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 this, because I believe one can apply philosophical arguments to dispute the existence of BBs.
Like the brain-in-the-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 would be independent of the second law because it’s a logical consequence of past events being more reliably known than future (chaotic) events. I’m sure the authors 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 can 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’.
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