I was just wondering if gravity was instintanious or not? Ie, if the sun suddenly vanished would all the planets fly off into space instantly or would it take a few minutes for the lack of gravity to reach the earth. Does gravitational interaction have a speed? Is it faster than light?
Sorry if I unitentionally killed any kittens…
It is largely believed that gravity travels at the speed of light. However, it’s insanely hard to do experiments to confirm this prediction. Within the last several months, a measurement was conducted which the experimenters claimed was inconsistent with instantaneous gravity, and consistent with a gravity speed of c. It involved measuring the light aberration from a quasar due to Jupiter’s gravitational field. However, their results have not been totally accepted.
It travels at the speed of light. It was measured directly for the first time recently:
http://www.space.com/scienceastronomy/gravity_speed_030107.html
Thanks !!
Tee Hee!
Mmmmmmmmmm…gavititties!
They tried to measure the speed of gravity. Some folks believe they just measured the speed of light by mistake, giving a fairly predictable result.
I think the jury’s still out.
I remember reading somewhere, quite a while ago, that an experiment had been done where the precise location of the sun’s centre of gravitational attracton (from Earth’s point of view) was measured, and it was found to be 8.whatever minutes “in front” of the apparent visible centre of sun. The conclusion was, of course, that gravitational force must propagate instantly (or at least much much faster than light).
Does anyone else know about this? Is it for real, or crackpot ramblings (possible, since I frequent a number of science news sites that occasionally link to crackpotty stuff)? If it’s for real, it would seem to settle the issue…?
Maybe there is disagreement about that experiment, but I think most scientists agree that gravity travels at c. We have observations that show binary pulsars losing energy through gravitational wave emission. As far as I know, the theory behind gravitational waves is pretty well established, and it predicts that gravity propagates at c.
Considering the accepted equation for gravity
F = G * m1 * m2 / r^2
Would the measured rate of gravity not depend on how you changed one of the variables?
For the space.com article, they measured how light was bent by
Jupiter. To me that seems like its begging the question. The light was bent at the speed of light? The original post asks about the sun instantaneous disappearing, but that’s not possible. In my logic (however flawed) the force measured is going to decrease as fast or as slow as the mass is changing.
In my opinion, Kopeikin and Fomalont have failed to address this issue.
Gravitational waves were predicted by Einstein’s General Theory of Relativity (capitals or not?) and there is experimental evidence to support their existence. However, their speed is not necessarily that of light. It is unlikely to be higher, but could be less due to other effects.
That would be the case if the graviton had a rest mass, for instance. I don’t know if anyone has looked into this. I certainly don’t know how you’d go about it. But you can’t assume it to be zero without evidence. It now that it seems likely that the electron neutrino has a rest mass and a lot of people had assumed not.
Also, various theories suggest that gravity may not “follow the same contours of space” as light. i.e. it may take a different route from A to B, resulting in a different perceived speed.
Take a look at http://www.livingreviews.org/Articles/Volume4/2001-4will/node29.html
The thing is, there’s an explicit time dependence that you left out. You have to use the values of m1 and r at the retarded time, not the current time, if gravity travels at a finite speed.
Here is some news in support of the assertion that what they measured in September proves nothing definitive. Just thought I’d update. The speed of gravity is one of those remaining great big questions. Another favorite of mine is the debate over whether or not information is destroyed inside a black hole. I also am waiting for instant propagation of information to be achieved with that “spooky at a distance” stuff Einstein was scared of. . .
DaLovin’ Dj
Relevant quote:
Wiggle your gravitities at me baby…
Hey, I didn’t name the thread. . .
. . . but I can’t resist! The speed of Gravitity can be expressed using the following formula:
E = MC-Cup
*dalovindj is correct. Gravitity is the force that acts on breasts over time, pulling them downward.
There is a significant equation which is:
BR/A = antigravitity.
Yes, people have considered this and there are good observational limits on a graviton mass as a result.
Quoth puggyfish:
Fact of the matter is, we don’t know for certain that any particle, photons included, has exactly zero mass. The best we can do is to put upper bounds on the mass. For instance, the photon has a mass less than 210[sup]-16[/sup] eV (for comparison, an electron is 5.1110[sup]5[/sup] eV, and the neutrinoes are believed to be around 1 eV or so). It so happens that our current best bounds on the graviton mass are nearly a million times lower than our best bounds on the photon mass, at around 5*10[sup]-22[/sup] eV. But then again, the gravitational waves we typically deal with have much lower frequencies than the electromagnetic waves we typically use, so it’s still possible for the graviton mass to be more significant than the photon mass, even if it’s smaller.
As for assuming particles to be massless: Up until a few years ago, the best we knew about neutrinoes was that their mass was less than about 3 eV (some folks disagreed with those experiments, in which case the bound was about 20 eV). It’s simpler to assume zero mass, than it is to assume that they have a mass a hundred thousand times less than the next-lightest particle, so that’s what we assumed. Now, of course, it turns out that they do have a mass, so we have to change those assumptions. The same argument holds true for the photon and the graviton: Barring any evidence to the contrary, it’s simplest to assume that they’re massless. We can’t prove it yet, and we’ll probably never be able to prove it, but it’s reasonable.