Alright, it’s been ages since I had any General Rel, so I’ll probably make a fool of myself. But here goes.

If you’re looking for Gravity Waves the lowest order component you can look for is the one associated with the Gravitational Quadruople Moment.

Gravity waves are analogous to electromagnetic waves (radio, light, you know – that stuff), but a *lot* weaker, and still not successfully detected, despite much ingenuity on the part of the searchers. One reason is that gravity is a *much* weaker force than electromagnetism. (Actually, I’ve always hated that statement – you’re comparing apples and oranges, but let it pass for now.) Another reason is that in electromagnetism you can detect the Dipole Moment radiation, which dies off less rapidly with distance.

You make gravity waves the same way you make EM waves – you wiggle something up and down, or otherwise accelerate it. You can break down the resulting radiation into components that die away at different rates with distance. If you jiggle an atom up and down the lowest order term would be the Monopole Moment term – but it’s zero, because the effect ofjiggling the positive nucleus exactly cancels the effect of jiggling the negative electrons. But the Dipole Moment term is nonzero, because the electrons and protons move in opposite directions *and* have opposite sign charges. The dipole radiation drops off with the square of the distance – the infamous “inverse square” law. There is a higher order term, the quadrupole moment term, that is also nonzero (so are all the higher-order terms), but this one drops off with the inverse fourth power of the distance, so it’s a lot smaller.

In the case of gravity waves, the dipole moment term cancels out – jiggle two masses against each other (or, more likely, have one rotate around the other, like the earth and the sun) and the two masses move in opposite directions, but they have the same “sign” – all mass is positive, and there ain’t no negative mass (I know all your objections and special cases – can 'em. They don’t count)-- so the dipole term is zero. That means that the lowest order contribution is the quadrupole term, which is not only intrisically smaller in magnitude than the dipole term, but dies off *much* more rapidly with distance.

People doing experiments to try to validate string theory would love to be able to detect quadrupiole radiation to see the gravity waves, but the detectors have to be built to see quadfrupole forms of radiation and have to be able to see a VERY weak signal. The biggest problem is noise and interfering effects due to non-gravity wave sources (like trucks rumbling by on a nearby highway).