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B Boltzmann Brain

  1. Mar 9, 2016 #1
    A coworker of mine keeps mentioning Boltzmann Brains. I was never really interested until yesterday when reading Timeline of the Far Future on Wikipedia, which states the ultimate fate of the universe could be a Boltzmann Brain. Is this a theory scientists believe could really materialize?
     
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  3. Mar 9, 2016 #2

    Chalnoth

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    The Boltzmann Brain is used to exclude models: if Boltzmann Brains are more common than real brains in a particular model of the universe, then that model is not likely to be correct.
     
  4. Mar 9, 2016 #3
    More common than real brains or more probable do you mean?
     
  5. Mar 9, 2016 #4

    Chalnoth

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    As in the number of Boltzmann Brains > the number of real brains. If a model predicts that, then it can't be right.

    This argument originally came up with the simple model of the universe as a thermal fluctuation out of equilibrium. The idea there is that when a system is at equilibrium, there are random deviations from that equilibrium. Here you might imagine the universe as a giant box. Within this box, most every time you look, there will be nothing but empty space. But every once in a while, a random drop in entropy will produce something.

    The problem with this is that small drops in entropy are exponentially more common than large drops in entropy. So if you only look at the box those times that there is, say, a galaxy in the box, nearly every time there will be only a single galaxy. Two galaxies will be absurdly rare compared to just one galaxy.

    Similarly, a single star system will be far, far more common than having two star systems, let alone three or an entire galaxy of stars.

    A single planet, one that is temporarily at a high enough temperature to support life, will be far more common still. A single person that imagines observing an external universe will be even more common than a whole planet. Just a disembodied brain that thinks it exists will be the most common thing that thinks it exists.

    You might say, "But brains and galaxies don't just pop out of nothing: they form from matter that was around earlier." But that just pushes the problem back: the entropy was always lower in the past. It's harder to form the early universe from a thermal fluctuation than it is to form the present universe!

    This line of reasoning shows that there had to be something special about the early, low-entropy state of our universe. There's a fair amount of theorists who are working on possible solutions to this problem. Currently there are a lot of models that avoid this problem, but we don't yet know which (if any) of them are correct.
     
  6. Mar 9, 2016 #5

    Chronos

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    Boltamann brains can be avoided in any number of ways - one being the many worlds approach in quantum mechanics. This is, IMO, a rather messy solution. All those branching alternate realities would appear to make the bulk pretty crowded in short order. It's sort of like worrying about the universe going poof by decaying out of a false vacuum state. If you take the Boltzmann brain paradox seriously, you also risk falling prey to false vacuum psychosis. I find solace in the idea some events are so wildly improbable they simply never occur in a finite amount of time - of course, that opens you up to the prospect the universe is not past eternal. Either way, I have yet to encounter an altogether convincing and elegant solution to the Boltzmann brain dilemma.
     
  7. Mar 12, 2016 #6

    mfb

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    Why not?
    I have a model predicts that the majority of brains does not use "Chalnoth" as user name. Does the model have to be wrong because you are not in this majority?
    Even worse, where is your proof that you are not a Boltzmann brain? It seems unlikely as your memories make sense (which seems to be unlikely for a Boltzmann brain), but you cannot exclude it.

    From a philosophical point of view, a model that doesn't have a majority of Boltzmann brains sounds nicer, but that is not a scientific argument.
     
  8. Mar 13, 2016 #7

    Chalnoth

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    Except observation confirms that I'm not in the majority. I really don't know what you're trying to argue here.

    Nearly all Boltzmann brains will have disordered observations. Most will see nothing but random noise. Almost none of them will see an ordered world that follows natural laws.

    One way to see this is to note that there are ways to tell pretty conclusively whether or not you are dreaming. If you are dreaming, then physics generally doesn't work, and there are very simple ways to test this. For example, you can hold your nose and try to breathe. In a dream, you'll still be able to breathe because your brain doesn't do a very good job of simulating the physics of holding your nose. Another method is to look at writing: in a dream, writing tends to change if you look away then look back.

    But even dreams are a hell of a lot more ordered than anything you'd get in most any Boltzmann Brain scenario, as such brains are not at all constrained by the limits that our brains operate under.

    It's true that there's no absolute proof that we are not Boltzmann Brains. But that's a philosophical argument, not a scientific one. Science doesn't deal with absolute proof.
     
  9. Mar 13, 2016 #8

    mfb

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    So where would be the problem of being in a minority of non-Boltzmann brains?
    That's why I said "it seems unlikely" - but there will be a few Boltzmann brains where memories make sense, at least for the short period of their existence. And those Boltzmann brains will rule out that they are Boltzmann brains - incorrectly.
     
  10. Mar 13, 2016 #9

    Chronos

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    If a Boltzmann Brain were not in an ordered state, it would not be much of a brain, would it? We may be digressing here in quibbling over the nature of a Boltzmann Brain. This excerpt from http://arxiv.org/abs/hep-th/0405270 sums it up nicely, IMO:

    "A century ago Boltzmann considered a “cosmology” where the observed universe should be regarded as a rare fluctuation out of some equilibrium state. The prediction of this point of view, quite generically, is that we live in a universe which maximizes the total entropy of the system consistent with existing observations. Other universes simply occur as much more rare fluctuations. This means as much as possible of the system should be found in equilibrium as often as possible.

    From this point of view, it is very surprising that we find the universe around us in such a low entropy state. In fact, the logical conclusion of this line of reasoning is utterly solipsistic. The most likely fluctuation consistent with everything you know is simply your brain (complete with “memories” of the Hubble Deep fields, WMAP data, etc) fluctuating briefly out of chaos and then immediately equilibrating back into chaos again. This is sometimes called the “Boltzmann’s Brain” paradox."

    I find Chalnoth's comments compatible with this logic
     
  11. Mar 13, 2016 #10

    Chalnoth

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    The brain itself may be in a very ordered state, but it still isn't likely to "observe" order (observe in quotes because it's not actually observing anything).

    This is why I made the comparison to dreaming: you can reliably determine whether or not you're dreaming by testing how physics behaves. The only real obstacle to this is that it rarely occurs to us to actually test whether or not we're dreaming (if you really want to do this, one way is to get into the habit of testing yourself for dreaming regularly while you're awake).

    I certainly hope so! Andy Albrecht was one of my professors. I always admired his clarity of thinking when it came to theoretical physics.
     
  12. Mar 13, 2016 #11

    Chronos

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    I was merely acknowledging I appeciate where you were coming from.
     
  13. Mar 13, 2016 #12
    I asked my girlfriend about this, how do you know whether or not an observation is real or imaginary?
    She said 'You know'?
     
  14. Mar 25, 2017 #13

    Sorry for hijacking an old thread, but I feel I need to respond so the members who search the forum about this topic find some better insight.

    This is why: https://arxiv.org/abs/1702.00850 [Sean Carroll - Why Boltzmann brains are bad]

    I waited for your response and eventual opinion in the other thread because I respect it and if you haven't got any, that's ok. Just making sure that it is mentioned what is wrong with this line of thinking.
     
  15. Mar 27, 2017 #14
    Theres an assumption in the BB argument that a brain is more likely to thermally fluctuate into existence than a entire universe. but how sure can we can be of this statement? What if you don't need an entire universe to fluctuate but instead just a universe seed? perhaps that seed is less complicated than a brain and could thermally fluctuate into existence then it would seem Boltzman would be right all along. What's wrong with the reasoning here?
     
  16. Mar 27, 2017 #15

    PeterDonis

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    And then what? The seed itself is not a universe or a brain. If the answer is that in 14 billion years or so it's going to develop into a universe that contains brains, then your hypothesis is not the Boltzmann Brain hypothesis, and in fact is an argument against it.
     
  17. Mar 27, 2017 #16

    Chalnoth

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    A "universe seed" by definition has lower entropy than a brain (for the reason that a whole universe has lower entropy than a single brain, and an early universe has lower entropy than a late one). As long as the probability of a fluctuation is proportional to an exponential function of entropy, the brain is going to be far more likely.

    Basically the way out of this is to provide a coherent argument that the probability is not simply proportional to an exponential function of entropy.
     
  18. Mar 27, 2017 #17
    Which is very tough, of course, but there's something else which is controversial when discussing what can fluctuate. Sean mentions it in the article, and the term is ergodicity. All these issues are based on a pretty unproven assumption that everything obeys Boltzmann's ergodic theorem which is almost 150 years old.
     
  19. Mar 27, 2017 #18

    Chalnoth

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    I think this boils down to the question of unitarity. If the fundamental laws of physics are unitary (which many theorists feel is a requirement for causality), then the system follows Liouville's Theorem, which is closely related to ergodicity. Ergodicity and Liouville's Theorem are not exactly the same, but I think it's the latter which is more related to this subject.
     
  20. Mar 27, 2017 #19
    Could you please explain why in your previous post you mentioned that the current universe has a lower entropy than a brain?

    Isn't entropy the measure for the number of microstates and by definition a bigger object would have more, so the entropy would be higher in the mentioned example?
     
  21. Mar 27, 2017 #20

    Chalnoth

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    The way to do this properly is not to just look at the entropy of the brain in isolation, but rather compare its entropy to the maximal entropy of the system in question. A brain is a much smaller difference in entropy compared to empty space over the same region than our current universe is from empty space over the whole universe.
     
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