Rejected for the following reason(s):
No LLM generated, heavily assisted/co-written, or otherwise reliant work.
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Rejected for the following reason(s):
Yes, biological Boltzmann brains have an insanely high mass and complexity penalty. If you have Boltzmann brains at all, biological ones can be ignored.
It's incredibly much more likely to get non-biological brains that persist for a long time, and perhaps some of those think that they are biological entities.
Once you take that a bit further, you get the idea that the penalty is on computational complexity of supporting the subjective experience, rather than complexity of the subjective experience directly. It's close to the simulation hypothesis, but with simulations carried out on random computers instead of designed ones.
However, we have no idea what sort of fundamental physical processes are necessary for subjective experience, nor how likely they would be in any sort of universe that supports Boltzmann brains at all. Ours obviously does not, and only wild extrapolation over ridiculously larger physical scales weakly suggests that maybe something like our universe might eventually do so.
To take the "ubiquitous Boltzmann brain" hypothesis seriously, you have to assume that we (or at least you the reader since I may not actually exist) are not actually experiencing true physical laws. So that leaves nothing except trying to extrapolate over a distribution of all possible physical laws - a Tegmark IV multiverse - supporting all possible simulations weighted by whatever mathematical rules are most likely to support subjective minds with your subjective experiences.
However, this is pretty quickly self-defeating since the least complex system of physical rules that supports having subjective experiences similar to yours is that they are real, without the added complexity penalty of some completely different physical substrate supporting computation that simulates a universe that merely emulates what you have a memory of experiencing.
So I don't take the "ubiquitous Boltzmann brain" hypothesis seriously at all.
Epistemic Status: Speculation. I am not a physicist, and I am disagreeing with a consensus that includes people like Sean Carroll and Sir Roger Penrose. Please treat this as a "I notice I am confused" post rather than a definitive claim.
I. The Standard Nightmare
The standard story about Boltzmann Brains goes like this:
The universe is expanding and cooling. Eventually, we hit the heat death. But because of quantum mechanics, the vacuum isn't perfectly empty; it’s a soup of fields fluctuating randomly. Given an infinite amount of time, these thermal fluctuations will produce particles. Given even more time, they will produce structures.
According to the laws of thermodynamics, the probability of a fluctuation decreases exponentially the bigger it gets.
A human brain requires a small fluctuation to produce. A solar system needs a bigger one. The universe needs a much, much bigger one.
Therefore, the argument goes, it is exponentially more likely for the vacuum to fluctuate into a single, disembodied brain (complete with false memories of reading a blog post) than it is to fluctuate into a whole solar system containing a real brain. And it is exponentially more likely to fluctuate into a single solar system than into the universe we observe.
If we accept the Copernican Principle—that we are typical observers—we should expect to be the most common type of observer. The math says the most common type of observer is a lone brain floating in the void, hallucinating its existence for a microsecond before freezing to death.
This is the Boltzmann Brain Paradox. It is usually deployed as a reductio ad absurdum to argue that the universe must not last forever, or that our understanding of vacuum fluctuations is wrong.
But I want to challenge the middle step. I’m not sure the math actually favors the lone brain.
II. The Complexity Penalty
The consensus view focuses heavily on mass. A brain weighs ~1.5 kg. A solar system weighs 2×1030 kg. The fluctuation required to gather a solar system’s worth of mass in one place is significantly rarer than the fluctuation required to gather 1.5 kg.
But mass isn’t the only variable in entropy. Arrangement matters.
For a Boltzmann Brain to form, atoms need to randomly drift together and assemble themselves into neurons, synapses, and blood vessels, all in the exact configuration of a thinking being, complete with the electrical gradients required for consciousness.
This is an incredibly "fussy" state. If you misplace a few trillion trillion atoms in a solar nebula, you still have a solar nebula. If you misplace a few atoms in a brain, you have a tumor or a stroke or just dead meat.
The complexity penalty for a brain is massive. You aren't just paying for the mass; you are paying for the hyper-specific, low-entropy configuration of that mass.
Now consider a solar system. Solar systems are comparatively messy. They form spontaneously whenever enough hydrogen drifts together. You don't need to arrange the atoms precisely. You just need a gravity well and a whole lot of gas. Physics does the rest. The gas collapses, the center ignites, the disk accretes, planets form.
To get a Boltzmann Solar System, the universe doesn't need to fluctuate a completed, clockwork star system into existence. It just needs to fluctuate a cloud into existence. Or, perhaps, smaller fluctuations of gas just need to drift near each other until gravity takes over.
Is the exponential penalty of gathering 1030 kg of messy gas actually higher than the exponential penalty of arranging 1026 atoms into a precise, functioning neural network?
I haven’t seen the calculation, but I’m not convinced the brain wins.
III. The Observer Moment Multiplier
Even if we grant that a single Solar System is harder to make than a single Brain, we have to look at the yield.
A classic Boltzmann Brain exists for a moment. It has one observer-moment, or perhaps a few seconds of them, before the hostile vacuum kills it.
A Boltzmann Solar System, once formed from that initial gas fluctuation, lasts for billions of years.
Let’s say the probability of a Boltzmann Brain is 10−X and the probability of a Boltzmann Solar System is 10−Y. Even if Y>X (making the solar system less likely), does it outweigh the fact that the Solar System produces 1020 observer-moments, while the Brain produces 1?
If the "yield" of observers in a solar system is N, we shouldn't be comparing P(Brain) vs P(System). We should be comparing:
P(Brain) vs P(System)×N
Given that life tends to fill available niches, N can be astronomically large. The chance of intelligent observers evolving in a solar system is small, sure—but it's vastly higher than the double-exponential improbabilities we're dealing with in vacuum fluctuations. Once you pay the initial entropy cost to get the star and the rock, the biology comes "free" relative to the cost of assembling a brain out of pure vacuum.
IV. The Starless Sky
If this hypothesis is true, it changes the picture of the "Typical Observer."
We are no longer worried about being "freak observers"—disembodied minds with random, chaotic experiences. That theory was always self-defeating anyway (if you are a Boltzmann Brain, your memories of physics are hallucinations, so you have no reason to believe the physics that predicts you are a Boltzmann Brain).
Instead, the typical observer lives in a Boltzmann Solar System. They are evolved biological beings. Their memories are real. Their understanding of local physics is correct. They have bodies, history, and reliable cause-and-effect.
But there is a catch.
A galaxy is still a much, much larger fluctuation than a solar system. The mass difference between a star and a galaxy is vast (1011 stars). The probability penalty for fluctuating a whole galaxy into existence is insurmountable.
So, if most observers live in Boltzmann Solar Systems, they should look up at the night sky and see... nothing.
Just their own sun, their own planets, and a field of endless, absolute blackness. Maybe a few other random particles drifting in from the heat death void. But certainly not the billions of galaxies and the Cosmic Microwave Background that we see.
V. Conclusion
Physicists have spent a lot of time calculating the mass penalty of fluctuations, but I haven't seen enough work on the complexity penalty.
If the difficulty of assembling a working brain out of the void is greater than the difficulty of assembling a raw gas cloud, then the "disembodied brain" problem might just be a math error. We might be able to discard the solipsistic nightmare of being a lone brain.
However, we still have to explain why, if we are typical observers, we see a universe teeming with galaxies rather than a lonely, starless void. But at least in that scenario, we have real eyes to look at it with.