
Boltzmann Brain - 6581
https://en.wikipedia.org/wiki/Boltzmann_brain
======
AgentME
I find this question really fun: "Am I a Boltzmann brain, or a real human?".
If there exists both a human and a Boltzmann brain with your current mental
state, then it doesn't make sense to try to answer this question with a single
answer. Your line of reasoning will be identically executed in both, so your
line of reasoning can't result in a single answer and be correct.

Instead, one way to address the confusion about whether you're a Boltzmann
brain or not, is to realize that in the case you're a Boltzmann brain, none of
your thoughts or decisions matter at all. Even if you admit there's some
probability that you're a Boltzmann brain, there's no use factoring it into
your decision processes besides as a curiosity. Your decisions only matter in
the case that you're an actual human, so that's the only possibility worth
considering while coming up with decisions. (Similarly, if a coin was going to
be flipped in a moment, and a bunch of things depended on which way the coin
landed, including that you were going to be immediately executed with no
possibility of escape if the coin landed on heads, then all your plans for the
future should be exactly the same as if you expected the coin to land on tails
with 100% certainty. It's the only path that any of your decisions matter.)

~~~
jdkee
What if you are a recurring sequence of Boltzmann brains pooping into
existence, lasting mere seconds but correctly temporally sequenced as to form
a continuous consciousness?

~~~
klodolph
If you like these thought experiments, consider that I’m not actually
responding to your comment, but I am actually mashing the keys randomly and it
only appears to form a response by coincidence. We are not actually
communicating with each other.

~~~
dsr_
You are not mashing the keys, because nothing happens except that God wills it
so. It just happens that God keeps making miracles that you have interpreted
as the effects of your actions on the world.

~~~
dsr_
This is the first time I've been downvoted for paraphrasing David Hume :)

[https://en.wikipedia.org/wiki/David_Hume](https://en.wikipedia.org/wiki/David_Hume)

------
irjustin
I've always enjoyed this thought experiment and the loosely related one - Last
Thursdayism[0].

[0]
[https://rationalwiki.org/wiki/Last_Thursdayism](https://rationalwiki.org/wiki/Last_Thursdayism)

~~~
trhway
> is the idea that the universe was created last Thursday, but with the
> physical appearance of being billions of years old.

or it may have been just restored from a backup (with some bit rotting and
missing pieces as evidenced by dark matter/energy and other inconsistencies)

~~~
mr_toad
The dreams you recall are just artefacts of the restore process initialising
your mind.

~~~
mannykannot
I actually considered that as a realistic possibility -- that memories of
dreams form as you wake up -- but I think that is didproved by people talking
and moving in their sleep in ways consistent with what they report about their
dreams when they wake up. PET and NMR scanning might be able to resolve the
issue, if it has not done so already.

~~~
speakeron
Lucid dream research done in the 1970s had subjects signalling from within a
dream using eye movement.

[https://www.researchgate.net/publication/258201870_Lucid_dre...](https://www.researchgate.net/publication/258201870_Lucid_dream_verified_by_volitional_communication_during_REM_sleep)

------
jancsika
Why is it not a Boltzmann rock, or a Boltzmann commemorative coin, or
something else inanimate like that?

I thought the whole point was to give a simple metaphor for a measuring the
probability that a mundane object came into being based on the widely accepted
history of the universe that modern physics predicts. As opposed to just
coming into its current state instantaneously based on extremely unlikely
random events among its constituent atoms.

Or put differently (based on the Wikipedia article), if the math behind a
modern physics theory means that it's more likely the given object's existence
was due to random fluctuations then that's a strike against adoption of that
theory.

But if you make the object of interest a brain you get all this bikeshedding
about consciousness and how do the regular brains know they're not Boltzmann
brains, etc.

It'd be like introducing me to your new programming language's syntax by
showing me the code for a quine instead of "hello world." What is the value in
doing such a thing?

~~~
AgentME
There are similar thought experiments around complex objects suddenly coming
into existence. They're not very controversial. The Boltzmann brain thought
experiment is interesting because it raises the possibility that there's some
chance that you might be a Boltzmann brain without realizing it. It's very
hard to try to figure out what the correct way to reason about this
possibility is. Properly reasoning about whether you're a Boltzmann brain, and
whether you should care about that, probably involves an interesting argument
around decision theory and the anthropic principle.

~~~
Zenst
I think the chance of being a Boltzmann brain is zero if you can write down
"I'm human" on a bit of paper, paper in the room, go to another room and then
return to the original room and read the paper saying exactly what you
initially wrote. Whilst the memory of writing it may well be false, the paper
will not be.

Now, a Boltzmann universe would cater for that - spooky thoughts at a distance
:).

~~~
kromem
How about a Boltzmann simulation?

No physical anything at all, just organization of energy according to various
rules that abstract a physical reality. A pure information based existence
that depends on no ontological physicality.

~~~
glenstein
This is interesting, but wouldn't 'organization of energy' be physical too?

~~~
Zenst
I'm leaning towards looking at this akin to the Matrix, whilst your in the
Matrix, if you have never been outside then reality and what is real is
totally different from perspective. As nobody has proven they have been
outside this Boltzmann Brain aka Matrix, nobody can disprove it. Very much
like most religious icons you could say.

------
gus_massa
Most of the arguments in favor of the Boltzmann Brain are written before the
discovery of the Hubble's law (1929) when most people thought that the
universe was essentially eternal and static. Even Einstein was trying to use
the cosmological constant to save the model of a eternal static universe.

After the discovery of the Hubble constant, the Big Bang and Inflation, it's
more razonable a low entropy starting point.

~~~
raattgift
> arguments in favor of the Boltzmann Brain are written before the discovery
> of the Hubble's law (1929)

I'm pretty sure the Boltzmann brain paradox is 20th century, likely
originating with John D. Barrow and Frank Tipler -- they discuss it in e.g.
chapters 6 and 10 of their 1986 book on the anthropic cosmological principle,
although afaict the actual words "Boltzmann['s] brain[s]" first popped up in
the early 2000s.

Dyson, Kleban and Susskind (DKS 2002, [https://arxiv.org/abs/hep-
th/0208013](https://arxiv.org/abs/hep-th/0208013)) is firmly after the
widespread acceptance of the lines of evidence pretty much compelling an
accelerated expansion of the universe, for example. Albrecht & Sorbo actually
talk about brains and bodies and so forth in 2004 [https://arxiv.org/abs/hep-
th/0405270v2](https://arxiv.org/abs/hep-th/0405270v2) : in section "C." they
discuss some of the history of the paradox, briefly.

Without a finite age to the universe -- this comes from Lemaître in 1927,
rather than Hubble in 1929 -- there is no particular reason to assume that
there is a low entropy state in the past. A positive cosmological constant
creates more empty space (all of which is very high entropy, because of the
indistinguishability of subregions of any region of truly empty space) and
thus demands a much lower entropy before the dark energy dominated era. It's
the low entropy of the past that's the source of concerns about Boltzmann
brains. And a hot big bang is even worse. How did the universe end up in such
relatively low entropy? And it must be low entropy rather than a hot dense
system in equilibrium, because otherwise one runs into the problem of
_suppressing_ Boltzmann brains.

cf. Carroll's deck at [https://www.slideshare.net/seanmcarroll/the-origin-of-
the-un...](https://www.slideshare.net/seanmcarroll/the-origin-of-the-universe-
and-the-arrow-of-time) starting at or before slide 21. A second deck at
[https://www.slideshare.net/seanmcarroll/what-we-dont-know-
ab...](https://www.slideshare.net/seanmcarroll/what-we-dont-know-about-the-
beginning-of-the-universe) starting at slide 5 is slightly more technical,
with references at the bottoms of slides later in the deck. Suppressing an
"entropy catastrophe" is very modern hard work for physical cosmologists!

> save the model of a eternal static universe

He didn't know the Raychaudhuri equations (1955, shortly after his death) or
the focusing theorems they support, only the Jeans equations (1902), and was
trying to suppress a collapse via Jeans mechanisms. Had he known about
Raychaudhuri's expansion scalar, who knows where he would have went -- perhaps
he might not have discarded his small _negative_ cosmological constant in the
aftermath of evidence for the expanding universe, but rather might simply have
accepted a small positive value as consistent with the evidence and theory,
and we wouldn't have had to wait half a century to get it back in the minds of
cosmologists. On the other hand, perhaps we would still have had to wait for
the discovery of the _accelerated_ expansion to get to a small positive
cosmological constant as a parsimonious explanation.

Finally, this is a really nice way to connect Raychaudhuri and the CC: [Ellis
2007]
[https://www.ias.ac.in/article/fulltext/pram/069/01/0015-0022](https://www.ias.ac.in/article/fulltext/pram/069/01/0015-0022)

------
kelnos
PBS Spacetime did a fun video on this:
[https://www.youtube.com/watch?v=nhy4Z_32kQo](https://www.youtube.com/watch?v=nhy4Z_32kQo)

~~~
julienchastang
Excellent video. Thank you.

------
_bxg1
It's always seemed dubious to me that people try to make probabilistic
statements about an event that we only have one sample of, and whose causes we
can only speculate about

~~~
alexpetralia
Exactly - probability (and by extension, statements such as "more likely")
only make sense when we have a reference class of similar events for which we
have iterated trials (frequentism), or iterated trials we can reasonably
hypothesize about (hypothetical frequentism).

Statistics and probability was not designed for one-off events with specious
reference classes. The more speculative the reference class, the more we
should doubt any claim of "more likely".

In the case of the Boltzmann brain, given we have no evidence of this ever
occurring thus far, we can at very least claim it does not have a clear
reference class that we can identify empirically.

------
empath75
I found the argument that this was self-refuting fairly convincing.

If you're a boltzmann brain, then anything you think you know about anything
is illusory and based on nothing at all, including the idea that you could be
a bolzmann brain.

I think this also applies, although less convincingly, to most theories that
the world is illusory or a simulation.

In any case, it doesn't matter because you'll stop existing momentarily.

~~~
icandoit
I'd be careful assigning meaning depending on a relative time frame.

After all the Sun will swallow the earth in about 7 billion years. I like to
think the things we do matter independent of that fact.

~~~
roywiggins
Sure, but you have causal effects in your general vicinity that will persist a
little while, not least effects that come back to change the originating
brain.

Boltzmann brains have no past and approximately no future _at all_.

~~~
icandoit
Yes, but I experience meaning independently of those effects. I think that
most people think those effects are what create the experience of meaning.

Boltzmann brains experience meaning too. Not that is does them any good in the
way that it does for biological system.

It's not like we have a planet rich in meaning and then the sun swallows the
earth and all that meaning retroactively disappears. It just exists in the
past, not the future, and I think that is sufficient.

I always thought it was funny when people thought that the heat-death of the
universe had moral implications. Like that killing someone means they never
existed, or something.

~~~
glenstein
I have maddeningly had this exact conversation with a friend, and I can't help
but think there's something they're just not understanding about meaning.

I don't think this view is uncommon, and I think people get there because
they're just not thinking straight, but I don't know what they feel is at
stake that they have to hold onto, that gets 'lost' if you accept that meaning
doesn't persist through time forever.

------
dang
A thread from 2016:
[https://news.ycombinator.com/item?id=12152658](https://news.ycombinator.com/item?id=12152658)

2014:
[https://news.ycombinator.com/item?id=6999074](https://news.ycombinator.com/item?id=6999074)

~~~
jv22222
Someone should make a "List of repeating ideas that come up on Hacker News
with every new HN Generation" post.

It would also be interesting to know how often a "New HN Generation" came in
to being.

~~~
pmiller2
IMO, there's not a lot of repetition. It's just that certain individual posts
get repeated over and over.

------
croo
There is a sci-fi book about scanning a human brain/conciusness to computer
and testing wether it recognizes that the computer gets turned off between the
tests. Hint; it doesn't as it does not change state. Based on this experiment
one can create a machine, upload his mind then destroy the machine because
there will eventually be somewhere sometime a state of the universe where the
state of the machine will proceed a step. So you can make a simulated world in
that computer and be anything in that world, even god.

Great mindfuck scifi, wish I could tell it's title but now it's clear the
author read about this Boltzmann Brain theory...

~~~
alach11
Permutation City, perhaps?

[https://en.wikipedia.org/wiki/Permutation_City](https://en.wikipedia.org/wiki/Permutation_City)

~~~
croo
That's the one! Thanks I'll re-read it :)

------
lemonoftroy
[http://boltzmannburrito.com/](http://boltzmannburrito.com/)

------
jackhalford
> Some cosmologists believe that a better understanding of the degrees of
> freedom in the quantum vacuum of holographic string theory can solve the
> Boltzmann brain problem.

All the evidence right now points that I am a Boltzmann Brain. It would be
amazing for cosmologists to challenge this.

------
JohnJamesRambo
This is one of my absolute favorite ideas/theories.

------
cletus
Mandatory pointer to Isaac Arthur's collaboration on this, Part 1 [1] and 2
[2].

I too find this a fascinating thought experiment.

[1]: [https://www.youtube.com/watch?v=9UfQb_-
XAuY](https://www.youtube.com/watch?v=9UfQb_-XAuY)

[2]:
[https://www.youtube.com/watch?v=GrK9EaQRp2I](https://www.youtube.com/watch?v=GrK9EaQRp2I)

------
DethNinja
This is an interesting topic. Although I don't have much knowledge in this
area, randomly arranged atoms forming Boltzmann Brains and keep observing
random bits handed out by universe doesn't seem all that likely to me.

My main reasoning is due to how we tend to classify the knowledge coming from
the observed universe, and how logically stable it is.

Imagine this, you are Boltzmann Brain and track a randomly arranged
consciousnesses. You sleep, go to work, eat, and sleep again as a human being.

Main problem is that this kind of existence is very stable for a Boltzman
Brain, as in when you wake up you see yourself in the same universe (or at
least a very similar one).

Now you have to ask why you are classifying these randomly arranged bits
handed out by universe as a human life, but not as an alien one. Or why not
you are imagining you are something supernatural like a god? Just shift the
bits handed out by universe to you and next time you wake up, you will be a
completely different being. It seems to me that this should be possible if you
were a Boltzmann Brain and existence would be extremely chaotic.

It seems to me that there is more to existence and consciousness than just
reading and classifying random bits handed out to you by universe. It seems to
me that logical stability is extremely important for our kind of existence and
I don't see how Boltzmann Brain theory would possibly keep the universe
logically stable.

~~~
admax88q
I think you're not fully understading the theory of the Boltzmann brain. A
Boltzmann brain would not exist for any extended period of time. They only
exist for an instant, they don't "track" any sort of existence or
conciousness, they merely pop in to existence for an instant and then
disappear the next.

Imagine your current mental state as a snapshot. By pure luck some atoms could
arrange into the exact same state as your brain is right now. And for that
moment, it would be you, it would have your full set of memories. It would be
indistinguishable from your current existence.

And then the next moment it would cease to exist.

But there could be infinite many such brains popping in to existence and then
disappearing at every instant, with every conceivable configuration of
state/memories.

------
mhh__
When it gets to seriously deep questions like this (possibly not such with
this one because it's fairly abstract) I can't help but feel that we don't
know enough about the universe to start pontificating.

We are still doing a lot of science (especially in the last 30 years) in our
own solar system let alone understanding the whole universe.

------
RoutinePlayer
This is way too much thinking for a Friday ... call me a Boltzmann brain all
you want, but I'm having beer!

~~~
goatlover
Not if you're a Boltzmann brain.

------
stevebmark
This is about equally as ignorant, incorrect, and useless, as thinking
infinite universes means all infinite possibilities will happen.

> Given enough time, every possible structure is formed via random fluctuation

NO. That is not what infinity means. I hate that this garbage made its way
into anyone's lexicon.

~~~
paulrouget
Can you expand on that?

If a state is possible, isn’t an infinite amount of permutations will form
that state? Isn’t that the definition of a non zero probability?

~~~
ducttapecrown
If you pick a real number at random, what's the probability it will be
rational?

~~~
paulrouget
Not zero. I think the proper way to define it is “almost never” when infinite
are involved:
[https://en.wikipedia.org/wiki/Almost_surely](https://en.wikipedia.org/wiki/Almost_surely)

I’m not sure if that means it will for sure happen or not, if the draw happens
an infinite amount of times.

~~~
drdeca
In the standard measure theory formalization of probability, that outcome set
does have measure exactly 0.

Perhaps we could use a variation on in which somehow supports some
infinitesimal values, but if one wishes to keep both countable additivity and
have the distribution be uniform in the sense of translation invariant, then
we run into difficulties.

------
dmead
[https://en.wikipedia.org/wiki/Ego_the_Living_Planet](https://en.wikipedia.org/wiki/Ego_the_Living_Planet)

------
RocketSyntax
This inspires me even more to learn about boltzmann machines. The `why`
lighting up the limbic brain

------
state_less
If a Boltzmann Brain that begets more Boltzmann Brains is possible, it would
be more likely, since as soon as you had one, you'd have a whole lot more.

Similar for other forms with this property. A form that when it does appear is
then more likely to return again.

~~~
visarga
self replicators do, such as all living things

~~~
state_less
Sure do. They tend to get beaten up by random things and lose their form, but
reforming soon enough helps keeps them around.

------
aj7
Bad counting in the probabilities.

------
infradig
There is no such thing as a Boltzmann Brain, the mere idea of it is ludicrous.
Get over it already.

