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Second law of thermodynamics and the macroscopic world

  1. Sep 22, 2015 #1
    if second law of thermodynamics is emergent and not fundamental, in the same way color is emergent but atoms themselves are colorless, then perhaps black holes do not carry entropy or can violate the second law? second law of thermodynamics is shown to exist in the macroscopic world but how do we know it applies to the quantum world and below? maybe it is possible to violate the second law with black holes.

    if black holes carried no entropy, how would this change QG HEP and string theory research?
     
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  3. Sep 23, 2015 #2

    Andrew Mason

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    The second law of thermodynamics is a statistical law that applies only to systems with large numbers of particles: ie the macroscopic world. It does not apply to individual atoms. It applies to large collections of atoms.

    So perhaps you could clarify your question about black holes.

    AM
     
  4. Sep 23, 2015 #3

    Demystifier

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    First, black holes are macroscopic, so it is very reasonable to think that they carry entropy and obey the second law.
    Second, quantum laws describe not only microscopic systems, but macroscopic systems as well.
     
  5. Sep 23, 2015 #4
    what would be the ramifications if black holes carry no entropy due to the no-hair thereon
     
  6. Sep 23, 2015 #5

    Demystifier

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    In that case I would say that black hole has internal entropy which cannot be seen from the outside in its hair.
     
  7. Sep 23, 2015 #6
    in what way is entropy a meaningful concept inside a black hole?
     
  8. Sep 24, 2015 #7

    Demystifier

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    I never understood arguments why this might not be meaningful. Just because some observers cannot see this entroppy does not mean that this entropy does not exist. After all, an observer may jump into the black hole and thus observe it.
     
    Last edited: Sep 25, 2015
  9. Sep 24, 2015 #8
    in what way does entropy exist inside a black hole?
     
  10. Sep 24, 2015 #9
    One (hypothetical) way entropy can exist inside a Kerr or Kerr-Newman black hole is via Hawking absorption at the inner event horizon-

    'http://www.researchgate.net/publication/230923684_Entropy_of_Kerr-Newman_Black_Hole_Continuously_Goes_to_Zero_when_the_Hole_Changes_from_Nonextreme_Case_to_Extreme_Case [Broken]' by Liu Bo Liu & Wen-Biao
    (or download the paper via http://bit.ly/O1L2qD)
     
    Last edited by a moderator: May 7, 2017
  11. Sep 24, 2015 #10

    Demystifier

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    For example, a hot cup of tea has some entropy. If you throw this cup down to the black hole, then this entropy must exist inside the black hole.
     
  12. Sep 24, 2015 #11
    hot tea has entropy b/c water molecules. inside a black hole those water molecules might transform into something that the concept of entropy doesnt apply anymore
     
  13. Sep 24, 2015 #12

    Demystifier

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    Like what? That would contradict the second law of thermodynamics and possibly some miscroscopic fundamental laws like Liouville theorem of classical mechanics or unitarity of quantum mechanics.
     
  14. Sep 24, 2015 #13

    Demystifier

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    Let me also add the famous quote by Sir Arthur Stanley Eddington:
    "If someone points out to you that your pet theory of the universe is in disagreement with Maxwell's equations—then so much the worse for Maxwell's equations. If it is found to be contradicted by observation—well these experimentalists do bungle things sometimes. But if your theory is found to be against the second law of thermodynamics I can give you no hope; there is nothing for it but to collapse in deepest humiliation."
     
  15. Sep 24, 2015 #14
    if second law isn't fundamental but emergent contradicting it isn't a problem. what are the microscopic degrees of freedom of a black hole and how does adding water molecules increase its entropy?
     
  16. Sep 25, 2015 #15

    Demystifier

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    It is a problem whenever there are many degrees of freedom. The second law is the most robust law in physics, despite the fact that it is emergent. See post #13 above.

    Nobody knows that with certainty, but the validity of the second law should not depend on it. For instance, the microscopic origin of the second law is usually formulated in terms of atoms and molecules, but the fact that atoms are emergent (from electrons and quarks) does not influence the validity of the second law.

    In the same way as adding water molecules increases entropy of anything else. You can add water molecules to the acid, to the core of nuclear reactor, to the neutron star, whatever. The water molecules may suffer a violent transformation when you do that, but they will still increase the total entropy of the system.
     
  17. Sep 25, 2015 #16
    you are answering my questions but i happen to think different. all the examples you give of water molecules increasing entropy are all examples of systems that have the same degree of freedom, atoms.

    the microscopic degree of freedom of blackholes are unknown. water molecules may not influence bh degree of freedom
     
  18. Sep 25, 2015 #17

    Demystifier

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    You are mistaken; the neutron star does not have atoms.
    https://en.wikipedia.org/wiki/Neutron_star
     
  19. Sep 25, 2015 #18

    Demystifier

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    They do not need to influence them to increase the entropy of the total system - the initial black hole + the new water molecules.
     
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