I read the wikipedia article, but I just don’t get it…
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The underlying idea is that the laws of physics perfectly permit any collection of atoms to form some kind of structure capable of sapience, purely by happenstance. It’s vanishingly unlikely, but very much possible.
You can’t tell if you’ve actually experienced your whole life, or if you’re just a weirdly shaped lump of molecules in an asteroid belt somewhere that just randomly came into existence with all of its memories included.
The real gimmick of the theory is that if the universe is infinite, then there are infinitely many places for that random formation of consciousness to occur, so it’s actually _likely_ that you’re a Boltzmann brain rather than, well, you.
So the Boltzmann brain (BB) is a thought experiment. First let’s clarify what a thought experiment is,then we go on.
A thought experiment is an experiment that you do not perform, but you think what would happen. It’s basically a tool to test ideas (if this were true then this would happen), or to explain/describe an idea.
There are a lot of popular thought experiments. For example the trolley problem is one. Or Scrödinger’s cat. The common thing is that using such an “experiment” you can think about the world: how it works, how it should work.
Now the BB is also a thought experiment in a way. Boltzmann said that if the universe exists for ever, then eventually a fully formed human brain will pop up out of nowhere. Why? Because forming a human brain from atoms (just by random assembly of atoms) is very unlikely but not impossible. And every unlikely things happen if you have enough time for it.
Boltzmann used a brain in his example, but it could have been any complex thing: a car, a mobile phone (if it had been invented). Then it would be the Boltzmann-car or the Boltzmann-phone.
So the BB is basically a toolkit to think about the universe for physicists. If they come up with a new theory, they can test it by asking a question: if this theory is true, would it allow forming brains (or cars or phones) out of nothing? And how likely? Once in every hundred years? Once in every trillion years? And because in the real universe we do not see brains popping out of nowhere, a theory that would allow it frequently is a bad theory and ruled out.
Given the opportunity, stuff in space will spread out, on average. With a large enough collection of stuff, over a large enough of time, everything will always* spread out and eventually there will be no usable energy left. If you drop a pile of legos on the ground, they will basically be a random pile that wants to be as close to the ground as possible.
Except, not exactly always. The key word here is randomness. Atoms sometimes, on a very small scale, do not obey this rule. Quantum fluctuations are also responsible for this, but very, very rarely, a bunch of atoms out in space can spontaneously clump up and create random stuff.
The Boltzmann brain is a hypothetical scenario/thought experiment in which these random fluctuations in space spontaneously create a brain. The odds of this are freakishly low, but according to our current understanding of physics, not impossible.
There are some hypotheses that the big bang could have been an even rarer fluctuation in which a lot of random matter decided to get into one spot at once.
This would be like you dumping a box of legos onto the ground and it perfectly falling to assemble a perfect 1:1000 scale model of the Burj Khalifa complete with functioning elevators.
Who knows, maybe you are a Boltzmann brain and all of your memories are the result of random fluctuations, and a second from now the brain will die because the fluctuations didn’t bother making lungs or a heart.
PS: why is this question voted down – is it not on-topic here?
The Nebula had electrical discharges.
Those discharges, over time, replicated the effect of synapses firing in a brain.
The result was consciousness without a body.
It’s an extension of the brain-in-a-jar thought experiment.
The idea of that is there is no way to tell if you are actually what you think you are (a human with senses experiencing the real world) or a brain in a jar somewhere whose sensory impressions are being piped in with wires or something.
How could you tell the difference if all your tests have to go through the possibly faked senses?
One layer of abstraction back, we have the idea that there aren’t even any senses piped in, just the artificial memory of having seen something. After all, when you “see” something, it isn’t instant – there is a delay while various bits of processing are done to get it into a form your brain can handle.
Ok, so we now have a conscious entity with no real connection to the universe, just memories of it, and can see how its possible to have that experience be indistinguishable from our own.
Peel back the abstraction one layer further – do you need the brain? You could be a simulated brain running on arbitrary physical hardware.
Peel back one step further – does the simulated brain need to be built, or can it spontaneously self assemble? Sounds ridiculously unlikely, and is, but ridiculously unlikely doesn’t mean impossible. In the same way the lottery could be won with 1, 2, 3, 4, 5, 6 (and that ticket is no less probable than any other), the atoms in the universe could just happen to be arranged in a way that produces a consciousness that is identical to you in this moment.
If you want to go even further, Greg Egan wrote a book called Permutation City that takes time out of the equation. It’s a bit trippy.
A Boltzmann brain is a hypothetical brain-in-a-vat, floating in the vast emptyness of space, that came about by sheer chance. A bunch of atoms were just flying around, randomly collided and formed this brain-in-a-vat. It formed spontaneously only 1 minute ago, and in only 10s it will break apart as tenuously as it was formed. This brain contains ‘memories’ that never actually happened. This brain believes it lives in a world called earth, and that it’s currently reading something called a ‘comment’ on ‘reddit’. It has formed a theory about the universe. But all it’s theories and beliefs are wrong, as the universe it sees is fictitious. None of it is real.
The Boltzmann brain limits how much you can appeal to chance in your cosomological model. Over-relying on chance is tempting. If the universe is eternal, even the most unlikely events are bound to occur eventually. For example, suppose you have a theory for what caused the big bang. It goes something like this:
* The universe, on average, tends to towards higher entropy (more spread out and colder)
* However, changes towards lower entropy states are not impossible, just unlikely.
* The universe is eternal, so even the most unlikely events will happen eventually
* Eventually, the universe will experience another unlikely fluctuation into a low entropy state (smaller and hotter).
* Following this sudden contraction, the universe will evolve toward higher entropy again. And the cycle repeats.
* Conclusion: The big bang was a momentary fluctuation of an existing universe to a lower entropy state.
Ok, so the big bang was a chance fluctuation of entropy? This model has a problem: Boltzmann brains.
In such a model, a Boltzmann brain would be a FAR more common occurence than an entropic fluctuation the size of the big bang. So if this model is true, then it also follows that you are most likely a Boltzmann brain. The world you see is completely fictitious, any conclusions you derived from it are pointless, and you’re about to non-exist in 3…2….1…..
So, there seems to be a limit to how much random luck you can put into your model of the universe. If you rely on it too much, you would get Boltzmann brains. And then the most likely situation is that the universe you see is not even real so any model you have is bogus.
In the vast emptyness of space, there is a chance that two opposite particles randomly come into existence, such as an electron and a positron. They have opposite charges and will usually quickly eliminate each other and then cease to exist. These random occurances happen throughout all empty space.
So if space is big enough (first assumption) or time goes on for long enough (secondary assumption), then these random occurances could actually form something tangible when a bunch of them happen in the same space at the same time. Well, as long as there isn’t something else influencing where and when these random occurances happen and they are truly random (second assumption).
So if the universe is big enough, or even theorized as infinite, OR time is infinite, then we can expect a brain to randomly be created at some point in time, in a random location of empty space. It’s the same idea as “1000 monkey typing randomly on 1000 type writers will eventually produce Shakespeare.” A brain with memories, thoughts, ideas, everything. This brain could then immediately die, or cease to exist, but the point is that these would exist due to pure randomness. That’s a Boltzmann Brain. And you could be one.
The part that breaks this down the most for me is that if I am a Boltzmann Brain, then my memories and experiences would also be random – so I cannot apply the random universe I am experiences to the explanation of my own creation. That’s circlular logic.
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