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Gravity waves, how big are they?

  1. Jul 24, 2012 #1
    Hi, I was thinking about gravity waves and how big leading theorists believe they may be in size of wavelength? I saw a video where Steve Gerricks (sp?) of NIST in Colorado, shows how the NIST clocks are so accurate and at different elevations and in different geographic locations, so that he can detect how the earth's spherical shape deforms and compresses using the time differences between clocks of varying altitudes. So a higher up clock beats a bit faster since its further from the gravitational mass of the Earth. This seems like a gravity wave to me. But clearly it is a definition thing and I am wrong.
    What about LISA and her 3 500,000km spaced satellite array.. are they meaning to find gravity waves larger than 1,000,000km in length? I wonder what the longest wavelength it can pick up is, with its triangular array...

    Thoughts, correction? Thanks.
  2. jcsd
  3. Jul 24, 2012 #2
    First of all, gravitational waves are totally different from the cause of clocks at different heights to measure different times (not really totally different, but different enough to not want to confuse the two).

    Second, the wavelengths of gravitational waves are really really small. And I mean tiny. It depends on what caused them, but LIGO (the largest currently operating gravitational wave detector) is designed to measure the gravitational waves emitted by, amongst other things, black hole collisions. The gravitational waves involved have wavelengths a fraction of the width of an atomic nucleus (that's tiny).

    The reason you use massive detector arms is that the longer the arm is, the more the combined wave at the light detector will be changed by the tiny gravitational wave. If you're confused I'd try looking up the workings of Laser Interferometers like the ones used at LIGO and LISA; if it is ever built.
  4. Aug 6, 2012 #3
    A gravity wave will have a wavelength that is the product of the speed of gravity (assumed to be equal to the speed of light) and the period of whatever is emitting the gravity waves. For example, the earth emits gravity wave radiation as it orbits the sun. The period of earths orbit is 1 year so the wavelength of the gravity waves emitted is 1 lightyear.

    The reason this is extremely difficult to detect is that in order to detect it you have to measure the differences in gravity from a particular source on 2 or more parts of you detection apparatus. For example, suppose you were trying to detect earths gravity waves using a detector at proxima cantari. First of all it would be quite a feat to detect earths gravity at all from such a distance. Secondly, you have to measure the difference in earths gravity at each node of your detector, this difference will be orders of magnitude less then the actual value of earths gravity. If your detector is 186282 miles long and one end is pointed toward earth, the other end pointed away. The further end is seeing gravity that is 1 second older then the near end. Earth moved during that 1 second so earths gravity at the 2 ends of the detector is pulling in slightly different directions. That difference is what a gravity wave detector is trying to detect.
  5. Aug 7, 2012 #4


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    You are confusing wave length with amplitude.

    The wave length depends on time scales involved at the source - binary black holes directly before merging have a frequency of something like f=10kHz, this corresponds to wave lengths of lambda = c/f = 30km.

    We are far away from those sources, therefore the amplitude of the waves are tiny here - of the order of 10^(-20) or less relative change in the distance between objects. If your detector has a length of 1km, this corresponds to a length change of 10^(-17)m.

    NGO (possible replacement for LISA) with ~1 million km length between the spacecrafts is sensitive to gravitational waves in the range of some Hz or less, this corresponds to orbiting objects with a larger separation.

    You do not have to measure the gravity at all (cannot be done due to the equivalence principle), or the different distance to the object (way too tiny), gravitational waves are more like electromagnetic waves: You cannot measure the individual charges on the surface of sun, but you can clearly measure their radiation.
  6. Aug 7, 2012 #5
    That's really interesting, I had no idea. I knew how gravitational wave detectors measured gravitational waves but all the stuff I had read seemed to say that the waves themselves were just as small as the difference they were detecting between the arms (which now that I know the truth, doesn't quite fit).

    Thanks guys :) And sorry to the OP for giving the wrong information.
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