in my mind the visual metaphor of a forest of walking brains. The unsettling concept of disembodied brains has taken me back to that analogy. ‘Boltzmann brains’ are observers stripped to an absolute bare minimum — their consciousness.  


What is a brain anyway? A tool conceptually and causally integrated into a physical state with the ability to sense and adapt, an entryway between what is and what is not. I have another image in my head: that of cephalopod-like giant brains propelling their appendages bestowed with a sense of beauty and complexity. Boltzmann brains may be pair-produced from photons and gravitons, provided that the existence of an elementary graviton particle is verified.


A Boltzmann Brain is a configuration of matter that is (along with its local environment) as close as possible to thermal equilibrium, while still qualifying as an intelligent observer

Sean Carroll

If we understand processes to be going forward and backward in time, Boltzmann brains could reach consciousness both ways and experience the flow of time when it goes forward and when it is reversed. To bring them to life, we would only need local subsystems to behave ergodically, not the universe as a whole. Sean Carroll wrote the following dialogue between two scientists on why Boltzmann brains are bad:

W : I am worried that our best cosmological model predicts that typical observers are Boltzmann Brains. 

S: Why? Sure, there may be a huge number of BBs in this universe, but I’m not one of them, so what do I care? 

W : How do you know you’re not a Boltzmann Brain? 

S: I don’t seem very brain-like. I have arms, legs, etc. The environment around me seems pretty dramatically far from equilibrium. 

W : Fine. But in a large, randomly-fluctuating universe, even if you are not a minimal-fluctuation observer (a BB), it’s overwhelmingly likely that you are in a state that represents the minimum possible fluctuation away from equilibrium, conditioned on whatever features your local environment has. Call these “Boltzmann Observers” (BOs). Even granting that you have arms and legs and are sitting in an office, it’s still overwhelmingly likely that all of that has randomly fluctuated into existence out of equilibrium. How do you know you’re not a Boltzmann Observer? 

S: Because I can see that I’m not! I can go out and, for example, observe the cosmic microwave background, which is evidence that the universe was in a much lower-entropy state several billion years ago, which isn’t what I would expect if I were a BO. 

W : But even if you are observing the microwave background, you’re not necessarily seeing the leftover photons from the Big Bang. You’re seeing a radiation field entering your telescope right here in your nearby environment. In a fluctuating-universe scenario, it’s certainly possible to see such photons, but it’s overwhelmingly likely that they randomly fluctuated into existence, without any connection to an earlier low-entropy state. 

S: Well, that scenario makes a very reliable prediction, as Feynman basically pointed out: all I have to do is look in my radio telescope again, and see whether 16 or not the background radiation is still there. If what I observed already is just a random fluctuation, there’s no reason for it to persist over time; in fact it’s incredibly unlikely. So let me wait a second before checking again and... Nope. There’s the microwave background. I still seem to be in a thermodynamically sensible environment. I am not a BO. 

W : You don’t have the right to conclude that. In a randomly-fluctuating universe, just as almost all observers are minimal-fluctuation BBs, and almost all observers with arms and legs and microwave backgrounds are minimal-fluctuation BOs subject to those macroscopic constraints, it’s also the case that almost all of the observers with arms etc. who believe they have just waited a few seconds and failed to observe any evidence of the surrounding equilibrium are also random fluctuations with just those properties. You can conditionalize on any macroscopic information you like: in a randomly-fluctuating universe, it remains overwhelmingly likely that you are at a local minimum of entropy that evolved by chance out of equilibrium. 

S: But everything I know and feel and think about the world is what I would expect if I were an ordinary observer who has arisen in the aftermath of a low-entropy Big Bang, and nothing that I perceive is what I would expect if I were a random fluctuation. 

W : And in a randomly-fluctuating universe, the overwhelming majority of people who would say exactly that are, as a matter of fact, random fluctuations. That’s why those kinds of cosmological models are bad. 


Although the existence of Boltzmann brains may be vastly improbable, it doesn’t make it impossible. They may inhabit one part of the Universe while we, walking brains, live in another. There may be other scenarios. Could the extremely rare occurrence of random fluctuations have resulted in the formation of only one Boltzmann brain that has given shape to all matter? 


If ultimately, the sort of thing that the Universe holds within is a brain with furrows and ridges where signals ricochet, one might also ask where the rest of the body is. If even the most abstract idea has a sort of feeling attached to it, and without it, an idea has no resonance, no meaning, the organized manifestation of the life of feelings might as well have resulted in the shaping of a Boltzmann heart. 


Are we, ourselves, part of a simulated reality or born out of a Boltzmann fluctuation? We would then expect not just Boltzmann brains, Boltzmann hearts, Boltzmann galaxies, and, who knows, Boltzmann universes to eventually fluctuate into existence. Such a suggestion would put us on even shakier ground. Therefore it should be rejected, Sean Carroll argues, because it is cognitively unstable. 

The nightmare in the most extreme case envisaged is that exactly what you firmly believe yourself to be doing now, in this very instant and in every last detail, is being replicated infinitely many times by infinitely many identical avatars of yourself scattered around the infinite multiverse, every one of them believing themselves to be the one and only true you. What is more, each of them including almost certainly you, consists of nothing more than a disembodied brain that exists fleetingly in what is otherwise fluctuating chaos.

Julian Barbour, The Janus Point, p.77

This is not much different from the uneasiness felt by philosophers in the past. Long ago, the Chinese philosopher Zhuang Zi dreamt that he was a fluttering butterfly (needless to say that a butterfly strikes me as a better alternative than a disembodied brain). When he woke up, he felt suddenly lost. He no longer knew if he had dreamt that he was a butterfly, or a butterfly dreamt that it was he. For all that we know, we could be frogs in a meditative state, dreaming that they are humans, looking for ways to jump out and escape their unavoidable environment.


All this flow of time that we experience and these various bodies that we feel, these different thoughts that agitate us, are perhaps only illusions. We believe we see spaces, figures, movements in our dreams. Who knows if this other half of life where we think we are awake, the French philosopher Blaise Pascal wrote, is not another sleep a little different from the first, from which we wake up when we feel we are sleeping?


If we are allowed to dream that we are butterflies and live our waking lives as human beings, I feel there is more to us than our physicality. The stuff that we are made of is a kinetically nonlocal energy that transcends dreams and reality. In the end, we are left to wonder whether stars, galaxies, filaments are pictorial representations of a Boltzmann brain or physical manifestations that do not presuppose the existence of a brain. At the basis of it all lies the fundamental question of quantum ontology. It appears more likely that our thoughts randomly fluctuate in our heads. We are quantum-theoretical beings, not randomly-fluctuated physical observers.


My brain keeps hidden, in its folds, metapatterns, circles, and lines moving poetically, enigmas that do not lend themselves to reason, leaps forward and beyond. Circles contract and expand. They regulate the flow of my thoughts, forming snakes eating tails, drawing the start and end of Wheeler’s Universe. Inside circles, the snow falls into eternity, making the Universe quieter. Wells, too, are circles opening their mouth into the bowels of the Earth. What’s an idea whose time has come? Ensō. 

(Quote by Thich Nhat Hank)

(Quote by Thich Nhat Hank)

Humankind is not a circle with a single center but an ellipse with two focal points: facts are one, and ideas are the other.

Victor Hugo

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