Dark energy and entropy

Discussion in 'Physics & Math' started by Magical Realist, May 19, 2023.

  1. Magical Realist Valued Senior Member

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    How can entropy exist in a universe that is expanding at an accelerating rate? Does this not suggest an infinite supply of dark energy?
     
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  3. DaveC426913 Valued Senior Member

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    Entropy doesn't really have much to do with how much energy is available.
     
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  5. exchemist Valued Senior Member

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    Entropy is a measure of the unavailability of entry to do work or, equivalently, of the degree to which energy is spread out randomly and cannot be collected together again.

    Why would you think an expanding universe somehow prevents energy spreading out randomly?
     
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  7. DaveC426913 Valued Senior Member

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    Some of these things - especially entropy - are very slippery concepts. There are many ways to look at entropy and many definitions, but yeah, available work is one way.

    A star system has low entropy. A cloud of dust and gas - with the same mass and average temperature but evenly distributed about the volume of space has high entropy.

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    "Dark energy" is really just a placeholder for "whatever it is that is causing the universe to expand". It is acting uniformly across the volume of the universe - IOW, effectively evenly distributed and not concentrated anywhere. That means it does not lower the entropy.


    In fact, it can be argued that DE actually raises the overall entropy of the universe:

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    Last edited: May 20, 2023
  8. exchemist Valued Senior Member

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    Blasted spellchecker: energy, dammit, energy, not "entry".
     
  9. Magical Realist Valued Senior Member

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    I perhaps took too literally the metaphor of a watch winding down being what entropy is like. So I assumed more energy would keep the watch going forever. But apparently not. It has more to do with the distribution of energy as you point out.
     
  10. exchemist Valued Senior Member

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    Exactly.
     
  11. Seattle Valued Senior Member

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    Think of a glass of water and you put in a drop of dye. It's going to be distributed in the water pretty soon and it's never going to be a drop again.
     
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  12. Guestfornow Registered Member

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    If entropy is increasing because of universal expansion, why does the clumping (or contraction) of matter increase entropy?

    Recall that a black hole has maximal entropy.
     
  13. exchemist Valued Senior Member

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    I’m not sure whether conversion of gravitational potential energy to kinetic energy would involve an entropy increase. But once k.e. starts being converted to heat I think the entropy would increase.

    P.S. I’ve found a link that discusses exactly this: https://math.ucr.edu/home/baez/entropy.html

    The hint is that a clump of matter is not a closed system, because it radiates.

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    Last edited: May 21, 2023
  14. Guestfornow Registered Member

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    Yes, so entropy increases around the hot clump, while the entropy of the particles clumping together (becoming more solid) decreases, so the 2nd law holds.

    I tried discussing the points in my first post with ChatGPT 3, and it ended up telling me that because the universe was initially very hot, entropy was very high (!).
     
  15. exchemist Valued Senior Member

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    Does that mean you were checking of we were bots here, then?

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    But more seriously, it does seem to have become something of a sport asking ChatGPT science questions and getting often nonsensical replies. It rather leads one to question how accurate the damned thing may be in other domains of knowledge. Of course one expects it to get better over time but perhaps there's a bit of hype around it just now.

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  16. Guestfornow Registered Member

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    I don't know that there are any real clues in what I've posted about entropy--such as, what the hell is it--but, do you think there's an obvious connection between gravitational entropy and information?

    Given that, in the theories such as Shannon's, it's about lost information or 'unavailable' information. Particles are hidden when they collapse into a 'regular' shape. Perhaps that gives you another hint. Perhaps its why a black hole's entropy is "expressed" as the sum of Planck areas on its "surface".

    p.s. perhaps I should have a stab at what entropy is. It's a relation and it depends on knowing things about the state of a system. That's one thing Baez makes fairly obvious with his back-of-the-envelope analysis.

    Because energy has different forms and because entropy is an energy-relation, entropy has different forms too. For example, thermodynamic entropy is not the same animal as gravitational entropy, or von Neumann entropy. The common thing with all forms of entropy is that they are all energy-relations.

    What about information entropy, how is that an energy relation? The answer seems kind of simple but it really isn't.
     
    Last edited: May 21, 2023
  17. exchemist Valued Senior Member

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    What do you by particles being hidden when they collapse into a regular shape?
     
  18. Guestfornow Registered Member

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    I mean, take the planet we're living on. How many particles can you detect or know about, below the surface?

    Sorry, I should have posted: when a lot of particles collapse into a clump, most of them are hidden.
     
  19. exchemist Valued Senior Member

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    Whether you can see them or not does not affect entropy.
     
  20. Guestfornow Registered Member

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    So not knowing the state of particles inside an horizon, doesn't "affect" entropy? I can't really parse that.
    If you know the surface temperature of a solid body, what do you know about the internal temperature?
     
  21. DaveC426913 Valued Senior Member

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    Well, all of them. I "detect" them by standing on a scale. It tells me - in pounds - how many there are at an distance of approximately 3860 mi below me.
     
  22. Guestfornow Registered Member

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    Ok. So can you connect that number of miles, 3860, to the number in pounds the scale tells you?
    If you can, what's the entropy measure?

    What do those numbers tell you about the state of the particles?
     
  23. Guestfornow Registered Member

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    What do you think of Steven Hawking's proposal from 1976?
    Isn't Hawking saying that particles can be hidden by gravitational collapse? Then why does the formation of a gravitationally bound system of particles not hide particles behind an horizon?
     

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