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Force based explanation of gravity waves still possible?

  1. Feb 12, 2016 #1
    It's quite chilling that gravitational wave has been detected..

    can force based explanation of gravity waves still be possible? How? or does it prove 100% that GR is true or gravity is really spacetime curvature?
  2. jcsd
  3. Feb 12, 2016 #2


    Staff: Mentor

    What do you mean by a "force based explanation"? There's no such thing as gravity waves in Newtonian physics.

    No; a scientific theory never gets "proved 100% that it is true". But it's good confirmation that GR makes valid predictions in a regime where we don't have previous experimental data, namely the regime near and at black hole horizons.

    GR does not say that gravity "is really" spacetime curvature. It models gravity as spacetime curvature. Scientific models don't make any metaphysical claims about what something "really" is. They just tell how to make predictions about what we will observe.
  4. Feb 12, 2016 #3
    I meant gravity as a force instead of spacetime curvature.. can any force based theory explain gravitational waves?

    GR models gravity as spacetime curvature.. so GR is spacetime curvature. Isn't this logic wrong..

    Anyway.. when one claps the hands.. would there be any gravitational waves produced.. no matter how miniscule and how far can this wave travels (theoretically)?
  5. Feb 12, 2016 #4


    Staff: Mentor

    The only "force based" theory of gravity that I'm aware of is Newtonian gravity, and it can't explain gravitational waves, so I would say no.

    I didn't say "GR is spacetime curvature". I only said "GR models gravity as spacetime curvature".

    If you clap them the right way, in principle, yes, you could produce gravitational waves. But they would be many, many orders of magnitude smaller than the weakest waves we can detect now or in the foreseeable future.
  6. Feb 12, 2016 #5
    What do you mean the right way? How must one clap to produce it.. and would the gravitational wave reach Pluto?
  7. Feb 12, 2016 #6


    Staff: Mentor

    In order to generate gravitational waves, a source must have a time-varying quadrupole moment. Ordinary clapping, just moving your hands back and forth along a single line, would only produce a time-varying dipole moment, which won't generate any gravitational waves. You would have to change the direction in which your hands moved from one clap to the next.

    In principle, yes, but please read again what I said about how weak the waves would be.
  8. Feb 12, 2016 #7
    In 1600 no one has forseen radio and electromagnetic wave.. and even the time of Maxwell.. they say em wave has no use. So is it possible to have gravitational radio maybe a million years from now? or 100 years.

    Back to GR as spacetime geometry. GR is model as spacetime geometry. You said gravity as a force is back in Newtonian days.. what I meant was gravity as a force like weak field or maybe as gravitons.. meaning there is really a force of gravity that is not spacetime geometry. So can gravitons produce gravitational wave without geometry warping?
  9. Feb 12, 2016 #8


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    There's no way to predict what will be possible a million years from now. 100 years from now? Not going to happen.

    Neither of these are models of gravity as a force.

    No. Gravitational waves are waves of spacetime geometry warping. That is what they are made of.
  10. Feb 13, 2016 #9
    Oh. You mean in quantum field theory.. there is no force? But why are they called 4 fundamental forces??

    What other models can produce gravitational waves that are not waves of spacetime geometry warping?
  11. Feb 13, 2016 #10


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    Only colloquially or in popular science. It would be more correct to refer to four fundamental interactions.
  12. Feb 13, 2016 #11
    I always thought that the whole purpose of postulating particles like photons, gravitons, gluons, etc is to characterize them as force carriers (ie. photons are force-carriers for EM, gluons are force-carriers for Strong force, gravitons are force-carriers for Gravitational force, etc)

    What is a graviton supposed to represent if not a force-carrier for Gravity?
  13. Feb 13, 2016 #12


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    This is a bit complicated. Of course, it's right that "force" is a difficult concept in relativistic physics, and you should get used to the field concept. In the case of gravity many people stress the geometric interpretation, i.e., gravity as the dynamically evolving pseudo-Riemann space-time manifold. Nevertheless it's (at least for me as a theoretical nuclear/particle physicist) simpler to think about gravity as a relativistic field with the pseudometric as gauge fields.

    Of course, operationally, as any field also the gravitational field is defined by the motion of test particles, and indeed that's how most textbooks on the subject starts. As an introduction I like most Landau/Lifshitz vol. II. The development of general relativity starts with the motion of test particles in the gravitational field. The unique characterization of gravity in comparison to the other fundamental interactions (described as the strong and electroweak interactions in the Standard Model) is that all test particles with the same initial conditions move along the same trajectory in the gravitational field, no matter what's their (invariant) mass or their material. As it turns out this leads to a description of the motion of such testparticles along geodesics in a curved spacetime manifold, and that's where the geometric reinterpretation of the gravitational gauge field as Riemannian pseudometric comes from. The equation of motion, however indeed is not so much different from the relativistic motion of a particle in an external field, and in this sense for this case you can indeed think in terms of a locally acting external force (as in electromagnetics, where you can think of this situation of a charged particle moving in an external electromagnetic field as the motion of the particle subject to the electromagnetic Lorentz force).

    Gravitational waves are also pretty much analogous to electromagnetic waves. The main difference is that as a non-abelian gauge theory gravity is a self-interacting field, i.e., the Einstein equations of motion are non-linear even without matter present. Gravitational waves were, however, predicted by Einstein working with the linearized field equations, and there it's pretty much just a wave equation for the assumed to be small deviation of the gravitational field from the "vacuum solution", i.e., the Minkowski metric. The manifestation of a gravitational wave to test particles is similar to that of the electromagnetic wave field on test charges. The only difference is that the multipole expansion of gravitational waves starts with the quadrupole term, because the gravitational field is a spin-2 field rather than a spin-1 field (both massless).

    So you can very well think about the action of the gravitational wave field on matter (among them the LIGO detectors) as the action of a force, and this is indeed use to detect them with these very sensitive interferometers.
  14. Feb 13, 2016 #13


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    There is a way (which I believe has some limitations - more later) of viewing gravity as a field that propagates in a flat space-time. This field has the property that it distorts rulers and clocks - ALL rulers and clocks. Note also that this sort of distortion doesn't fit well into a "force" model of gravity - other forces don't distort clocks and rulers. We observe "gravitational time dilation" but we do not observe similar effects from other fields.

    I probably should dig up some of the relevant references from Einstein and MTW to provide more history on this approach. I'm sure I posted a link to a reference to a paper by Straumann in another thread that also takes a similar approach. But I don't have the time right now to dig up the references, and I probably won't bother unless someone asks, there's not much point in digging up references that nobody reads.

    What happens is that the underlying "flat" space-time in this theory turns out to be physically unobserveable with actually clocks and rulers (such as the clocks and rulers either based on the SI standard, or idealized from the SI standard in a compatible manner), because gravity affects everything - no clock or ruler that we can make is immune to its effects.

    Onto the limitations of the theory as I see it - this is something that I haven't seen discussed rigorously, surprisingly. If one consider topological issues, getting rid of curvature in this manner globally becomes problematical. For instance, the Earth is widely accepted to be round, approximately a sphere - which implies that it's curved in the sense I'm talking about. Over a small, or perhaps even a large region (perhaps even as large as a hemisphere), one can imaging constructing a set of hypothetical rulers that shrink in just the right way to be consistent with a "flat Earth". But it's harder with such a "flat Earth" theory to explain why it has the topology of a sphere - for instance, why one winds up back at one's starting point if one moves in the same direction. If one move in the same direction on a plane, this doesn't happen. So the "fields in flat space-time" model of gravity might have some problems explaining, for instance, black holes, if one takes it really seriously.

    Keeping these limitations in mind, it might be easier to imagine that gravity waves are some sort of field that distorts clocks and rulers if one is not familiar with some of the arcane aspects of differential geometry. This is a recognized approach with some precedent in the literature, and MTW claims that phsicists can and do use this approach on occasion. It is however, not widely taught in introductory classes as the primary way to understand gravity. Almost all serious treatments I've seen takes the geometrical approach. I believe that the limitations I mentioned may be one of the reasons for this.
  15. Feb 13, 2016 #14


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    As Orodruin said, this is pop science, not science.
  16. Feb 26, 2016 #15
    Gravitoelectromagntism also predicts gravitomagneticwaves. Gravitoelectromagntism model describes gravity as force. Gravitoelectromagntism is approximation to GR ,so I am not sure is it enought accurate to be used for describing gravitionalwaves that have been observed.
  17. Feb 26, 2016 #16


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    No, gravitoelectromagnetism (GEM) describes some aspects of gravity as a force. It does not describe all aspects of gravity as a force, any more than regular electromagnetism describes all aspects of EM as a force. GEM does not describe GEM waves as a force, just as regular EM does not describe EM waves as a force.
  18. Feb 26, 2016 #17
    You are right about that.
    GEM-theory is based on electricfield, electrimagneticfield, gravitationalfield, gravitimagneticfield (instead of spacetime curvature) and forces that these fields cause.
    But I think that is what original poster meant by force based explanation.
  19. Feb 26, 2016 #18


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    "Forces that these fields cause" are part of the GEM and EM theories, but not all of them.

    He specifically asked if there was a force-based explanation of gravitational waves, not just "gravity". Just because GEM and EM theories have "forces" in them, it does not follow that they are a "force-based explanation" of GEM or EM waves, since the waves are aspects of GEM and EM that are not modeled as "forces" in those theories.
  20. Feb 29, 2016 #19


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    Well, I think the original poster did not know enough to phrase his question clearly. What I interpreted him to be asking was whether gravitational waves can be interpreted as propagating disturbances in a field, in the same way that electromagnetic waves are interpreted as propagating disturbances in the electromagnetic field.
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