Hi All

Maybe someone has an answer to this.
How fast is gravity?
In other words if the sun vanished, how long before we on earth would feel this. Would we feel it instantly or can gravity only travel as fast as the speed of light so would we have 8.5 minutes or there abouts before we would feel the suns absence

The speed of gravity is the same as the speed of light, apparently - see here for an article on this.

I wondered about this... is there such a thing as a 'speed of gravity'? It is, after all, an invisible attravtive force between ANY object and another.

I often thought that the speed of gravity was dependant upon the bodies interacting. My reason being, on Earth, (acceleration due to) gravity is 9.8m/s-m/s. On the Moon, it is different, as on any other larger-than-me body, anywhere else. Would this figure remain proportional to my distance if I was e.g. 1,000 miles above Earth? If I head in a straight line towards e.g. the Sun, I will meet the La Grange point whereby my attraction to the Earth is equal to the Sun. Then, go a little farther again, and the attractive force towards Earth is outnumbered by the Sun, therefore I will be attracted from then on by the Sun.

Confusing really!

How fast is gravity?
In other words if the sun vanished, how long before we on earth would feel this. Would we feel it instantly or can gravity only travel as fast as the speed of light so would we have 8.5 minutes or there abouts before we would feel the suns absence

Yep, its the speed of light and we'd have to wait for 8.5 minutes before it got dark and we went flying in a tangent.

I wondered about this... is there such a thing as a 'speed of gravity'? It is, after all, an invisible attravtive force between ANY object and another.

I often thought that the speed of gravity was dependant upon the bodies interacting. My reason being, on Earth, (acceleration due to) gravity is 9.8m/s-m/s. On the Moon, it is different, as on any other larger-than-me body, anywhere else. Would this figure remain proportional to my distance if I was e.g. 1,000 miles above Earth? If I head in a straight line towards e.g. the Sun, I will meet the La Grange point whereby my attraction to the Earth is equal to the Sun. Then, go a little farther again, and the attractive force towards Earth is outnumbered by the Sun, therefore I will be attracted from then on by the Sun.

Confusing really!

You seem to be confusing the acceleration due to gravity with the speed of gravity, totaly differnt things. The way to look at it is this. If the mass of an object chagnes, at what speed with the effect of that change propagate. A dramatic example would be if the sun where to dissapear, how long would it take for the graviational field as felt at the earth to see the effect of this change and for earth to shoot off into space?

If you believe Einstein (and lets face it there are good reasons to do so) then the answer is C.

BB

Thanks lads. Just one more question..

Is gravity
a wave
a particle
a wave and particle
a warp in spacetime
or all of the above?

Thanks lads. Just one more question..

Is gravity
a wave
a particle
a wave and particle
a warp in spacetime
or all of the above?

It is certainly a warp in spacetime and hence there is also such thing as a gravity wave (a moving ripple in spacetime) but I'm not at all sure there is a graviton. Time will tell!

Gravity usually travels at the speed of light.
If the change in the gravitational field is weak to middling in strength then it updates itself at the speed of light.
If it is a very drastic or "sudden" change, the word speed isn't that well defined. In which case there is no speed of gravity.

Thanks for all the help lads.

Son Goku just on your last comment, are you saying that if the sun dissappeared,(drastic) that gravity would not travel at the speed of light. I dont get what you mean by speed isn't well defined..Speed is surely the distance traveled over time...

There is no need to speak of the Sun hypothetically 'vanishing' instantaneously. It is actually losing mass at the rate of around 4 million tons every second, converting it to c^2's worth of energy per ton. In theory, this will not be continuous, but in quantum amounts, and each quantum change in the Sun's mass, and therefore gravitational pull, will propagate at the speed of light.

Consider if you had 2 probes in orbit around the Sun, one close in, the other much more distant. Say they both can measure their orbital motion (and accelleration) accurately enough to calculate the Sun's mass from Kepler's law. The inner probe transmits this mass information to the outer one. At the instant the outer one receives the data, it compares the Solar mass so received with its own calculation at that moment. Now the Sun's mass has reduced in the time taken for the signal to travel between the 2 probes, and hence between the 2 calculations, but if gravity only travels at the same speed as the signal then the orbital motion of the 2 probes should be related to the same Solar mass. On the other hand if gravitational changes were propagated instantaneously then the calculation performed by the outer probe would give a result of a lower Solar mass.

Hi Trevor,

There was some discussion on gravity here recently at www.irishastronomy.org/boards/viewtopic.php?t=4050 which you might find interesting if you havent read it already.

Cheers,
~Albert

Son Goku just on your last comment, are you saying that if the sun dissappeared,(drastic) that gravity would not travel at the speed of light. I dont get what you mean by speed isn't well defined..Speed is surely the distance traveled over time...

If the Sun disappeared gravity would travel at the speed of light as it isn't a drastic enough process.

For processes that are drastic enough speed isn't defined because how can you say how fast a piece of spacetime is travelling?
Speed is distance travelled divided by time taken. However this concept makes no sense in relativity because we can't separate out what is space and what is time, there is only spacetime.

A gravitational wave is a travelling "redefinition" of time and space.