how fast is gravity?

[arthur_einstein]

If I switch on a nuclear reactor on earth, at what speed would the (small) effect of the reduction of the earth's mass travel out to the rest of the universe?

I seem to remember information cannot travel faster than light, so that puts a top end on the speed. I just wonder if gravity is a speed of light kind of thing, or maybe much slower. Has anyone measured it?

[Ringo]

First off, I don't know the answer, and I really need to get back to the office, but, quickly...

I learned, as we all did, the nothing can exceed the speed of light. But gravitational attraction is regarded as instantaneous.

There's a fellow named Tom Van Flandern at the University of Maryland who published an article in Physics Letters A (12/21/98) that claims the speed of the propagation of gravitational force must be at least twent billion times faster than the speed of light. Whoohoo! That's fast!

That's all I've got time for now. Try a web search on that guy's name.

[Earl Snake-Hips Tucker]

I have no idea.

But here is what I've gathered:

Gravity, under our current model, does not emit any particles (or at least no one has found any). Instead, it is a warping in space-time caused by mass. The common model is a stretched rubber sheet, with a bowling ball representing the sun, then various other fruits and seeds of varying sizes representing the planets. Bowling ball = big warping. Fruits and sees = smaller warping. That's a two dimensional (sorta) model. Gravity does the same thing, but in three (or four, or more, who knows?).

So. . .

It would appear that "gravity" would travel at the same time as the mass causing the warping.

[SingleDad]

Gravity, like all the other forces, travels at the speed of light. As we orbit the sun, we're responding to the gravity "emitted" ~8 minutes ago by the sun. Similarly, when you open your eyes, it doesn't take 8 minutes to see the sun; you see the photons it emitted 8 minutes ago. So, in that sense, response to gravity is "instantaneous".

Generally speaking, nothing that can carry information can travel faster than the speed of light. Since one could (in theory) modulate gravity waves by moving large masses relative to one another, you can be pretty sure on general principles that it can't travel any faster than the speed of light.

Of course, if you could show experimental evidence that some information-carrying phenomenon could travel faster than the speed of light, then I'm going to suck up to you because I would love a free trip to Stockholm.

[Perderabo]

Here is the answer from the Relativity FAQ:

http://math.ucr.edu/home/baez/physics/grav_speed.html

[Gilligan]

Since the question’s been answered already, I just wanted to throw in a personal story. Back in high school in the 70’s I used to wonder this same thing. This being B.C. (before Cecil) I wrote to Isaac Asimov and asked him. He wrote me back and said pretty much the same thing Mjollnir and SingleDad did. I still have the letter, and his business card which he enclosed.

[bibliophage]

Just as the photon is the electromagnetic field particle, it has been hypothesized that gravity has a "gravitational field particle", called the graviton. If it exists (and it hasn't been detected), it would have (probably) zero rest mass and would (probably) propogate at the speed of light, just like the photon. See Graviton Question (http://www.fnal.gov/directorate/public_affairs/letters/graviton_1.html)

[Chronos]

As others have already pointed out, gravitational interactions travel at exactly the same speed as electromagnetic interactions such as light. One example of a way that you might use gravity to transmit information, is this: Suppose you have two rigid, parallel wires with a massive bead on each, like this:
------O------

------O------
Now, we move one of the beads:
------O------

---------O---
This will cause the other bead to move, so as to be as close as possible to the first
---------O---

---------O---
However, it will not respond in this manner until a short time after we move the first bead, that time lag being equal to the time it takes light to travel that distance.

Another fun fact: Anything, not just gravity and light, that has some definite speed must travel at the speed of light. Things that don't travel at c, like sound, must always travel relative to some medium.

[Padeye]

Maybe this is because of my limited understanding of relativity but would the reactor experiment cause no net change in the earth's gravitational field/space warping? Matter is not being destroyed, only converted to energy.

[DougC]

- - - No advanced education, but it seems to me that if you accept the concept of a black hole absorbing all energy, then gravity must be working faster than the energy absorbed. - MC

[DougC]

- - - No advanced education, but it seems to me that if you accept the concept of a black hole absorbing all energy, then gravity must be working faster than the energy absorbed. - MC

[wevets]

If gravitational force is propagated by a particle (graviton), then wouldn't that require some revision of current thinking on black holes? Would black holes then emit light, because the photons carrying the light would be travelling as fast as the gravitational force that would pull them back into the black hole?

[SingleDad]

Padeye: Immediately, you're correct. You're merely converting the energy of matter to a different form of energy; both have identical graviation.

However, "energy" energy converts quickly to heat, whereas "matter" energy generally stays put. Once you convert the energy to heat, it will soon radiate away, reducing the mass-energy of the earth.

MC: The gravity of the black hole "warps" space-time. The propagation of this warpage is at the speed of light, but once warped, space-time stays warped. Once a particle enters this already warped space-time, at a certain point all of its possible futures lead to the singularity at the center.

[emarkp]

There's a fellow named Tom Van Flandern at the University of Maryland who published an article in Physics Letters A (12/21/98) that claims the speed of the propagation of gravitational force must be at least twent billion times faster than the speed of light. Whoohoo! That's fast!
Unfortunately, it's crap. I don't think he's associated with any University either. I found his paper and it makes sense intuitively, but he apparently disregards much of general relativity in proving his point. I read some of his posts on Usenet (via deja.com) and I concluded that his reasoning is incorrect. I think if you do an altavista/google search for his name, you'll find a webpage that refutes him.

[JonF]

There's a fellow named Tom Van Flandern at the University of Maryland ...

Herre's a link to the article: The Speed of Gravity - What the Experiments Say (http://www.ldolphin.org/vanFlandern/gravityspeed.html). I analyzed this paper in some depth during a thread a few months ago .. involving the V word ...

The major problem in the paper is that he's mixing relativistic analysis with Newtonian analysis; although relativity includes Newtonian mechanics, and Newtonian mechanics is sufficiently accurate for the vast majority of practical analyses, when you try to use some of the relativistic equations and some of the Newtonian equations in one model, the result is meaningless. He assumes a central force field (Newtonian) and then tries to analyze that situation using relativity.

The entry in the Relativity FAQ that's already been posted indicates how this approach is incorrect.

[FreakFreely]

Mass warps space, directly proportional to the amount of mass. The instant that mass becomes energy, the warpedness(?) of space changes. Speed of light constraints are only relevant for something traveling through space.

If two 10lb balls are orbiting each other in space, and one of them magically radiates a pound of nuetrinos or something, at that instant the mutual gravitational relationship, based on the warpature of space, is different.
Hence, the orbits of the balls are changed instantaneously

The "information" about the weight loss doesnt have to travel to the other ball, at whatever speed. Where the ball is, thats information, speed limits apply. Where the ball is gonna be, thats instantaneous.

[Silo]

How fast is Gravity? Throw your friend off a building and see how long it takes him to hit the ground. As he thuds into the ground the electrostatic field (on the ground) reacts much faster than the gravity does. :D

[Achernar]

"The "information" about the weight loss doesnt have to travel to the other ball, at whatever speed. Where the ball is, thats information, speed limits apply. Where the ball is gonna be, thats instantaneous."

FreakFreely, with all due respect, what the heck are you talking about? It sounds like you're saying that position is a quantity, whereas velocity is not, or there's some fundamental mathematical distinction between them.

I remember that I used to assume that Gravity was instantaneous, too. But then I realized that I'd been making an assumption, and that many of those Scientists who do things like this are a lot smarter than me. It's my understanding that, as SingleDad and Chronos have mentioned, Gravity is right now believed to travel at c, but that sensors do not yet exist accurate enough to show this experimentally. I am rather sure that a speed limit on Gravity does not conflict with black hole theories. Just because Gravity travels at the speed of a photon does not mean that it's subject to all the same laws as a photon.

Also, about the whole destruction of mass thing. When you fire up a nuclear reactor, as has already been pointed out, you're not decreasing the gravitational pull of the Earth one iota. The thing is, and I think I have this right, mass and energy don't get converted between each other. Mass and energy are equivalent. To say a particle has m mass or mc² energy are the same thing. Photons do have mass. I guess you could even have two photons orbiting each other. Another form of what we call energy, other than photons, is kinetic energy, the energy of something moving. But you remember from Special Relativity that when you speed something up, it gains mass? Well then, when you give an object Kinetic Energy, you are in fact giving it mass. Again, mass and energy are equivalent.

[FreakFreely]

The gravitational interaction between A and B is entirely the warping of space due to thier respective mass. Gravity is the warping of space, not an effect of it. The warping of space by mass is not "information" that needs travel through space, it is space itself. Hence, the amount of time necessary for a change of mass to affect the warping of space is precisely the amount of time necessary for the transition between mass to energy.

Ball A, having a change in mass, does not send a message to Ball B "change your orbit". Ball B changes its orbit immediatly because the space-warping through which it travels is changed immediatly by the mass change of A. If Uncle Albert is right, why do we need to dream up some "gravitons" to transmit gravity? When a light beam is warped around a gravity field, do gravitons travel to the light beam and bounce back? You gotta be kidding.

[JonF]

Ball A, having a change in mass, does not send a message to Ball B "change your orbit". Ball B changes its orbit immediatly because the space-warping through which it travels is changed immediatly by the mass change of A.

Sorry, that's information. If what you claim is true, 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? That is certainly information in the Shannon sense, even if you don't think that it's information in the everyday sense {grin}.

If Uncle Albert is right, why do we need to dream up some "gravitons" to transmit gravity?

There's no need for gravitons in today's formulation of General Relativity. But ...

General Relativity has been tested and investigated to an incredible degree. The Standard Model of particle physics (quantum mechanics) has also been tested and investigated to an incredible degree. Both produce absolutely and incredibly precise and unquestionably correct results except in extremely unusual situations where they both must be taken into account. In those situations, the results are not merely incorrect, they're meaningless. Something is wrong, and figuring out what is wrong and doing something about it is the[/i] unsolved question in today's physics.

It seems most likely that gravitons are part of what is needed to unite the two theories.

When a light beam is warped around a gravity field, do gravitons travel to the light beam and bounce back? You gotta be kidding.

Yup, it's almost certain that gravitons travel to the light beam and back. Nope, not kidding. There's no requirement that the universe "make sense". It's been proven that, when quantum interactions are being considered, the universe absolutely does [b]not "make sense" in terms of our common sense and everyday experiences.

One common illustration is two ice skaters throwing bowling balls back and forth. It's easy for us to imagine the effect; whenever a skater throws or catches a bowling ball, their motion changes. But this is purely "repulsive force"; a skater that throws a bowling ball recoils from the thrown ball, and a skater that catches a bowling ball picks up some motion in the direction the ball was moving. So the skaters move apart.

I don't know of any real-world analogy that illustrates how gravity (an attractive force) or electromagnetism (attractive or repulsive depending on the charges involved) is transmitted by the exchange of "particles". But that's not a failure of the scientific model; it's just an indication that quantum stuff is really, really, really weird. It's weirder than a snake's suspendors.

[Chronos]

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.

[FreakFreely]

....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?

[scr4]

Sorry, that's information. If what you claim is true, 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?

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 (http://www.ligo.caltech.edu/) and TAMA300 (http://tamago.mtk.nao.ac.jp/tama.html) 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.)

[CurtC]

FreakFreely wrote:
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...
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.

[Chronos]

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.

[Achernar]

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:

3. 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.

[Earl Snake-Hips Tucker]

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."

[JonF]

I don't think I understand the point of the gravity "propagating."

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? (http://math.ucr.edu/home/baez/physics/grav_speed.html) 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.

[JonF]

... 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.

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 (http://boards.straightdope.com/sdmb/showthread.php?threadid=25603).

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.

[FreakFreely]

... just what FF's background is?...


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?

[JonF]

Is it feasible, in theory, to cause major fluctuations in the mass of an object in space?

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.

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 ... Since "warpedness" is a property of space, that will change immediatly ...

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?

[JonF]

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 ...

[JonF]

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

From Ask the Astronomer: Special & General Relativity Questions and Answers (http://einstein.stanford.edu/gen_int/relativity/qanda.html):

"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 (http://www.ncsa.uiuc.edu/Cyberia/NumRel/GravWaves.html):

"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 (http://www.lisa.uni-hannover.de/shared/gravinfo/gravitywaves.html):

"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 (http://kestrel.nmt.edu/raymond/ph13xbook/node127.html):

"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."

[Chronos]

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 :) ). 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?

[avalongod]

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.

[JonF]

gravity is still a big mystery...we know what it does but not how

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.

[Chronos]

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.

[FreakFreely]

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.

[Achernar]

Quoth FreakFreely:
Is there some quantum mojo that allows two bodies to be at rest and accelerating at the same instant?

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.

[JonF]

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?.

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?

My reading would suggest that the subject is still open.

If I may ask, what have you read?

[FreakFreely]

Well, from the excerpts you posted. Allowing for the possibility that I have bong-water for brains, it seems to me that one or another of the Cited Ones rather contradict each other, don't your think? In the interests of avoiding bloating post syndrome, I wont quote.

As for the experiment, I guess I have to further assume for the stated masses A and B to be at relative rest AND accelerating towards each other is that they didn't exist until that very instant. That would do it, I guess. Other than that, I must confess, I can't. At rest,AND accelerating. Perhaps I'm too dim to digest the paradox, but there it is.

One of those quotes set me off, though:

If a mass propogates gravity waves, which behave in a similar fashion to light waves, i.e, thier relative speed

THEN

if Godawfulhuge Mass A is acclerating in my direction, might I get blue-shifted gravity waves? And vice-versa?

If we could in fact make two massess suddenly exist, if I follow you, then the amount of time that must pass before any mutual gravitational exists is directly tied to the speed of light, hence the speed of gravitational propagation.

In that case, would the waves meet at the mid-point between the two masses? Would their mutual gravity begin at that point, or does it take long enough for both waves to reach thier opposite number? And if gravity is waves, does it have wavelength? Interference phenomena?

[Koffing]

FreekFeely wrote:

As for the experiment, I guess I have to further assume for the stated masses A and B to be at relative rest AND accelerating towards each other is that they didn't exist until that very instant. That would do it, I guess. Other than that, I must confess, I can't. At rest,AND accelerating. Perhaps I'm too dim to digest the paradox, but there it is.


No paradox. Throw a ball straight up. At its maximum height, the ball's velocity is zero. Gravity is the only force acting upon it, so the acceleration is 9.8 m/s^2 down.

[Chronos]

Yes, gravity waves behave like other waves: they can interfere, they'll be Doppler shifted if the source is moving (not necessarily accelerating) relative to the receiver, etc. Note that not all gravity is gravity waves; if a mass is just sitting there, it'll be gravitating, but not waving. Something (position, distribution, amount, etc.) about the mass has to change to produce waves. The wavelength is just the propagation speed (c, in this case) divided by the frequency with which the system is changing.

As to an object at rest, acceleration just means that it's not going to stay at rest. If you're stopped at a light, and step on the gas, then at the moment you step on it, you're still at rest, but you're accelerating, but at any later moment, you're not at rest anymore.

[JonF]

it seems to me that one or another of the Cited Ones rather contradict each other, don't your think?

No. They do indeed explain things somewhat differently, but I do not see any contradictions. They all agree that gravitational effects propagate at "c".

Incidentally, I slightly resent your implication that I am employing an "Appeal to Authority" fallacy. I suspect that you do not understand the fallacy. I do not claim that those authorities are correct because they are authorities. Rather, their writings are a sample of current physical thinking that seem to me to be reasonably clear to the non-expert reader.

I guess I have to further assume for the stated masses A and B to be at relative rest AND accelerating towards each other is that they didn't exist until that very instant.

I hope this has already been adequately addressed by other posters. I suggest that you expend some though towards the difference between velocity and acceleration; if you don't understand the difference, you're going to have great difficulty holding your own in any discussion of physics.

In that case, would the waves meet at the mid-point between the two masses?

Yes, but I think that the only effect would be interference between the waves.

Would their mutual gravity begin at that point, or does it take long enough for both waves to reach thier opposite number?

It takes long enough for both waves to reach their opposite number. Gravitational waves travel at the speed of light in vacuum. No information may be transmitted faster than the speed of light in vacuum. If the mutual gravity began at the point where the waves meet, then information could be transmitted at twice the speed of light in vacuum.

Earlier on you showed that you are aware that the gravitational "force" between bodies is "really" (in general relativity) just bodies reacting to the local curvature of space-time. A gravitational wave is a change in the local curvature of space-time. A body cannot react to a change in the local curvature of space time until that change arrives at the locality of the body.

[FreakFreely]

Incidentally, I slightly resent your implication that I am employing an "Appeal to Authority" fallacy. I suspect that you do not understand the fallacy.



Of course I understand! It similir to Ad Homonym, like when an arguments sounds just like a reasonable one...but it ain't. This thread on gravity speed has been dandy mental gymnastics, good for brain fiber! However, it has set one hook that still annoys me. A mass can be at rest and accelerating at the same instance? No way, Joseph. That sort of says that "going to move" is the same as "is moving". No, no, (1.0 times ten to third)times no! Of course, I recognize that your arguments don't necessarily depend on that being true.

Please e-mail a serving of crow when the first gravity wave is definitively detected. I can wait.

[Koffing]

An addendum to what I previously wrote:

FreakFeely wrote:

As for the experiment, I guess I have to further assume for the stated masses A and B to be at relative rest AND accelerating towards each other is that they didn't exist until that very instant. That would do it, I guess. Other than that, I must confess, I can't. At rest,AND accelerating. Perhaps I'm too dim to digest the paradox, but there it is.


No paradox. Throw a ball straight up. At its maximum height, the ball's velocity is zero. Gravity is the only force acting upon it, so the acceleration is 9.8 m/s^2 down.

The point of this is that the ball and the Earth are accelerating toward each other, but their relative velocities are zero.

[JonF]

A mass can be at rest and accelerating at the same instance? No way, Joseph. That sort of says that "going to move" is the same as "is moving". No, no, (1.0 times ten to third)times no!

Wow.

You are wrong.

Apparently you made it to graduate school without taking (and learning something from) basic calculas and a basic physiscs course that included Newtonian mechanics.

It appears that teaching you about your error is too large a task to take on here; several other posters have pointed out your error and offered simple explanations and scenarios from everyday experience. II suggest that you ask an undrgraduate who has taken calculas and basic physics.

Please e-mail a serving of crow when the first gravity wave is definitively detected. I can wait.

Pardon? I have never claimed that gravity waves do not exist. Detecting a gravity wave and measuring the speed of a gravity wae are two different propositions (though possible to do in the same appropriate experiment).

[CurtC]

JonF wrote:
Detecting a gravity wave and measuring the speed of a gravity wae are two different propositions (though possible to do in the same appropriate experiment).
Not to take FF's side, but wouldn't any experiment which observes the speed of gravity necessarily be measuring gravity waves? A wave would be the propagation of some change in the local gravity field.

[Chronos]

Well, measuring the speed of gravity waves would (probably) require detecting the durn things, but it would be possible to detect them without measuring the speed, so yes, they are two separate problems. And yes, any propagation of information using gravity could be considered a wave, although it wouldn't necessarily be sinusoidal with a nice, well-defined wavelength, etc. It could be just a pulse.
On another note, although gravity waves have not been detected, evidence for their existance has been. Pulsars are spinning down, loosing both kinetic energy and angular momentum, and gravity waves are the only method proposed consistent with the observations. Further, there have been some observations of the effects of gravity waves on galactic dust distributions; some would even go so far as to call this a detection (so they're using a whole galaxy as a detector. You have a problem with that? :) ) The fact that we've never detected them with man-made apparatus just means that we aren't trying hard enough.

[JonF]

What Chronos said.

As indirect evidence, from the LIGO Home Page (http://www.ligo.caltech.edu/) Fact Sheet:

"More specifically, LIGO has the possibility to:

Verify directly general relativity's prediction that gravitational waves exist.

Test general relativity's prediction that these waves propagate at the same speed as light, and that the graviton (the fundamental particle that accompanies these waves) has zero rest mass.

Test general relativity's prediction that the forces the waves exert on matter are perpendicular to the waves' direction of travel, and stretch matter along one perpendicular direction while squeezing it along the other; and also, thereby, test general relativity's prediction that the graviton has twice the rate of spin as the photon.

Firmly verify that black holes exist, and test general relativity's predictions for the violently pulsating space-time curvature accompanying the collision of two black holes. This will be the most stringent test ever of Einstein's general relativity theory."

Note that they list existence and propagation speed as two items ...

[arthur_einstein]

being a bit of a physics dunce, i am really pleased with what i have learned from the debate. ta all. i think i still believe that info cannot travel faster than c. ta einstein. the implications of this being otherwise seem too horrible to contemplate. i still struggle with how gravity/distortions in space time is transmitted. i am sure i ma not alone. this board is great though. ta again