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Hopefully not too stupid question about gravitational waves

  1. Aug 7, 2009 #1
    I've been trying to get my head around relativity etc and struggling :-(
    However there's one thing that I can't find much information about at an interested-layman level - gravitational waves.
    They're described as having a frequency etc (being waves) and travelling at light speed. But if they have a frequency then presumably they can have varying frequencies. What would this mean for a gravity wave? Would it have a different amount of energy? Exert a different amount of pull? Could red shift alter the effect of gravity? What about amplitude - is that fixed? Could gravitational waves interfere with each other?
    Or am I taking the whole 'wave' thing too literally?

    Apologies for adding to all the noob questions here, but it's got me stumped!

  2. jcsd
  3. Aug 7, 2009 #2


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    Gravitational waves can have any frequency (in theory) just like any other wave. The actual frequency depend son their source - for something like two orbital bodies (eg merging stars) the frequency is twice the orbit.
    So given the possible range of orbital times for stars there is only a range of generated gravity waves - the detectors are built for this range.
    The amplitude of the wave depends on the mass of the objects colliding - more impressive events (like merging massive black holes) give bigger amplitudes and are more likely tobe detected.
  4. Aug 7, 2009 #3
    I'm with you so far, almost - thanks for the help!

    In a merging-star system like you mention, where would the waves originate? From the centre of gravity or some from each star? Is any piece of 'falling' matter radiating gravitational waves?

    So would gravity be effected by interference or a version of red shift? Are there discrete packets of it, similar to photons?
  5. Aug 8, 2009 #4


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    It's the frequency as it would be for any type of wave: the number of wave crests that pass per unit time. The frequency of a gravitational wave is determined by whatever process produces it.

    Gravity waves would be effected by red shift in the same way that photons are, and yes, we believe they do exist in discrete packets we call gravitons (which would be massless, spin-2 particles: photons are massless spin-1 particles). But we don't yet know for sure what the quantum theory of gravity is.
  6. Aug 8, 2009 #5
    So... if gravity is effected by red shift, does that mean that it's effect is lessened at large distances? Do we have to take that into account in any of our models, or are the distances so great that the effect is swamped by other effects?

    Are gravitons thought to be affected by gravity as photons are? I assume not... but why?

    Sorry - I've got far too many questions; can anyone recommend a book? I'm a maths graduate so I don't mind equations but I haven't done physics since A-level.

    Many thanks for all the answers so far!
  7. Aug 8, 2009 #6
    Current theory is that they act just like light does in all these classical things, including being affected by gravity. (Personally I'm not so sure current theory is completely right in some of this, but we won't go there).
  8. Aug 8, 2009 #7
    So gravity is effected by gravity - the top of my head just fell off.

    Hang on. If gravitons are effected by gravity in a similar way to photons, then how could they get out of a black hole? But if they can't get out of a black hole then a black hole wouldn't have any gravity, and then the gravitons could get out no problem. My head hurts.
    Last edited: Aug 8, 2009
  9. Aug 8, 2009 #8
    Yikes, now there's an irritating question. I'm glad I showed criticism of current theory in my last post! I suspect the gravitational energy of a black hole exists above the black hole, so maybe it would be explained somehow along those lines. In any case I have seen papers talking about gravitational lensing of gravity waves. I'll watch this thread to see what the answer is.
  10. Aug 8, 2009 #9


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    What's important here is not to confuse "gravity waves" with "gravity the force". Gravity waves do not carry the gravitational force, they carry information about changes in the gravitational field. In other words, if you were to erect a barrier between the Earth and Sun that blocked gravity waves produced by the Sun from reaching us, the gravitational attraction we feel towards the Sun would still be left.

    The same is true for black holes. Gravity waves cannot escape the event horizon, but the gravitational field of the black hole remains.
  11. Aug 8, 2009 #10
    Below is an extract from Physics FAQ-
  12. Aug 8, 2009 #11


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    No, it doesn't quite work that way. It's the traveling gravitational waves that are affected by redshift, which might be seen as ripples in the underlying gravitational field. The overall shape of the gravitational field is what determines the strength of gravity, not the wavelength of ripples running across it.
  13. Aug 9, 2009 #12
    Thanks for all this - it is starting to make more sense. So far my understanding is as follows:
    The force of gravity isn't transmitted in waves, but fluctuations in that force can be seen as waves.
    As a result, black holes capture gravitons, but this doesn't effect whether they have gravity.

    Am I ok so far?

    If there is a lag between an object moving and the gravitational field at an arbitrary distance changing, then would the gravitational field of a moving body appear different to the gravitational field of a static body?

    If there is a minimum wave size (a single graviton) then does this mean that there is a minimum difference between gravitational field strengths?
    If so, then could a movement be of the kind which produces gravitons, but involve masses / orbits so small that they can't produce a graviton?
  14. Aug 9, 2009 #13
    With one slight correction. Gravity waves contain no rest mass (so they can travel at c), but they do contain energy (otherwise they wouldn't exist) so when they are captured (by a black hole or just turn into heat when they shake up some matter), then they add to the mass of that black hole or ordinary matter that captured them, increasing the gravitational and inertial mass of that black hole or ordinary matter. But that increase in gravity of the capturing mass is not because it was gravity waves in particular that were captured, its just because energy was added to that object, making it more massive.
  15. Aug 9, 2009 #14
    Actually, neither the photon nor the postulated graviton are themselves effected by 'gravity'. Such effects are due to the difference between emission source and observer -- velocities, time dilation, etc.

    In General Relativity, both photon and graviton follow geodesic paths through a 'warped space-time'. There are other ways of explaining this but this is the current mainstream view. There are problems with the 'warped space-time' model -- not so much with the results the math produces but more with the confusion it causes in interpretation.
  16. Aug 9, 2009 #15
    "In General Relativity, both photon and graviton follow geodesic paths through a 'warped space-time'."
    Is that because of their lack of mass, their sort-of-infinite speed (I know that's not quite right but I tend to think of it that way), or are the two properties so inextricably linked that it's both?
  17. Aug 9, 2009 #16
    Just to expound, here, I think you put quotes around 'gravity' to make clear that there are a layman's definition and a scientific definition of the word 'gravity'. I'm sure you're saying here that in current theory, the scientist's definition is that 'gravity' is a synonym of 'the warps in space-time', so using that definition, gravitons & photons are affected by gravity.
  18. Aug 9, 2009 #17
    Everything follows those geodesics. Baseballs, too. Both the light and baseball follow a straight line as seen by a local observer, because that local observer follows that warp, also. A remote observer can see that warp, though.
    Last edited: Aug 9, 2009
  19. Aug 9, 2009 #18
    Only massless 'particles' follow what might be interpreted as 'straight-line' for the geometry of the 'local space-time warp'.
  20. Aug 9, 2009 #19
    You're right, i should have qualified that "straight line" I said to show it was a strictly local thing. Poor wording on my part. An astronaut in a spacecraft without windows can rightly consider himself not accelerating. A local 'straight line' and 'geodesic' have different meanings. For the OP: A geodesic is simply the free-fall path of an object. Geodesics are relative. A photon (and, theory says, a gravity wave) travel on a null geodesic, which means they are right between the concepts of space-like and time-like geodesics.
    Last edited: Aug 9, 2009
  21. Aug 9, 2009 #20


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    The photon and the graviton are affected by gravity just like everything else is. All other matter fallows geodesic paths through warped space-time as well. That's just how GR is formulated.

    The difference in the relationship between the energy and momentum of photons and gravitons makes for some differences in how they gravitate, and in how they produce a gravitational field. But they still gravitate, and respond to gravity.
  22. Aug 9, 2009 #21
    They appear to be effected by gravity yes. That does not confirm they 'create' or 'produce' a gravitational field themselves. Gravity is a phenomenon of organized matter for sure, but MAY not be one for energy (unproven).

    Chalnoth, you may be right but your statement is presented as fact -- but that has not be as yet shown.
  23. Aug 9, 2009 #22


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    If it were not true that radiation produces gravitational fields, then our observations of the CMB would be way, way off, as in the early universe, radiation was the primary energy density.
  24. Aug 9, 2009 #23
    Yes, they might. There is no proof that gravity -- as we know it today -- existed at and prior to the time of last scattering. An no real need for it as far as I know -- but again that doesn't mean your are wrong Chalnoth -- only that it is not proven fact. Also:

    Is not true. The effect on the massless photon is twice what would be expected from the Newton formulation. This means that there is something different about photons and normal matter when it comes to gravity. The entirety of the difference is still in question.
  25. Aug 9, 2009 #24


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    Well, I suppose you could say there's no proof if you completely ignore how well the CMB, Baryon Acoustic Oscillation, and primordial nucleosynthesis data match with theory.

    That just says that Newton's theory is incomplete. In this case it's incomplete because it doesn't consider the gravitational effect of momentum. General Relativity perfectly predicts the deflection of photons (to within current experimental bounds), so I don't know how the difference can still be in much question.
  26. Aug 9, 2009 #25
    Yes, the CMB, Baryon Acoustic Oscillation, and primordial nucleosynthesis data match with theory and do appear to fit together -- but it is not clear to me what role gravity really has in doing this. None that I can see.

    Also, General Relativity should match the deflection of photons -- it was designed to do so.
    Don't see how that makes it 'complete' either. Problems it has include:

    1) Its deterministic -- on purpose. Developed that way by the accepted philosophy of the day before quantum mechanics.

    2) Has a fixed three spatial and one time dimension. Very unlikely from what we know or rather suspect today.

    3) Descriptive more than predictive. You need to be able to specify the geometry and symmetry of the problem before you can calculate. Likely all models will have this limit to some degree.

    4) Little or no mechanism to explain 'how gravity works'. This leaves the understanding limited by the understanding of the mathematics -- very few.
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