How Fast Is Gravity?

[Phanerothyme 12]

I think you might have misunderstood the question, which has been answered as well as we are able at the moment.

 

It's probably the speed of light, but given that we don't yet know how gravity is transmitted, we don't know for sure.

 

If it was as slow as that, we'd have measured it by now, surely?

 

I think that, in one way or another, gravity is propagated through something analogous to 'entanglement' - thus is instantaneous.

[Cyclone 10]

surely it is like asking the speed of the acceleration of a car, the question doesn't make sense. Or is the question "how fast does gravity act?"

The question makes perfect sense, does the force of gravity propagate instantaneously, or with a velocity. The force is what creates the acceleration, not the act of acceleration itself.

 

http://en.wikipedia.org/wiki/Gravitational_wave

 

The model of relativity predicts that gravity has a propagation speed, unlike the newtonian model which assumes instantaneous transmission.

[Cyclone 10]

If it was as slow as that, we'd have measured it by now, surely?

 

I think that, in one way or another, gravity is propagated through something analogous to 'entanglement' - thus is instantaneous.

 

How would we measure it though?

[llamatron 10]

The question makes perfect sense, does the force of gravity propagate instantaneously, or with a velocity. The force is what creates the acceleration, not the act of acceleration itself.

 

http://en.wikipedia.org/wiki/Gravitational_wave

 

The model of relativity predicts that gravity has a propagation speed, unlike the newtonian model which assumes instantaneous transmission.

 

so the latter "How fast does gravity act?"!

[truman 10]

I think it would, but my A level physics is too far in the past to quickly work it out.

 

To reverse your question though, since the light from the sun does take about 8 minutes to reach us, and if gravity were instantaneous, wouldn't we appear to be orbiting a patch of space a little bit to the side of where we see the sun???

 

I don't just mean the position but also the shape/regularity of the orbits? The planets would be "off centre" if you see what I mean..

[Cyclone 10]

I don't just mean the position but also the shape/regularity of the orbits? The planets would be "off centre" if you see what I mean..

 

But we wouldn't see it that way, because the light would also be off centre, the sun isn't actually where we see it, that's where it was 8 minutes ago.

 

The orbits aren't simple circles anyway, but I don't think orbital mechanics needs to make any allowance for the speed of propagation of gravity, but equally I don't think that's because it's instantaneous.

[Phanerothyme 12]

How would we measure it though?

The same way we measure gravity everywhere. Gravimeters?

[Cyclone 10]

so the latter "How fast does gravity act?"!

 

Yes, I suppose that is better worded, the OP made it clear what he was asking though I think even if the title wasn't quite correct.

[Cyclone 10]

The same way we measure gravity everywhere. Gravimeters?

 

I think you'll have to explain a bit more, what would be the design of an experiment that would determine the speed of gravity (using gravimeters) and what would the predicted measurements be if gravity were a) instantaneous, b) bound by the speed of light.

 

Slightly aside from the above if gravity propagates instantaneously (or FTL) then it provides a means to create paradoxes by sending information FTL. All you need to do is manipulate matter in one place, measure that manipulation gravitationally in another and you've broken causality.

[HeadingNorth 11]

The same way we measure gravity everywhere. Gravimeters?

 

What we measure is the strength of the force produced by gravity. We can't, as yet, measure the speed of the subatomic particles that cause gravity - indeed, we can't prove they exist, but it's the widely accepted hypothesis.

[Cyclone 10]

I think Phan is suggesting that we can measure the direction of gravity (we can) and compare that to the direction of the visually observed gravitational inductor (ie where the moon is). If there's a measurable difference then there's a difference in propagation speed.

 

I don't think it's practical to execute that experiment though, we can't actually see the centre of the moon and I suspect that the distance is too small, it's only a 2 second delay between the surface of the moon and the earth.

[Phanerothyme 12]

I think you'll have to explain a bit more, what would be the design of an experiment that would determine the speed of gravity (using gravimeters) and what would the predicted measurements be if gravity were a) instantaneous, b) bound by the speed of light.

 

 

Well, A bag of sugar on a set of scales will weigh slightly less when the moon passes overhead and slightly more when exactly underneath. Or the nearest laboratory equivalent.

 

So you just need very accurate masses, and very accurate scales, and lots of measurements.

 

The position of the moon can be determined to a pretty good degree of accuracy, and given the distance, it's actual position (as opposed to its observed position) will be very slightly different. This distance can be calculated, and thus you'd expect the gravitational peak (the moment of least/most weight in your standard mass) when the centre of the moon and earth line up through your experiment, visibly if gravity propagates c or slightly ahead of the observed moon if it propagates instantaneously.

 

You'd probably want to account for all the other major masses in the solar system in your calculations and expectations too.

 

This is an experiment that might better be carried out at a lunar Lagrange point rather than on the surface of the earth, where the local gravitational field will blot everything else out.

 

I just think of the rubber sheet model, when I think about gravity.

Edited February 27, 2012 by Phanerothyme