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Can gravity waves be focussed?

  1. Sep 22, 2006 #1
    Assuming LISA does find graviational waves in the future, is there any way in theory to focus these gravitational waves?

    I know that gravitational lensing of distant galaxies can focus their EM spectra as in the Abell clusters, but can this same gravitational lensing be achieved on gravity itself? Or any other devised manner in theory?

    Also, this brings us to an idea I thought of where there may be local areas of space that exhibit a pocket of abnormally high gravity from a specific direction in space. Of course this hinges on the above questions, so if anything its just an interesting fictional idear.
  2. jcsd
  3. Sep 22, 2006 #2
    Very interesting subject. I hope someone could answer it.
  4. Sep 22, 2006 #3
    Thanks lightarrow, I'm very curious to have it answered also too. SpaceTiger you are needed! *shines giant ST stencil up into the clouds*
  5. Sep 23, 2006 #4
    I don't think there are any accepted theories that expect gravity to bend of "focus". Just a "fictional idear" or two.
  6. Sep 23, 2006 #5


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    I thought we'd been throuh all this before?

    Anyway, in empty space, gravity waves follow null geodesics, just like light does.

    Gravity waves should not be confused with "gravitational forces", which behave differently. Gravity waves act just like light waves - gravitational "forces" are analagous to electrostatic forces, though the analogy isn't perfect. "Gravitational forces" do not follow null geodesics, for instance the Earth is attracted towards the current position of the sun, not the retarded position.

    Light travels slower in a physical medium because of dielectric effects. Dielectric effects require dipoles - a positive and a negative charge. There aren't any gravitational dipoles (unless one postulates exotic matter), so one can't build the gravitational equivalent of a dielectric.

    This leaves one main possible way to make a lens for gravitational waves (NOT gravitational forces). I think the required approach is reasonably obvious if one understands the rest of what I just said. In the interest of inspiring people to actually think about what I said, I'm not going to spell it out.
    Last edited: Sep 23, 2006
  7. Sep 23, 2006 #6
    You must be having a bad day, please don't reply to my posts on days when you are not happy.
  8. Sep 24, 2006 #7


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    I'm not sure why you think I've had a bad day.

    I'm pretty sure we have been through some of this before. If you dig around, you can probably even find some of the old posts. It could have been with someone else, but the old posts should still be there in any event.

    I thought (and still do think) that it's reasonably obvious what (other than passing through a dielectric) than can bend light, something that can also bend gravity waves, and anything else that follows a null geodesic - at least in theory.

    Sorry we don't get along better, but I've found that threads with you do tend to get rather chaotic, this thread seems to be a typical example.

    Still, I don't want the thread to get too far off track, and it looked like some other people were interested, so I thought I'd drop a few hints to nudge the thread into the right direction.

    The key technical points I did want to make I made, these are the facts that in GR, gravity waves travel at 'c' and follow null geodesics, just like light does, and the fact that gravity waves are different from gravitational attraction in this regard.
  9. Sep 25, 2006 #8
    You make good points zapper and I've always found your posts insightful. It just bothers me that you use a condescending tone and put people down. Can't you shine without being mean?
  10. Nov 22, 2006 #9

    Chris Hillman

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    Virtues of the discovery method

    Hi, Chaos,

    Oh dear! I had to read pervect's reply to your question several times before I decided that you were probably referring to this:

    If we delete the single word "actually" (probably inadvisable, since open to interpretation as sarcasm, although I think that would be reading much too much into pervect's post), I think that pervect's post is fulling in keeping with the time-honored pedagogical principle to the effect that the goal of teaching should not be tell the student how to proceed, but to get him to discover for himself the right way to proceed. Teachers who can do this (it's incredibly difficult; c.f. "the Moore method", aka "the discovery method" in teaching mathematics) have given something the student something more precious than gold: "ownership" of the neccessary concept or technique.

    Hope I'm being helpful here in trying to pollute troubled waters with petroleum products.

    Chris Hillman
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