How fast is gravity?

[cicatrix]

This is amazing discussion! Nobody on Earth knows the answer for sure, but it is amazing nevertheless.

Understanding gravity is a door yet to be opened. There are several theories or hypotheses but none of it are without flaws.

[Iskierka]

The galactic CoM is moving a miniscule angle over solar orbital periods, it will not have a significant effect stronger than the sun's own.

Reference frames is the question to ask. If we assume its behaviour is similar to light, then the reference frame is always the observer's, so there would be no issue with "where is it actually being pulled to?". However, it makes the 8 minute delay extremely hard to measure. You can't ask if we're being pulled towards its visual position or actual to confirm or deny, because those are actually one and the same - both gravity and light have been emitted and travelled out to the earth's orbit at the same rate, and the angle they arrive at the earth at is their emitted angle. As gravity would always be measured at c, if we assume it's like light, we can't measure its local velocity variations. This leaves that we must measure the differentials of the strength of gravity, which is related to how the sun's motion deviates from a simple complimentary orbit to earth's. Effectively, we'd have to measure the redshift or blueshift of gravity, and similar properties, same as how we observe light to make conclusions about distant objects. However, this is caused by other planets, largely Jupiter, in their own orbits, which is an incredibly subtle effect, and I find it doubtful we'd be able to measure the changing strength of the sun's gravity meaningfully, so we would not be able to make conclusions based on our own solar system. These observations might be able to get useful data if we can observe somewhere generating strong gravity waves, two orbiting neutron stars or black holes, but I don't know if we've actually seen any close enough to home to measure.

So in the end, I find it very doubtful that we know about the propagation of gravity, truly - people theorise it's limited to c, but as far as I last heard, there is yet to be evidence to show such, and I'm not convinced we know how it would affect our own solar system to make predictions of that, either.

Edited February 5, 2015 by Iskierka

[Sirrobert]

This is amazing discussion! Nobody on Earth knows the answer for sure, but it is amazing nevertheless.

Understanding gravity is a door yet to be opened. There are several theories or hypotheses but none of it are without flaws.

I love those discussions, where that tiny fragment of information I read some time ago can still add something

[Mr Shifty]

This site explains both how general relativity works to eliminate the 'propagation delay' problem and what experimental observations have been made of gravity's speed. It's been measured to within 1% of the speed of light:

In the simple newtonian model, gravity propagates instantaneously: the force exerted by a massive object points directly toward that object's present position. For example, even though the Sun is 500 light seconds from the Earth, newtonian gravity describes a force on Earth directed towards the Sun's position "now," not its position 500 seconds ago. Putting a "light travel delay" (technically called "retardation") into newtonian gravity would make orbits unstable, leading to predictions that clearly contradict Solar System observations.

In general relativity, on the other hand, gravity propagates at the speed of light; that is, the motion of a massive object creates a distortion in the curvature of spacetime that moves outward at light speed. This might seem to contradict the Solar System observations described above, but remember that general relativity is conceptually very different from newtonian gravity, so a direct comparison is not so simple. Strictly speaking, gravity is not a "force" in general relativity, and a description in terms of speed and direction can be tricky. For weak fields, though, one can describe the theory in a sort of newtonian language. In that case, one finds that the "force" in GR is not quite centralâ€â€it does not point directly towards the source of the gravitational fieldâ€â€and that it depends on velocity as well as position. The net result is that the effect of propagation delay is almost exactly cancelled, and general relativity very nearly reproduces the newtonian result.

While current observations do not yet provide a direct model-independent measurement of the speed of gravity, a test within the framework of general relativity can be made by observing the binary pulsar PSR 1913+16. The orbit of this binary system is gradually decaying, and this behavior is attributed to the loss of energy due to escaping gravitational radiation. But in any field theory, radiation is intimately related to the finite velocity of field propagation, and the orbital changes due to gravitational radiation can equivalently be viewed as damping caused by the finite propagation speed. (In the discussion above, this damping represents a failure of the "retardation" and "noncentral, velocity-dependent" effects to completely cancel.)

The rate of this damping can be computed, and one finds that it depends sensitively on the speed of gravity. The fact that gravitational damping is measured at all is a strong indication that the propagation speed of gravity is not infinite. If the calculational framework of general relativity is accepted, the damping can be used to calculate the speed, and the actual measurement confirms that the speed of gravity is equal to the speed of light to within 1%.

[WestAir]

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.

This is what PackledHostage is questioning. How can something interact with its surroundings when the information telling the Universe how it should interact is trapped inside a singularity. Once you fall into an event horizon, all physical space-time directions bring you closer to the center. (I.E, the actual singularity is ALL AROUND YOU like the walls of the garbage compactor from Star Wars. No direction brings you back to the Universe, all directions bring you closer to the center). So how can an object stuck in this cosmic garbage compactor send a note to you or I telling us its mass, spin, or charge?

K^2 gave a pretty decent excerpt a few months back explaining the answer, but it went over my head. The fact is, however, that saying "Gravity isn't affected by spacetime" isn't a valid retort, because we know that it is.*

*Except in this instance, obviously.

Edited February 5, 2015 by WestAir

[Sirrobert]

This is what PackledHostage is questioning. How can something interact with its surroundings when the information telling the Universe how it should interact is trapped inside a singularity. Once you fall into an event horizon, all physical space-time directions bring you closer to the center. (I.E, the actual singularity is ALL AROUND YOU like the walls of the garbage compactor from Star Wars. No direction brings you back to the Universe, all directions bring you closer to the center). So how can an object stuck in this cosmic garbage compactor send a note to you or I telling us its mass, spin, or charge?

K^2 gave a pretty decent excerpt a few months back explaining the answer, but it went over my head. The fact is, however, that saying "Gravity isn't affected by spacetime" isn't a valid retort, because we know that it is.*

*Except in this instance, obviously.

Because spacetime bending is the thing that causes those effects.

Compare again to a ball on a trampoline. Even if nothing can climb out of the hole made by the ball, there is still a hole that effects everything around it

Gravity having a speed limit just means that, when you put the ball on the trampoline, it takes some time for the rest of the fabric to deform (for a trampoline, that'd be the speed of sound for that fabric)

[PakledHostage]

K^2 gave a pretty decent excerpt a few months back explaining the answer, but it went over my head. The fact is, however, that saying "Gravity isn't affected by spacetime" isn't a valid retort, because we know that it is.

I remember that thread as well and I agree that the answer in that thread was something of a "field goal" for me. He did give another more succinct explanation a page or two back in this thread, however. The answer seems to be (I am paraphrasing) that the mass that collapses to form the black hole was there before the collapse and was already warping spacetime, so the information about how much mass was present before the black hole formed already existed outside the black hole and doesn't have to get out. The information persists after the black hole forms in the manner in which the black hole's mass continues to warp spacetime outside of the event horizon. Whether that mass is concentrated into a "cosmic trash compactor" of a singularity or not, no mass has been added and not much changes for an observer sufficiently far away to be unaffected by tidal forces or the event horizon itself. Feel free to correct me if I've misunderstood anything; I am sure that there are many subtleties to this topic.

P.S. Reading K^2's response, I am also reminded of the thought experiment that I read many years ago about what happens to our solar system if the Earth were turned into a black hole by a mad scientist.

[SolarLiner]

P.S. Reading K^2's response, I am also reminded of the thought experiment that I read many years ago about what happens to our solar system if the Earth were turned into a black hole by a mad scientist.

Did it too, and it turns out, that not much will be affected. Locally everything will be sucked, but the Moon, instead of orbiting the Earth, will be orbiting a black hole. As simple as that. If you turn the Earth into a black hole, mass will be conserved, and so will the gravitational well be. On the end, we're all dead, but the rest of the solar system will not care.

However, if matter comes by and enough of it is sucked by the newborn black hole, then it'll gain mass and start growing, sucking even more things, expanding its gravitational well. If enough matter has been added to the black hole (in a short enough amount of time, else it'll be consumed because of the Hawking radiation), the newborn black hole will eventually eat the Sun, gain even more mass, and start to be a problem to nearby interstellar objects.

[WestAir]

Because spacetime bending is the thing that causes those effects.

Compare again to a ball on a trampoline. Even if nothing can climb out of the hole made by the ball, there is still a hole that effects everything around it

Gravity having a speed limit just means that, when you put the ball on the trampoline, it takes some time for the rest of the fabric to deform (for a trampoline, that'd be the speed of sound for that fabric)

It's not that I'm saying gravity can't go fast enough to escape, I'm saying there's no linear path of space that gravity can follow to escape. It's stuck in a closed system. I'm imagining it like if someone placed a 1,000,000 ton marble on a trampoline and it just punched a hole into the fabric. The fabric will return to a flat state, and the information (weight) the marble would normally convey can't escape. As I understand it, that's a fair analogy. The inside of an event horizon is like the inside of a balloon - all directions a graviton can travel will stay inside the balloon because there's no linear track that leads outside of a balloon.

Edited February 5, 2015 by WestAir

[Sirrobert]

It's not that I'm saying gravity can't go fast enough to escape, I'm saying there's no linear path of space that gravity can follow to escape. It's stuck in a closed system. I'm imagining it like if someone placed a 1,000,000 ton marble on a trampoline and it just punched a hole into the fabric. The fabric will return to a flat state, and the information (weight) the marble would normally convey can't escape. As I understand it, that's a fair analogy. The inside of an event horizon is like the inside of a balloon - all directions a graviton can travel will stay inside the balloon because there's no linear track that leads outside of a balloon.

Except that a black hole still doesn't punch a 'hole' in spacetime. It's still a dip. Just a very deep dip.

Your theory seems based on the fact that a graviton is somehow effected by gravity (gravity being the thing that stops everything else from escaping). But a graviton IS gravity, it's not EFFECTED by gravity (like a photon would be).

You can't draw a parallel between a force carrier (graviton) and a regular particle (photon) (false on this claim)

Edited February 5, 2015 by Sirrobert

[Mr Shifty]

You can't draw a parallel between a force carrier (graviton) and a regular particle (photon)

Except that photons are precisely 'force carriers' for the EM field.

[Sirrobert]

Except that photons are precisely 'force carriers' for the EM field.

Really? Well scrap that scentence than. I'm nowhere near an expert either, I gues this shows that.

[WestAir]

Except that a black hole still doesn't punch a 'hole' in spacetime. It's still a dip. Just a very deep dip.

Your theory seems based on the fact that a graviton is somehow effected by gravity (gravity being the thing that stops everything else from escaping). But a graviton IS gravity, it's not EFFECTED by gravity (like a photon would be).

You can't draw a parallel between a force carrier (graviton) and a regular particle (photon) (false on this claim)

Absolutely true. Gravitons are not affected by gravity, but they still propagate through space. If space is bent into a closed loop, there is no vector that a wave of gravity can travel that leads out. Gravity and space are two different things, you can't say gravity doesn't follow a coordinate system in linear space-time.

Edited February 5, 2015 by WestAir
Incorrect spelling.

[Sirrobert]

Absolutely true. Gravitons are not affected by gravity, but they still propagate through space. If space is bent into a closed loop, there is no vector that a wave of gravity can travel that leads out. Gravity and space are two different things, you can't say gravity doesn't follow a coordinate system in linear space-time.

But it's gravity that's bending space in a closed loop in the first place. So would that make this a chicken and egg problem?

[WestAir]

I have absolutely no idea. I'm obviously wrong (because black holes exist), but I can't configure how I'm wrong. If gravity weren't affected by space, then we'd see all sorts of weird anomalies (For instance, we'd still be feeling the gravity of Galaxies outside our observable Universe, because the expansion of space would have no effect on the propagation of the gravity waves of those Galaxies) On the flip side, if this weren't the case, Earth would orbit the Sun where it were 8 minutes ago (In fact, we orbit where the Sun is, not where it was 8 minutes ago when its light left).

Long post made shorter: I'm confused.

[K^2]

Because spacetime bending is the thing that causes those effects.

Compare again to a ball on a trampoline. Even if nothing can climb out of the hole made by the ball, there is still a hole that effects everything around it

Ball on a trampoline is a false analogy. Do not rely on it too much. For starters, ball on trampoline cannot recreate black hole condition. While changes on trampoline surface do propagate at a finite speed, these are not affected by self curvature. Gravitational waves are.

If we insist on decomposing interaction into virtual particle exchange, it is better to look at electrostatic interaction. After all, black hole can have electric charge, and we can actually do Quantum Electrodynamics in curved space. Here, the trick is to realize that virtual particles can follow space-like curves. They are not bound by speed of light rules. There is a big difference in force propagation and propagation of change in force. Later carries information and is limited to c. Former, might as well be instant. Of course, "instant" is a frame dependent concept. It is all just math trickery when we describe forces as particles.

[WestAir]

K^2,

So then a gravitational field is static and what propagates at C is information containing changes to that field? In other words, once a field is made there no information exchange necessary unless a change is made? Am I following that correctly?

[*Aqua*]

While reading this thread I came upon the question how bended spacetime would behave different compared to "unbended" spacetime.

Let's take this picture which is common to illustrate bended spacetime. But illustrations on that topic are often simplified and show some details wrong. My question is, do both marked lines have the same length? (If we use a folding yardstick or similiar.)

[Sirrobert]

While reading this thread I came upon the question how bended spacetime would behave different compared to "unbended" spacetime.

Let's take this picture which is common to illustrate bended spacetime. But illustrations on that topic are often simplified and show some details wrong. My question is, do both marked lines have the same length? (If we use a folding yardstick or similiar.)

http://i.imgur.com/SFgCwft.png

Thinking outloud here:

The 2 lines are the same 'distance' in spacetime, which is how time dilation works? Emphasis on question mark, I have no clue if that's actually how it works.

@K^2: Ok, that does make things a little clearer. Quantum physics is weird

[pellinor]

My question is, do both marked lines have the same length? (If we use a folding yardstick or similiar.)

http://i.imgur.com/SFgCwft.png

The are not. Far from the middle, the geometry gets more and more flat (i.e. like euclidean geometry). And in euclidean geometry concentric circles can not have the same circumference.

The lines sketch a coordinate system. Just like longitude and latitude on earth, the grid lines do not have the same length. In fact, you can recognize a curved space by the property that it does not allow a coordinate system in which all grid lines have constant length.

[Bill Phil]

The two lines would have the same measure using a yardstick. But that's because the yardstick would get smaller too...

[*Aqua*]

Ok, so the distances get shorter the further you are in the gravity well. Does that also mean that light travels "faster" in a bended space compared to an unbended spacetime? Or does it get "shorter", too? Or does time dilation make it up (as space is compressed, the time needed to pass a certain distance becomes longer -> they cancel out each others effect)?

[pellinor]

The two lines would have the same measure using a yardstick. But that's because the yardstick would get smaller too...

If this was true, then that part of space would be flat. See, supposing radial symmetry, you can always put your coordinates in a way such that all radial lines are the same length. Now suppose the other lines were also the same length (within some region of the space). Then the coordinate grid defines a length-preserving mapping (i.e. an isometry) between a flat space and this region. Since an isometry preserves curvature, the above space with the 'yardstick' distance measure would be flat. All the circles would have the same length. Hence, the geometry you are describing is that of a straight cylinder.

If this is what the picture wants to illustrate (which I doubt), then it would not be a good choice of illustration.

[Bill Phil]

It's perceived to be the same length, but the two lines aren't in reality.

[pellinor]

Ok, so the distances get shorter the further you are in the gravity well. Does that also mean that light travels "faster" in a bended space compared to an unbended spacetime? Or does it get "shorter", too? Or does time dilation make it up (as space is compressed, the time needed to pass a certain distance becomes longer -> they cancel out each others effect)?

Actually, in a radial coordinate system (see the north pole of the earth), those distances are supposed to get smaller een in a flat space. What the image shows is that they get smaller at a different rate than a 'flatlander' from the outside would expect.

I'd like to point your attention to a different viewpoint: geodesics. On this surface, what happens if an ant starts in some direction and then walks 'straight on' (suppose gravity is negligible) ? Just like the latitude circles on the earth, the circles in the illustration are not geodesics. So if you start tangential to a circle and walk without turning, your radial coordinate increases. Far away from the center, curvature converges to zero and your path converges to a straight line.

Since a geodesic is the analogue to a straight line in curved space, in general relativity light travels on geodesics. So, if we send a ray of light close to this curved region of space(time), it is bent towards the middle. If one source sends multiple rays into the middle region, they will cross again on the other side. For rays of light, this effect is called gravitational lensing.

Feel free to give feedback if you can't follow, I am probably doing this too fast and should give more intermediate steps.

Edit: a small comment to the above illustration in general: I think what it illustrates really well is geodesics. Its main weakness is that people think of gravity as an extra force on top of the geometry, pulling things 'down', which is just plain wrong. The model would work just as well if you turn the surface 90° into the vertical, like the mouth of a trumpet. In general relativity, light rays (and other trajectories) do not bend because there is some magic force pulls them away from a straight line, but just follow the concept of 'walking straight on without turning', which is the same as going on a 'locally shortest path'. General relativity does not know a gravitational force at all. Gravity is encoded in the curvature of spacetime, and trajectories just need to start with some initial condition and then 'walk straight on without turning left or right'.

Edited February 5, 2015 by pellinor