disproof of boltzman brain?

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Eebster the Great
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Re: disproof of boltzman brain?

Postby Eebster the Great » Thu Dec 06, 2018 2:05 pm UTC

The universe in this model is infinite, not a single point. But due to inflation, a region that was once extremely small (though not literally a single point) is now larger than the entire observable universe. This inflationary event may be the low entropy event we are discussing. In any case, the idea is that the universe is usually at or near equilibrium and only rarely fluctuates away from equilibrium. Entropy never decreases in classical mechanics. You will, in your whole life, never observe a case in which total entropy decreases. It is effectively impossible and is a violation of the second law of thermodynamics. However, it is not literally impossible, and in an infinite universe, it happens infinitely many times. But there is no reason entropy dropping gradually is more probable than it dropping quickly. At every step, it is overwhelmingly more likely that entropy will increase. You cannot set things up so entropy just coasts downhill; it always rises.

Pfhorrest, large universes would be more likely to naturally evolve brains than small universes would be, sure. But they would also be more likely to evolve Boltzmann brains, and more of them. The fact that these brains occur more frequently than natural ones doesn't change in a large event.

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Re: disproof of boltzman brain?

Postby Pfhorrest » Thu Dec 06, 2018 5:57 pm UTC

Eebster the Great wrote:Pfhorrest, large universes would be more likely to naturally evolve brains than small universes would be, sure. But they would also be more likely to evolve Boltzmann brains, and more of them.

This phrasing doesn't make sense to me. In the sense of "universe" I was using, the classical image of a Boltzmann brain that just pops into being from nothing without any surrounding context is a "universe", and in the looser sense of a Boltzmann brain you seemed to be using in the post I was replying to, "naturally evolved" brains are still a kind of Boltzmann brain.

Like, one possible event is that out of the thermal equilibrium of the infinite true universe, energy just happens to arrange itself into a brain for a moment. That's the classical image of a Boltzmann brain.

Another possible event is that out of the thermal equilibrium of the infinite true universe, energy just happens to arrange itself into some other low-entropy state, which then evolves over the course of its return to equilibrium, and over the course of that evolution, a brain comes into being. You seemed to be talking about this, and saying that this also counts as a Boltzmann brain.

Yet another possible event is that out of the thermal equilibrium of the infinite true universe, a whole lot of energy arranged itself into a massively low-entropy state, which then evolved over the course of tens of billions of years as it slowly returned to equilibrium, generating everything we know, stars and planets and life with brains, along the way.

If the brains that occur in that second case still counts as Boltzmann brains, then the "naturally evolved" brains of our universe are just an edge case of Boltzmann brains too, and you really have this spectrum of spontaneous low-entropy events from ones the size of brains to ones bigger than our observable universe. The smaller ones are going to be more likely to happen, of course. But if brains are more likely to occur in the bigger ones than in the smaller ones, which seems plausible, then that seems like it negates that, as far as where you're likely to find a brain. If there are many more small random things spontaneously popping into existence across infinity all the time, but many many fewer of them ever become (or come to contain) brains than the big ones, and the big ones tend to contain many many many brains each, then the unlikelihood of a brain occurring in a small fluctuation could outweigh the greater likelihood of those small fluctuations occurring, leaving most brains most likely occurring within big fluctuations, like the universe that we know.
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Re: disproof of boltzman brain?

Postby Eebster the Great » Thu Dec 06, 2018 8:42 pm UTC

I mean that a large universe which develops into a vacuum with a surprise brain in the middle is still more likely than a large universe with such low entropy that not only do you get a brain, but also planets and stars and stuff. Because the former is far higher entropy.

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Re: disproof of boltzman brain?

Postby ucim » Fri Dec 07, 2018 2:40 am UTC

Eebster the Great wrote:I mean that a large universe which develops into a vacuum with a surprise brain in the middle is still more likely than a large universe with such low entropy that not only do you get a brain, but also planets and stars and stuff. Because the former is far higher entropy.
Is it?

In the latter case, all you need is a random (particle distribution in the) universe, whose (small finite number of) laws generate life, which generates Brians, or perhaps brains. How many (from the set of possible) laws will lead to the generation of brains? Probably (IMHO) more than random fluctuations of the Void will spontaneously generate a brain. (Or a Brian)

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Re: disproof of boltzman brain?

Postby tomandlu » Fri Dec 07, 2018 3:07 pm UTC

ucim wrote:
Eebster the Great wrote:I mean that a large universe which develops into a vacuum with a surprise brain in the middle is still more likely than a large universe with such low entropy that not only do you get a brain, but also planets and stars and stuff. Because the former is far higher entropy.
Is it?

In the latter case, all you need is a random (particle distribution in the) universe, whose (small finite number of) laws generate life, which generates Brians, or perhaps brains. How many (from the set of possible) laws will lead to the generation of brains? Probably (IMHO) more than random fluctuations of the Void will spontaneously generate a brain. (Or a Brian)

Jose


I don't think anyone's claiming that the evolution of the universe, including life, is anti-entropy - just the initial change from total entropy to low entropy (which then allows everything else to happen). That said, I too am struggling with the notion of a large-scale anti-entropy event, but which is essentially unorganised (creation), vs a small-scale anti-entropy event that is highly organised (a BB), but I think that's my puny brain, rather than something wrong with the BB principle.
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Re: disproof of boltzman brain?

Postby doogly » Fri Dec 07, 2018 3:14 pm UTC

What do you mean by "Organized" here?
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Re: disproof of boltzman brain?

Postby Xanthir » Fri Dec 07, 2018 3:46 pm UTC

ucim wrote:
Eebster the Great wrote:I mean that a large universe which develops into a vacuum with a surprise brain in the middle is still more likely than a large universe with such low entropy that not only do you get a brain, but also planets and stars and stuff. Because the former is far higher entropy.
Is it?

It is, by virtue of the simple fact that it's a *much* smaller amount of energy that had to locally-entropy-reverse into that tiny inflationary point, versus *all* the energy that had to do the same to form our full universe. Every quantum of energy you add to the event decreases its likelihood of occurring in the first place (exponentially, iirc); a vacuum+brain is *vastly* smaller than the mass-energy of our physical universe!

Once you set up a physical universe like ours, it may be inevitable that brains develop that are capable of thinking about themselves and having memories and such, versus the clearly ad hoc/random nature of a brain arising in a void with built-in memories of a life, but the starting probabilities of such a physical universe versus the brain universe are so mind-bogglingly tilted toward the brain-void universe that, as far as many of our attempts at cosmology can tell, the brain-void is going to happen *way* more often than brains in a physical universe capable of developing them normally.

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Re: disproof of boltzman brain?

Postby ucim » Fri Dec 07, 2018 4:43 pm UTC

tomandlu wrote:just the initial change from total entropy to low entropy (which then allows everything else to happen)
What is "total entropy" and how does it even apply to a universe that doesn't exist yet?

Xanthir wrote:
ucim wrote: Is it?
It is, by virtue of the simple fact that it's a *much* smaller amount of energy that had to locally-entropy-reverse into that tiny inflationary point, versus *all* the energy that had to do the same to form our full universe
But also the bigger the universe that is created, the more likely it is that, within that universe, there will be pockets that develop into brains. It would happen due to the (small number of) natural laws, rather than the (huge number of) boundary conditions that are needed to support a BB.

Xanthir wrote:In conclusion, Last Thursday-ism appears to be the most statistically-correct religious stance of all.
Heretic! It's last Wednesday-ism!

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Re: disproof of boltzman brain?

Postby Xanthir » Fri Dec 07, 2018 5:44 pm UTC

ucim wrote:
Xanthir wrote:
ucim wrote: Is it?
It is, by virtue of the simple fact that it's a *much* smaller amount of energy that had to locally-entropy-reverse into that tiny inflationary point, versus *all* the energy that had to do the same to form our full universe
But also the bigger the universe that is created, the more likely it is that, within that universe, there will be pockets that develop into brains. It would happen due to the (small number of) natural laws, rather than the (huge number of) boundary conditions that are needed to support a BB.

Sure, but again, Every. Single. Quantum of Energy. you add to the entropy fluctuation that becomes a universe exponentially decreases the likelihood of it occurring. A bigger universe is *fundamentally* more unlikely than a smaller one, before you even start doing the math for figuring out the chances of brain-o-genesis!

Basically you're not doing the math hard enough if you think that pockets of brains in gigantic energy-full universes end up being more likely than a small brain in a void. (Or at least, many respected cosmologists who *have* done the math seem to currently agree that, based on our current understanding of things, it's way more likely we're a BB than a big universe.)

Also, I'm not quite sure if you got this point or not from your wording, but the BB hypothesis doesn't need to support a brain, like long-term or anything. It's sufficient to have a universe that conjures a brain into existence for a bare moment, just long enough to meaningfully exist and process the most recent part of the thought you are currently thinking. Prior existence is irrelevant; conjuring the brain with pre-built memories of prior existence is *far* more likely than conjuring the brain with sufficient life-support and hallucinatory inputs to make it actually *have* a prior existence. (Giving the brain particular memories is basically just rearranging some of the energy making it up; it doesn't increase the cost of the universe. Giving the brain a long-lived existence *does* increase the cost of the universe.)

Most brains will *not* have coherent pre-built memories, and will pop into being insane and inchoate before their entropy fluctuation dissipates and they dissolve back into the high-entropy soup of background particles. And of course, most fluctuations won't produce a brain at all; a Rubik's cube in a void is probably more likely than a brain, being smaller and less massive and thus requiring a smaller entropy fluctuation to produce.

The BB hypothesis is just saying, basically, that if you list *all possible entropy fluctuations* in order of how likely they are to occur in an infinite universe with infinite time, then "a brain in a void with pre-built memories of an existence in a meaningful universe exists for a moment before dissolving" is way, way, way closer to the front of the list than "a large universe enjoys a stable existence for billions of years, eventually producing a brain (plus the body, biosphere, planet, star, etc surrounding it) that experiences a meaningful existence".
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Re: disproof of boltzman brain?

Postby Eebster the Great » Fri Dec 07, 2018 9:10 pm UTC

It is not the case that a larger universe is more likely to develop a brain, because the issue is not just size, but entropy. A very large universe that is at thermal equilibrium will not produce brains in any way except by spontaneous fluctuations, because it's in equilibrium already, so nothing else can happen. The question is not volume or mass but entropy.

In order to produce a universe that, even if largely composed of diffuse plasma (which, to be fair, is still very far from equilibrium), also has lots of structure, up to galaxy superclusters, you need to start with something that has very, very low entropy. In order to produce a universe of the same size (or bigger, or smaller, or whatever) that can only produce mostly diffuse plasma plus a brain or two, you only need to start with something that has kind of low entropy. That's really all there is to it. A lot of structure is necessary to evolve a brain, and that beginning will always be more structured than the brain itself, because that's how thermodynamics works.


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