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B Entropy is greater in galaxies or pre atomic era

  1. Apr 7, 2016 #1
    1. I understand an expanding gas has increasing entropy and at a cosmic scale the universe is an expanding gas...sort of.

    2. back before the universe was cool enough to form atoms it would seem to be very disordered, ie a high temperature universe of a plasma made of nuclei and elementary particles in random motion and lots of energy.

    The temperature dropped until atoms formed......and then galaxies.

    3. Galaxies seem more ordered than a hot plasma of elementary particles

    How do these three phenomena add up to increasing entropy??

    thanks any ideas.
     
  2. jcsd
  3. Apr 7, 2016 #2

    Orodruin

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    "Seems" is not a very precise physical statement and the conclusion is not as obvious as you might think. You also have to take into account that the hot plasma was much smaller than the current universe. It is not entropy per unit volume, it is the comoving entropy.
     
  4. Apr 7, 2016 #3

    PeterDonis

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    Only if you leave gravity out of your calculations. In the presence of gravity, a galaxy has higher entropy than a hot plasma of elementary particles occupying the same volume with the same overall mass. More generally, in the presence of gravity, clumped matter has a higher entropy than matter of uniform density.
     
  5. Apr 7, 2016 #4

    Orodruin

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  6. Apr 7, 2016 #5

    Orodruin

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    As John shows in the link, this is actually not true. However, the energy lost when the matter is contracted has to go somewhere ...
     
  7. Apr 8, 2016 #6

    PeterDonis

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    Yes, a better statement would be that, in the presence of gravity, clumped matter plus the radiation the matter emitted while it was clumping has higher entropy than matter of a uniform density. The key point is that the clumping process increases the overall entropy.
     
  8. Apr 8, 2016 #7
    just scan read the Baez article - awesome. bookmarked and printed.

    thanks all replies, question answered.

    off topic is the author Baez any relation to Joan Baez the folk singer??

    random physics trivia - Olivia Newton-John of Grease fame is the granddaughter of a Nobel prize winner in physics.
     
  9. Apr 8, 2016 #8

    Orodruin

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  10. Apr 8, 2016 #9
    cool, they are cousins;

    "His physicist uncle, Albert Baez (inventor of the X-ray microscope and father of singer and progressive activist Joan Baez), "

    smart family. for a smart guy tho he needs to employ a website designer for his home page, it aint pretty.
     
  11. Apr 8, 2016 #10
    Well it at least a good resource for information, which was the original idea of the internet
    Nobody was expecting social media and cyber-bullying and so on.
    'Pretty' websites were not even a concept, but what is pretty depends on the observer.
     
    Last edited: Apr 8, 2016
  12. Apr 8, 2016 #11
    off topic but I believe the internet has made the human race on average dumber; too much it's just a way to trawl for information to reinforce individual confirmation bias.
     
  13. Apr 8, 2016 #12
    Yeah I guess it is, but a discussion in a new topic about that might be interestng.
     
  14. Apr 8, 2016 #13

    Orodruin

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    The actual topic seems exhausted. Thread closed.
     
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