The speed of gravity?

JackKelly

Ok, firstly, my knowledge of physics is limited to the leaving cert (in 6th year at the mo) so what i say mightn't make any sense! But anyway, obviously if you let go of an object (on earth) it will fall due to gravity,but is there not a speed in to which the "gravity" reaches the object and forces it down?Or even a minute delay from the time a person lets go of the object,and the time gravity takes over.(although i presume the force of gravity is constanly there, so there wouldn't actually be a delay, no?).

So, presuming gravity works at the speed of light -and im totally out of my depth here- and it's also true that gravity has the abillity to slightly bend light. But if the two are travelling at the speed of light, how can one have an effect on the other?

Well anyway,can anyone help me out here?

[Edit]lol, when i say, the speed of gravity, i mean, the speed of its effect i suppose, not 9.8m/s![/EDIT]

Sangre

Well, gravity doesn't have a speed afaik, it just simple is. You said that is there a delay between when you let go and gravity takes over but you must remember gravity never stops having an effect on something, its just that when you let go the force you exert on it is gone and the force of gravity brings it to earth. The earth doesnt reach up and grab it with gravity and pull it back down.
I think the best way to explain it is that gravity is everywhere, it doesnt have to move anywhere so it does not have any speed.

Of course, my knowledge is limited to an a1 in physics so this is just what i would see as a logical result.

David19

I wouldn't have as much knowledge of physics as other people here but anyway.
There's no speed that an object has to attain for gravity to affect it. Your computer is at zero speed and gravity is affecting it. Its staying on the desk, not floating off into space.
There's no delay either, gravity is there all the time.
Gravity doesn't have any speed, it doesn't go at the speed of light.

Zulu

TimAy wrote:

obviously if you let go of an object (on earth) it will fall due to gravity,but is there not a speed in to which the "gravity" reaches the object and forces it down?

No - not really. Gravity is always acting as a force on the body in question. If you are holding the object - you create a force equal but opposite to gravity until such time as you release the body. Then the only force is gravity, and the object starts to accelerate towards the ground.

TimAy wrote:

Or even a minute delay from the time a person lets go of the object,and the time gravity takes over.(although i presume the force of gravity is constanly there, so there wouldn't actually be a delay, no?).

No delay. As soon as the opposite force is removed the body starts to accelerate.

TimAy wrote:

So, presuming gravity works at the speed of light -and im totally out of my depth here- and it's also true that gravity has the abillity to slightly bend light. But if the two are travelling at the speed of light, how can one have an effect on the other?

Ok, you're getting deeper into this now. Depending on the way the forces are acting, a massive gravitational pull (say a black hole) can alter the path of a "packet" of light. The fact that they are travelling at the speed of light is irrelevant at this point. It's all about the directions. If this massive gravitational force is acting at right angles to the light, then it would pull the light in that direction.

Hope this helps...

JackKelly

that would mean, that gravity is built in to the fabric of the universe no?
which would mean that, if gravity was just there -a part of the universe- everysingle piece of matter would would be stuck together. I know that , in theory, everything in the universe IS infact attracted together.

Also,the force of gravity diminishes (sp?) with distance, so would that not have something to do with the gravity having to travel to an object, instead of just being there?

When i say the speed of gravity, i mean for example, say a planet was just created - out of the blue- in space (stupid, i know!), now what would be the time (minute) that any object around it, would take to be grasped by its gravity and pulled towards it?

im confused

daveirl

This post has been deleted.

Sangre

TimAy wrote:

Also,the force of gravity diminishes (sp?) with distance, so would that not have something to do with the gravity having to travel to an object, instead of just being there?

No, think of gravity as an elastic band between two objects, any two objects in the universe. As the distance is increase the tension of the band is reduced therefore so is the attraction between them until it is negligible

When i say the speed of gravity, i mean for example, say a planet was just created - out of the blue- in space (stupid, i know!), now what would be the time (minute) that any object around it, would take to be grasped by its gravity and pulled towards it?

Instantenous, think of it like dropping a round ball into a pond, the water would be displaced immediately and displacement would be immediate and so would the affect of gravity. Of course no one has ever done this or will ever because you can't create matter out of nothing. It can only be theory.

Crucifix

It may help to imagine gravity like a field around something with mass. And the further you get from said mass the weaker the field is. So it isn't really travelling.
As for how long it takes gravity to interact with a new particle/planet, instantaneous I suppose.

JackKelly

Zulu wrote:

No - not really. Gravity is always acting as a force on the body in question. If you are holding the object - you create a force equal but opposite to gravity until such time as you release the body. Then the only force is gravity, and the object starts to accelerate towards the ground.

Yea, i getcha. But say that, instead of considering gravity as always being there (which i know it is), if you compare gravity to the light from the sun, if the sun were to instantaniously stop giving out light,the light would take however long to reach use(because of the limit of the speed of light),giving a difference in time from when the sun actually stopped giving out light, and to when WE see the sun stopping in give out of light. So if it were the same with gravity, say the sun dissapeared!would the time it takes for the force of the sun acting on the earth to dissapear (or the gravity to stop having an effect on us) be instantaneous, or the same time as the light stops?

{edit}just thinking, wouldnt that mean, if the sun were to - in theory- disappear, we would spin out into space, and out of orbit, before we even saw the sun disappearing?{edit}

sorry, if that is hard to understand!

Sangre

I get what you're trying to say but you can't compare the scenarios.
You can make light stop so you can measure the speed.
But can not just make mass disappear or appear, we're pretty much stuck with what we've got in the universe so any affect gravity has is not going to change.

Of couse Im sure there is some quantom law contradicting this
Think there is a Physics teacher who posts on boards somewhere.

Crucifix

AFAIK, as soon as the mass is gone, the gravity is gone.

David19

TimAy wrote:

Yea, i getcha. But say that, instead of considering gravity as always being there (which i know it is), if you compare gravity to the light from the sun, if the sun were to instantaniously stop giving out light,the light would take however long to reach use(because of the limit of the speed of light),giving a difference in time from when the sun actually stopped giving out light, and to when WE see the sun stopping in give out of light. So if it were the same with gravity, say the sun dissapeared!would the time it takes for the force of the sun acting on the earth to dissapear (or the gravity to stop having an effect on us) be instantaneous, or the same time as the light stops?

sorry, if that is hard to understand!

It'd be instantaneous. Gravity doesn't travel. Its always around an object like a planet or star.
Also in your previous post, everything doesn't stick together because gravity is actually the weakest of the four main forces over a short distance.

Sangre

Ok think of it this way, if the mass could disappear, it would just be like a force being removed, it would be like your hand holding a ball let go, all the other forces start acting on it.

Say two people are pulling a ball by strings two miles long, so there is me pulling a string two miles long with a ball on the end connected to another 2 mile string being pulled by B. If B disappears we dont have to wait for his 'force' to catch up, I just suddenly fall on my ass because there is no oppossing force.

See what I mean? Its not like gravity has to catch up, it just stops being, the other forces continue as normal and the masses move accordingly.

JackKelly

i still dont quite get it, at some stage, was that mass not created from energy? (big bang), so at some stage, at the moment the mass was created - did the gravity not have to travel?

Say, then, (again, this is probably contracdicting some obvious rule) that the universe was matterless. No planets or anything. Then gravity wouldnt be there no?So gravity only rears its head inthe presence of mass?So where is it the rest of the time? Would it have to be buried in "the fabric of the universe", i dunno, like a dimension or somthing?

NotMe

TimAy wrote:

that would mean, that gravity is built in to the fabric of the universe no?
which would mean that, if gravity was just there -a part of the universe- everysingle piece of matter would would be stuck together. I know that , in theory, everything in the universe IS infact attracted together.

Yes, exactly. For every two bodies/atom/particles in the universe gravity pulls them together.

Also,the force of gravity diminishes (sp?) with distance, so would that not have something to do with the gravity having to travel to an object, instead of just being there?

No, that just means that for example there is a greater force of attraction between you and the Earth than between you and the Sun.

When i say the speed of gravity, i mean for example, say a planet was just created - out of the blue- in space (stupid, i know!), now what would be the time (minute) that any object around it, would take to be grasped by its gravity and pulled towards it?

im confused

Well that's impossible so who can say. In reality planets are created by gravity - rocks and dust etc are pulled together to form the planet.
If a planet did just appear out of nowhere then I assume it would have an immediate effect on everything around it.(everything in the universe in theory).

The most important thing to realise is that gravity is a force between two objects, not a force that one object has on another. ie. you are pulling on the Earth as much as the Earth is pulling on you. It's just that the Earth is much bigger (greater mass) so you move towards it and not vice versa.


*edit* I see there's been 9 more replies since I started typing this

JackKelly

Sangre wrote:

Ok think of it this way, if the mass could disappear, it would just be like a force being removed, it would be like your hand holding a ball let go, all the other forces start acting on it.

Say two people are pulling a ball by strings two miles long, so there is me pulling a string two miles long with a ball on the end connected to another 2 mile string being pulled by B. If B disappears we dont have to wait for his 'force' to catch up, I just suddenly fall on my ass because there is no oppossing force.

See what I mean? Its not like gravity has to catch up, it just stops being, the other forces continue as normal and the masses move accordingly.

(lol, didnt imagine this board would move so fast)

Yea, i never looked at it that way.Again though, is that not limited by the speed of light. If you could see the other person, would you not fall , before seeing him dissapear?

Sangre

TimAy wrote:

i still dont quite get it, at some stage, was that mass not created from energy? (big bang), so at some stage, at the moment the mass was created - did the gravity not have to travel?

Say, then, (again, this is probably contracdicting some obvious rule) that the universe was matterless. No planets or anything. Then gravity wouldnt be there no?So gravity only rears its head inthe presence of mass?So where is it the rest of the time? Would it have to be buried in "the fabric of the universe", i dunno, like a dimension or somthing?

No, it doesnt have to travel, ever. Its not something that can travel, it doesnt have mass to move, it is just a force. Look again at my example instead of thinking about planets.

I would assume that gravity is always there as a force but its effects are only visual once mass is put into play. Stop trying to confuse yourself with matterless universes, you might aswell start studying the physics of planets in an alternative universe where gravity is reversed or something.

NotMe

TimAy wrote:

(lol, didnt imagine this board would move so fast)

Yea, i never looked at it that way.Again though, is that not limited by the speed of light. If you could see the other person, would you not fall , before seeing him dissapear?

Yeah if the sun was to just disappear we would feel the effects of the loss of gravity before the seeing the light disappear.
(I put that really badly )

*edit* It takes ~8 minutes for the light to get from the sun to the Earth.

Sangre

TimAy wrote:

(lol, didnt imagine this board would move so fast)

Yea, i never looked at it that way.Again though, is that not limited by the speed of light. If you could see the other person, would you not fall , before seeing him dissapear?

Well I cant see 4 miles so I couldnt tell you
Yes, seeing him is irrelevant, it would be like saying I wouldn't fall because the lights are off and I cant see him. What happens is that his force stops, so the force of gravity from the earth becomes the strong force and I fall on my ass. There is no time between this.

It is not limited by the speed of light because it is not traveling, think of gravity as a spiderweb in the universe connecting everything. It doesnt move between objects.

Capt'n Midnight

Speed of Gravity eh ?
The acceleration of the earth caused by the sun's gravity is directed at where the sun is, not where it was 8 minutes ago. this means that gravity is much, much faster than light otherwise you would be able to detect the angle between the sun is and where it's gravitational effect appears to come from.

Zulu

Gravity isn't light. It is a force, like friction. It dosen't move anywhere, it just is. If there is mass - there is gravity.
A body has a mass. It dosen't take time to get that mass - it just has it.
Think of gravity as similar - it just acts on a body.

chakotha

From undergrad physics and Big Bang books I think Einstein's General Relativity explains gravity not so much as a classical force like electric field but as deformities in the structure of space and time in the vicinity of mass.

So there should be no actual speed of interaction - spacetime at the surface of the earth is already curved in the favour of the earth as the Earth is by far the body with more mass than any small loose objects above the surface - so a small light object when dropped will follow the path of least resistance and move along the curve in spacetime towards the Earth

Abdiel

As somebody already mentioned Gravity is a force that acts between two masses:

F = G*M*m/r*r

G is the universal gravitational constant, M and m are the 2 masses and r is the distance between them.

You have raised one interesting point in that if the sun instantaneously disappeared we would notice the effects gravitationally before we would actually see the sun disappear - of course all the other planets gravity would instantaneously effect us if the sun disappeared like that. Jupiter with a mass of 1000 times smaller than the sun would be the next main gravitational attractor. But we would have no heat or light so we'd all freeze anyway

Zulu

...as for the sun disappearing - it takes 7seconds (correct me here if I'm wrong) for light to travel 1 AU (distance from earth to sun).
The gravational pull would be released instantly. Earth would be slung off on a tangent (same effect as a sling-shot). We probably wouldn't notice anything for 7 seconds, but after that - it would be very dark and cold. What happens to the moon? - thats an equation I'm not bothered to work out.

As for Jupiter - it would only have a marginal effect, as it to would be slung out to space, and chances are, in an different direction.

Capt'n Midnight

Zulu wrote:

...as for the sun disappearing - it takes 7seconds (correct me here if I'm wrong) for light to travel 1 AU (distance from earth to sun).
The gravational pull would be released instantly. Earth would be slung off on a tangent (same effect as a sling-shot). We probably wouldn't notice anything for 7 seconds, but after that - it would be very dark and cold. What happens to the moon? - thats an equation I'm not bothered to work out.

As for Jupiter - it would only have a marginal effect, as it to would be slung out to space, and chances are, in an different direction.

The moon would still orbit us and we would not notice the difference since the earth has a slightly greater attraction than the sun for both. yes it would be colder (use lots of green house gases and nuclear power) and tides would be monthly rather than daily

chakotha

As somebody already mentioned Gravity is a force that acts between two masses:

F = G*M*m/r*r

G is the universal gravitational constant, M and m are the 2 masses and r is the distance between them.

Yes but that's Newton's classical equation which relativity showed is an approximation that holds true in non-extreme situations. It doesn't account for light rays being bent or black holes and the like.

Newton's equation still implies a field that is simply always there between any two objects that have mass. There is no start to the interaction. Relativity explains it in terms of spacetime curvature.

Abdiel

Indeed, glad somebody mentioned that fact i.e. gravity doesnt actually effect light as such (as it has no mass) but has an effect on light by bending the spacetime through which it travles.

chakotha wrote:

Yes but that's Newton's classical equation which relativity showed is an approximation that holds true in non-extreme situations. It doesn't account for light rays being bent or black holes and the like.

Newton's equation still implies a field that is simply always there between any two objects that have mass. There is no start to the interaction. Relativity explains it in terms of spacetime curvature.

cyberbob

Zulu wrote:

... it takes 7seconds (correct me here if I'm wrong) .

ok

google wrote:

The distance from the Earth to the Sun varies because the Earth's orbit about the Sun is elliptical.At it's closest, the distance is 91,402,000 miles and it's farthest distance it is 94,512,000 miles.This gives an average distance of 92,957,000 miles. Light travels at 186,282 miles per second. Dividing the average distance by the speed of light gives 499.01225 seconds which is 8.3168708 minutes.

Zulu

No point in getting into too much theory people - this is 6th year LC physics, we don't want to confuse the poor guy.

Suffice to say - gravity dosen't have a speed.

ecksor

My reading of physics suggested that it does have a speed and that if the sun were to suddenly disappear then we'd remain in orbit for approximately 8 minutes (not 7 seconds) before the effects of the sun's gravity disappeared. That was certainly Einstein's take upon gravity and nobody seems to have pointed out that Einstein was incorrect on this thread. Is there new evidence to contradict Einstein?

David19

ecksor wrote:

My reading of physics suggested that it does have a speed and that if the sun were to suddenly disappear then we'd remain in orbit for approximately 8 minutes (not 7 seconds) before the effects of the sun's gravity disappeared. That was certainly Einstein's take upon gravity and nobody seems to have pointed out that Einstein was incorrect on this thread. Is there new evidence to contradict Einstein?

Hmm seems like you're right.

http://www.newscientist.com/article.ns?id=dn3232