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This is pretty silly. The ``paradox'' only arises because of a confused notion an apparent incongruity between the second law of thermodynamics and the subjecti

• deleted 13y ago
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• ubernostrum 13y ago
  (I am not a cosmologist or particle physicist, but my thesis did require me to delve into both, and was defended in front of someone who was a particle physicist) It doesn't require confusions about the second law. It does require some very specific conditions which may not obtain in the actual universe. It requires, for example, that the universe can eventually reach thermodynamic equilibrium, that the universe in this state can persist over incredibly long time periods, that at least some types of particles are stable enough to exist over such a time range, etc. And then starts considering whether situations that we normally associate with states where free energy and the ability to do work exist could arise occasionally as a result of random collisions of particles, possible quantum effects, etc. The key is that the second law is statistical in nature. It describes a tendency, wherein macroscopic entropy-decreasing events in a closed system are merely incredibly improbable. Or, more simply: entropy-decreasing events can occur, they are just so improbable/rare compared to entropy-increasing events that we should expect not to observe them. In that context, the Boltzmann brain and other strange things are at least not outright impossible.
  • gajomi 13y ago
    I think I completely agree with you. As you point out, one expects that there is a small but finite probability to have Boltzmann brain-like things pop up in the universe. And if it did happen it would violate the second law, which is making a statistical statement. So my problem with the article (perhaps your comment should help to clarify) is that it attempts to start a discussion, involving some kind of anthropic principle/natural selection to resolve a paradox that doesn't exist in the first place. The body of the article states: >The usual resolution of the Boltzmann brain paradox is that we and our environment are the products of a long process of natural selection, which can produce complex and improbable outcomes without violating the laws of thermodynamics. >The Boltzmann brain paradox is that any observers (self-aware brains with memories like we have, which includes our brains) are therefore far more likely to be Boltzmann brains than evolved brains, thereby at the same time also refuting the selection-bias argument. If our current level of organization, having many self-aware entities, is a result of a random fluctuation, it is much less likely than a level of organization which only creates stand-alone self-aware entities. The authors of this article somehow turn a correct argument of the expectation value of the number of Boltzmann brains in an ensemble of universes to a claim that most brains in these universes must be Boltzmann brains. They do this without mentioning at all how to calculate the probability of non-Boltzmann brains (assuming the distinction is even valid). TO deal with the feeling of existential anxiety that they are left with at the worry they might be Boltzmann brains they then invoke a natural selection+ anthropic principle argument. As far as I can tell has very little to do with the concept as it was introduced by the cosmologists.
    • ubernostrum 13y ago
      I suspect, but cannot be sure, that a lot of the handwaving about the second law is due to the common misconception that evolution somehow violates the second law. It's unfortunate but common, especially among people who refuse to accept evolution, so I'm not surprised to see it being mentioned.