Boltzmann brain (opens in new tab)

The point isn't that they can be preserved; you're right that they probably wouldn't be. But all it takes is a single instant where "you" are randomly "conjured" into being: your Boltzmann brain has all your memories and everything. Even if that brain dissipates a small fraction of a second later, for that tiny amount of time, you would believe in the world as we all know it.

And the bizarre thing about this is you can't say "oh look, I've been in existence long enough to type this sentence, so I and my memories and reality must be real, and not just a random spontaneous formation of a brain", because at every single instant, you could be that brand-new Boltzmann brain, never before formed, and the bits where you thought "oh look, I've been in existence long enough..." are just the memories spontaneously implanted into that brain.

It's kinda wild. I'm not sure I buy it as a realistic possibility, but it can be fun (or terrifying) to think about.

A mere trillion years is a tiny fraction of the time it would take for a brain to form like this spontaneously. This concept arose when people generally thought the universe was in a generally steady state and had existed forever. Even though generally say that entropy always increases if you wait long enough you might see reversals if you wait periods of time proportional to something like e^N where N is the number of particles in the system. So it's weird that we're in this low entropy solar system, but you'd expect something like that to happen every once in a while across infinite time. But since a brain has many less particles than a solar system [citation needed] you should expect brains to form from spontanious entropy reversals much more often than galaxies do. There's a principle called the "incompressability of phase space" that means that a low entropy solar system with a low entropy brain is necessarily much less likely than just the low entropy brain, because entropy reversal across an entire solar system is just so mind boggling unlikely.

Of course nowadays the cosmic redshift, etc, make us think that the universe is not eternal but began a short few billion years ago and will end in a big crunch or big rip long before we expect a single Boltzman brain to arise through the random walk of particles.

If time is infinite then the argument from exponential complexity doesn't really work as a counterargument to Boltzmann brains. It might take Rayo's number of years plus five for a good measure but it is still bound to happen with absolute certainty.

The point is not that it's likely (or practically possible) that such a brain would come into being spontaneously. The point is that it is (mathematically) still more likely than some other theories about the formation of the universe (namely those that give it the order we see, low entropy, by "miraculous" happy accident), thus showing that those theories are most likely nonsense, by reductio ad absurdum. (Those other theories can't be true, otherwise we might as well find brains in space.)

It's about quantum mechanics and the fact that "empty space" is not really empty. Particles do pop into existence (from nothing), according to QM, so there's a non-zero probability for any "pattern" to pop into existence. Sort of like if you have an infinite number of coin flips then at some place and time you'll land on heads a million times in a row, no matter how unlikely. And for any million-bit sequence you're guaranteed to hit it too. So a human "brain" is just a pattern that's likewise guaranteed to be "encountered".

A similar concept is how the first replicator RNA/DNA got created as the beginning of life. If RNA can exist in large numbers of random sequences, then a sequence that can replicate itself only has to "happen" once and then life is started and will never slow down but will grow in complexity, as long as the environment can support it.

I'm with you. There are forces that would prevent random particles from being in any configuration "at random".

Example: At random, 2x2x2cm ice cube in center of sun sized star. You can't get from here to their because an ice cube will never form next to heat.

In some alternate reality where atoms materialize in random configurations out of nothing maybe, but that's not our reality where atoms interact with other atoms and that interaction prevents the vast majority of combinations. It doesn't matter if time is infinite.

> How would a brain form spontaneously without those random fluctuations tearing the constituent components apart?

>> Over a sufficiently long time

Think of it this way: you've probably heard about the million monkeys on a million typewriters will eventually produce Shakespeare? Ultimately something deeply meaningful like that is just a sequence of symbols and you generate enough randomo sequences of symbols, one or more of them will be literature.

A Boltzman Brain is basically that but for a quantum system. In the far future, it's suspected the Universe will undergo heat death and eventaully reach thermodynamic equilibrium but that doesn't mean nothing happens on a quantum level. A chaotic system as otherwise thermodynamic equilibrium is still capable of having temporary, localized order without violating entropy.

So over any sufficiently long period of time (we're talking 10^100000+ years here), you'll get all sorts of interesting arranagements of matter. One of these is a localized reality where it or being in it are capable of sentience.. They won't have any conception of the Universe outside of this but, for a time, they will be able to contemplate their reality.

Think of it another way: take a bucket filled with random elements, mix it and tip it on the floor. Do this enough times and one of those bucket pours will be sentient life.

I agree with your initial argument about why we aren't Boltzmann brains, but I don't think that the same argument follows regarding the simulation hypothesis. A simulation would imply a set of rules being set up, of which your senses, and the reality they experience, are an emergent property. That is, there are rules, and your senses are generally consistent with whatever they are.

With a Boltzmann brain, there's no reason whatsoever to think that any senses are consistent from one moment to the next or that any action can predictably yield any reaction. It would all just be random.

The falsifiable part of something like the simulation hypothesis has to do with nested sets. If the reality we exist in is capable of producing everything that would be necessary for a conscious being to experience a convincing reality within its available resources and computational power, then the fact of consciousness is not sufficient to conclude that the reality in which we operate does not exist inside another reality with the same capability to produce an internal model of reality sufficient for supporting consciousness.

As you said, it does not definitively tell us anything about the reality in which that simulation exists other than that it has the same property that we experience of being capable of hosting a nested reality sufficient for supporting consciousness.

Although we can't specify particular aspects of that reality, we are capable of mathematically representing potential properties of universes that have the capability to host a nested simulation. None of this provides certainty, but it provides a basis of exploring possibilities, and actual understanding requires methods beyond the hypothesis of a larger reality.

Physics provides us a lot of ideas about the potential for the nature of reality and methods for testing and falsifying them but is not in itself sufficient. We don't need to find all the answers in one line of inquiry. The holographic principle of string theory is one example of a type of simulation existing inside a larger reality but far from the only one.

Also, any line of inquiry that applies probabilistic arguments to evolution-based questions is already off in the weeds. The brains in our bodies didn't arise as a result of pure random organization, but developed incrementally in a direction that maximized reproductive fitness. There is no reason to think that brains appearing in isolated space at random, complete with memories, would ever be a thing.

So it's simply not meaningful to ask about the relative odds of these two positions.

My chief complaint is that it’s functionally equivalent to belief in God. It strikes me as fundamentally the same instinct but reinterpreted in a manner palatable to the “i’m very logical and rational” crowd.

I think there's an even more interesting implication of Boltzmann brains. The non-local universe is infinite in both time and space and when you're dealing with infinities of time and space two axioms become true. Everything that can occur has to occur and nothing can occur only once. So if intelligent life was able to form in our local universe we would have to assume higher intelligence spontaneously arose, AKA Boltzmann brains, in the non-local universe and exists infinitely.

That's a good argument for why it's not worth spending a lot of time being concerned about your own ontological status, but it's not really an argument against the concept or the math behind it. It's a conclusion one could derive from some fairly non-controversial assumptions, and if we're agreeing that the conclusion is wrong, the problem with the chain of logic and math needs to be found.

I don't think that's actually a problem with the Boltzmann brain concept; I think it's just a reason why we don't want it to be the actual representation of reality.

I've chosen not to care; if I'm a BB, or even just a character in a simulation, my life and choices still have meaning to me -- whatever I actually am -- and that's what I've decided is important.

Well no, that's not the reason why we're not Boltzmann Brains. A Boltzmann Brain is perfectly capable of believing that it is sensing reality in exactly the same way that your brain believes so.

It is worth reading the section "Modern reactions to the Boltzmann brain problem" in the article to understand why the Boltzmann Brain is a useful thought experiment.

A lot of religions can be interpreted in such a way as to essentially say "the entire real world is just a very complex simulation being played out by the Divine"; there there is no real world to know about, just thoughts in the Mind of God.

The point of descarte is that you dont know anything about anything, and in my opinion, all of his bad arguments begin when he starts stating his certainties, except for his first one.

> It’s all unfalsifiable.

That might make it unscientific (in Popper's philosophy), but not impossible or implausible.

This is exactly what a Boltzmann Brain would say...

Their argument hinges on one particular claim:

> As unlikely as it is for a BB to form at all, it’s drastically more unlikely for additional things to simultaneously form around it. And even if additional things did form around the BB, it is not especially likely they will be the kind of stable and sensible objects a brain could even perceive. Therefore, the evidence we have is more likely supposing we are OOs than supposing we are BBs.

which doesn't ring true to me. Assuming that universe is indeed dominated by BBs, it's not at all clear to me that any observations we could possibly make "is more likely supposing that we are OOs". While the number of BBs "with decorations" would be dwarfed by the number of BBs without, it is still entirely feasible that there are many more such BBs than there are OOs.

I also found their argument as to why all observations shouldn't be considered hallucinations (including "over time", "history of" etc) as a matter of probability to be incomprehensible.

I love that this addresses that probabilistic confirmation is fundamentally intransitive in Bayesian epistemology, and bumps in probability don’t necessarily propagate through multiple probabilistic links (Evidence -> BBU and BBU -> BB doesn’t mean Evidence -> BB).

Really well put-together and careful main line of argumentation IMO

Philosophists... bringing us the hard science since never.

I am mostly playing devil's advocate

> and reality does not collapse immediately

How do you know that reality does not collapse immediately? At any given instant you could be a fresh brain that just came into existence, all your previous memories which imply a life lived up to this point also formed in that same instant.

The point is that you, dear reader, could be the Boltzmann Brain, and that would mean that all your memories spontaneously came into being, giving you the illusion of a past history, and it would also mean that you will dissipate again shortly after; there is no continuity, and we're not all BBs. 'We' are all just figments of your imaginary imagination, conjured momentarily into memory and then lost again.

For a Boltzmann brain there is no real past or future - your reality does indeed collapse immediately and you'll never know it; the idea that 'reality doesn't collapse immediately' is not a verifiable fact, because the only evidence you have to the contrary is encoded in and perceived by your brain...

If you are a Boltzman brain then you are born with memories you have (that haven't really happened) and you have no future (because the next moment you'll collapse).

You could even live kind of a "life" by randomly popping into existence once every million years in some differrent galaxy, experiencing one planck time and collapsing (and the only thing that connects the instances to each other is that the next instance by random chance has memories consistent with the previous instance).

They could even appear in non-chronological order.

I don't think it's likely, but it's more likely than having the one randomly generated brain experience stuff "in real time".

Here's a counterexample: https://kundor.github.io/Finding-Waldo/

See also this fantastic podcast (if I remember the episode correctly, although it doesn't matter much as they're all good and he talks about the subject frequently because it's tightly related to his day job)

https://www.preposterousuniverse.com/podcast/2022/06/06/200-...

We fixed the title (it was originally "Boltzmann Brian")

let's not elaborate on those fundaments though

Here's a different way of thinking about it. There are two things that are both very plausible: One - based on your direct personal experience - is that you are a non-boltzmann-brain-human living a normal life on earth. Two - based on well-accepted science - is that it is MUCH more likely for you to be a boltzmann brain than not.

Great; these two things are seemingly inconsistent. Which means one must be false. But if either of these is false, it's surprising! Because one is based on our direct experience of ourselves, as you have pointed out, and the other is based on well-established science. So what's interesting about Boltzmann brain (and similar) is that it shows that one part of our body of knowledge must be false. And this ought to motivate us to investigate exactly what it is that we have wrong.

I don't think anyone wants to convince you that you're a Boltzmann brain. This is more a thought experiment. The fun is to try to explain on logical grounds why this isn't a viable option.

Not sure why you're so hostile toward this. No one is saying that you're a BB; it's a thought experiment on the nature of reality.

And if you were a BB, you probably wouldn't know it, and how you experience yourself is irrelevant. That's kinda the point. The problem with the BB thought experiment is that it isn't falsifiable, at least not with techniques we have now.

It's impossible to logically disprove a theory like this, because the entirety of your thought process would itself be a random fluctuation, never mind the inputs on which it is based.

The problem is that if you start with the reasonable assumption that you objectively exist and that your observations are valid, the model of the universe derived from those observations (or at least some otherwise viable models) includes prevalence of Boltzmann brains.

You can always rely on HN to produce a software guy with a unique hypothesis about how the universe and/or the economy actually works.

That's fun. Where did the universe that is collapsing inwards come from, and what is the CMB? Do you have a mathematical treatment of this?

I think the main issue is that the event horizon of a black hole only exists relative to an observer. They're not physically 'real' objects but just theoretical mathematical limits we can calculate. They're 'where the escape velocity becomes c'. As an observer approaches a black hole the apparent event horizon shrinks away from them, in a way directly analogous to a geographical horizon.