Should we take Boltzmann Brains seriously? If not, why not?

I recently read an academic paper on Boltzmann Brains (3 Dec. 2025), jointly authored by David Wolpert, Carlo Rovelli and Jordan Schamhorst. Under their names are listed 5 institutions, in the following order:
 
Sante Fe Institute, Sante Fe, USA.
Centre de Physique Theorique de Luminy, Marseille, France.
Department of Philosophy, The Rotman Institute of Philosophy, London, Ontario, Canada.
Perimeter Institute, Ontario, Canada.
Physics Deparment, University of California, Santa Cruz, USA.
 
For those who don’t know, Boltzmann Brains (BBs) are a hypothetical, similar to the brain-in-a-vat idea I referenced in the addendum to my last post. But this idea has more relevance to that post than just that analogy, which will become obvious as I proceed.
 
The first sentence of their Abstract provides as good a definition of a BB as you’re likely to find.
 
Are your perceptions, memories and observations merely a statistical fluctuation arising from the thermal equilibrium of the universe, bearing no correlation to the actual past state of the universe? Arguments are given in the literature for and against this “Boltzmann brain” hypothesis.
 
There is an argument that if the universe exists for long enough in a heat death, then the probability of a fully functioning brain just coming into existence as a quantum fluctuation will become greater than zero (even one). If this hypothesis is true, then you don’t know if you’re a BB or not.
 
The paper discusses previous arguments against BBs, which the authors find circular, and propose what they call a more rigorous approach. They use the language of logic and probabilities to support their arguments and discuss previous ones. I will do my best to avoid that, because I believe one can apply philosophical arguments to dispute the existence of BBs.
 
Like the brain-in-a-vat hypothesis, I don’t believe anyone actually thinks they’re a BB, yet we’re told that we need to take them seriously. The authors devote an entire section of their paper to How and Why Intuition Leads Us Astray, basically arguing that we can no longer trust ‘common sense’ in the wake of quantum physics and relativity, both of which defied longstanding tenets of common sense. However, I think there are important differences; not least, the matter of evidence.
 
I’ve written previously that science is all about explanations. Newton’s theory of universal gravitation, not only explained why objects fall to Earth, but why the Earth orbits the Sun. Darwin and Wallace’s theory of evolution explained the diversity of species and why species geographically isolated evolved differently from each other. It’s very important to appreciate that neither of these theories explained everything in their respective fields, and consequential discoveries and hypotheses have expanded them without completely overturning them.
 
My biggest bone of contention with the BB hypothesis is that it doesn’t explain anything that’s not already explained more effectively by existing theories, and certainly adds nothing new to our knowledge of the Universe. In fact, it’s arguably a hypothesis to explain everything away.
 
In the case of relativity theory, it explained events hitherto unexplained, like the precession of the perihelion of Mercury’s orbit, and predicted new phenomena like clocks slowing down at relativistic speeds. Quantum mechanics (QM) explained previously unexplained phenomena like black body radiation and the photo-electric effect, while predicting new phenomena like anti-particles and entanglement, both confirmed by experiment, even if the experiments were performed over half a century later. There is no experiment I’m aware of that can confirm the existence (or non-existence) of Boltzmann Brains, and not surprisingly, the authors don’t entertain that possibility.
 
One of the arguments you rarely hear these days, but I’ve heard expressed by Carl Sagan, among others, is that there is a unity within science across disciplines. For example, our theories of cosmology don’t contradict our theories on biological evolution, though they could. Some might argue that relativity and QM don’t play nice (euphemistically speaking), but that demonstrates a gap in our knowledge rather than a reason to scrap either one of them.
 
The BB hypothesis, by comparison, infers that all these interactions between evolution, genetics, cosmology and quantum phenomena are all figments of our minds, because there can be no history. Take the literal everyday experience of night and day having a causal relationship arising from the rotation of the planet we all live on. How does the BB hypothesis explain that? To borrow a term coined by Sabine Hossenfelder, I would call it ‘ascientific’. Basically, it’s a thought experiment with mathematical underpinnings.
 
I won’t elaborate on the authors’ arguments in detail, partly because they’re very esoteric. Nevertheless, I’ll cherry-pick in an attempt to convey the gist of them. One of their points is that if entropy can be time symmetrical, meaning explicitly that entropy can increase into the past as well as the future, then BBs are likely because they are complex by definition, therefore require very low entropy. The very last part of that sentence is mine, not theirs, but I assume that’s the conclusion based on time-symmetrical entropy, which is their argument supporting the BB hypothesis.
 
Then, if I understand them correctly, the only reason we believe entropy is time asymmetrical (only increases into the future) is because of the second law of thermodynamics. I don’t think anyone would argue that entropy and the second law are not related, but their relationship to the asymmetry of time is not necessarily causal. In other words, I think there are other factors to time irreversibility, which I discuss below.
 
My understanding of entropy is that it’s fundamentally probabilistic, and this is reinforced by Erwin Schrodinger and Richard Feynman, both of whom used a pack of cards as their analogy for entropy. The more you shuffle a pack of cards the less likely you are to return it to its original state. That’s a simplification, but I think it’s a good analogy for anyone who is not a physicist (which includes me, btw). The thing is that the equation for entropy effectively looks at all the states that a system can have, and it will always increase in time (in a closed system). In an open system with an external energy source, it can be reversed or you can have increased ‘complexity’, which is what you have with the Earth and its relationship with the Sun as an external energy source (refer Paul Davies, The Cosmic Blueprint, 1987).
 
The authors spend a lot of time on the second law of thermodynamics, and in particular, they claim it’s essential to the role of memory. Specifically, they argue that we wouldn’t have memories without the second law. As discussed below, they basically argue that only the second law gives us an asymmetry in time, and without that asymmetry, memories of a past can’t exist.
 
In fact, they specifically point out that our belief in the accuracy of our memories is premised on the assumption that our knowledge of the past is more accurate and reliable than our knowledge of the future because we intrinsically accept the asymmetry of time. They then make the point that we make this assumption with no knowledge of thermodynamics, with the inference that this knowledge should give us pause regarding the validity of memories.
 
They then give analogies of the craters on the moon and footsteps in the sand, as providing physical ‘memories’ of previous events, similar to memories in our physical brains. Below is their own conclusion to this argument.
 
In fact, these examples do have one striking feature in common: all of them seem to rely on the second law of thermodynamics. The temporal asymmetry of these memory systems, and by extension of our human brains, seems to reflect nothing more than the simple fact that entropy increases in time. Indeed, the precise reason we are so very sure about some of our memories seems to reflect the ironclad nature of the second law.


In other words, the reason we seem to have so much confidence in our memory ultimately resides in the fact that those memories rely on the second law.
 
I confess I have problems with this argument. For a start, I’m not convinced that entropy and the second law is what gives us asymmetry in time. I think the collapse of the wavefunction in QM is what provides the asymmetry in time. To quote Paul Davies (out of context):
 
The very act of measurement breaks the time symmetry of quantum mechanics in a process sometimes described as the collapse of the wave function…. the rewind button is destroyed as soon as that measurement is made.
 
I agree that overall entropy increases with time for the entire universe, but that doesn’t mean it’s the cause of the asymmetry in time we all observe and is often recorded, irrespective of individual memories. And I haven’t mentioned how chaos theory seems to break the symmetry of time, given that a chaotic event can’t be repeated, with a coin toss being the most obvious example, and weather being the most commonly observed. Chaos ensures that the future is ultimately indeterminable, even when the past is recorded, so you have an asymmetry in time without the second law.
 
Right at the end, the authors have a section titled, Discussion, which is really a summation of their arguments. Basically, they argue that the BB hypothesis without the second law is just as valid as the PH (past hypothesis), meaning there is a past. In fact, they argue that before the cosmological discoveries of the 20th Century (which indicate a history for the entire universe), the BB hypothesis could not be dismissed without assuming the second law. To quote:
 
…the only reason we have for assigning more credence to the PH or even to the second law than we assign to the BB hypothesis is the credence we assign to our cosmological observations concerning the big bang. This means in turn that before the early 20th century, when we first made those cosmological observations, we should have treated the BB hypothesis as just as probable as the second law. (My emphasis)
 
They argue that the common acceptance of PH over BB ‘currently in vogue’ (their words) is ‘naïve’. I think there is another way of looking at it, which, no doubt, they would consider even more naïve. What if time asymmetry is a fundamental feature of the Universe and the second law and entropy are a logical consequence arising from that asymmetry in the form of increasing probabilities over time, like the shuffling of a pack of cards? If that was the case, then surely the BB hypothesis would be dead in the water.
 
If the second law and increasing entropy are a consequence of the asymmetry in time, and not its cause, then memory is independent of the second law, because it’s a logical consequence of past events being more reliably known (recorded) than future (chaotic) events. I’m sure the authors would find this naïve as well, yet the entire universe seems to obey this law: the past is observable and the future is not.
 
To quote Freeman Dyson:
 
“We do not need a human observer to make quantum mechanics work. All we need is a point of reference, to separate past from future, to separate what has happened from what may happen, to separate facts from probabilities.”
 
The ‘point of reference’ being the intersection of the past and future light cones for every observer in the Universe, whether it’s a BB or not. However, a BB is the subject of a thought experiment, whereas you and I create the actual thought. Isn’t that ironic? Actually, it's self-referential, if one was a BB.

 
I can’t help but think of my last post and what Curt Jaimungal would have made of my arguments. Based on what he’s previously argued, I expect Curt would contend that I only ‘believe’ I’m not a BB, and that I don’t actually ‘know’. But here’s the thing: if I’m wrong about NOT being a BB, then I must be wrong about everything else I claim to ‘know’.
 

If everything is belief, what is reality?

This arose from a video I watched of Curt Jaimungal’s where he takes Neil deGrasse Tyson to task for saying: “Don’t say I believe anything.” Curt then says (to the viewer, not Neil): “Your wincing is justified.” But we don’t know the context, and maybe Neil was saying that in response to a specific issue, where he was possibly being told what he ‘believed’. If that’s the case, it’s different to Curt’s inference that Neil doesn’t believe anything at all.

Having said that, there is another video where Curt criticises Neil's position concerning the role of philosophy, where Neil takes the position a lot of physicists do, that there is no role for philosophy in science, whereas I argue that theoretical physicists practice philosophy even if they don’t know it (Curt makes exactly the same point). 

However, I have an issue with Curt’s position, when he elaborates (in the first video) that we only have varying degrees of belief, implying there is nothing we can say that is true with complete certainty. I’ve taken issue with this before with people less qualified than Curt. I’m one of those who makes a distinction between things I know and things I believe, between facts and theory. I think this distinction is important, otherwise everything is up for grabs. And, in a way, this is already happening, when Donald Hoffman argues that there is no objective reality. For example, “There is no objective moon or spacetime that exists even when unperceived… 

It’s a consequence of my go-to metaphor that there is an island of knowledge surrounded by a sea of ignorance, where science is the island and philosophy is the shoreline, and they interact and inform each other. As John Wheeler pointed out, as the island grows, so does the shoreline, therefore the interaction will never stop. This in itself is a philosophical position, consequently a belief.  

It's obvious to me that scientists have beliefs, and I have no dispute with Curt over that particular point, which is what he spends the first few minutes of his video elaborating on, using philosophical quotes like, “Most contemporary philosophers characterize belief as a propositional attitude.” (Emphasis in the original.) Meaning, in effect, it can be updated later when one finds new evidence, as Curt explicitly points out. In other words, it’s conditional, which I expect all scientists agree is the case for all scientific theories. This is a logical consequence arising from there being no theory that can explain all phenomena – they all have limitations. Therefore, all scientists have beliefs about phenomena that a particular theory can’t explain.

But my point is that this doesn’t rule out facts and truths that an existing theory covers, based on empirical evidence. And this was Neil’s point in another video, where he claims that empirical evidence will always overrule philosophy, and I agree. In the same way that empirical evidence will overrule a scientific conjecture or hypothesis, which is basically the scientific method in a nutshell.

I wrote a comment on Curt’s post, where I referenced Bell’s Theorem, and how Alain Aspect demonstrated (by experiment) that it proved Einstein wrong about entanglement (refer my post discussing Brian Greene’s interview with him). Basically, I contend that Aspect demonstrated something that was true (quantum entanglement), therefore, I claim it’s no longer a belief, but something we know. But that has since led to other beliefs that physicists and philosophers of science argue over. For instance, Sabine Hossenfelder argues it’s evidence of superdeterminism, not non-locality (instantaneous influence) as Aspect would argue.

So I contend there is a dialectic between science and philosophy, between facts and theory. In the same comment, I argued there are mathematical truths like there being an infinite number of primes, which I contend transcends belief. Many argue that this is still a belief (including, possibly, Curt) but my response is that, where you have something that can be either true or false, and it’s demonstrated to be true, it becomes a fact and no longer a belief. Another example I give, is that the Earth goes around the Sun or it doesn’t – it’s either true or false – and if it’s true, it’s a fact, therefore something I claim to know.

Now, you might argue this is a belief, based on what others have told me, which is correct. But here’s the thing: look at the alternative, which is that the sun goes around the Earth, which was believed for millennia, and a classic case of a propositional belief being updated with new evidence. The new evidence is not just astronomical data but a deeper understanding of reality and the nature of gravity in particular. So it’s similar to the example I gave of Alain Aspect, where empirical evidence provides facts.

In the case of our theories on gravity (first Newton’s, then Einstein’s), they not only provide an explanation for the Earth’s orbit around the Sun, but a lot of other phenomena that were previously inexplicable or even unknown - black holes, anyone? Curt would argue that it’s still a belief, only with a very high degree of confidence. But if I go back to my metaphor: it’s become part of the island of knowledge, and I can’t see it being cut adrift back into the sea of ignorance.

My other point is that this knowledge, or fact, or truth, then becomes the basis for new beliefs, otherwise I can’t see how science can progress. Curt would argue that this in itself is a belief, because it’s a philosophical position (see below), and I can’t argue with that. So, even claiming there are truths in mathematics, for example, is a philosophical position, therefore a belief. Does this lead to a contradiction? I don’t think believing in truths axiomatically means there are no truths, which is an even greater contradiction.

After watching the video a second time, I wrote another comment on Curt’s post, where I attempt to address this point:

Towards the end, Curt provides a counter viewpoint that scientific theories are contingent, which most, if not all, scientists would agree. However, my point is that a scientific theory needs to contain some 'truths' otherwise it's useless. I don't believe there are no 'truths' in the world; just that they all have applicable limitations. I'm talking about 'truth' in an epistemological sense; something we know, even though it started out as a belief. I don't believe in epistemological relativism. Obviously, that's a philosophical position, so axiomatically, a belief.

But, leaving aside scientific beliefs and truths, and whether the latter even exist, there is another extreme viewpoint that Curt addresses, where he quotes Bas van Fraassen:

“I believe many things, I believe I had a father, that I wrote a book, that we are still in America, and not on a flying saucer… Could I be wrong? Are there circumstances under which I would correctly give up some of these beliefs? Certainly.”

(I could make exactly the same assertions, except I live in Australia.)

And the ‘circumstances’ are, when I am dreaming, but to question one’s reality to that extent (when awake), I would argue, lies on the road to madness. We would normally expect, if someone genuinely questioned these ‘facts’, they would need psychiatric help. In other words, taking extreme examples of where truth could be questioned opens a Pandora’s box, usually known as insanity.

But what if the whole pack of cards comes crashing down, and we learn that we live in a simulation? As more than a few people claim, therefore believe. I don’t see how that changes the reality I deal with every day, which can literally be a matter of life and death. That’s my litmus test for reality – it can kill me.

And this brings me back to Donald Hoffman, who doesn’t believe we live in a simulation, but nevertheless believes everything we see and interact with is a fabrication of the mind. He even argues that the 3 dimensional space we ‘perceive’ is in fact, 2 dimensional, despite all the evidence to the contrary, and despite the fact that it would be impossible for one to live in a 2D world in the same way that one can live in a 3D world – we would fall apart, for a start.

I wish to give an example of something I ‘know,’ which is utterly trivial, yet transcends belief. I know that I can’t breathe without air. But here’s the thing: air exists in space and time, whether perceived or not. I don’t ‘believe’ it ceases to exist when I stop interacting with it, or when I take off my ‘headset of my own making’ (to cite Hoffman). Does that mean, if I could ‘take my headset off’, I’d stop breathing? Even when we are dreaming, experiencing a simulacrum of ‘our own making’, we continue to breath, because the external reality continues to exist, even while unperceived.

 

Addendum: This is one of those posts where I keep going over arguments in my mind. So, in response to Bas van Fraassen, one could argue that you don’t know if you’re a brain-in-a-vat, which is a well known philosophical conundrum, even though I don’t know of a philosopher who actually believes that. Nevertheless, there have been a number of sci-fi movies involving characters having false memories. Ones that come to mind are Ghost in the Shell, Dark City, City of Lost Children and even Blade Runner. There is also the Hyperion series by Dan Simmons dealing with people having their brains jacked, not dissimilar to the Matrix movies.

However, in real life we can verify our memories with physical records and through our interactions with others who have had the same experience. This is how I claim we know if something we remembered was a dream or not. So yes, we can have false memories, though usually not about what country we live in or whether we have a father, or if I’m the author of this blog.

In my childhood, there are 2 events that stood out and were bizarre enough to make me wonder if they were just a dream or not. Remember, in my original post, I explicitly reference dreaming as the ‘circumstances’ that Bas van Fraassen mentions as exceptions where his beliefs could be wrong.

One of the events was a fight between my parents that never happened, so it must have been a vivid dream (I need to point out that my father never ever hit my mother, even in the dream). The other was an event that occurred when I was 7 or 8 years old, and were sounds that came out of the walls of the room I was lying in, while I was sick in bed. This is a very strong candidate for a dream except my father was a witness, and was even able to recall it decades later.