He assumed that the speed of gravity (the speed at which gravity waves travel, and can thus affect other bodies) was the same as the speed of light, but until recently, it hadn't been actually measured. Now it has.

I was bothered by the following grafs, though.

Fomalout and Kopeikin said their results are accurate within about 20 percent.

Knowing the precise speed of gravity is important to physicists testing such modern ideas as the superstring, which holds that fundamental particles in the universe are made up of small vibrating loops or strings. It also affects some basic space-time theories.

There is a difference between accuracy and precision. Accuracy means that it's in the ballpark, while precision indicates that we can measure the value to a large number of decimal places. It's possible to be accurate, and not precise, and to be precise, and uttely inaccurate (i.e., precisely wrong). As an example, if I said that the speed of light was 1.3578456457746456456 meters per second, I would be giving an extremely precise answer, that wasn't within six orders of magnitude of the correct answer.

Results that are accurate within only twenty percent are hardly precise, and in fact, don't provide full confidence that the speed of light is indeed the speed of gravity.

We can never know that for sure, of course, no matter how precisely we measure. It might be that the speed of gravity is exactly 99.99999999999% of the speed of light, rather than equal to it, and we may never know, for sure. Similarly, the exponent in the universal law of gravitation might actually be 2.00000000000000000000000000001 rather than exactly two (so it's not exactly a function of the distance between the bodies squared), but Occam's Razor indicates that that's pretty unlikely, particularly since there are sound theoretical and geometric reasons to think that it's truly a squared law (having to do with the nature of spherical fields).

Similarly, if the two speeds are measured to be within twenty percent, it's likely that they're truly the same (particularly since there's no reason to think they're not), and the likelihood will increase as we get better measurements, with less error.

Color me sceptical, but I find it a bit convienient that the speed of gravity is the same as the speed of light, when light was used as the basis to measure gravity. Big surprise there that they turn out to be nearly the same eh?

20% isn't an impressive degree of accuracy, but AFAIK this is the first time that anyone has even come close to measuring the speed of gravity. (Correct me if I'm wrong- I'm not a physicist and don't even play one on TV) The first solar neutrino experiments were only accurate to within about 67% of what is now thought to be the number of neutrinos emitted by the sun, but they nevertheless provided important data about the nature of stellar fusion reactions.

As for the fact that gravity propogates at the speed of light- c is the universal speed limit as far as we know, so it isn't an unreasonable estimate for the propogation speed of something that's just really, really fast. Personally, I've always been a fan of some form of quanta-based gravity, so my money is on a very high percentage of the speed of light rather than c itself.

John --

Light (actually microwave radar) is used as the basis to measure the speed of fighter aircraft. Yet they do not turn out to move anywhere close to the speed of light.

Your post does not make much sense.

Additionally, gravity affects light. This is well understood even if the exact numbers are still in flux. Since nothing is known to exceed the speed of light on the macro scale there is , in fact, no better way to make the measurement than by light.

Actually, there's a lot of controversy right now over the meaning of Kopeikin's result-- you can follow developments on the moderated Usenet group sci.physics.research. Everyone agrees that it's *consistent* with general relativity, in which the speed of gravity is c; but many big names in the field now say that he hasn't actually measured anything depending on the propagation speed of gravity. The sticking point seems to be the precise nature of the alternative hypothesis that he's testing against, and there are many subtleties.

Kopeikin's findings are being contested by authorities such as Clifford Wills, reportedly one of the world's foremost experts on experiments proving SR and GR. Even prior to release of the findings to the general media, Wills rejected Kropeikin's paper on the observations during peer review of the Astrophysics Journal. Print media such as The New York Times and most others have failed to update the story, but it appears likely that Kopeikin's findings will not stand. See updates in (the journal) nature.com, Jan. 16 and in NewScientist.com, Jan. 17. Physicist Peter Van Nieuwenhuizen at Stony Brook calls the findings "complete nonsense." (Nature)