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Why do bodies attract each other?

  1. Oct 22, 2014 #1
    I just wanted to clear a little doubt of mine.
    The second law of thermodynamics says that every physical system wants its entropy to be increased, the universe being a physical system should be doing the same.
    But , then why would two bodies want to attract each other (gravitation) even though the above process decreases the entropy(randomness) hence violating the above stated law.
    Am i interpreting it the wrong way or is sth wrong with the theory?
     
  2. jcsd
  3. Oct 22, 2014 #2

    Nugatory

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    Staff: Mentor

    You are interpreting it the wrong way.

    Most of the explanations of entropy on the web and in the popular press are greatly oversimplified; there's more to it than just "randomness". Two idealized point masses can move under the influence of their gravitational attraction more or less indefinitely without any change in entropy at all.

    As for why masses attract gravitationally... We don't know. It is a fact that they do, and it is a fact that Newton's law of gravitation describes this phenomenon very well, and it is a fact that Einstein's general theory of relativity works even better. But why? All we can say is that that's how the universe we live in works.
     
  4. Oct 23, 2014 #3
    That's a really deep and tough question. As far as I understand, It is now believed that gravity doesn't necessarily decrease entropy, and in fact, very high gravity objects such as black holes are actually very high in entropy. In other words, the black hole has hair. Although gravity might seem like it decreases entropy, if you go into all the math, it is not necessarily so. I don't understand it well enough to give a better answer, but I think it is still a field of current research. Maybe you should ask Roger Penrose; I think he has some ideas on the question.

    According to Bekenstein and Hawking, a black hole has the maximum amount of entropy for a two dimensional area given by the event horizon. I assume, as masses attract gravitationally, the system moves closer to this black hole limit.
     
  5. Oct 24, 2014 #4
    Thank you for your explanation.
    But still the concept of entropy doesn't satisfy me. It would be great if you could suggest me some resources to understand the same.
     
  6. Oct 24, 2014 #5
    Hi Akash, Welcome to PF,
    The question you have put forward is really an interesting one :D but what missing in it is the correct understanding to about the second law of thermodynamics. In thermodynamish language, the law states that a cold body can't heat up a warm body. Heat naturally wants to flow from warmer to cooler areas. Heat wants to flow and spread out to areas with less heat. If heat is going to move from cooler to warmer areas, it is going against what is “natural”, so the system must put in some work for it to happen.
    Similarly its the natural tendency of a body to attract other, and so they do.
    They would violate the law of thermodynamics when they shall disobey Sir Issac Newton or Einstein. I hope you got the point.:)
     
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