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Does gravity indeed travel in waves?

  1. Mar 6, 2015 #1
    A massive star has a huge gravitational influence far far away from itself.

    It goes Supernova.

    Does the gravitational effect it has on its most outer objects still remain until the change in gravitational attraction has reached them at c?

    If so then does the gravity indeed travel in waves?
  2. jcsd
  3. Mar 6, 2015 #2


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    Until mass ejected by the supernova goes past a remote object, the remote object sees no change in the gravitational attraction from the center of mass of the mass that was, and still is, there.
  4. Mar 6, 2015 #3

    It's a field, and is limited by c. it's static on it's own.

    Yes it takes time for the change to reach whatever. The moon is 1 light second away. If the moon disapered it would take one second for that change to have physical significance here on Earth.
  5. Mar 6, 2015 #4
    Thank you

    But that means that gravity travels independent from its source.

    How? Can someone elaborate
  6. Mar 6, 2015 #5


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    Do you think that if you shine a flashlight at the moon and then turn off the flashlight, the beam of photons heading for the moon will suddenly disappear because you turned of the flashlight?
  7. Mar 6, 2015 #6
    No I don't.

    But I see light. I can observe its passing.

    I'm just trying to picture gravity "travelling" in such a fashion
  8. Mar 6, 2015 #7


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    Does picturing ripples on the surface of a pond help?
  9. Mar 6, 2015 #8
    No. I can see them

    Could this travelling gravity be detected?
  10. Mar 6, 2015 #9


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    ...which actually happens right after a time consistent with c-speed propagation, since the first ejecta to go past are in the form of radiation emitted by the SN event !
    - not very satisfying I suppose but still... Or is this misconstrued ?
  11. Mar 6, 2015 #10


    Staff: Mentor

    Yes. That's what gravitational waves are. We have already detected them indirectly (in the binary pulsar observations), and we expect to detect them directly eventually.

    Note, however, that this "traveling gravity" is not the same thing as what you ask about in your OP. In the scenario in your OP, the gravity felt by an observer at a distance from the supernova only changes when mass or radiation ejected by the supernova passes him on its way out. Nothing has to "travel" to him; the change is purely local.
  12. Mar 6, 2015 #11
    I'm perplexed by 2 things

    1. How fast gravity travels

    2. If it travels in waves, how gravity isn't subject to thermodynamics. That is to say, a set amount of gravity isn't diluted by how many objects or how much mass it holds
  13. Mar 6, 2015 #12
    But the decrease in mass or at least the spread of the mass of the Star would alter the orbit
  14. Mar 6, 2015 #13


    Staff: Mentor

    You need to first get clear about what "gravity" is and what aspects of it can "travel" at all. If by "gravity" you mean the Newtonian "force" of gravity produced by a static object, that doesn't "travel" at all; it's a static force. (And this view of gravity as a "force" isn't really correct in GR anyway; it's just an approximation that works in certain scenarios, but doesn't give a full picture of what "gravity" is.) But if you think of "gravity" as spacetime curvature, then gravity "traveling" just means changes in spacetime curvature propagating. Those changes in spacetime curvature propagating are gravitational waves, and they propagate at the speed of light.

    I'm not sure what you are trying to say here.
  15. Mar 6, 2015 #14


    Staff: Mentor

    Yes, after the ejected mass passes the object in orbit on its way out. Up until that time, the object's orbit does not change--all of the mass is still inside the orbit, so it all still acts the same as far as the orbit of the object is concerned. (In this particular scenario, "gravity" in GR works the same as Newtonian gravity does.)
  16. Mar 6, 2015 #15


    A Star has a fixed mass and therefore a definable gravitational pull. But that pull is not diluted whether it has 1 orbiting planet or 100.

    The net "force" is the same
  17. Mar 6, 2015 #16
    Are waves not dispersed with the Supernova as the mass of the remaining Star is decreased
  18. Mar 6, 2015 #17


    Staff: Mentor

    Yes, this is true.

    No. At least, not in the idealized case of a spherically symmetric explosion; all of the mass lost by the star is carried away by ordinary matter and radiation that is ejected. If the explosion is not symmetric, some gravitational waves may be emitted, but the energy carried away by them will be miniscule compared to the mass/energy carried away by ejection of mass and ordinary radiation.
  19. Mar 6, 2015 #18
    This is what confuses me when people say gravity is measurable. It's measurable yet infinite.

    A gravitational field could hold a theoretically infinite number of objects and its capacity for more is unblemished.

    This seems at odds with how I understand thermodynamic systems
  20. Mar 6, 2015 #19
    Sorry just realised, the respective centre of mass relationships the various orbiters will have with Star do in fact have an increasing effect the more mass there is
  21. Mar 6, 2015 #20


    Staff: Mentor

    Perhaps the problem here is that you are thinking of the field as having to "hold" each object separately. It doesn't. All it does is create spacetime curvature, and it only has to do that once. All the objects "held" in the field are simply responding to the same spacetime curvature, which they do anyway--that is, they move through whatever spacetime geometry there is, whether there's a gravitating mass there or not.

    How so?
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