how fast is gravity?

Factual Questions
21

Yeah, I used to think that gravity was faster than light, too, else how could a black hole have gravity? There’s an interesting twist to this: In addition to mass, black holes can also have electric charge (and magnetic charge, if such a thing exists). This means that a black hole can exert an electric force on an outside object as well. Now, here’s the fun part: Just as gravity is believed to be vectored by gravitons, electric forces are vectored by (drum roll please) photons. Hence, a charged black hole is exchanging photons with any other charged objects in its vicinity. How can this be? The answer is that the rule you’ve heard that “no particle can escape a black hole” isn’t quite right. No real partical can do it, but a virtual particle, such as a photon or graviton vectoring their respective force, can, subject to the restriction that it does so in such a way that it can’t transmit any information about the interior of the hole other than total charge and mass.

As to the nesessity of gravitons, when we’ve already got a theory of gravity without them: Nothing in GR alone predicts the existence of gravitons. However, for gravity to be consistent with the Standard Model of particle physics that correctly describe all of the other forces, it’s got to exist. It’s just another example of a duality, no more problematic than the wave-particle duality of photons, for example.

22

**
…what prevents us from wiggling ball A in just the correct way to make ball B change it’s orbit in time to the latest gangsta rap song?..**

Good taste and physics. Disgustubis non disputandum est, but as to the physics…

wiggling aint gonna get it. You’re gonna have to cause the mass of ball A to fluctuate rythmically to cause the warped space of the A-B system to fluctuate rhythmically, hence therefore to cause the orbit of B to etc. etclly.

Other than radical acceleration/deceleration (and I do mean radical!!) is there another means to increase/decrease the mass of our balls?

23

Exactly, and since the warping of space only propagates at the speed of light - or so we think - information can only travel at the speed of light.

In fact, if you move any mass, the disturbance propagates as a gravitational wave. If two massive objects - neutron stars or black holes - orbit around each other, the energy radiated as gravitational waves may not be negligible. Still, they are extremely weak. There are attempts to observe them, such as the LIGO and TAMA300 project, but nobody has succeeded yet. (I can see TAMA300 from my office window, by the way - what little of it that’s above ground.)

24

I’m learning a lot about what I don’t know from this thread, but I can see that if you move ball A back-and-forth in a coded pattern, then the “warpage” of space at ball B will be affected by it, and you could decode the information. The mass doesn’t need to change, only move. If B can tell its movements faster than the speed of light, there will be a lot of surprised scientists.

25

Is anyone else wondering right about now just what FF’s background is? He seems to be pretty comfortable tossing around terms like “disturbance of space”, but when it comes to the question of transmission of information, it’s like he’s talking a different language from the rest of us.

26

I admit that when it comes to Modern Physics, I’m an utter newbie, but I was just thinking about something. All of us (or at least many of us) are aware of these two facts:

  1. Gravity can be used to transmit information.
  2. Information can not be transmitted faster than c.

And I think that people are assuming that from these two statements follows:

  1. Gravity can not travel faster than c.

Now in normal circumstances, I would agree, that 3 does follow from 1 and 2. But Quantum Physics is anything but normal. Would it break any well-established laws of Relativity or anything, if Gravity were instantaneous, but that the change brought about by Gravity were so small that we could never measure it? And I do know that there are theoretical, not just practical, limits on how accurately things can be measured (IE The Uncertainty Principle (No, I do not know how to spell Heisenberg)).

Let me try to give an example. Approximately 17 posts up, Chronos gave the really neat explanation of the two beads. Now suppose they were one light-second away from each other, and suppose that the theoretical lower limit in the uncertainty of the position of the top bead is one nanometer. That means you can’t tell that it’s moving until it’s moved one nanometer. Now if you assume that gravity is instantaneous, you can compute how fast that the top bead will react to the bottom bead’s motion. What if you did work it out, and you found out that no matter where you moved the bottom bead, it would take at least one second for the top bead to move one nanometer? Then, the information itself isn’t really being transmitted instantaneously, since there’s no way to measure it. Does that example make any sense? And if so, please punch holes in it.

27

An object has a gravity well, and that gravity well travels at the same speed as the object right?

I don’t think I understand the point of the gravity “propagating.”

28

We’re talking about the time lag between cause and effect. If something caused the Sun to somehow disappear instantaneously, we wouldn’t know until eight-something minutes later. If the Sun started accelerating in any direction at any rate, the effect on the Earth’s orbit would not happen until eight-something minutes later.

At the risk of complicating things further, there is a valid question involving “non-moving” objects. If changes at the Sun are indeed not felt until eight-something minutes later, how does the Earth know to orbit around the Sun if it doesn’t know where the Sun is right now? One can show mathematically that if gravity is a central force field, as it is in Newtonian mechanics, and if the effects of gravity do not propagate at infinite speed, then the Earth’s orbit (and all orbits) are unstable. There are stable orbits; at least stable over the time Mankind has been observing the universe, and there is good reason to believe there are orbits stable over longer periods. Therefore one of those two assumptions is incorrect. In the paper referred to at the beginning of this thread, van Flandern concluded that the second assumption is incorrect (although he didn’t put it the same way I did). In Does Gravity Travel at the Speed of Light? the authors point out that, in General Relativity, gravity is not a central force field, although in some situations it may be modeled as a non-central force field.

29

General statement: The Standard Model (quantum mechanics) includes and is compatible with Special Relativity, which in turn includes “you can’t transmit information faster than light”. It’s General relativity that is inconsistent with the SM. So, the top level answer to your scenario is “you can’t transmit information faster than light. So there!”. We do have reason to believe that quantum efects which do not and cannot carry information can be transmitted faster than light; see EPR Paradox and Local Variables.

Bu t to address your specific example … the quantum limits on certainty apply to paired variables. If you can come up with an appropriate measurement system, you can measure the position of that bead to essentially arbitrary precision, maybe even to totally arbitrary position; you just have to accept that the more precise your position measurement is, the less precise is your momentum measurement. Quantum Mechanics does not say (today) that there is a limit on how precisely you can measure the position of a bead.

30

Darkly, he lurks through the sub-basement of Graduate School, shivering with caffeine. Submitted numerous posts of profound significance and shattering insight, only to have his genius cruelly mocked by message board smarty-pants. Torn and humiliated, he stumbles blindly from the keyboard…

imHop: If gravity is nothing more than another way to say “warped space”, and gravity is directly a product of mass, if the mass changes, the gravity changes at the same instant that the mass changes.

So…if mass B orbits around mass A at roughly a light hours distance. Mass A suddenly loses x amount of mass, poof! Since “warpedness” is a property of space, that will change immediatly, the orbit of B changes accordingly. Of course, anything traveling through space is limited to light-speed. But does that same limitation apply to space itself? I think maybe not. That would mean the orbit of mass B would not be affected by the space it occupys until one hour after the mass change of A. What kind of experiment might test such an idea? Feed sun the a gazillion tons of jelly donuts all at once? Is it feasible, in theory, to cause major fluctuations in the mass of an object in space?

31

We do not have any practical method today for experimentally measuring the speed of gravitational propagation, and we
do not have any practical method today for causing major fluctuations of an object’s mass in short periods of time, unless you count 'elementary particles" in a particle accelerator.

You have not yet offered any spport, such as plausible arguments or calculations or references, to support this assertion. You do obviously like this assertion {grin}, since you’ve made it several times. You are, of course, disagreeing with the vast majority of people who have studied this very situation in great depth. Some support woudl be interesting.

Let’s try a thought experiment.

Assume a universe empty expect for:

Two objects which have mass (but not a lot of mass), object A and object B, one light-minute apart. Objects A and B are currently at rest relative to each other at this instant, although they are slowly accelerating towards each other due to gravitational attraction.

A comparatively massless observer, much closer to object A than one light-minute at this point in time, and capable of measuring any change in the acceleration of object A.

A comparatively massless actuator, close enough to object B (and strong enough) to physically push on and move object B.

The observer and the actuator have previously agreed that pushing object B towards object A stands for Morse Code dash, and pulling object B away from object A stands for Morse Code dot, and moving object B back to its original position (relative to the actuator) stands for Morse Code space.

Now start the experiment. The actuator moves object B towards object A three times, returning it to its original position after each push. This takes significantly less than a minute.

Object A’s acceleration will change six times (once for each push of object B and once for each returning of object B to its original position).

Has not the actuator transmitted information to the observer, the letter “O” in Morse Code? How long does it take this information to travel to the observer? How do you square your answer with General Relativity?

32

Oh, FreakFreely, I am assuming that you realize that the mutual gravitational attraction between two objects depends both on the masses of the objects and the distance between the objects …

33

I had some extra time, so I dug up some reference information:

From Ask the Astronomer: Special & General Relativity Questions and Answers:

“Does gravity travel at the speed of light?
We don’t know, because gravitational waves are so weak that we cannot detect them in order to time them. However, gravitational radiation is described very accurately by Einstein’s theory of general relativity and in particular his field equation for gravity. The basis of this theory is that the velocity of gravity is limited to the speed of light.
A very precise study of the so-called Taylor-Hulst binary pulsar system shows that the system is losing energy at exactly the rate prescribed by Einstein’s field equation for gravity. This would only be the case if this equation represented the essential physics of the phenomenon of gravity with high fidelity. The key assumption is that, like all other forces, gravity travels at exactly the speed of light. I don’t know if this is compelling evidence to the non-physicist, but to me it does. In the future, we may be able to test this directly if we can figure out some way of detecting under laboratory conditions a gravitational wave.”

“If nothing can travel faster than light, why does gravity take affect instantly?
Woah! Who said that gravity operates instantaneously? There is no observation that justifies this ‘Newtonian’ statement, and lots of evidence from special and general relativity that gravity obeys the speed limit set by the speed of light. The dynamics of our solar system, with light travel times up to several hours, would be very different if gravity acted instantaneously. The calculation of planetary positions and their mutual gravitational influences would lead to different predictions for where the planets are in the sky at a specific time, especially for the planets beyond Jupiter where the light travel times are the longest.”

“Can gravity waves be used to carry information?
In principle they can, but the engineering required to detect modulated gravity waves is formidable. Many natural phenomena also produce modulated gravity waves.”

"How does gravity travel through space?
Mathematically, according to general relativity, it takes advantage of the fact that the curvature of space-time is mathematically defined by 10 more curvature terms than are constrained by the portion that is controlled by matter. In a previous question and others in the ‘Physics – Relativity’ archive, I have mentioned how Einsteinian gravitational forces are DEFINED as the curvature of space-time. But, this curvature in 4-dimensional
space-times is specified by 20 distinct terms. Only 10 of these are defined by the local distribution of matter which
is mathematically found by solving Einstein’s equation of gravity. The other 10 terms in the full ‘Riemann
Curvature’ tensor define how the space-time outside the massive body respond to the presence of the mass, and define a ‘source-free’ solution for gravity. This only happens in space-times with more than 4 dimensions, which is why gravity does not exist as a force in space-times of dimensionality of 3.

That is the mathematical reason why gravity can ‘travel’ through space. In general relativity, gravity and space-time are EXACTLY THE SAME THINGS BY DEFINITION. It is impossible, within general relativity, to separate gravitational fields from the fundamental properties of space-time. This is like trying to define what a computer is without its software."

From Ripples in Spacetime:

“Predicted in Einstein’s General Theory of Relativity, gravitational waves are disturbances in the curvature of spacetime caused by the motions of matter. Propagating at (or near) the speed of light, gravitational waves do not travel “through” spacetime as such – the fabric of spacetime itself is oscillating.”

From Theory of Gravitational Waves:

"In 1916 Einstein found out that the General Theory of Relativity (“Geometrodynamics”) predicts the existence of gravitational waves. The changes in the gravitational field introduced by the movement of large cosmic masses do not propagate instantaneously at arbitrary distances from the source, but with finite velocity. In the Theory of Relativity this is the speed of light c.

Gravitational waves are perturbations in the curvature of spacetime propagating with the velocity of light. They are caused by accelerated masses."

From Effects of Relativity:

“One potentially observable prediction of relativity is the existence of gravitational waves. Imagine two stars revolving around each other. The gravitational field from these stars will change periodically due to this motion. However, this change propagates outward only at the speed of light. As a result, ripples in the field, or gravitational waves, spread outward from the revolving stars.”

34

What does it mean for a signal to travel instantaneously? It means that the sending of the signal and the receipt of the signal are simultaneous. However, according to special relativity, which is as close to being a certainty as it’s possible for anything in physics to be, simultanaity isn’t defineable: If, in one reference frame, two events are simultaneous, then in a frame moving to the right, the event furhte right happened first, and similarly for a reference frame moving to the left. I don’t think that most fols are too comfortable with the notion that the motion of one mass might cause changes in the motion of a second, before the first one moved.

FreakFeely, I didn’t mean any offense to a fellow graduate student, I was just a bit puzzled. Total ignorance of relativity I’m familiar with, this being the state of the vast majority of people. Knowledge of relativity I am likewise familiar with, as this is the state of the people with whom i customarily work. It’s just that you seem to display an interesting combination of ignorance and knowledge-- Which still places you as rather more knowledgeable on the subject than most people.
If it helps you to visualize the error in your argument, picture a waterbed (actually playing around with a real waterbed would be better, but I realize that a graduate student might not have access to same :slight_smile: ). If you press down on part of the bed, the shape of the bed changes over the whole surface, to some degree. Now, if you remove your hand, the surface will flatten out, but it won’t do it instantaneously-- you’ll get ripples travelling out to the rest of the bed at some definite speed. Similarly, changes of some sort in a massive object will cause ripples in space-time that will travel out at the speed of light. Better?

35

Keep in mind all of this is purely theoretical.

As I understood from reading a number of physics text (and perhaps some people more directly involved in physics can correct any misinterpretations) gravity is still a big mystery…we know what it does but not how.

36

There certainly are some mysteries in gravity. We have a mathematical model (general relativity) for how it works, and that model works wonderfully well in many many many situations, but not quite in all. That model includes the effects of gravity propagating at the speed of light in vacuum. We do not have any direct measurements of the sped of gravitational propagation. We do have some measurements that almost certainly would come out differently than they do if gravity propagated at some other speed.

Because any model that replaces or extends general relativity must make the same predictions as general relativity in those many many many situations that have been directly checked, it is unlikely that such a model will include gravity propagating at some other speed.

37

The folks who say that gravity is a mystery are just biased to particle physics (no offense, Karen). What they mean is, that the other three forces are fairly well explained by the standard model of particle physics (as I understand it, the strong force could still use some work), but that there is (as yet) no explanation for gravity in the standard model. This does not mean that we don’t have an explanation for gravity, it just means that we don’t have the same explanation for gravity as we do for the others. As JonF said a few posts above, General Relativity does a good job explaining gravity, but it doesn’t mesh well with the Standard Model. Personally, I’m inclined to believe that GR is more satisfactory than the standard model, and that we need to bring the Standard Model in line with relativity, but then, I’m biased, too.

38

One goes to message boards for interesting conjecture, if I want the Recieved Wisdom, I go to school. I been to school.

JonF, thanks for your concern, I am indeed aware of that stuff about gravity is mass divided by distance, etc. Something about an apple, wasn’t it? Isaac Nuetron, or some such? Isn’t he the guy who poisoned some German guys cat?

You experiment is intriguing, but the premise is a bit awkward. Two masses are at rest relative to each other at this instant, but slowly accelerating… Well, now, which is it? Is there some quantum mojo that allows two bodies to be at rest and accelerating at the same instant? To paraphrase Mark Twain, thats too many for me, I fold.

Have doggedly pursued trying to absorb the implications of your posted Authorities. (I may not be smart, but I’m obstinate!) My reading would suggest that the subject is still open. Thank you for your evenhandeness, it speaks well for you intellectual honesty. Unless I misunderstood.

39

Quoth FreakFreely:

Yes, except it’s not quantum mojo. It’s basic Calculus. If a body is at rest, then v = 0. If it’s accelerating, then a = v’ != 0. There’s nothing conflicting about these two statements. A function can be zero with a non-zero derivative. It won’t be like that for long, but when it’s an “instant” you’re talking about, there’s no contradiction.

40

Not particularly complicated, and there’s no need for quantum mechanics. I was merely specifying initial conditions and acknowledging reality.

Is it not obvious that it is posslble for two bodies to be at rest relative to each other at the instant that the experiment starts? That’s the specification of the initial state.

Is it not obvious that, in the situation I described, the two bodies will feel mutual gravitational attraction and accelerate towards each other?

If I may ask, what have you read?