When an object pop's into existence I presume that it will begin to curve space outwards from it's centre at the speed of light. But I was wondering, how far does gravity stretch?

If a ball was to pop into existence would it's gravitational presence be felt in due time all across the universe? Or is there a limit, decided upon desity, mass, that would give a limitation to it's gravitational limit? ... For instance, if a ball has a mass of 300 hundred pounds (or whatever) would there be a limit to how for it's gravitational force reaches out into the universe, shall we say, 10 lightyears... or would it's gravitational presence bend space at the speed of light that would have implications on every object in the universe, and would thus stretch out to the very ends of the universe itself?

There's no reason to suspect that there is any theoretical distance limit.

Does it matter?

Well, why wouldn't it matter? I'm curious you see. That's the reason.

The gravitational force follows an inverse square law at large distances. i.e.

F is proportional to 1/r²

where r is the distance between two objects mutually attracting each other.

Even if r is very very large, F never quite goes to zero. Therefore, everything in the universe is attracted to everything else. The effects of gravity never go to zero, although they do become very small at large distances.

As for changes to gravity, those most likely propagate outwards at the speed of light from where the change took place.

F is proportional to 1/r^2

What happens when r^2 is 0?

edit: On second thought, maybe it would be impossible to be at r^2=0. Maybe if you were in the dead center of hollow sphere. I would think that you would just hang there, but how would it actually work? Mathematics breaks down at this point because you can't divide by zero. I suppose "you" couldn't be at the exact center of mass, because parts of you would extend beyond that precise center. But there does exist a center where r^2=0 regardless of it's size.

The gravitational force follows an inverse square law at large distances. i.e.

F is proportional to 1/r²

where r is the distance between two objects mutually attracting each other.

Even if r is very very large, F never quite goes to zero. Therefore, everything in the universe is attracted to everything else. The effects of gravity never go to zero, although they do become very small at large distances.

As for changes to gravity, those most likely propagate outwards at the speed of light from where the change took place.

Excellent. I have an idea that gravity has a dual usage! 1: To keep everything in order (NATURALLY)! 2: To record the events of everything that happens in the universe into the tapestry of space-time itself. You see, if gravity lasts forever, and nothing can catch up with the speed of gravity (the speed of light), then the events of everything in the universe will be recorded into the tapestry of space-time that will travel to the very edges of the universe in a sphere like manner.

Now the reason I brought up a ball popping in existence in my first post, is that if a ball was to pop into existence for 2 seconds, then pop out of existence. It's presence would be recorded, FOREVER unHINDERED, into the tapestry of space time. It's gravitational field would be two light seconds long in each direction from it's centre. When the object dissapears, a signal will be sent out telling space-time to straighten itself, but since nothing can travel faster than light, the two light seconds of the gravitational curves caused by the object's mass in the first place will forever travel across the universe, and in it will reside the timeline of the disappearing baseball, and possibly the actions (movements) of that ball.
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if r^2=0, wouldn't that mean that we are only dealing with a single object? besides, i recall some rule about two things not being able to be in the same place at the same time. bleh, too early. :bugeye:

The r=0 problem never arises in practice.

Take a hollow sphere of mass M and outer radius R. Measure r from the centre of the sphere outwards. Then, the gravitational field strength due to the sphere at distance r > R is:

GM/r²

where G is a constant.

But for r < R, the gravitational field strength turns out to be zero. There is no gravitational force from the sphere on any object placed inside it. The inverse square law does not hold inside the sphere.

More generally, the inverse square law only holds once you get a reasonable distance away from any object. Up close, or inside the object, the force of gravity doesn't go as 1/r².

I don't know why, but I like the idea that gravity records every event that occurs in the universe. It sounds like one of those whacky ideas that you read about in science magazines!

Can this idea be applied to more than just changing mass. What about interactions between atoms or thermodynamical processes, etc...

When an object pop's into existence I presume that it will begin to curve space outwards from it's centre at the speed of light. But I was wondering, how far does gravity stretch?

If a ball was to pop into existence would it's gravitational presence be felt in due time all across the universe? Or is there a limit, decided upon desity, mass, that would give a limitation to it's gravitational limit? ... For instance, if a ball has a mass of 300 hundred pounds (or whatever) would there be a limit to how for it's gravitational force reaches out into the universe, shall we say, 10 lightyears... or would it's gravitational presence bend space at the speed of light that would have implications on every object in the universe, and would thus stretch out to the very ends of the universe itself?

hey AntangonistMuslim, other than solar system events that pi=]urporgt to link peturbations with e force of grvtiy what evidence is there that the force of gravity extends out side what is observed to be the limit.?

If we applied quantum heory to ur observations we opwuld say that any forces exteranl to the solar system were unobserved, which is a polite way of saying the force of gravity is observably nonlocal or external to the solar system.

Theory is really nice. In fact Ilike theory. I have one problem with gravity theory, or theories. I have never heard of any solar system stellar gravity affect being mesured from the nearest star (approximately 5 light years) outward. The asssumptions are that gravity has an inverse distance limit. Stellar object external to our solar system have no experimentally current observed affects.

It is gravity mesurements and assumptions that led to the early postulations of "dark matter" due to excessive velocities in the outer statrs and galaxies in clusters and galaxcies etc. The excessive measured speed , assuming the speeds measured were as claimed should have flung these fast outer bodies out of the orbit of the galaxiy or clusters that were scrutinized. The presence of dark matter was postualted , partially, as an explanation of these measured excessive velocities, as that mass force holding the too-fast object to the observed orbits.

What say you?

I don't know why, but I like the idea that gravity records every event that occurs in the universe. It sounds like one of those whacky ideas that you read about in science magazines!

Can this idea be applied to more than just changing mass. What about interactions between atoms or thermodynamical processes, etc...


You can also read about it in Dirk Gently, Holistic Detective Agency books (Douglas Adams). According to Detective Gently, you could solve a crime by interregating :m: a single atom of a table leg. So this idea clearly has real world applications in crime fighting :p

You can also read about it in Dirk Gently, Holistic Detective Agency books (Douglas Adams).

Also in Hitchhikers Guide to the Galaxy where in the Total Perspective Vortex, the whole entirety of the universe can be drawn forth from a piece of fruitcake.

It seems to me the discusion in this thread has turned in to nonsence. since r^2 can never equal 0, that should close the case. I'm surprised the moderator have'nt closed the thread.

Well now, aren't we hostile? As a matter of fact, I'm sure the discussion can continue on many points. And if it doesn't then it will slowly fade away into oblivion. Perhaps to be ressurected in years to come by our descendants. By the way, r^2=0 was a side topic, not the main one.

And I was about to ask about r < R.

James,

I know that you're not saying that gravity doesn't exist within the sphere, are you? Does it just not follow the inverse square law? Is there a different law that it follows? I know we haven't delved very deep beneath the surface of our own little rock here, but are there any deviations in 1/r^2 once one delves into the crust? Or must we drill deeper to detect a noticable difference?

I just thought of the perfect example. I had been thinking of miners in mineshafts, but of course they don't get very deep. What about Jupiter? The pressure gets more and more as you go deeper and deeper. If gravity didn't exist within the sphere then why would it act this way? Is it only within solid objects that it can be said? What about objects that can pass through "solid" objects. Are they affected by gravity within the sphere?

Interesting sidenote, I read somewhere that it's possible that the center of the gas giants is made up of a gigantic diamond. If that's true, just wait till we find a way to dig them out. Bye bye, jupiter.

http://www.hawken.edu/ptra/archive/msg01176.html

gravity inside a hollow sphere.

If space and time are quantized, then I imagine there would eventually be a limit...

invert nexus:

I know that you're not saying that gravity doesn't exist within the sphere, are you? Does it just not follow the inverse square law? Is there a different law that it follows?

It follows the law which says "gravity = 0". There is no gravitational force on an object placed inside a hollow sphere. Another way to look at it is that the sphere pulls in all directions on any object placed inside it, and the pulls in opposite directions cancel out, so there is no net force.

I know we haven't delved very deep beneath the surface of our own little rock here, but are there any deviations in 1/r^2 once one delves into the crust? Or must we drill deeper to detect a noticable difference?

The precise variation of the value of g, the acceleration due to gravity, inside the Earth, depends on the density of the various parts of the Earth (crust, mantle, core etc.). If the Earth had uniform density, g would increase from zero at the centre of the Earth uniformly and linearly to its value at the Earth's surface. In other words, inside the earth, g is approximately proportional to r instead of being proportional to 1/r².

Ok, I get you. Gravity doesn't exist in a hollow sphere. But it does exist in a solid sphere. It just doesn't increase at the same rate. Thanks James. Thank you too, Boris. Good link.

Edit: And it would be like gravity increases until the surface (from space) then decreases to the center. Where is the line of demarcation? Probably too intangible to determine?

Edit Again: I suppose the change would come when you start to have enough mass over your head to counteract the mass below. How does mass above you pull at you. Gravity comes for the center of mass, but is there a center of the mass above you?

For a uniform sphere, the line of demarcation is the surface of the sphere.

For Earth, it's fuzzier because it's not uniform. The core is denser than the crust.

Gravity only appears to come from the center of mass. There is actually a gravitational interaction between every particle of yourself, and every particle of Earth.

Pete has a points. For a homogenouos sphere it can be determined that g=GM(r)/r2. M(r) being the mass within of the sphere with radius r. The total mass outside that radius is cancelling each other. Since M(r)=4/3 * rho * pi *r^3 it follows that g=4/3 * G * rho * pi() * r. Or the gravity inside a homogenouos sphere is directly proportional to the distance to the centre.

Indeed Earth hasn't exactly a uniform density:

http://pubs.usgs.gov/gip/interior/

Now with powerfull calculation tools like Excel it is fairly easy to calculate the gravity inside the Earth. Just try it using the numbers in the bottom table:

http://pubs.usgs.gov/gip/interior/table.gif

Now where would gravity be the strongest and how strong? You'd be surprised.

Classically, the field strength would only apprach zero, but never quite get there.

Bringing Quantum Mechanics in, though, the force is actually propagated by the exchange of particles called gravitons. As the distance from the mass increased, the illusion of continuity would break down, I guess, because you would only receive a graviton once in awhile. Not sure how this would work out.