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Could gravitational waves form interference patterns?

  1. Sep 19, 2009 #1
    If gravitational waves formed interference patterns, or just simply interfere as normal waves, couldn't that result in gravitational anomalies in absence of mass? Destructive interference could be thought of as negative gravity(dark energy), leading to expansive "forces", whilst constructive interference could emulate the presence of mass, i.e. dark matter. And perhaps the current shapes of galaxies are due to some resonance effect.

    Sorry if I'm reviving some old discussion, or got the wrong forum section. I got this idea after reading an article about dark matter on physorg and I'm just curious if anyone has pursued this train of thought before, if it's a valid one. My knowledge is thoroughly limited and my approach is "by the ear", so I may have just proposed something very silly.
  2. jcsd
  3. Sep 20, 2009 #2
    If you swing a lump of matter about from side to side you may be able to create something that you could classify as a gravitational wave otherwise I can't see how gravity waves to produce a gravity effect could ever work.
  4. Sep 21, 2009 #3
    I wasn't talking about using such an effect for engineering purposes, though now that you mention it, there is some potential to it. Confinement technology for fusion power comes to mind. And gravity guns, of course.

    It just seems to me like a simple explanation for dark matter/energy.
  5. Sep 21, 2009 #4

    The stochastic gravitational wave background has yet to be detected and the stringent limits on its amplitude seems to precludes it from being a source of 'dark matter' gravitational potential.


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  6. Sep 21, 2009 #5
    The original post is about
    One wouldn't get interference patterns from stochastic (def: ruled by the laws of chance)sources wouldn't one?
    The sources would have to be coherent.
    ergo, unless one could achieve coherent sources (gravitationally phase linked) then a stationary gravitational interference pattern would be impossible?
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  7. Sep 22, 2009 #6


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    But the stochastic background could act like dark energy, only far too weak to explain the measured effect.
    Dark matter, of course, demands some pretty strange standing waves. It doesn't seem possible, but I haven't read anything about such an idea.
  8. Sep 22, 2009 #7
    By dark matter/energy, I am refering to positive and negative gravity, in the geometrical sense, i.e. expansion and contraction of spacetime. I guess the choice of name for these phenomena is quite biased, which is why most people are looking for particles and measurable energy. Also I believe the gravitational waves would be longitudinal in this case.

    Standing waves do come to mind when trying to visualise, but I don't think they'd be possible on a galactic scale, at least not in a perfect form that would make them very visible(so to speak) over long periods of time. Rather, I'm thinking of periodic occurrences of positive/negative interference strong enough to have a significant effect. When you average them out on the biggest scales of time and space, the energy should be conserved, since they'd cancel each other out. Then again, the universe might be expanding, or contracting.

    Unfortunately, I lack the mathematical knowledge to formulate this properly and it might be that this could only be proven by computer simulations. Have there been any attempts to simulate gravitational waves, in the geometrical sense I have been describing?

    I've tried googling the idea, but this thread comes up first, followed by a lot of articles about gravitational wave detectors and explanations of how their laser interferometers work. The closest thing I could find is this: http://en.wikipedia.org/wiki/Gowdy_solution
  9. Sep 22, 2009 #8
    We have interference in light.
    we have stationary waves in all sorts of situations - light, sound water...
    I have never heard of negative light
    I have never heard of negative sound.
    I have never heard of negative water waves.
    Why do you think I should accept negative gravity from interference?
    If waves arrive in phase they construct.
    If waves arrive in antiphase they cancel ie zero. They don't go negative.

    Then sort it out
  10. Sep 22, 2009 #9
    Pardon the brief reply but have only Iphone available. I also had this same question a few months ago but was too busy to pursue-plenty of time now! I'm also not professionally involved with physics and simply viewed this conceptually. I can't add to the proposition which you put forth but thought I might respond to couple of the comments that you received. First,to the issue of the potential for both constructive and destructive interference,wouldn't all waves except those exactly out of phase at least preserve their inherent curvature of space-time ? Certainly those exactly in phase would,under this line of reasoning, amplify their mass and those exactly out of phase could extinquish their mass. Unless there is a required directionality to space-time curvature, all other wave interactions would preseve some curvature and therefore preserve some mass.

    As to the issue the miniscule (prob not the correct word) amplitude of gravitational waves and the liklihood of the requisite interacting waves,doesn't the sheer immensity of potential sources (esp within galaxies) provide sufficient opportunity for the proposed interactions.
  11. Sep 22, 2009 #10
    The 'sheer immensity of potential sources' would mean, on average, they cancel. hence zero effect
  12. Sep 22, 2009 #11
    I'm talking about longitudinal waves, spacetime compressing and expanding. At least, that's my understanding of what a gravitational wave is.

    Light and water waves(surface ones) are transverse, there is a 0 on the plot at which there is no light or the water is level, otherwise both up and down signify work being done on the medium(if there is one, for EM radiation). But if you think of sound waves as pressure waves, longitudinal, there is such a thing as negative pressure, work being done by the medium.

    So, if positive gravity is work being done on spacetime by massive bodies, negative gravity would be work done by spacetime on massive bodies. This does entail an elastic fabric of the universe, of course. Not quite the same thing as aether, though.

    Well, I start my country's equivalent of a college physics/maths major in October. If no one else does so before me, in 3 years max I should come up with a clear conclusion, maybe even a paper.
  13. Sep 22, 2009 #12

    Jonathan Scott

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    Gravitational waves do not have any longitudinal component. They are basically transverse quadrupole. See the Wikipedia article on http://en.wikipedia.org/wiki/Gravitational_wave" [Broken] for a nice description including animated graphics.
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  14. Sep 22, 2009 #13
    I did skim that article several times, apparently missing the part about polarisation on each occasion. I guess I confused curvature with density. The AWT people must be getting to me.

    Then again, doesn't the analogy still hold? Aren't spacetime curvature variations from gravitational waves negative, as well as positive?
  15. Sep 23, 2009 #14
    'Negative' just means the pressure is below the norm
  16. Sep 23, 2009 #15
    If the pressure/curvature is below the norm and we assume an elastic medium, then, in absence of a source of deformation, the medium will return to that norm. The force of the medium returning to normal is, relative to the force that deformed it, negative.

    The way I see it is, in that 2D rubber surface analogy popularly used to explain how mass deforms spacetime, positive gravity is downwards, objects drawn to the deformation(a valley), while negative gravity is upwards, objects going away from the deformation(a hill). In the pressure analogy, negative pressure would be positive gravity, objects sucked in, and vice versa.

    A gravity wave in 2D would be that rubber film going up and down, between negative and positive, peaks and troughs. I'm assuming the same happens in 4D.
  17. Oct 9, 2009 #16
  18. Oct 9, 2009 #17
    This was the next thing that bugged me after thinking about this idea. Are particles/force fields and spacetime separated? Because I don't see how else would strain, or measurable change, occur in grav wave detectors, like Weber bars.

    If a region of spacetime is distorted, then aren't any interactions occuring in, or near the distortion, distorted proportionally, so that the laws of physics stay the same? For instance, say we have two oppositely charged particles situated just outside each other's EM field, so that they don't affect each other. If spacetime contracted between them, say by the influence of a GW, would that lead to electromagnetism kicking in and drawing the particles closer together? If that were to happen, wouldn't it imply that electromagnetism takes place either at a different speed, or in a separate dimension?

    I guess this is more of a relativity question. I just can't figure out how spacetime distortion could be "seen"(detected), as opposed to being inferred geometrically. I feel it's like a cog trying to see the entire mechanism of which it belongs to.
  19. Oct 9, 2009 #18
    The idea behind general relativity is that all gravity is caused by a distortion in the space-time field. So you can see already see the force caused by distortions in space-time field. Just drop an apple and watch it fall.

  20. Oct 10, 2009 #19
    Wouldn't I be seeing just the effect of the distortion? I'm talking about what a "naked" distortion would look like, a GW.

    Let me reformulate my example. Two oppositely charged, massive particles are situated at a certain distance at which their inertia barely overcomes the electrostatic force. OK, so that would probably be a pretty big distance, but let's assume a GW of the appropriate wavelength/amplitude passes between them and briefly compresses spacetime(increases curvature?) over that distance.

    The distance between the particles shrinks, in a relativistic sense. But does that mean the electrostatic force can now overcome inertia, drawing the particles together in that brief moment of compression?
  21. Oct 13, 2009 #20
  22. Oct 14, 2009 #21
    A-ha, I think I get it. So is it correct to say that the non-relativistic one is caused by the rest mass of particles, while the relativistic one is caused by the invariant mass? Maybe caused isn't the right word here. Does it have anything to do with some ratio between the two masses? I.e., relativistic effects occur when the invariant mass becomes greater than the rest mass. I'm also guessing GW can't be generated by momentum alone, that is, photons won't cause them.

    I really need to get a good book on GR...
  23. Oct 14, 2009 #22
  24. Oct 14, 2009 #23
    I did read a pretty good wikibook on SR(I think it's called just that), but I stopped once I reached a chapter pertaining to electromagnetism. I figured it'd be better to first study that separately and then in the context of relativity. Didn't really read the maths thoroughly either.

    What really stuck with me is the whole everything-depends-on-observer thing, which probably got over-inflated by my penchant for philosophy(read 'pointless meandering'). So I think my main error was to think of particles and forces as observers.

    I still feel like I'm on some sort of right track, though. Luckily, I just got a neat algebra book, which should knock some sense into me and allow me to properly study GR.
  25. Oct 22, 2009 #24


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    Spread a bucket of sand over a pond and what do you see? Certainly not calm water!
  26. Oct 22, 2009 #25


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    A better analogy might be to consider a huge gravity wave passing through the Earth, from trough to peak. Since the spacetime would increase and decrease it's curvature within the volume of the Earth, I would suspect that the Earth would expand and contract accordingly due to the Gravitational Constant changing during the wave cycle. I don't know if GW actually changes the distance between particles, only that it changes the curvature which effectively changes the mass of the particles involved causing the Earth to shrink or expand under it's own gravity.

    The idea of this thread is also interesting, and my gut feeling is that steady state gravity is a much larger force for such things as lensing effects than would be interferece of gravity waves since gravity waves would look more like gravity "noise" due to the various interfering frequencies (like using white light in a dual slit experiment) and this noise would be completely random around something like a galaxy or galactic cluster not so much contributing to lensing but instead contributing to bluriness of the virtual images. Still, I congratulate you on an interesting thought.
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