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I On gravity and the conservation of energy

  1. May 11, 2018 #26

    JMz

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    Yes, from the gravitational field.

    "Originate" is either a deep philosophical question (Why does gravity exist at all?) or a simple one: Because the Einstein field equations, or something like them (even Newton's), hold, and they say that mass creates gravity.
     
  2. May 11, 2018 #27

    jbriggs444

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    One can adopt a less causal interpretation. They say that mass (or, more generally, the stress energy tensor) is associated with gravity. Not necessarily that it creates gravity.

    Correlation is not the same as causation.
     
  3. May 11, 2018 #28

    JMz

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    Quite so, but I'm not sure that bringing the semantics of cause into an I-level thread is philosophically simpler. ;-) And the "creates" verb is at least consonant with MTW's "[mass] generates [gravity]".
     
  4. May 14, 2018 #29
    The behavior of bodies moving in gravitational fields like these are very similar to how electrically charged objects interact. A very light negatively charged "asteroid" will swing by a very light, positively charged "Earth" exactly the same (the attractive forces will have the same form, 1/r^2, and magnetic interactions due to the fact that charges are not stationary will be very small).

    But somehow that interaction would not look strange to you:

    "The others all have a fairly limited range, they're many times stronger than gravity, and they each have a defined carrier particle (or two) that we're aware of. Conservation of energy is pretty straightforward with these, so no big deal."

    Why gravity looks stranger that electromagnetism to you?
     
  5. May 14, 2018 #30

    JMz

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    BTW, this seemed an odd thing to say: Electromagnetism has exactly the same range as gravity. This is usually stated to be "infinite", but the point is that both drop off as 1/r^2 (cosmological corrections aside), so their ratio is constant. As @nikkkom points out, you seem to be asking, for gravity, about something you already accept for electromagnetism.
     
  6. May 15, 2018 #31

    Demystifier

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    In the Newtonian limit all gravity is encoded in ##g_{00}(x)##, which is more like "compression" (of time) than like curvature.
     
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