I’ve read a couple of interesting threads here on the speed of gravity, but can’t for thr life of me remember what that speed is. Gettin’ old, I guess, but I seem to remember a comparison to light speed.
Anyway;
What limits the speed of a mass in space due to the attraction of gravity? If you could take a (lead?) ball jillions of miles out in space, away from a massive object, and release it, how fast would it eventually travel, and what would limit that speed? If space were a true vacuum, would the velocity of the ball approach light speed? Gravity limits, and uses up the energy (inertia) of objects moving away from each other, right? So what limits the energy (gravity) of objects moving toward each other? I know that the force of gravity, as forces go, is relatively weak, but given time two massive objects in space should eventually attain something close to the speed of gravity.
Does “the speed of gravity” have a name? 
Is this an “expanding/shrinking” universe question?
Peace,
mangeorge
Umm, no, you’re thinking about this all wrong. Remember the phrase “escape velocity”?
Take your lead shot on the surface of the Earth. If it is shot out into space at escape velocity or higher (ignoring the atmosphere), it will never return. Reverse the scenario. The lead shot is waaaay out there and is attracted into the Earth. When it hits, it’ll be moving at escape velocity (i.t.a.).
So, without any further momentum being imparted, escape velocities limit the speed. Only in the case of black holes does escape velocity and speed of light get tied together.
Ftg is correct regarding the speed of objects attracted to one another. However, gravity itself does have a veliocity, and it is equal to c. Thus, if the Sun were suddenly made to disappear, for instance, our orbit wouldn’t “feel” the loss of gravitational attraction for approximately 8 minutes.
Ok, I have to bite. Because dangit, I do not know!
ftg, your answer hints at a black hole being able to pull at the speed of light, but you don’t say that it can actually do it. Can it? If not, why?
Sort of. The event horizon of a black hole is essentially defined as the distance from the black hole at which the escape velocity equals c. As such, an object freefalling towards a black hole will not reach c until it passes the event horizon, at which point it effectively disappears from the universe.
Why should that be the case if gravity is nothing more than a curvature of spacetime?
The only (AFAIK) published measurement of the speed of gravity is still somewhat controversial.
Gravity is supposed to travel at lightspeed, but as far as anyone’s measured it, it could propagate at 227,000 km/sec, or 300,000,000 km/sec.
The Speed of Gravity - What the Experiments Say
Gravity and light are somewhat different animals, and while it can be said that gravity “radiates” in a sense, it does not propagate in the same way that light does, so talking about the “speed” of gravity is a somewhat different concept than the radiant “speed” of light or other EM phenomena if you’re trying to apply a metric to the comparison.
Assuming that gravity propagates at c, the curvature of spacetime will also propagate at c. If you’re thinking of the bowling-ball-on-a-rubber-sheet analogy, think of the rubber sheet having a finite response time, so if the bowling ball is suddenly lifted, stuff far away from it doesn’t know until the wave of the lifting action passes.
I’ll try to ignore the irony of the basic question being addressed in an a priori assumption. 
Here’s the thing: that rubber sheet is modeling spacetime. Why should the fabric of spacetime be limited by a velocity, which is a combination of space and time?
If gravity is a curvature of spacetime, then what’s going on in those 8 minutes? Say 4 minutes have passed since the sun vanished. Is it accepted wisdom that the earth is still orbiting normally, as if the sun were still there? If so, take a snapshot of the entire system at this moment, the 4 minute threshold. What can be pointed to as the cause for the earth’s orbit? If you did a Newtonian analysis on what the earth’s path should be at that moment, you’d be way off. (But then again, matter / energy can’t just disappear, so I could accept a response of “that’s a meaningless thought experiment”, as unsatisfying as that would be.)
I have a reall hard time with the concept of gravity based on accepted wisdom. I like the rubber sheet analogy a whole lot. It is poetic in its simplicity. But the analogy appears (to me) to be fundamentally opposed to two main doctrines:
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Gravity travels at speed c (or whatever speed…the analogy seems to imply action at a distance)
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Gravitons are the particle aspect of gravity
If there is such a thing as a graviton, then it must be possible to shield those particles, thus making anti-gravity suits/ships/carpets/whatever possible. Why is it that gravity cannot be shielded, if it is particle in nature?
Another question: If gravity travels at the speed of light, how can light possibly be affected by gravity? Gravity can’t catch it. :dubious:
Clearly I have trouble with these concepts. 
I think the OP was getting two ideas mixed up. The “speed of gravity” as I understand it refers not to how fast an object will end up falling under gravity, but to how fast the force of gravity is propagated - i.e. if the sun were to vanish, how long before the Earth reacted by flying off at a tangent.
If gravity were to act instantaneously then surely causality would be violated, since information is supposed to be forbidden from travelling faster than light. I’d say that having the Earth fly out of it’s orbit is *probably *some information that we could use to deduce that the sun had vanished…
Well, the way I see it, say you have a constant stream of light crossing the “rubber sheet”. It takes 8 minutes (or whatever) for the sheet to deform, with the deformation travelling at the same speed as the light. So all the light ahead of the point where the deformation happens continues unperturbed, whereas all the light behind that point follows the curved path. Right?
Several different issues are being discussed as if they are one. Hence the confusion.
The idea of the “speed of gravity” is the idea that changes in mass will propagate a change in the gravitional field at speed c.
In the rubber sheet analogy, pretend the sheet is made of a very stiff foam rubber, rather than the usual balloon-type membrane. Now imagine a large area of free space, or flat foam rubber. Magically we set our bowling ball in the middle and the ball begins slooowly squashing into the foam. As the ball sinks, we see a slowly expanding region of curvature, and at any given radius within that region we see the curvature increasing over time.
Eventually the ball sinks to equilibrium and the region of curvature stops growing and the curvature at any point in the region stops changing. That’s the steady-state situation typical of the real world.
Now magically remove the bowling ball. The region of curvature begins to shrink as the foam rebounds and the well eventually disappears and everything is flat again.
We can measure the speed at which the edge of region of curvature expands and contracts. That’s the “speed of curvature” for the particular stiffness of foam rubber we used.
Now back to the real world. The “speed of gravity” is the speed at which mass-caused curvature propagates through free space. In some sense, space has a “stiffness” which gravity works against. And the speed of gravity is only meaningful if the mass and hence curvature is changing over time. Which generally doesn’t happen.
Going back to our foam rubber, if a ball is rolling across the plane, it experiences the instantaneous curvature at its present location. The rate at which the curvature threshold is moving (the “speed of gravity”) is immaterial. Whether the ball is rolling faster or slower than our speed of curvature, it’s still affected at each moment by the curvature present at that moment at that point.
Now IF the ball is outside the region of curvature and is going faster than the speed of curvature, then a bowling ball dropped behind it will never affect it. Likewise if it’s traveling at exactly the speed of curvature. The ball has a “head start” over the advancing wave of curvature and the curvature never catches up.
But another ball traveling at exactly the same speed in a different direction may well roll through the region of curvature and be greatly affected by it. Whether the curvature is changing or is steady-state doesn’t matter; either way a ball traveling through the curved region is affected by the curvature.
The speed of the ball and the speed of the curvature are two very different ideas.
Make sense??
You guys are right. I was confusing the concept of the speed of gravity.
One more thing, though;
If I were to send the ball out at 25,001 mph it would escape Earth’s gravity, never to return. But at what point could I stop the ball (0 mph) and have it not be affected by Earth’s gravity? How far out does the influence of Earth’s gravity reach? Just how big is that rubber sheet? It’s infinite from our POV, right? Just like light, it would disperse but not cease to exist.
The ball will always be affected by Earth’s gravity, no matter where in the universe you put it. Of course, the effect is almost completely overwelmed by the gravity of other objects in the universe, but if it were just the Earth and your ball, no matter where you put it, if it has 0 velocity relative to Earth, it will eventually fall to it.
Cool. That’s what I thought, and part of my initial confusion. So when it did contact Earth’s surface, it would be traveling at escape velocity. Not c.
You guys are so smart!
Technically, the influence of Earth’s gravity stretches all the way out to about 4.5 billion light years — the distance at which, only now, the formation of the Earth is being physically “discovered”. That’s in principle anyway. In reality, it becomes undetectable pretty quickly, as the Earth’s gravitational field strength tails off to the point where it is overwhelmed by those of other objects.
The Sun’s mass alone is about 330,000 times that of the Earth. Since gravitational field strength goes as mass over the square of distance, this means you have to be about 580 times closer to the Earth than you are to the Sun in order for their gravitational tugs to be comparable. But at a range of only one light year, the distance ratio between the two is already indistinguishable from 1 — and therefore the Sun’s gravitational force overwhelmingly predominates, by a factor of 330,000.
Not to mention that Jupiter’s mass is over 300 times that of Earth as well. So even if you could distinguish the Sun’s gravity from non-Sun gravity at that distance, the first thing you’d discover would be Jupiter, then the rest of the gas giants, then maybe, maybe, dinky little Earth emerging from the noise.
In fact, to a pretty good approximation, the solar system is just the Sun and Jupiter. Everything else is a bit of residual fluff that those behemoths couldn’t grab onto.
Wow. I was not aware of that and never gave that particular question “speed of gravity?” any thought.
How did they ever measure the speed of gravity? Or is this just a thought experiment so far? Any links to the underlying reasoning and / or experiments would be greatly appreciated, as this is a very interesting issue.
Something, someone, somewhere, may find interesting is that the gravitational force vector points to the actual position of the Sun rather than its retarded position. (Where the sun was 8 minutes ago). If this were not true stable orbits would not be possible. This is what forced Newton to postulate that gravity propagated instantaneously. (Action at a distance)
How about this, from New Scientist:
It bears repeating that Kopeikin’s measurement of the speed of gravity is still somewhat controversial.