From the site Perderabo mentioned in the post, “How fast is gravity?”:
This conflicts with what this site (Choose either the Swiss original site or the North American mirror site. Both are exactly the same and in English.) says.
Is the first site mistaken? Is the second guy confused?
None of the experiments in the link you posted have to do with the propegation speed of gravity.
Nobody is saying that gravitational force is too weak to measure. However, to measure the propagation speed of gravity, you need to change something and measure the time delay of its effect on another object. The “change something” is the first tricky bit; you can’t turn off gravity, so all you can do is move a massive object, which doesn’t have a very big effect. Then you have to measure the effect on another object which is far enough that there is a measurable time delay.
I’m not even sure if there are/were attempts to do this. There are attempts to detect gravitational waves, but most people think current such projects don’t have enough sensitivity to succeed. In a couple of decades though, who knows?
Padeye:
[reword question]What I meant was that the first site said we did not have the “technological capacity”, but the other said Archemedes could’ve done it. What’s the straight dope?[/reword question]
scr4:
I congratulate your ability to understand the nature of my inperspicuous posts. Thanks for the answer.
Gravity probe B is supposed to test some of general relativity. I believe that gravity waves traveling at the speed of light are a consequence of the general theory of relativity.
http://einstein.stanford.edu/gen_int/faqs/faqs.html
Yeah, that’s a great page.
I suppose Archimedes could have done that, but it’s pretty unlikely that he would have thought of it; them ancient Greks weren’t big on doing experiments. Thought experiments they liked.
However, WoodThrush, nowhere on that site is there any mention of the time that it takes the gravitational effect to travel from the test mass to the apparatus. That’s what the first site is talking about.
The apparatus from the second site is not suitable for measure how fast a change moves from one mass to another. The apparatus just doesn’t react fast enough; it takes minutes for it to change from one stable state to another stable state. It takes on the order of 0.000000001 second for a change in gravitation to move from the test mass to the apparatus.
The big problem with measuring the propagation of gravity is that all the matter in the universe is already pulling every other piece of matter.
At one time, all the matter was within an infintesimally small point. When the Big Bang occurred, there was no “gravity barrier” to overcome. If there were, then matter that broke that barrier would continue forever with no slowing down since it was “out of range” of the rest of the universe’s matter’s gravity and had no friction, since it’d be the first matter to exist wherever it traveled.
AWB:
Concerning that idea on a “gravity barrier”, you might want to read this… it starts out a little dull, but it gets better as scalar fields are introduced…
Quoth scr4:
The current best bet for gravitational wave detection is the LISA mission, which will consist of three separate satellites, orbiting in the shape of an equilateral triangle, with separations of about 10 million kilometers. Unless there’s something seriously wrong with General Relativity, we’re certain that LISA will detect gravitational waves; the only debate is whether we’ll detect anything interesting, and if so, what. As for when it’ll be launched, funding does not currerntly exist 
Yet another thing to lobby your congressmen for, boys and girls!
As for the ground-based attempts at detection, the best prospect is the LIGO progect, which has been going for several years now, and has thus far only made a few possible detections, all of which may well just be flukes. The problem isn’t so much sensitivity per se, as it is noise: Any sort of vibration will show up on a grav-wave detector (An attempt a while back here at Montana State could pick up Richter 4 earthquakes halfway around the world).