How fast is gravity?

[Darnok]

Lets say I just increased mass of Sun 10 times, light needs over 8 minutes to travel to us, how long it takes for the gravity to pull Earth into Sun?

[Sirrobert]

I can't remember exactly where I read it, but I read/heard that gravity 'information' travels at the speed of light like everything else.

So it'd take 8 minutes for the orbit to be effected

[Starwhip]

Yes, Sirrobert has it: however gravity works (I think we're still not entirely sure, but hey, it could be headline news next week!) its footprint travels at the speed of light.

[RainDreamer]

Well, I tried your scenario in Universe Sandbox, the answer is 22.5 days for the earth to reach sun.

http://i5.minus.com/i68EylHF2rtdZ.png

Edit: I misunderstood the question. Still, seeing earth get slingshoted out of the system was fun.

Edited February 4, 2015 by RainDreamer

[Darnok]

But if gravity speed is same as light then it should have same structure and we should be able to study it in similar ways?

[RainDreamer]

I don't think we can study a curvature in spacetime the same way we study photons travelling through space and time. But I am just an amateur at this.

[N_las]

But if gravity speed is same as light then it should have same structure and we should be able to study it in similar ways?

The "speed of light" is not actually a property that has anything to do with light. It is the conversion factor between time and space in spacetime. Light just happen to go at the fastest speed possible, which is equvalent to that conversion factor. Anything with no rest mass will move with that speed. So the fact the speed of light is the same as the speed "gravity is traveling" doesn't mean they have similar structure.

[Ten Key]

Sounds like you're talking about gravitational waves?

[Sirrobert]

It has been theorized that there is some form of particle that carries gravitational energy yes (like a photon carries light), called Graviton. However, as it is now, we have no way of detecting that particle

This video (or more accruatly, this playlist) explains that a little:

[PakledHostage]

So the fact the speed of light is the same as the speed "gravity is traveling" doesn't mean they have similar structure.

Something I have always found fascinating is how information about how much mass a black hole contains "gets out" if gravity travels only at the speed of light?

I have read various explanations for how it does so, including some written by members of this forum, but they all seem a bit "hand wavy". A recent paper by Stephen Hawking suggested that matter that falls in leaves a "signature" outside the event horizon that would convey information about changes in the black hole's mass over time, but that doesn't explain how information about how much mass was present at the time of its gravitational collapse gets out if that information can only travel at the speed of light.

Does anybody here want to take a stab at explaining this conundrum in layman's terms?

[Sirrobert]

Something I have always found fascinating is how information about how much mass a black hole contains "gets out" if gravity travels only at the speed of light?

I have read various explanations for how it does so, including some written by members of this forum, but they all seem a bit "hand wavy". A recent paper by Stephen Hawking suggested that matter that falls in leaves a "signature" outside the event horizon that would convey information about changes in the black hole's mass over time, but that doesn't explain how information about how much mass was present at the time of its gravitational collapse gets out if that information can only travel at the speed of light.

Does anybody here want to take a stab at explaining this conundrum in layman's terms?

What? That makes no sense.

Just because 2 particles travel at the same speed, doesn't mean they have the same properties. So just because light can't escape a black hole, doesn't mean gravitons can't either.

Gravitons ARE gravity, they don't get sucked in BY gravity

[Starwhip]

There's also the tiny complication that the force of gravity does not seem to be confined to three dimensions...

[Sirrobert]

There's also the tiny complication that the force of gravity does not seem to be confined to three dimensions...

Wait, what? How does that work?

Or do you mean wormholes?

[Rakaydos]

Wait, what? How does that work?

Or do you mean wormholes?

More like 10 dimention string theory. Gravatons are a closed loop and not bound to the skein, and so can actually jump between manifolds.

In theory, Dark matter could potentially be real matter in a nearby universe, that cant interact with out own except with gravity.

[PakledHostage]

So just because light can't escape a black hole, doesn't mean gravitons can't either.

Gravitons ARE gravity, they don't get sucked in BY gravity

Black holes warp spacetime sufficiently that nothing can escape, not just light. My question has nothing to do with the nature of how gravitational information is transmitted (whether it be transmitted by gravitons or what have you).

If information about an object's mass only travels at the speed of light, then it too should not be able to escape because information is something and we already established that nothing can travel faster than light or escape a black hole.

In the Stephen Hawking paper I mentioned earlier, he proposes that matter falling into a black hole never actually crosses the event horizon; that it only spreads out and never crosses. That yeilds a plausible explanation for how we can measure how much a black hole's mass changes over time, but how does the information about how much mass collapsed to form the black hole in the first place get out?

You may think it is a stupid question (and I admit that I am not an astrophysicist so it may well be a stupid question), but I take it from the fact that I have yet to read a good answer anywhere to mean that the answer isn't obvious.

[B787_300]

as I understand it gravity obeys the galactic speed limit like everything else. there is also talk about gravity actually having measurable "waves" that can be used to sense things that have or are going on in the universe.

[Mr Shifty]

You may think it is a stupid question (and I admit that I am not an astrophysicist so it may well be a stupid question), but I take it from the fact that I have yet to read a good answer anywhere to mean that the answer isn't obvious.

It might be related to the other (non-obvious) question of how anything falls into the black hole in the first place, i.e. how do black holes form. As a clock gets nearer and nearer to the event horizon, its ticks get further and further apart because of time dilation. From an external frame, it can never actually cross the horizon; it slows to frozen just outside. (Actually, it apparently gets red shifted so much that it's invisible to outside observers.) This, as far as I'm aware, is a real paradox in black hole theory with some somewhat unsatisfactory explanations for the phenomenon.

[MockKnizzle]

Well to an outside observer that's the case, but from the point of view of the thing actually crossing the horizon the passage of local time doesn't appear to change and you can fall across the horizon just fine.

[Bryce Ring]

Well to an outside observer that's the case, but from the point of view of the thing actually crossing the horizon the passage of local time doesn't appear to change and you can fall across the horizon just fine.

I think you got it backwards.

from the outside observer the thing falling in will appear to travel close to the speed of light as it falls in. to the object falling in the speed will appear to be even greater due to time dilation effects affecting perception of time.

for those that say nothing can ever fall in. are stuck in time reference frames.

its a bit like the riddle about the three men booking a hotel for a football night crash pad. they each pay 100 for the 300 per night room and head on up to the room to get ready for the game. the manager heard the clerk and reminded him that the room is only 250 for football night, so the clerk walks up to give back 50. but on the way up he cant do the math on how much to give each man. so he decided to give back just 10 each and keep 20 for himself. on the way down he is still trying to do the math. he figures. "they each paid 100 initially, and I gave them back 10 each so they all paid 90 each now. so 3 times 90 = 270...... and the 20 in my pocket makes ?

objects fall into a black hole. those falling in start to travel at very fast speeds. as a result they experience time dilation that makes them to the outside observer move slowly (this time dilation does not affect their velocity as a whole, it only affects the hands on a clock they hold and every orbit of every atomic particle in them)

time on the clock they are holding to those falling in may appear to be normal. but as they observe the outside world, it may appear to be in a state of fast forward. and as that is , the black hole coming to them begins to happen even faster, it may appear to happen faster than the speed of light, but it didn't really, it was just that they were slow in perception.

Edited February 5, 2015 by Bryce Ring

[K^2]

Something I have always found fascinating is how information about how much mass a black hole contains "gets out" if gravity travels only at the speed of light?

In simplest terms, the information "gets out" before stuff actually falls into the black hole. The total mass of the black hole, and its effects on its vicinity, is the same for extra mass being inside the event horizon or just at the event horizon. And black hole itself is expressed as series of conserved quantities (mass, charge, angular momentum, etc) just as if it was an elementary particle. So where the mass enters black hole is absolutely irrelevant.

This kind of goes back to the Sun changing mass 10x question. It can't just suddenly change. That mass has to come from somewhere. And that has a lot of interesting consequences in relativity. Yes, gravity propagates at the speed of light, but a lot of things wouldn't change even if it was instant.

P.S. Yes, if you want to describe gravity in terms of gravitons, you end up having trouble with black holes. But then again, you end up with a whole lot of problems even before that, because quantizing gravity is not a trivial task.

[Mr Shifty]

There's an interesting discussion here:

http://physics.stackexchange.com/questions/21319/how-can-anything-ever-fall-into-a-black-hole-as-seen-from-an-outside-observer

[VincentS]

I'm no physicist, but it seems to me that something like http://en.wikipedia.org/wiki/Cauchy%27s_integral_formula would do the trick. Cauchy's integral formula states that for some functions (holomorphic functions), there is no difference between knowing the function value on a full disk and on the disk boundary. Hence, if some similar property holds for gravity, there is actually no need for the information inside the horizon to travel outside because the horizon information is already equivalent.

[stupid_chris]

Black holes warp spacetime sufficiently that nothing can escape, not just light. My question has nothing to do with the nature of how gravitational information is transmitted (whether it be transmitted by gravitons or what have you).

If information about an object's mass only travels at the speed of light, then it too should not be able to escape because information is something and we already established that nothing can travel faster than light or escape a black hole.

In the Stephen Hawking paper I mentioned earlier, he proposes that matter falling into a black hole never actually crosses the event horizon; that it only spreads out and never crosses. That yeilds a plausible explanation for how we can measure how much a black hole's mass changes over time, but how does the information about how much mass collapsed to form the black hole in the first place get out?

You may think it is a stupid question (and I admit that I am not an astrophysicist so it may well be a stupid question), but I take it from the fact that I have yet to read a good answer anywhere to mean that the answer isn't obvious.

Alright, here's a way you could see it then:

You know the analogy of how the curvature of spacetime can be physically represented by a trampoline? Heavy objects put on it will curve it and objects will be attracted to it because they "fall". Well it's the same for spacetime. Except spacetime in that case is the trampoline itself. If you have a big bowling ball at the center of the trampoline and you throw a bunch of marbles in they'll start falling towards it. If you add a second bowling ball in the middle, it'll act as a twice as massive object. In that case, the force on the trampoline will propagate at the speed of sound as any force in a physical object does. That's the same idea here. The curvature of spacetime will "spread" at the speed of light, because that's the speed limit. But the curvature of the trampoline is not affected by how curved it is unlike the marbles that will be attracted twice as much, and it's the same for spacetime. The curvature is not affected by it's own precense unlike light and matter.

And then again, you don't get the information of how massive a black hole is from the black hole itself. You estimate it by looking at how it affects it surroundings. It doesn't mean any information is leaving the black hole.

[Beowolf]

The reason arguing about gravity is so much fun is it's made of crazy.

I read a recent article about this. Here's what I can remember...

The article said we "know" that changes in gravity propagate at lightspeed. So if you hooked up a warp drive to something supermassive, then dropped out of warp 10 light seconds from Earth, it'd be 10 seconds before Earth started be accelerated by the object's gravity. Makes sense, or at least seems consistent with the rest of relativity and the way spacetime seems to work, right?

But we also "know" that orbits don't work that way. The planets aren't being pulled towards where the center of the Sun was >8 minutes ago, but where it is "now", somehow bypassing relativity. Planetary orbits wouldn't be stable over millions of years, otherwise.

Conclusion: We are certain that we don't know how gravity works.

Disclaimer: I couldn't find a link to the article just now, so it's possible I misunderstood or even dreamed it. For entertainment purposes only.

[KerikBalm]

But we also "know" that orbits don't work that way. The planets aren't being pulled towards where the center of the Sun was >8 minutes ago, but where it is "now", somehow bypassing relativity. Planetary orbits wouldn't be stable over millions of years, otherwise.

And where was the sun 8 minutes ago? What is your inertial reference frame?

How much does the galaxies CoM deflect the sun from its inertial reference frame? how much does it deflect the planets? what is the difference between those?

I'd like to see some math to back those claims up