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Gravity is different

  1. Nov 19, 2011 #1
    From Greene's pop science book 'The hidden reality', I got this:

    Could someone explain that I bit further? Why and how is in the gravitational field an arbitrarily large amount of energy stored?

    What about the potential energy of the two approaching particles, does it not decrease, while the kinetic energy increases?

    Greene says this arbitrary gravitational energy supplied the early universe the necessary energy to inflationary expand. How?

    thank you
  2. jcsd
  3. Nov 19, 2011 #2

    Simon Bridge

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    As the kinetic energy increases, potential energy decreases.
    Since it starts out at zero, then potential energy goes negative. This is what he's talking about.

    Classically: if two point masses can get arbitrarily close to each other then the energy gets arbitrarily negative. Usually real objects hit each other's surface before that happens.

    This is because gravitational potential energy is the work done to separate masses by infinite distance.

    One of the puzzles of the formation of the Universe is where all this matter came from - it cannot come from nothing and each gram accounts for a great deal of rest-mass-energy.

    One of the arguments is that when you get matter you also get gravity and thus gravitational potential energy. The energy of matter is positive and the energy of gravity is negative - sum them up to get zero. So everything can come from nothing!

    But you should be careful of pop-science descriptions - the authors tend to like to blow your mind at the expense of being accurate. It's supposed to be for stimulating your mind but not for drawing conclusions.
  4. Nov 19, 2011 #3
  5. Nov 19, 2011 #4
    I see, thanks
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