# The Naked Scientists Forum

### Author Topic: what is the speed of Gravity  (Read 6044 times)

#### Lampiao

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##### what is the speed of Gravity
« on: 31/03/2003 14:03:18 »
(1) Recently (New Scientist I think) it was reported that scientists had measured the speed of gravitational propogation by some rather complex method involving the gravitational lensing of light around a distant galaxy, or some such proceedure.
Why such a complex method?
In the 1970's I believe that the Apollo astronauts had dropped of lazer redlectors on the moon so that the moons distance can be measured to a few meters distance.
The moon (and earth) orbit around the common centre of mass which is 1/80th the distance from the earth to the moon (because the moon is 1/80th the mass of the earth). Which is 3,000 miles from the earths centre, and 237,000 miles from the moons centre.
The earth travels around the sun at 18.5 miles per second.
So the information about the common centre of mass takes  about one and a half seconds to propogate to the moon at the speed of light, in which time the earth has moved some 27 miles along it's orbit.
Hence is gravity propogates at the speed of light, then the orbit of the moon is 27 miles out from the expected position - and this "error" is along a tangent of the circle comprising of the earths orbital path
(2) If gravitational propogation is at the speed of light - what about abberation (as occurs with light) - the moons orbital speed will make the gravity as if it comes from a slightly different direction -this will act to "increase" very slightly the orbital speed - why is the moon still with us?
(3)gravity is a two way thing - the moon pulls on the earth as the earth pulls on the moon - but if there is a 1.5 second time difference due to the propogation speed between the two objects, the angle at which the gravity pulls each is slightly different due to where each body "sees" the other at that time- that angular difference will lead to a couple - again the resulting force will have a component not in direct line with the true mass centres.
[?]
« Last Edit: 19/05/2007 00:36:25 by ukmicky »

#### NakedScientist

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##### Re: what is the speed of Gravity
« Reply #1 on: 01/04/2003 20:17:31 »
A good point well made, but does the fact that both objects are spheres and essentially homogeneous serve to prevent the 'couple' effect you mention. In other words the gravitational field is constant, albeit slightly bent ?

What do you think ?

#### Lampiao

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##### Re: what is the speed of Gravity
« Reply #2 on: 02/04/2003 08:50:28 »
Regardless of the moon/earth being a sphere the gravitational attractions acts as a vector force with a definate direction as if the mass was totally concentrated at a point. F=GmM/d^2

#### NakedScientist

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##### Re: what is the speed of Gravity
« Reply #3 on: 02/04/2003 16:39:29 »
But clearly, as you say above, the moon is still there, and I'm not aware of any data that says the moon is slowly escaping from its orbit, so for a stable situation to exist there must be a force countering the couple applied by the orbital velocity - do the movement of the oceans on the planets surface have an impact ?

TNS

#### chris

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##### Re: what is the speed of Gravity
« Reply #4 on: 06/04/2003 16:22:46 »
On the subject of the moon - do you lot believe that the moon landings actually took place? There was a rather disturbing documentary on C4 about how the whole thing was 'staged'. Evidently approaching the moon is not as easy as one might think owing to large amounts of debris in space. Does anyone know any more about this?

Chris

#### Lampiao

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##### Re: what is the speed of Gravity
« Reply #5 on: 07/04/2003 08:50:05 »
Yes the Moon is still there - so this implies that gravity, or it's effects are transmitted instantaneously, not at the speed of light.
If gravity was propogated at light speed then I would expect to see the couple effect and abberation. Have I got it wrong.
Why has it been so difficult to obtain a gravitational propogation value?

#### Quantumcat

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##### Re: what is the speed of Gravity
« Reply #6 on: 21/04/2003 14:48:25 »
About the staged thing .... surely something as gigantic as the first man on the moon could not have been "covered up" as it was. So amny people were involved, I don't think that all of them could have kept thir respective mouths shut ... I'm sure you'l find that "documentary" was just shown as entertainment.

#### Donnah

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##### Re: what is the speed of Gravity
« Reply #7 on: 21/04/2003 17:34:11 »
If, as Lampiao suggests, the astronauts left laser reflectors on the moon in the 1970s, anyone with an adequate telescope should be able to see for themselves evidence that at least one moon landing took place[?].

I do remember a movie, many moons ago, that suggested the whole thing was staged.  An interesting premise and an entertaining story, but in reality just a Hollywood fabrication (complete with the big bad government[}:)] trying to kill the astronaut protagonist whistle-blower[8D]).  Unfortunately I can't remember the title.
« Last Edit: 21/04/2003 17:35:00 by Donnah »

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##### Re: what is the speed of Gravity
« Reply #7 on: 21/04/2003 17:34:11 »