I read http://www.cnn.com/2002/TECH/space/11/19/blackhole.merger.ap/index.html
and am wondering what exactly “a burst of gravitational waves that could warp the fabric of space” and “The merger will be accompanied by an eruption of radiation and a burst of gravitational waves that will spread throughout the universe, causing ripples in the fabric of space, the astronomers said.” and
“The gravitational ripples could cause minute changes in the distance between any two points in the universe, they said.” actually mean.
That is, if such an effect hit earth today, what would it be like?
Its not yet known if gravity is constrained to the speed of light.
If it were so, you would just be minutely heaveir and then lighter (or pulled to the left and then right). Probably nothing that couldnt be detected without using an atomic clock.
While it is true that we don’t know that gravity travels at the speed of light, the idea is consistent with every thing we do know. For gravity waves to exist at all, gravity must be propagated at a finite speed. A nobel prize has been awarded for discovering pretty solid proof of gravity waves (indirect proof, but pretty solid) as it would be the only plausible mechanism for the reduction of orbital energy in some high mass short period binary stars. There is no (compelling) reason at the moment to think that gravity waves would move at any other speed than the speed of light.
Due to the expected characteristics of gravity waves, we would expect them to be damn difficult to detect (or notice as well). We have been directly trying to detect gravity waves for a number of years now. So far, it has been all negative. However, in only the last couple of years has the technology in the detectors been clever enough to give us a real chance of success with anything other than spectacularly large gravity waves.
If the galaxy mentioned in the article completed its black hole merger right now (or 400 million years ago, so it would appear as right now to us, and assuming that gravity propagates at the speed of light), unless there is a big anouncement from one of the labs trying to measure gravity waves anouncing success, you would be quite unaware of it. The effects would be so small here (and nearly everywhere else), that if you didn’t have equipment set up specifically designed to detect the effect, no one would notice.
For more background on this, do some searches for gravity wave detectors. You should be able to find some explanations of why gravity waves are so maddeningly difficult to detect, and that will also make very clear why no one would notice when they went by.
scotth has already addressed the speed and size of the effect. The character as described above is incorrect. Neither the magnitude nor direction of Earth’s gravity would be affected. Rather, all objects would be stretched in one direction and compressed in another. Some gravity wave detectors look for this by shining laser beams down the two legs of an L shape. When one leg is stretched and the other shortened, the interference pattern of the beams should change.