# Gravity Waves

What about the Earth-Moon system? There is already a ranging experiment. How much will Moon change distance for a gravity wave from a typical source?

Bill_K
What about the Earth-Moon system? There is already a ranging experiment. How much will Moon change distance for a gravity wave from a typical source?
According to Wikipedia, the expected amplitude for a gravitational wave is 10-20. With the Earth-Moon distance about 400,000 km, the distance change is predicted to be 10-12 m, or 1000 nuclear diameters.

According to Wikipedia, the expected amplitude for a gravitational wave is 10-20. With the Earth-Moon distance about 400,000 km, the distance change is predicted to be 10-12 m, or 1000 nuclear diameters.

From the Wikipedia article on Lunar laser ranging
As of 2002 work is progressing on increasing the accuracy of the Earth-Moon measurements to near millimeter accuracy, though the performance of the reflectors continues to degrade with age.[4]
That means the laser ranging measurement has to be at least 9 orders of magnitude more accurate to be able to discern a gravity wave from background noise.

What about the Earth-Moon system? There is already a ranging experiment. How much will Moon change distance for a gravity wave from a typical source?

Lunar laser ranging is typically thought not to have the required accuracy. However, planting a detector on lunar surface specifically designed for such a mission similar to my earlier discussion of seismograph on earth (in the 0.1 to 10 hertz range) could be advantageous to pulsar GW detection, especially since the moon is thought to be seismically quiet (compared to earth).
Here are some preliminary thoughts from a JPL workshop on that topic, placing it in the sensitivity range of LIGO and LISA, and even includes an idea for sensitive displacement detector using a free floating superconductor.
http://www.ligo.caltech.edu/~veronica/CaJAGWR/info/general/paik02.pdf
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OK, gravity waves aside, is it technically possible now to measure distance to the Moon using interferometer? If not using light, then at least radio waves?

With the Earth-Moon distance about 400,000 km, the distance change is predicted to be 10-12 m, or 1000 nuclear diameters.
OK, that falls somewhere in the X rays spectrum. Sounds promising.

My third question is about the gravity waves themselves. Do they affect light? I mean, does the light know it passed through a region of space where are gravity waves?

Suppose some advanced civilization put a laser in the space and a receiver very far away from it. Suppose also the space between them is filled with a bit of dust. Then, a gravity wave passes.
Scenario 1: A "wide" wave passes, causing the laser and the receiver change distance.
Scenario 2: A "narrow" wave passes, not affecting the distance between the laser and the receiver, but disturbing dust between them.

(I understand gravity waves are a bit like radio waves, that means they can have "shape". Am I correct? There can be region in space where there is a gravity wave and a region where it isn't. In particular, two orbiting neutron stars emit gravity waves mostly in their "equatorial" rotation plane and not at the "poles". The wave intensity angular characteristic is similar to that of the dipole antenna radiation pattern. Am I right?)

In scenario 1, we would see the passing wave on the interferometer, right.
Now in scenario 2. Will the wave be visible to the detector? I mean: the spacetime between the source and detector was disturbed, so the light has to be affected, right?

If that is the case then going back to scenario 1: what is the effect of the light disturbance for the distance measurement experiment? Does it amplify distance vibrations, does it damp them or is it negligible?

That said, could we see some effect of gravity waves in some distant stars light characteristics or in some exotic phenomena like gravity lensing pictures?

Bill_K
In particular, two orbiting neutron stars emit gravity waves mostly in their "equatorial" rotation plane and not at the "poles". The wave intensity angular characteristic is similar to that of the dipole antenna radiation pattern. Am I right?)
Thanks to the quadrupole nature of gravitational waves, dipole waves do not exist. This can also be understood as the result of momentum conservation. For a binary star system, the leading source term is a rotating quadrupole moment.

As I pointed out above in #14, there is radiation in the polar direction as well, although the polarization is different.

Bill_K
That said, could we see some effect of gravity waves in some distant stars light characteristics or in some exotic phenomena like gravity lensing pictures?
You haven't specified which directions the gravitational wave and the light are traveling, but if they're traveling more or less parallel to each other, the effect most easily observable at a distance will be that the light ray is deflected sideways.

PS - It's helpful to call them by their proper name, gravitational waves. Gravity waves are something else.

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Thanks to the quadrupole nature of gravitational waves, dipole waves do not exist. This can also be understood as the result of momentum conservation. For a binary star system, the leading source term is a rotating quadrupole moment.

As I pointed out above in #14, there is radiation in the polar direction as well, although the polarization is different.
OK, I understand, but please correct me if I'm wrong.

Gravitational waves, just as electromagnetic waves, have amplitude, frequency and polarization. Now: is it possible to arrange such an emitter that radiates waves in non-isotropic way? For example a setup, where the amplitude is maximal near the equator and zero near the poles?

You haven't specified which directions the gravitational wave and the light are traveling, but if they're traveling more or less parallel to each other, the effect most easily observable at a distance will be that the light ray is deflected sideways.
So if we happen to observe two merging stars at least one of is bright, then we would see the effects of the gravitational waves in the image? Have such experiments ever been performed?

Bill_K
Now: is it possible to arrange such an emitter that radiates waves in non-isotropic way? For example a setup, where the amplitude is maximal near the equator and zero near the poles?
The wave must have a polarization, and so from the viewpoint of the detector, the source must have a preferred direction - it must look asymmetric.

For any source that is cylindrically symmetric, an observer along the polar direction will NOT see a preferred direction, so there can't be any waves coming at him. E.g. if two stars collide directly, this is a cylindrically symmetric situation, and an observer situated right along the collision axis won't see any radiation.

For any source that is cylindrically symmetric, an observer along the polar direction will NOT see a preferred direction, so there can't be any waves coming at him. E.g. if two stars collide directly, this is a cylindrically symmetric situation, and an observer situated right along the collision axis won't see any radiation.
Thanks, that is the clear answer.

I am trying to learn GRT so I can answer questions for myself. But I might croak first, so I’ll ask here. That gravity wave interferometer they are building out in Richland, Washington - I obviously haven’t read all the technical papers on their web site, but I am pretty sure one I did read showed a spectrogram or PSD with search frequencies of 40 Hz and above. I understand that the behavior of the Taylor-Hulse binary is the only empirical evidence we have so far of gravity waves - and its orbital period is around 8 hours. Can anyone explain why they are looking at such high frequencies? Shouldn’t we be looking for ultra-low frequency waves with periods like 8 hours (or maybe half that for these type waves)? Do we expect stuff falling into a black hole to emit broad-band high frequency gravity waves? Thanks for any enlightenment.
With the recent discovery of gravitational waves, I have a question. Knowing the frequency and wave length of these waves, can a harmonic wave increase gravity in the sphere of influence and conversely can gravitational force be eliminated or reduced by generating a wave 180 degrees out of phase with the gravity wave. I suppose there are many wondering the same thing.

No. Gravity waves don't cancel the gravity attraction, just as light doesn't cancel magnetic attraction.

What gravity waves could cancel are tidal forces. In a pretty boring way.