Is gravity traveling at the speed of light?

[Kerbin Dallas Multipass]

I am utterly confused.

I read that the universe expanding at a speed higher than c. Because it's the actual spacetime continuum expanding which makes nothing travel faster than light.

I also read about gravitational waves being limited to c as top speed.

Is gravity "imprinted" in the space time continuum (so, is the effect instant) or is it gravitational radiation (which takes time to travel)?

Am I mixing things up here?

Let's say I was the flying spaghetti monster and I select the sun and click "delete". Would the earth continue orbiting a non existent body for 8 minutes and then fly straight?

[lajoswinkler]

Yes, after around 8 minutes it would carry on tangential into the darkness.

[relin]

Gravitational effects are not instantaneous. So for your example, the Earth would indeed keep orbiting for a few minutes before flying out along the tangent.

[K^2]

Yes, the gravitational perturbations propagate at the speed of light. Asking what happens if you just "delete" a planet is a bit pointless, because the very event is a violation of the known laws of physics, so asking what happens according to laws of physics if you break laws of physics has no meaning. But if you were to, say, accelerate the Sun all of the sudden, yes, Earth would only notice the change in 8-something minutes. This goes for both light and gravity.

The reason that universe can expand faster than c despite that is that it inflates locally. At no two nearby points does anything move faster than c relative to each other. But over a great distance, this adds up, allowing two remote objects to travel faster than c relative to each other. Gravity propagation is purely a local thing, though. So it can't exceed c.

Things get way more complicated once you go from small perturbation to something more complicated. We've been having a lot of discussion about warp drives lately, and that's just one example where things aren't so simple.

[Kerbin Dallas Multipass]

Hm thanks for your answers

Gravity is often explained as a property of spacetime. Spacetime just IS warped around a body, there is no need for propagating waves. Is that correct, too?

My follow up question would be: How can a black hole "transmit" gravity if nothing can escape the black hole?

How can a black hole "tell" spacetime that it's spinning? No Information can travel faster than c, and nothing traveling at c ever gets out of the black hole?

@K^2 Apologies for using godmode;)

[relin]

Gravity *should* propagate like a wave, since General Relativity predicts it. I don't believe we've actually witnessed gravitational waves, though, so it's still hypothesis at this point. K^2 would know better on that, since all of my books on astrophysics are aging rapidly.

Edited January 13, 2014 by relin

[Themohawkninja]

The reason why space is perfectly allowed to (and does) move faster than the speed of light, is because space is space, not mass. Spacetime has no mass, it is only affected by mass, and therefore can move faster than the speed of light.

Gravity should take place instantly, but if there is a lag, it is surely a very small fraction of a second.

[ZetaX]

Gravity should take place instantly, but if there is a lag, it is surely a very small fraction of a second.

No, as has already been said in the thread several times. A good guideline is that everything that could be used to send information (matter, light, magnetism, gravity, ...) is bound by the speed of light.

[Themohawkninja]

No, as has already been said in the thread several times. A good guideline is that everything that could be used to send information (matter, light, magnetism, gravity, ...) is bound by the speed of light.

I meant the effects of gravity on spacetime, which isn't automatically dictated by light.

[ZetaX]

Can you please be more precise with that¿

[WestAir]

I've asked this question before (How does a Black Hole transmit gravitons to objects outside the event horizon when it must accelerate past C to do so) and the answer was pretty unfullfilling. Either gravitons are allowed to violate causality, or our theory on gravity just isn't good enough to answer such a question.

[PakledHostage]

I remember that we even discussed it on these forums and that you were involved in the discussion, but I couldn't find the link. I looked for it earlier this evening. Do you remember when it was? Maybe it was before the April Kraken attack?

[nhnifong]

I've asked this question before (How does a Black Hole transmit gravitons to objects outside the event horizon when it must accelerate past C to do so) and the answer was pretty unfullfilling. Either gravitons are allowed to violate causality, or our theory on gravity just isn't good enough to answer such a question.

Perhaps it's a sort of bucket brigade where the gravitons are transmitted up the well a bit at a time and the relative speed between any two particles is still less than c.

[Z-Man]

My follow up question would be: How can a black hole "transmit" gravity if nothing can escape the black hole?

Hah! That is a very good question. My best attempt at an answer (studied this stuff way back at university, it really is weird):

For the outside observer of a hypothetical stationary black hole, there is no mass that causes the gravity of the black hole itself. Spacetime around the black hole, if you exclude the event horizon and an arbitrarily small bit of its neighborhood, just looks exactly like it would look as if there was a suitably formed mass in the area you excluded, but outside the horizon. In a way, it is like your mirror image: when you look at it, it just seems like there is another person in there behind the glass, but really, it's just photons reflected back forming that image. The black hole is just gravity that looks like there is mass there.

Now, of course, if you would smash the mirror or move away from it, the image would go away, so why does the black hole stay there with all its gravity even if the mass that caused it is gone?

Mathematically, it simply is so that the curved space outside the event horizon that you can observe is a stable solution, just like flat space is. It can simply exist on its own.

Perhaps easier to understand: Nothing escapes from the event horizon. Not even information. Not even the information that the whole mass that formed the black hole is gone. You are in the "FSM removed the sun" scenario, only that it removed the collapsing star (bit by bit) as it passed the event horizon. But we outside the black hole will never know, it takes infinite time for that information to get out to us.

The same goes for the rotation. A rotating black hole is just spacetime curved in the way the rotating collapsing mass was last shaping it.

And yes, you can calculate what happens inside the event horizon up to the final singularity. That does not help much here: whatever happens inside does not have any influence on the outside. If you are also interested in what happens when you fall into the black hole and why you still are spaghettified even though the mass is gone, replace "event horizon" with "singularity" in the above.

[NovaSilisko]

http://en.wikipedia.org/wiki/Speed_of_gravity

Lots of discourse on the subject on this wiki article. Interesting read. It does please me to see stuff in science that we're not quite sure of... keeps things exciting.

[Technical Ben]

I've asked this question before (How does a Black Hole transmit gravitons to objects outside the event horizon when it must accelerate past C to do so) and the answer was pretty unfullfilling. Either gravitons are allowed to violate causality, or our theory on gravity just isn't good enough to answer such a question.

There is a "catch all" for most black hole problems. All the matter is spread across the event horizon. As from some perspectives it takes infinite time to fall in, then you can say it's all there, spread thinly just above the event Horizon. Else we could possibly consider it a warping of spacetime, instead of something sending gravitons, and I'm not sure if spacetime is limited, so it could do things that gravitons cannot. Not sure which one is preferred/works currently.

[Skyler4856]

Remember gravity can be thought of like the depressions on a bed sheet. When i remove the massive object from the bed sheet it will take time to decompress.

[Monkeh]

Noob question: Would assuming that gravitons are massless or have negative mass or something, make it so they can escape the black hole? This question is annoying me, there must be a theory for how gravity gets away from these dark monsters...

[Ralathon]

Noob question: Would assuming that gravitons are massless or have negative mass or something, make it so they can escape the black hole? This question is annoying me, there must be a theory for how gravity gets away from these dark monsters...

Think of it this way: As you get closer to the event horizon time dilation makes time move slower and slower. This applies to the gravitational field as well, so the field can change slower and slower from the perspective of the rest of the universe as you get closer and closer to the event horizon. So the field on the event horizon itself is essentially frozen in time for the rest of the universe. It just hasn't had the time yet to dissipate since the star collapsed. For the same reason black holes can have rotation and electric charge. You're not feeling the black hole itself, just the fossil remains of anything that fell into the event horizon.

[Monkeh]

So it seems we need to know the origin of gravitons. As I understand it, (ha ha, it appears to me that I have just claimed to understand the science behind black holes....this may have been an error!), a black hole's event horizon is a finite distance away from the actual centre of the singularity so how do any gravitons escape at all. The one's 'behind' the event horizon have to pass through an area of infinite time and the ones on the 'outside' of the event horizon have to struggle through near infinite time to escape. Wouldn't the formation of a black hole not be felt gravitationally for a very long time as the gravitons attempt to break free of time's sticky grasp!

This isn't an issue for a collapsing star, but I understand, (there's that word again!), that tiny black holes are forming reasonably regularly form the quantum foam and then disappearing almost as fast, but what happens to their gravity, does it even exist or is it all held behind a 'time bubble' and kept from our universe all together?

I, quite obviously, have very little clue about this, but where do gravitons propogate from? The centre of the mass or from every part of the mass or from space just next to the mass or....?

[Z-Man]

a black hole's event horizon is a finite distance away from the actual centre of the singularity

Common and easy misconception. Inside of the event horizon, time and space (specifically, the radial component) swap places. If you are a bit outside, you have a finite distance to the event horizon and if you fall in, you reach it in a finite proper time. Once inside, the event horizon is suddenly a moment a finite time in your past. That is why you can't get back out. It would be equivalent to traveling back in time. And the singularity... well, sorry. It's in your future. All of your possible futures. The distance of the event horizon to the singularity is not a distance, it is a very finite time, and the event horizon comes before the singularity.

so how do any gravitons escape at all. The one's 'behind' the event horizon have to pass through an area of infinite time and the ones on the 'outside' of the event horizon have to struggle through near infinite time to escape.

Gravitons, should they exist, do not escape from inside the event horizon either. They don't have to:

Wouldn't the formation of a black hole not be felt gravitationally for a very long time as the gravitons attempt to break free of time's sticky grasp!

Ralathon puts it best: No no problem. The black hole freezes itself on creation.

This isn't an issue for a collapsing star, but I understand, (there's that word again!), that tiny black holes are forming reasonably regularly form the quantum foam and then disappearing almost as fast, but what happens to their gravity, does it even exist or is it all held behind a 'time bubble' and kept from our universe all together?

If such things happen (jury is still out), it would be like the pairs of electrons and positrons that constantly materialize and vanish again in the vacuum of QCD: their electric field is never felt directly, however it influences other electric fields and interactions in quantifiable ways.

[peadar1987]

Does that mean that the earth is actually orbiting the point where the sun was 8 minutes or so ago? How does this apply to stars that are moving very quickly, like those hypervelocity stars that were supposedly observed recently (should they have planets orbiting them)? Or am I just failing to get my head around frames of reference properly again?

[TheGatesofLogic]

The argument here is irrelevant in the sense that you are describing it. IF gravity is propogated by a spin-2 massless particle like the graviton then it would propogate at exactly c in vacuum, and would reduce to general relativity and conform to newtonian gravitation. the issue here is that a "particle" attributed to gravity would only be an arbitrary interpretation of a both wave-like AND particle-like state (the difference being the situation in which it is percieved) similar to the way photons are arbitrary in the sense that light propogates in a wavelike pattern AND a particle-like pattern as well. (the same thing happens to mass but on a far less noticeable scale). So what this means is that when we interpret gravity as a wave, as we must when considering such a situation as the escape of gravitation from a blackhole, we CANNOT simultaneously consider it to be acting as particles of propogation at the same time and as such interpret an interference between the two states because the two states cannot be percieved to exist simultaneously.

[Z-Man]

Or am I just failing to get my head around frames of reference properly again?

A bit. It's usually still safe to assume that in some frame of reference, the center of mass of a solar system is at rest at the origin and that the planets and star(s) revolve around that. How the whole system moves through some other reference frame is not relevant even at relativistic speeds. Unless the spacetime curvature is high enough that the concepts of center of mass and frame of reference break down. But even then, you can treat the system as mostly stationary and the external disturbance as something that moves relative to it.

If you want to treat star and planet in a coordinate system where they both move quickly, that can be done, too; and yes, Earth would then respond to the gravity field of the Sun from 8 minutes or so (time dilation etc.), but that is compensated by the gravity fields of the moving sun looking different; Newton's Law of Gravity no longer applies.

[K^2]

I meant the effects of gravity on spacetime, which isn't automatically dictated by light.

They are not instant either. Just like electromagnetic equations, Einstein Field Equations are local. Mass only affects space-time where it is located, and gravity propagates further via interaction of space-time with itself. So any perturbation propagates at the speed of light.

Does that mean that the earth is actually orbiting the point where the sun was 8 minutes or so ago?

Actually, no. I have no idea how to derive this result in proper GR, but at least for the case of interaction between planets and stars, using gravitoelectromagnetism is good enough. And just like in normal electromagnetism, there is a very cool effect. All of the fields are oriented in such a way that you feel attraction towards the point where the object actually is, rather than where it was with appropriate time-delay, so long as source travels at uniform speed. This is a very important result in electromagnetism and it carries over to gravity. If this wasn't the case, planetary systems would outright collapse, because the gravity fields would all come in at an angle, causing systems to use energy as if through a very strong tidal interaction.

Naturally, if the source suddenly accelerates, that's something you cannot possibly predict until fields propagate, so that's something that's going to have an effect on how things move. But constant velocity allows things to keep working as if all fields propagated instantly.

Edited January 13, 2014 by K^2