Is gravity traveling at the speed of light?

[Wahgineer]

I think my head just exploded. The only thing I can think of right now is: warp drive confirmed.

[peadar1987]

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.

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.

So, does this have implications for a satellite of a body in an elliptical orbit, where the velocity of the parent body will change significantly between aphelion and perihelion?

[K^2]

I don't know. That's a far more complicated problem. But it's a 3-body problem to begin with, so you expect some weirdness either way.

[Kerbin Dallas Multipass]

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.

Interesting, but I dont really get it.

They seem to differenciate between static gravity fields (gravitoelectric) which appear to have instantaneous effects, and gravitational waves (gravitomagnetic) which transmit pertubations of gravity at speed of light.

All this reference frame stuff is really giving me a headache, lol.

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.

When a black hole gains mass, the (spherical) event horizon gets bigger, doesn't it? So eventually everything that has already fallen into it will be inside the event horizon, not frozen in time near its surface. So i guess this catch all won't really work?

[Themohawkninja]

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.

So, if I understand it correctly (I just lightly read the wiki article that NovaSilsko posted), if the Sun were to disappear from existence right now, the Earth would still orbit the point where the Sun was for another 8-9 minutes (the time it takes for light from the Sun to reach Earth) as if the Sun was still there?

[WestAir]

Not only that, it would orbit where the sun SHOULD be were it not to disappear, as if its velocity had remained constant.

[YANFRET]

So, if I understand it correctly (I just lightly read the wiki article that NovaSilsko posted), if the Sun were to disappear from existence right now, the Earth would still orbit the point where the Sun was for another 8-9 minutes (the time it takes for light from the Sun to reach Earth) as if the Sun was still there?

Everything I have read on the subject agrees with that.

Also re someone's question about things crossing a singularity event horizon, remember that quantum virtual particles are disrupted by the horizon, giving the effect of X-Rays being emitted by the black hole when one part of a virtual particle pair gets sucked in and the other doesnt, whereas without the black hole being there they would anhilate each other. - from brief history of time.

[Themohawkninja]

Not only that, it would orbit where the sun SHOULD be were it not to disappear, as if its velocity had remained constant.

Wait... so the Earth technically doesn't orbit the Sun, so much as it orbits the Sun's position 8-9 minutes ago?

[K^2]

Again, you can't ask a question that starts with, "if the Sun were to disappear." For this to happen, entire theory on which gravity is based has to be completely wrong. You are asking a question based on a false premise, and therefore, it has no valid answer.

You have to have a change that is physical in order to ask what the physical consequences are going to be.

Wait... so the Earth technically doesn't orbit the Sun, so much as it orbits the Sun's position 8-9 minutes ago?

No, it orbits Sun's current position, as I have explained in an earlier post. And WestAir's point is consistent with that. Except for the Sun disappearing bit, which we can't take into account, because that is not itself physical.

[Themohawkninja]

Again, you can't ask a question that starts with, "if the Sun were to disappear." For this to happen, entire theory on which gravity is based has to be completely wrong. You are asking a question based on a false premise, and therefore, it has no valid answer.

You have to have a change that is physical in order to ask what the physical consequences are going to be.

Fair enough.

My other question remains valid then. That is to say, we don't orbit the Sun, so much as we orbit where the Sun was 8-9 minutes ago?

[YANFRET]

Again, you can't ask a question that starts with, "if the Sun were to disappear." For this to happen, entire theory on which gravity is based has to be completely wrong. You are asking a question based on a false premise, and therefore, it has no valid answer.

You have to have a change that is physical in order to ask what the physical consequences are going to be.

No, it orbits Sun's current position, as I have explained in an earlier post. And WestAir's point is consistent with that. Except for the Sun disappearing bit, which we can't take into account, because that is not itself physical.

Yeah I agree exactly this is correct.

[YANFRET]

Fair enough.

My other question remains valid then. That is to say, we don't orbit the Sun, so much as we orbit where the Sun was 8-9 minutes ago?

NO Because of how the field works.

The only scenario I can think of to illustrate how this works would be a supernova or something. An object in a very distant orbit of the pre-collapsed star would orbit the real point of the star, however the gravitational consequences of the collapse would be dilated at the speed of light. (But then so would all the other fire and brimstone so...)

Edited January 13, 2014 by YANFRET

[Themohawkninja]

I'm really confused.

If gravity propagates at the finite speed, then why do objects orbit their parent bodies as if gravity affected things instantaneously?

[YANFRET]

I'm really confused.

If gravity propagates at the finite speed, then why do objects orbit their parent bodies as if gravity affected things instantaneously?

Because they travel in accordance with the real distortion of space time. It's a good thing it's true or we would'nt be here.

Keep in mind we're also only here because the dice roll in the beginning made the physical laws the way they are. Any different and we might not be around to make the observation. It's just as likeley there was or is a universe where gravity is purple and goes 50kph but then there wouldnt be stars so no one to write a story about it. We just have to accept the universe as we observe it, there is no why beyond the mathematical explanation.

Where Humans get into troubble thinking about this stuff is we try to frame it in the context of our life experiences as large natural creatures on a planetary surface. Flowers wiggle when bees land on them and such. The universe at the level of detail we're discussing is more complex thatn the natural systems we typically encounter in daily life, but we try to fit everything into that framework so we can have a nice neat and tidy undertanding of our universe without contradictions with the world we can observe, which is problematic unless you just accept the math.

Edited January 13, 2014 by YANFRET

[Themohawkninja]

Because they travel in accordance with the real distortion of space time.

Right... and isn't gravity (at least Einsteinian gravity) by definition, the distortion of spacetime?

[K^2]

If gravity propagates at the finite speed, then why do objects orbit their parent bodies as if gravity affected things instantaneously?

Because a moving object generates gravity differently from a static object. The analogy is electric charge. If it's not moving, it just creates electric field. But a moving charge also creates a magnetic field, and a moving magnetic field induces its own electric field, and so on. As a result, the electric field lines point towards the current location of the object, rather than to where it was. Again, assuming the source is moving at a constant speed. If source accelerates, other weird stuff happens.

Right... and isn't gravity (at least Einsteinian gravity) by definition, the distortion of spacetime?

Related, but not the same thing. Space-time curvature is frame-invariant. Gravity is not. But if you know space-time curvature and have a chosen coordinate system, you can work out gravity. (Alternatively, you can look at gravity as a gauge field, which ultimately leads you to exactly the same conclusion, with coordinate system choice being your gauge choice.)

[YANFRET]

Right... and isn't gravity (at least Einsteinian gravity) by definition, the distortion of spacetime?

I'm gonna go get some coffee, you guys can take a swing

[Themohawkninja]

I sorta get what you're saying K^2, but I haven't learned anything about electrical charges (except for the basics like two charges, volts, amperes, watts), magnetic fields (except for north/south, auroras, and a basic knowledge of how Terrestrial field lines work/evolve), or frame-invariance (at all).

I think this will make a lot more sense in the next few years (I'm in my first year of college for astronomy).

[YANFRET]

I sorta get what you're saying K^2, but I haven't learned anything about electrical charges (except for the basics like two charges, volts, amperes, watts), magnetic fields (except for north/south, auroras, and a basic knowledge of how Terrestrial field lines work/evolve), or frame-invariance (at all).

I think this will make a lot more sense in the next few years (I'm in my first year of college for astronomy).

"If Coulomb propagation speed exceeded light, it might have implications for the speed of gravity propagation. In late 2012, experimenters the Istituto Nazionale di Fisica Nucleare,Laboratori Nazionali di Frascati in Frascati performed an experiment which indicated that there was no measurable delay in propagation of the force between a beam of electrons and detectors.[27] This was taken as indicating that the field seemed to travel with the beam of electrons as if it were a rigid structure preceding the beam. Though awaiting corroboration, the results indicate that aberration is not present in the Coulomb force.

In November 2013, Y. Zhu announced that he observed the speed of gravitational force, calculating the variations of the orbit of the geosynchronous satellites perturbed by the Sun. It is shown that the gravitational force of the Sun acting on the satellite is from the present position of the Sun. It indicates that the speed of gravitational force is much larger than the speed of light in a vacuum. From this observation and the recent experiments, the structure of the fields of a moving source (a body or a charge) is studied. A method to measure the speed of gravitational force in laboratory and a line to indirectly test the wavelengths of gravitational waves are presented. [28]"

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

[K^2]

"If Coulomb propagation speed exceeded light, it might have implications for the speed of gravity propagation. In late 2012, experimenters the Istituto Nazionale di Fisica Nucleare,Laboratori Nazionali di Frascati in Frascati performed an experiment which indicated that there was no measurable delay in propagation of the force between a beam of electrons and detectors.[27] This was taken as indicating that the field seemed to travel with the beam of electrons as if it were a rigid structure preceding the beam. Though awaiting corroboration, the results indicate that aberration is not present in the Coulomb force."

Erm... This very effect is derived from Maxwell's Equations in the Electrodynamics text by Landau and Lifshitz. That's precisely what I was talking about earlier. People really need to start spending more time learning theory. It's sad when publications like this actually happen.

Edited January 13, 2014 by K^2

[WestAir]

I think what K^2 is saying is that a change to the suns velocity will propagate along its gravity field at C. If the sun were to suddenly STOP moving, relative to the Galactic Disc, the information that it had stopped will propagate through its field of influence at C. Otherwise, all objects will assume its velocity has been constant and will orbit its current location. From an outsiders perspective this would look like gravity propagated instantly, rather than at C, because they'd always orbit where the Sun is rather than where it was x minutes ago.

[K^2]

Yeah, that's a good way of putting it.

[Themohawkninja]

I think what K^2 is saying is that a change to the suns velocity will propagate along its gravity field at C. If the sun were to suddenly STOP moving, relative to the Galactic Disc, the information that it had stopped will propagate through its field of influence at C. Otherwise, all objects will assume its velocity has been constant and will orbit its current location. From an outsiders perspective this would look like gravity propagated instantly, rather than at C, because they'd always orbit where the Sun is rather than where it was x minutes ago.

Well... the "why that is" may be way beyond my realm of knowledge, but that makes a lot of sense.

[Kerbin Dallas Multipass]

@WestAir

Where the sun IS is not regarded as "information".

That the sun has STOPPED its motion is regarded as "information".

Is that correct?

If so I still dont get it.

[ZetaX]

@Themohawkninja:

If you are ready to accept one of they key aspects of special relativity, this rough but different explaination mostly based on newtonian arguments might help:

All speed is only relative, i.e. A moving towards B is the same as B moving towards A in the frame of reference of A. Thus a moving solar system A cannot be distinguished from a nonmoving clone B from within (without an external frame of reference, you can't even define the difference). Therefore, the moving A and the nonmoving B will show exactly the same orbital behaviours. If gravity would work by where the central star was some time ago, the planets in A would try to circle around a star not even in their plane of ecliptic (or if the movement vector of the star is in the plane of ecliptic, the star would still not have been at a focal point that time ago), which is not possible. Thus they have to circle the actual position.