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I Boltzmann brains and other low probability events

  1. Jan 2, 2017 #1
    I am interested in your opinion with regards to expectation values of low likelihood events in QM. For example, Boltzmann brains are suggested as a problem in cosmology despite their probability being extremely miniscule.

    Is it realistic to expect Boltzmann brains and other low probability events to ever occur? What's your opinion on BBs in general?
     
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  3. Jan 2, 2017 #2

    PeterDonis

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    As it stands now, that would depend on one's interpretation of QM. The theory itself just predicts probabilities; it doesn't tell us what probabilities are "realistic". At some point we might come up with a more comprehensive theory that gives us a way to resolve this issue, but right now we don't have one.

    We already have several threads where BBs have been brought up; try the PF search feature.
     
  4. Jan 2, 2017 #3
    I already searched almost all threads where they have been brought up and I see no convention about their possible existence and other implications of it.

    What's your take on the topic?
     
  5. Jan 2, 2017 #4

    PeterDonis

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    That's odd since you have posted in at least one of them:

    https://www.physicsforums.com/threads/boltzmann-brains.894790/

    What I already said in post #2.
     
  6. Jan 3, 2017 #5

    bhobba

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    A load of rubbish - not because its impossible - it isn't - its just IMHO philosophical mumbo jumbo along the same lines as solipsism. Also consider according to inflation the universe inflated very quickly - that is NOT random behavior - its very orderly. Order emerges from chaos due to known physical laws eg slightly more dense regions collapse due to gravity - these laws work against random behavior even though the second law of thermodynamics says randomness is coming - true - but what happens in the mean time?

    There are much much more important things in physics we do know eg how U(1) local symmetry leads to Maxwell's equations and QED. Now that's simply plainly weird - how the hell can symmetry lead to electrodynamics. The thing is it does. Why is IMHO a very very deep mystery and much more important than Boltzmann's brains.

    Thanks
    Bill
     
    Last edited: Jan 3, 2017
  7. Jan 3, 2017 #6

    Demystifier

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    Maybe so, but I have published a paper about solipsism in a physics journal: :wink:
    https://arxiv.org/abs/1112.2034
     
  8. Jan 3, 2017 #7
    I was refering to other threads where I was not particapating, and even in those where I posted there seems to be little consensus about the paradox. And there are many questions which are related.

    1) Are BBs a certainty to happen in a spatially infinite universe?
    2) Are BBs even physically possibly, despite being statistically possible? Maybe we don't know enough about entropy and the early universe, like Bill mentioned in one of his previous posts, to give a definite answer if something complex and macroscopic can pop into existence - either from a soup of particles in the late universe or from "virtual particles" in vacuum?
    3) Cosmological problem - if the universe is eternal, BBs will become infinite in number. This is quite possibly the most controversial part, some posters have said that given infinite time BBs will outnumber humans and some have said that despite infinite time there is no guarantee that a single BB must happen.

    So there are many divided opinions, and given that I lack understanding in statistics, I wonder what stance is logical to take on the subject?

    Thanks in advance.
     
  9. Jan 3, 2017 #8

    bhobba

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    :smile::smile::smile::smile::smile::smile::smile::smile::smile::smile::smile:

    But I am sure you get my drift.

    Thanks
    Bill
     
  10. Jan 3, 2017 #9

    Demystifier

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    Of course. :woot:
     
  11. Jan 3, 2017 #10
    Infinity is not physical. Any theory that depends on or assumes the physical reality of infinity is twaddle. Concepts of infinite time, or infinite spatial extent of universe, or infinitely fine granularity of space and time, or infinite density, or infinite many-worlds, etc, are not allowable - even in philosophy, much less physics. To the extent that Boltzmann Brains and some other "low-probability events" do depend on infinity, they are meaningless. However you can speculate about BB's without invoking infinity. In that case the concept is not necessarily unphysical and some might think it meaningful.

    The word infinity is used in math and physics, but only as a placeholder or special symbol indicating some specific finite algorithm. For instance we talk about "the limit as x goes to infinity". That does NOT really mean "infinity". Instead it means "given any (small) delta > 0, it can be shown that the sequence or function approaches that close to a number we call the 'limit', for an identifiable corresponding (large) finite number N". Or, some other similar finite algorithm.

    Solipsism OTOH is a perfectly legitimate stance and may be used in a physical theory.

    Unfortunately some physicists, such as David Deutsch, have written that infinity is physical. They're wrong.
     
  12. Jan 3, 2017 #11
    Hi secur, could you explain why you think this? I can image a few possibilities. The universe either...
    1. doesn't exist
    2. is of finite size A
    3. is of finite size B
    4. is infinite
    To me, 2 and 3 seem the most arbitrary. As long as the universe exists at all, doesn't an infinite size seem the most natural? It also seems to be suggested by the evidence, like flatness of space.
     
  13. Jan 3, 2017 #12

    PeterDonis

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    This statement is not a statement of established mainstream science; it's a personal opinion. Please bear in mind the PF rules.
     
  14. Jan 3, 2017 #13

    PeterDonis

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    And they're questions to which we currently don't have established answers. Different people have different opinions and there's no way to test them. That's why I responded the way I did in post #2.
     
  15. Jan 3, 2017 #14
    If infinite in size, then infinite in time too; therefore, we have already had the heat death of the universe. I don't think BB's can emerge after the heat death.
     
  16. Jan 3, 2017 #15

    Stephen Tashi

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    How restrictive is the definition of a "Boltzman brain"? For example, by one definition, if the universe randomly entered an ordered state billions of years ago then, indirectly, the evolution of our own brains from that ordered state makes them a consequence of that random event. So our own brains are "Boltzman brains".

    By contrast, if we require that a "Boltzman brain" is a conscious entity that arises "immediately" from a random event, we are thinking about concepts such as a cloud of gas suddenly becoming a conscious entity.
     
  17. Jan 3, 2017 #16
    Normal brains require evolution as the intermediate step, while BBs come strictly from a fluctuation (sort of a shortcut).

    The real question is - is this really possible, despite statistics giving it a non zero probability. It is true that atoms can fluctuate into a set of positions, but to me it is debatable can organization into a structured macroscopic object emerge from chaos. It doesn't have to be a brain, though it's probably even more complex, it is debatable can chairs, cars etc. organize themselves from fluctuation. I think that we should get to know more about entropy before coming to extreme conclusions.
     
  18. Jan 3, 2017 #17
    That's fair. Do you have an opinion about this or you're agnostic until new evidence/theories emerge?
     
  19. Jan 3, 2017 #18

    PeterDonis

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    This question has already been answered. The answer is "we don't know".

    No.
     
  20. Jan 3, 2017 #19
    Of course I could explain it. It's not a personal opinion, it's a fact - of philosophy. Or, if you prefer, a fact of logic and reasoning, or of mathematical foundations. Obviously you can't prove it within the realm of physics. Unfortunately mainstream physicists are wrong on this issue, but can't be corrected, since their mistake can't be discussed. It's sort of a catch-22. Anyway, read Kant, Aristotle, Berkeley, Wittgenstein, Popper etc. They should convince you.

    BTW it's important to note, you can't prove the (entire) universe is finite, either! (Of course you might prove some portion of it, such as the visible part, is finite.) You can't assume the whole thing is either finite or infinite. See Kant's antinomies for the basic idea.

    Personally, I first learned it from my (mathematics) doctoral advisor, when we were reading Dirac's "Principles of Quantum Mechanics" (from a Functional Analysis point of view). There's been a lot of discussion about the problems of "infinity" in the field of mathematics foundations. Look up "constructivism", for instance. But that's philosophy also, just like foundational issues in physics, such as QM interpretations. QM interpretation is mostly philosophy, not physics.
     
    Last edited: Jan 3, 2017
  21. Jan 3, 2017 #20
    Sean Carroll has done some interesting work on this. He argues that the Everettian interpretation implies that the final equilibrium state of a de Sitter universe is truly static, i.e. no possibility for fluctuations of any kind. He then contrasts this with decoherent histories, which implies that there exists coarse grainings of the final state that are dynamical, and which therefore allow fluctuations. So the Boltzmann brains problem can be used as a testbed for different interpretations of QM.
     
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