Gravity question

[tomdemaine]

Does gravity have a maximum range? Say the universe had only two planets in it both earth sized and they were 50 million light years apart. Say also that they came into existence completly stationary. Would they move towards each other and eventually collide? How about 100 million? 100 trillion? Does the gravity from every object pervade the entire universe?

Also does gravity have a speed? Say the universe has one planet then another one is created somehow x light years away. Will the gravitational effect affect on the original planet be instantaneous or will it take f(x) amount of time?

[daryn]

gravity has no maximum range. in your example, yeah the two planets would eventually collide.

speed of gravity? fast, baby. way faster than c.

[Uethym]

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gravity has no maximum range. in your example, yeah the two planets would eventually collide.

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Correct, though *eventually* could also be nearly infinite.

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speed of gravity? fast, baby. way faster than c.

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Yes and no. The effect of a gravitational field is instantaneous, but changes in that field propagate outward at c. If the Sun suddenly shifted position, it would take ~8.5 minutes for the change in the curvature of space to reach us and pull Earth in a different direction.

Otherwise, you could send information faster than c by shifting heavy weights around to send gravitational Morse code, thus violating causality and creating mass hysteria.

[Lestat]

It's easier to think of gravity as a bowling ball on a very large soft mattress. If you place a ping-pong ball in the immediate vicinity of the bowling ball, it will be pulled into the depression bowling ball creates on the mattress. However, if the mattress is large enough, it is possible to place the ping-pong ball far enough away to where the bowling ball will have little to no effect on it.

So two earth sized planets seperated by a vast distance, would have no effect on each other even if they were the only two objects in the universe.

[tomdemaine]

Three responses and three different answers so far /images/graemlins/smile.gif

Lestat,
No effect or no significant/noticeable effect? Since I created this universe I'm prepared to wait an infinite amount of time for them to collide. How about two specks of dust 100 trillion light years apart? Or two electrons?

[cambraceres]

The effects of gravity are hard to measure, much harder than the other 3 forces. Gravity weakens over distance, but it's effect never completely dissappears. If you had two particles of arbitrary mass, very small or very big, and these were the only two particles present in the universe, they would collide in time, no matter the space between.

Cambraceres

[Lestat]

[ QUOTE ]
The effects of gravity are hard to measure, much harder than the other 3 forces. Gravity weakens over distance, but it's effect never completely dissappears. If you had two particles of arbitrary mass, very small or very big, and these were the only two particles present in the universe, they would collide in time, no matter the space between.

Cambraceres

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Why?

[Lestat]

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Three responses and three different answers so far /images/graemlins/smile.gif

Lestat,
No effect or no significant/noticeable effect? Since I created this universe I'm prepared to wait an infinite amount of time for them to collide. How about two specks of dust 100 trillion light years apart? Or two electrons?

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Ok, I see you're asking a much more intelligent question that your basic: What's with gravity?

I'm not the one to ask, but from what I think I know, gravity does travel at the speed of light. Also...

I don't see why two objects can't be placed far enough from each other to where they would have zero effect on each other over an infinite amount of time. I could easily be wrong about this however. I just don't see why they would have to eventually collide just because of infinite time. But hopefully, Cambracerers will answer that.

[cliff]

The electrons won't collide, right? At some point the electromagnetic forces overwhelem the gravitational forces and they repel. So they should reach an equlibrium distance right at the point that the stronger EM repulsion exactly cancels the weak gravitational attraction? Would they orbit each other?

[traz]

Because every object creates a gravitational pull, the strength of which depends on the mass of the objects and is inversely proportional to the distance. But as long as the mass isn't 0, there will be an attraction.

[daryn]

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Because every object creates a gravitational pull, the strength of which depends on the mass of the objects and is inversely proportional to the distance. But as long as the mass isn't 0, there will be an attraction.

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and attraction = unbalanced force = acceleration. lestat = wrong

[tomdemaine]

But were you wrong/joking when you said

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speed of gravity? fast, baby. way faster than c.

[/ QUOTE ]??

[daryn]

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But were you wrong/joking when you said

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speed of gravity? fast, baby. way faster than c.

[/ QUOTE ]??

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i don't think so. it's basically instantaneous, but i think it's just so fast that it can't easily be measured by science. that is only for the field to be set up though like the guy said. the force effect would travel at c.

[MrMon]

Gravity travels at the speed of light and works over all distances. But unless you have a very large mass or a very short distance, you can generally consider the effect to be zero, even though it does in fact exist. It's just so minimal as to be not significant. However, in a universe with only two masses, obviously it is significant, as it's the only field out there, no matter how much barely above zero it is.

[Cooker]

The initial velocity of the objects is important. You can easily put 2 objects in a universe in such away in which they will never collide, but the trajectory of each will be slightly different from the case where it is alone in an otherwise empty universe.

If you are talking about large objects like the Earth or a person and start them at rest, then they will eventually collide. If you are talking about electrons then the picture can be quite different, and since we really don't have a full theory of GR and QM no one can say for sure. I am sure there are cases where one can say for sure that they don't collide, but I don't one would be able to say for sure when they do collide.

Actually, for objects like electrons, even the idea of "collision" isn't really sensible since they will definitely not stick together. An electron isn't really like a ball.

[evil twin]

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Gravity travels at the speed of light and works over all distances. (http://www.newscientist.com/article.ns?id=dn3232)

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[billygrippo]

lenghty read on speed of gravity (http://www.ldolphin.org/vanFlandern/gravityspeed.html)

[Ace of Spades00]

Yeah that is somthing to think about. I beliveve that an object needs to perform an action to say and a chain reaction is formed to bring the planets togeather or apart. But i really dont know physics. and also no i dont think it has a speed.

[tolbiny]

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Because every object creates a gravitational pull, the strength of which depends on the mass of the objects and is inversely proportional to the distance. But as long as the mass isn't 0, there will be an attraction.

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and attraction = unbalanced force = acceleration. lestat = wrong

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Doesn't the force have to be sufficient to overcome an objects inertia?

[evil twin]

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and also no i dont think it has a speed.

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You might want to read some of the links above where it catagorically DOES have a speed then.

[evil twin]

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Doesn't the force have to be sufficient to overcome an objects inertia?

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No.

[daryn]

[ QUOTE ]
[ QUOTE ]
[ QUOTE ]
Because every object creates a gravitational pull, the strength of which depends on the mass of the objects and is inversely proportional to the distance. But as long as the mass isn't 0, there will be an attraction.

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and attraction = unbalanced force = acceleration. lestat = wrong

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Doesn't the force have to be sufficient to overcome an objects inertia?

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as evil_twin said, no.. F = ma , macroscopically speaking

[Buzz-cp]

F = G*m1*m2/r^2. G = constant, m1 = mass body 1, m2 = mass body 2, r = distance between the bodies, so yah, since all variables are defined (including r), then there is a value for F.

Buzz

[Cauldron]

Gravity isn't well understood. If you managed to somehow relate it to other forces you'd be famous for centuries.

[Silent A]

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Gravity isn't well understood. If you managed to somehow relate it to other forces you'd be famous for centuries.

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It's understood well enough to answer the OP's question.

Since the objects are "completely stationary' (with respect to one another) and are "earth sized" (therefore, quantum effects are negligable), the two objects will attract each other with a non-zero force and start to accelerate towards each other along a line between their centres of mass. With a little calculus you could even calculate the time required quite accurately since the speeds involved when they finally collide will be way below the speed of light (thus no significant relativistic effects).

here's my stab at the math:

M = mass of each object (kg)
r(t) = distance (m) after time t (s)
R = initial distance (m)

d2r/dt2 = -2GM/r^2

OK, not so simple. Anyone know a solution to this ODE?