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What is the relationship between gravity and energy?

  1. Nov 11, 2014 #1
    A question I have, the moon drags the oceans which can in turn be used to create tidal power on earth.

    Does this then mean that the energy of the Universe is theoretically infinite? As there is no shortage of masses moving around other masses than can have a similar effect.

    Is this, strictly speaking, gravity creating energy?
  2. jcsd
  3. Nov 11, 2014 #2


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    We don't know if the universe is finite.

    No, not even loosely speaking. It's kinetic energy. Gravity is just used to extract it and convert it into other forms of energy.
  4. Nov 11, 2014 #3
    Thanks for your response.

    But isn't the movement of mass without "consumption" of energy in space itself a kind of violation of Thermodynamics?

    I'm of course guessing not, but it would be great to know exactly why.
  5. Nov 11, 2014 #4
    Thank you.

    But this is what I can't fathom. The kinetic energy of the Universe - are we talking a separate thing from the conventional energy in terms of mass-energy equivalency?

    Is gravity itself a form of residual energy from the big bang?
  6. Nov 11, 2014 #5


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    I had removed the post to which Gaz1982 responded above in an effort to avoid the digression into cosmology
  7. Nov 11, 2014 #6
    Is Cosmology where this thread should be?
  8. Nov 11, 2014 #7


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    Gaz1982, in the case of tides on Earth the energy needed to move water around is taken from the kinetic energy of the rotation of the Earth and revolution of the Moon. It is not an infinite source of energy - it does get depleted. Due to tidal forces the days on Earth are getting longer, while the Moon is migrating outwards. Once the period of rotation of the Earth matches the period of revolution of the Moon there will be no more tides. The Earth will be locked in a permanently deformed position with the same side facing the Moon forever*.

    *not exactly true, as there are other bodies in the solar system to mess up with the simple picture, but the principle remains - the energy is not infinite
  9. Nov 11, 2014 #8
    Thanks. This is the key to my question. I couldn't work out where it was depleted. My primitive understanding of E=MC2 or Thermodynamics was telling me that there was no "free lunch". That something must be depleted somewhere. I'm still not getting the nature of the kinetic energy of the universe.
  10. Nov 11, 2014 #9


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    Not being able to see the post you originally responded to, I can't address the issue directly. Let me just say that describing the total energy of the universe is not something that has got a clear answer, as the very concept of energy in cosmology is problematic. Have a read through this to get a feeling for why it is so:
  11. Nov 11, 2014 #10
    I will. Thank you.

    My puzzlement is that my mind is conditioned to think in closed, thermodynamic systems, and I can't fit said kinetic energy into thought process
  12. Nov 12, 2014 #11
    Sorry, not sure where to post this question. In general relativity the curved spacetime is curving the path of light, but magnetism is also known to be able to bend light, correct? What is it exactly that links the bending of light to spacetime curvature? In other words, just because light is seen to bend, how is that observation explaining the reason behind it? Couldn't the same experimental observation not be explained by other forces, such as magnetism etc.?
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