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Gravity in relation to mass & energy

  1. Jul 29, 2008 #1
    I was thinking, a particle a certain distance from another would have a certain potential energy. Now the potential energy must be in the form of extra mass if im not mistaken (as its not kinetic, however does kinetic energy increase mass?) so therefore an object is at its maximum mass at infinity and at its lowest at 0.

    I can then conclude that the at the pre universe, the 'singularity' or whatever you call it would have had less mass than the current universe. Does this mean that the early universe had less energy than the present day?
  2. jcsd
  3. Jul 31, 2008 #2
    Was this too hard, or was i way off? My reasoning comes from the fact that you add energy to an atom in an excited state, its mass increases by E / c^2. Following this im concluding that GPE is stored in mass also. (disregard my previous post asking if kinetic energy increases mass, ive blatantly overlooked special relativity :)

    If this is correct then what i stated must be true, the pre big bang had less mass than it does today.
    Last edited: Jul 31, 2008
  4. Jul 31, 2008 #3
    When two particles approach themselves from infinity, the system of the two particles looses mass, in the form of energy radiated away. Where do this energy go? Of course it can't escape the universe, so the total energy of the universe doesn't change (at least, not for this reason).
  5. Jul 31, 2008 #4
    As gravitational waves?
  6. Jul 31, 2008 #5


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    GPE is negative stored mass. Kinetic Energy is positive. If the Objects are free floating, both parts change simultanely, conserving the total energy. If you bring the particles to rest wrt each other, you gain energy (from KE) in form of explosions, heat, or something usable. Radiate it away, and the mass of the System decreases. So, objects at rest in a gravitational field have less mass.
  7. Jul 31, 2008 #6


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    I've never heard that before. Where did you get it ?

    Does this mean as something accelerates in a gravitational field it loses (inertial?) mass ?
    Last edited: Jul 31, 2008
  8. Jul 31, 2008 #7
    I don't see others possibilities.
  9. Jul 31, 2008 #8
    Does it mean the planets lose continuously their mass because of orbital motion (acceleration)?
  10. Jul 31, 2008 #9
    Well they're not changing their GPE.

    Im not sure how to take this, are you saying this purely mathematical or realistic?
  11. Aug 1, 2008 #10


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    I'd say it loses rest mass and gains KE. Its total energy is conserved. This should be true at least in weak fields, when calculated in an inertial frame. Unfortunately, the people that would point out all the caveats are no longer around.

    Ok, if you bring two masses together, you gain energy. You can carry the energy away, and what's left ist a system with less energy (also less mass) than before. That deficit is called binding energy, which is negative. So I'm actually talking about mass defect here, not strictly potential energy.
  12. Aug 1, 2008 #11
    But what kind of mass is it? The number of particles does not change . Have particles less mass if they come nearer and why?
  13. Aug 1, 2008 #12
    I asked a question similar to this months ago.


    It went something like, if I lift an object does its mass increase due to increased potential energy? There was disagreement about whether a change in GPE was stored in the object, in both the object and the Earth, or "in the field", which puzzled me because I didn't know how an increase in energy -- which should also be an increase in mass -- could appear in a field and not concentrated in a particular location. Then someone posted a link to another thread


    which mentioned that in GR mass/energy is not even defined in a consistent way, or it depends on your state of motion, Killing vectors, etc...and it quickly went over my head.

    You might find these threads helpful. My impression is that there is no simple answer to this kind of question.
    Last edited: Aug 1, 2008
  14. Aug 1, 2008 #13
    No, their mass don't change. The mass is not only in the particles but in the field too.
  15. Aug 1, 2008 #14
    I think I know what you are getting at but how do you rationalize that this position holds up to the fact that what you call 'the field' is on the other side of the Einstein field equations?
  16. Aug 1, 2008 #15
    This is true only in case of electromagnetic field, because em-field disappears completly if partikels move very close together whereas gravitational field does not change.
  17. Aug 1, 2008 #16
    Sorry, could you please explain better what you mean?
  18. Aug 2, 2008 #17
    We can use object's mass (heat) to increase its GPE. In this case system has less mass - mass is converted to GPE - but if object falls GPE is converted back to mass (heat).
  19. Aug 5, 2008 #18
    You are right, gravitational field has mass too. And the question remains, how is this mass induced in "empty" space.
  20. Aug 16, 2008 #19
    Wait what? A gravitational field has mass?
  21. Aug 16, 2008 #20


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    This is a very entertaining thread. I'd like to throw in this - in GR, mass doesn't even couple to gravity. Mass is the source of the space-time configuration, so you could say that matter and energy couple to space-time, which acts like rails for the matter to move on. There is no interaction term between matter and the field in the Lagrangian or action of GR so no gravitons either.
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