Does Gravity Have Speed?

[Mardav]

Thanks for responses to my queries about galaxies. We now have posed a new question regarding the nature of gravity.
If the Sun was instantly taken away, would the planets instantaneously fly off, or would their orbits disintegrate in relation to their distances from the Sun. In other words, does gravity have speed of action comparable to the speed of light?

[steadystate]

Mardav,
Wouldn't it depend on just how you removed the Sun?
Gravity is slow acting, so I suspect it would take quite some time for the planets orbits to change. The new center of mass would be somewhere around Jupiter, with Saturn having the next biggest "impact". Maybe all of the planets would spiral into Jupiter, and make a tiny sun out of it? But this would take millions of years.

Then Jupiters Moons could bring forth new life!!!!

------------------
Steady State


[This message has been edited by steadystate (edited December 02, 1999).]

[Plato]

This topic has already been discussed some time ago now, may I point you to : http://www.exosci.com/ubb/Forum1/HTML/000016.html
Hope you find it interesting...

------------------
"If I have been able to see further, it was only because I stood on the shoulders of giants."
Isaac Newton

[ilgwamh]

I may be wrong but I'm almost certain that, according to the standard model, gravity works at the speed of light. It would be about 8 minutes until the earth flew off on a tangent if the sun just disappeared. But we would still see the sun during this 8 minutes. What would the planets do? I think it all depends on +their location when this happens.

Vinnie

Praise Jesus!!!

[Weitzel]

That's correct. Gravity travels at the same speed as light; it'd be about eight minutes before we noticed the effects of the Sun being removed.

Gravity waves are quite well established, though gravitons and quantum theories of gravity are not so much.

[samus]

gravity works instantly, because it is not a force, it is motion. more specifically, it is our motion through a curved fourth dimension. analyzing it by force and speed would imply that if you were to release something, it would take a small amount of time for it to begin dropping. this is not the case, as gravity acts instantaneously on all objects (in answer to your original question).

imagine a sphere, like the earth. if you were to send two objects off in different directions on the surface of that sphere, eventually they would come back together, not because there is some force pulling them together, but because that's the natural path of motion for both of them. now, flatten that sphere as you would a map of the world. it would appear from this vantage point that the two objects were being pulled towards each other, when in fact this is simply how they follow a straight path through an unseen dimension. this explains why all things fall at the same rate (9.8 m/s^2).

why are things with more mass heavier? good question, self. this is because when you carry something such as a book, you are actually accelerating it from it's natural path of momentum. because of the curvature of fourth dimensional space, the book's inertia would bring it down. but you exert sufficient force to prevent that path of motion, which is why you would get tired holding a heavy book in the same place for a long time, even though it seems to you that you haven't moved it at all.

in fact, the way we simulate the weightlessness caused in space is by flying a plane almost straight down at the earth (side note: the plane is called the "vomit comet"). also, the way to simulate gravity in a weightless environment is to accelerate at exactly 9.8 m/s^2. you would never know the difference. buy samus brand simulated gravity! just like the real thing!

[Crisp]

Hi Samus,

gravity works instantly, because it is not a force, it is motion. more specifically, it is our motion through a curved fourth dimension. analyzing it by force and speed would imply that if you were to release something, it would take a small amount of time for it to begin dropping. this is not the case, as gravity acts instantaneously on all objects (in answer to your original question).

even though I have no objections against the postulate that gravity reacts instantly (if someone can raise objections, then please mention there here ). I do have some doubts about your "proof"/conclusion for gravity interacting instantly. When you release something, it immediatelly starts falling because gravity is already working on it (you're just preventing it falls by exerting an equally large force on the object by holding it in your hand) - this has nothing to do with the speed of the interaction.

when in fact this is simply how they follow a straight path through an unseen dimension. this explains why all things fall at the same rate (9.8 m/s^2)

Hrm.. and I always thought all things on earth fall with an acceleration rate of 9.8 m/s^2 because of the earth's mass & distance to the center of mass...

which is why you would get tired holding a heavy book in the same place for a long time, even though it seems to you that you haven't moved it at all.

Hrm... I would go for the more simple explanation: gravity adds (negative/positive) potential energy to the book. You can do two things now:

• If you do nothing, the book itself will add kinetic energy (it accelerates) to return to its inital state of total energy zero.
• Or you can add potential energy yourself (= exert a force to keep it in place) that cancels the gravitational potential energy, giving the desired energylevel zero

This energy comes from your body, and whenever your body gives off energy (in the form of your muscles doing work on the book) it gets tired.

Bye!

Crisp


------------------
"The best thing you can become in life is yourself" -- M. Eyskens.

[This message has been edited by Crisp (edited January 05, 2000).]

[samus]

chris, i'm going to pretend that me and stephen hawking (and every other noted scientist on the subject for that matter) know what we are talking about here and try this one more time.

first, the curvature of time/space in the fourth dimension is a distortion caused by mass. this is why the motion is in the direction of that mass. larger mass causes a larger curve, ass seen by the earth, the moon, the sun, etc. but the size of this curve is decided by total mass, and not the masses of individual objects. in other words, large objects fall at the same rate as small objects on the same planet.

second, as just stated, all objects fall at the same rate on earth. if it were a force, then objects with larger mass would fall faster than those with smaller mass. this does not occur.

third, gravity requires no medium to work. all forces require a medium with which to transmit their energy, yet gravity works even through empty space.

fourth, gravity works the same through all mediums. there is no substance that can block gravity. if it were a force, physics would predict that there should be some substance which you could place between yourself and the center of earth to prevent or change the effects of gravity. there isn't.

fifth, gravity works the same on all things regardless of molecular makeup. if gravity were a force, this would require something in matter which attracts it to other matter. physics would then predict that some substances would have proportionally more (and some less) of this than other substances. but we see that in ALL cases, an object's weight ALWAYS corresponds with the force required to move that object 9.8 m/s^2.

and finally, we can reproduce gravity's effect perfectly through acceleration and gravity can be used to perfectly simulate acceleration. yet no force in the universe acts as gravity does.

i'm telling you, gravity is natural motion.

[Mardav]

Samus - Thanks for your clear and very convincing argument that gravity is NOT a force. Makes sense - but school physics courses teach that gravity IS a force. Perhaps we should get this corrected.

[Crisp]

Hi samus,

second, as just stated, all objects fall at the same rate on earth. if it were a force, then objects with larger mass would fall faster than those with smaller mass. this does not occur.

Could you explain why objects with a larger mass would fall faster if than those with smaller masses if you consider gravity to be a force ?

third, gravity requires no medium to work. all forces require a medium with which to transmit their energy, yet gravity works even through empty space.

And so do electromagnetic interactions. I don't think the requirement of a medium is a valid reason for an interaction not being a force.

Bye!

Crisp


------------------
"The best thing you can become in life is yourself" -- M. Eyskens.

[retlaw]

hello mr Crisp

Although I agree with your point that gravity is not a force. Your high school physics is lacking.

F=ma

a=F/m

In your hypothetical a larger mass would be accelerated at a slower rate.

Also the speed of gravity is equal to or greater than 20,000,000,000 x C. (According to Metaresearch.org) Although I would have accepted your premise that it is instantaneous.

Goodbye mr Crisps

[MacGyver1968]

You just replied to an 11 year old post.

[Dywyddyr]

You just replied to an 11 year old post.

And badly at at that!

Also the speed of gravity is equal to or greater than 20,000,000,000 x C. (According to Metaresearch.org) Although I would have accepted your premise that it is instantaneous.

Neither:
What the MetaResearch article actually says:

These causality problems would be solved without any change to the mathematical formalism of GR, but only to its interpretation, if gravity is once again taken to be a propagating force of nature in flat space-time with the propagation speed indicated by observational evidence and experiments: not less than 2x10¹⁰c

And note that that was written in '98.
More up-to-date information:
http://www.newscientist.com/article/...-revealed.html
http://www.nrao.edu/pr/2003/gravity/
Or even:
http://en.wikipedia.org/wiki/Speed_of_gravity

So far the speed of gravity is considered to be approximately c.

[wlminex]

. . . then we cannot talk about the "force" of gravity? . . . . isn't gravity (of earth) an 'acceleration' (d/t/t, e.g., 32 ft/sec/sec)? . . . I guess if you take the first derivative of acceleration, you get 'speed' . . . at a particulr point in time. So, how do we think (know=proven) that the 'speed' of gravity is c??

wlminex

[phlogistician]

hello mr Crisp

Although I agree with your point that gravity is not a force. Your high school physics is lacking.

F=ma

a=F/m

In your hypothetical a larger mass would be accelerated at a slower rate.

Except any schoolboy knows that:



so if we replace that F with m1.a, we can remove m1 from both sides of the equation, showing that the acceleration is related to m2.

IE, all m1's (dropped objects) will accelerate towards m2 (Earth) at the same rate, etc.

[phlogistician]

. . I guess if you take the first derivative of acceleration, you get 'speed' . . .

Er, no, you have that backwards.

The first derivative of speed is the rate of change of speed, ie, acceleration.

You would integrate acceleration to find the area under the curve, ie, the speed.

[wlminex]

. . . . been a while since I had calculus . . . .

[Walter L. Wagner]

Er, no, you have that backwards.

The first derivative of speed is the rate of change of speed, ie, acceleration.

You would integrate acceleration to find the area under the curve, ie, the speed.

Or conversely, the first integration of acceleration (with respect to time) is speed, and the first integration (with respect to time) of speed is distance.

[nietzschefan]

. . . then we cannot talk about the "force" of gravity? . . . . isn't gravity (of earth) an 'acceleration' (d/t/t, e.g., 32 ft/sec/sec)? . . . I guess if you take the first derivative of acceleration, you get 'speed' . . . at a particulr point in time. So, how do we think (know=proven) that the 'speed' of gravity is c??

wlminex

It doesn't make sense to me either. What about black holes?

[phlogistician]

. . . . been a while since I had calculus . . . .

Me too, like over 20 years.