# How fast is gravity?

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I didnt have time to read the whole thread but according to "the elegant univese" (String theory) Gravity acts at c.

i know it is in there, but i don't know where. i was going to check it to see if there was a better simple explaination than the one i gave.

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hold on didn't we have the same answer to the question? then why are you trolling?

He's not. He's pointing out that your answer boiled down to "it is this way because I say so." It's not an explanation.

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i didn't say "it's that way because i said so," i said that it cannot travel past c, therefore, it cannot be instantaneous. since almost all theoretical scientists that deal with gravitons say it travels at c, that would mean it takes 8 minutes. now lets get back on topic. :Þ

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Thanks swansont ..

Yes, the binary pulsar data is significant, but are you sure it indicates that the "the results are consistent with the speed of propagation being c with a high degree of precision"? I had thought it only showed the likely existence of gravitational radiation, which would occur at any speed less than infinite; that's what was said in early reports, back in the late 70's. Perhaps the greater accuracy of the data obtained by continued observations has allowed the determination that it must be c. (Note that even Newton was dubious about gravity acting "instantaneously", so I've been using "instantaneous" as short-hand for "much faster than c".) But if you're right, why did Fomalont and Kopeikin say "We became the first two people to know the speed of gravity" in New Scientist? And remember the many observed gravitational anomalies leading some physicists to postulate a "fifth force", cosmological constant, or similar. Most recent is the so-called Pioneer/Voyager anomaly, which is even more interesting than previous ones in being very short-range. Don't these oddities raise a doubt in your mind about such delicate observations as the tiny anomalies in distant neutron stars? Oh well, I'm just picking at nits - seems I lose this argument

Nevertheless let me briefly review the previous GR evidence to show it doesn't determine gravity speed.

Gravitational redshift: these observations have nothing to do with speed of gravity propagation, only the increase in potential at the nearer point of observation which causes the frequency "clock" to run slower. Note that the redshift could be computed simply by the difference in Newtonian potential energy (hbar omega / c^2) (GM/r2 - GM/r1) where omega is the frequency (assuming like Newton it's unchanged) and r1, r2 the radial distances from the center of mass of the two observations. If Newton had guessed photons existed, and their frequencies would change according to E = hbar omega, he could have computed the GR redshift result to the first order - which I think is still accurate enough (the higher order effects would become significant only very near Schwarzschild radius). Admittedly I'm not up on recent results observing redshift coming out of very strong fields such as white dwarfs.

Perihelion precession (and geodetic precession, gravitomagnetic precession): Mercury, being slow and in a weak, static gravitational field, would show virtually the same orbital precession no matter what gravity speed is. That was also true of the binary neutron star precession as of 1980, and probably is still true.

Bending of light rays near massive objects: doesn't it seem obvious that given the extremely short distance between the ray and the object that the speed of gravity difference would be far too subtle to detect? That was always assumed, until recently. Yet Fomalont and Kopeikin, examining bending of distant light past Jupiter, claim they can tell the difference between different speeds of propagation. Sounds fishy ..

Shapiro time delay of radio signals: measured most recently and accurately by GPS .. same considerations mentioned above apply. As far as I know, until F & K no one dreamed that such effects could be affected by gravity propagation speed, since they depend on being very near a more-or-less stationary massive object.

And so on .. but to avoid constantly repeating "unless things have changed since 1980" I guess I'll stop here

Thanks for the reference to questions about F&K's result - it really does sound flawed. fuhrerkeebs take note - gullibility might have its disadvantages!

Finally, to JHAC, Crash and others: please check out what I said above. You can't use theoretical predictions to prove experimental observations - it's the other way around. Once the observation is made, it proves (well, "strongly supports") the theory. Your arguments go something like this:

1. GR depends on a number of assumptions or principles, one of which is "gravity acts at c".

2. All the other principles have been proven by actual experiments on nature (the ultimate arbiter) therefore we trust GR to a high level.

3. Since we trust GR, <i>all</i> its assumptions must be correct, even those not yet observed.

4. Therefore gravity acts at c.

I hope you'll agree this "logic" isn't satisfactory.

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Yes' date=' the binary pulsar data is significant, but are you sure it indicates that the "the results are consistent with the speed of propagation being c with a high degree of precision"? I had thought it only showed the likely existence of gravitational radiation, which would occur at any speed less than infinite; that's what was said in early reports, back in the late 70's. Perhaps the greater accuracy of the data obtained by continued observations has allowed the determination that it must be c. (Note that even Newton was dubious about gravity acting "instantaneously", so I've been using "instantaneous" as short-hand for "much faster than c".) But if you're right, why did Fomalont and Kopeikin say "We became the first two people to know the speed of gravity" in New Scientist? And remember the many observed gravitational anomalies leading some physicists to postulate a "fifth force", cosmological constant, or similar. Most recent is the so-called Pioneer/Voyager anomaly, which is even more interesting than previous ones in being very short-range. Don't these oddities raise a doubt in your mind about such delicate observations as the tiny anomalies in distant neutron stars? Oh well, I'm just picking at nits - seems I lose this argument

Nevertheless let me briefly review the previous GR evidence to show it doesn't determine gravity speed.

[/quote']

The argument, as I understand it, is that gravitation radiation only occurs if there is retardation involved - the only way the angular momentum of the system can change (and be carried away by the radiation) is if the force is pointed to where the other object was, not where it is now, which is why gravity radiation can only be present for finite speed of gravity.

The rate of the radiation depends on the amount of retardation, which in turn depends on the speed of propagation. So by measuring the orbital decay rate, you indirectly measure the speed of gravity. And it agrees with GR.

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Thanks JHAQ, yes I see now how the binary pulsar example works. Turns out my old textbook (1980) mentions it also .. back then the data was more ambiguous but now it looks like it's been firmed up and is fairly definite. I found a reference from a "professional physics kook", Tom van Flandern, which does a great job of presenting some of the misgivings that have been rattling around in my head; he claims speed of gravity is >= 2*10^10 c. Makes a distinction between gravity radiation (which he admits the pulsar data confirms) and gravity waves, which he claims are unsubstantiated. Check it out, makes interesting reading:

http://www.ldolphin.org/vanFlandern/gravityspeed.html

Although I can understand his arguments I'm not in a position to evaluate them, much less defend them; give me a few months!

Another interesting reference seems to really debunk the Fomalont-Kopeikin result. Looks like they're the <i>only</i> guys who defend it, with a bunch of reputable physicists against them. It's another "cold fusion" episode. See

http://wugrav.wustl.edu/people/CMW/SpeedofGravity.html

It's interesting that this same question was addressed 2 years ago right here on this forum! It's one of these "bad pennies" that keep turning up. I must say we've taken a much more thorough look at it this time. See

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instantly......ive been bumping my gums all week about FTL....but that isnt the most mysterious thing about gravity

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Maybe you're right philbo1965uk, but having examined van Flandern's arguments, I'm figuring that he's wrong (and every other physicist since Einstein is right): gravity propagates at light speed. Admittedly, it remains an open question .. so, what do you say is the most interesting thing about gravity? Is it, perhaps, the question "<i>why</i> does everything attract everything else?"

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no it breaks the law of conservation of energy.It has no source.if you need me to give more read both Newton & Einstein they are just as flumexed.

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No known source.

Not the same as no source.

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ok but a little bit of a cop out. so what about magnetism.eg hold two hand size magnets with opposite poles that repel each other and push them together.as the law of cons/energy is proven correct you expend all your energy and tire.however the magnets maintain their force with no energy expended and do so for infinite time...no source.As you know perpetual motion cannot be probable because of the law of cons/energy.

or are you agreeing that obviously whatever drives gravity the same as magnetism is unknown so we cannot imply that or show that or measure correctly its speed in relation to light...and wouldnt it be on the bases of my earlier explanation of abberation safe to assume that what we know so far gravity appears to act FTL.would this be a fairer statement

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Not knowing something yet can hardly be called a "cop out". Whether or not we know of something bears no relation to whether or not it actually exists (although the converse state is obviously another matter).

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Not knowing something yet can hardly be called a "cop out". Whether or not we know of something bears no relation to whether or not it actually exists (although the converse state is obviously another matter).

example: God

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no it breaks the law of conservation of energy.It has no source.if you need me to give more read both Newton & Einstein they are just as flumexed.

Do you understand the concept of gravaitainal potential energy? Certianly gravity does not break the law of conservation of energy. In general relativty the conservation of enrgy isn't paraticularly important anyway.

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so what about magnetism.eg hold two hand size magnets with opposite poles that repel each other and push them together.as the law of cons/energy is proven correct you expend all your energy and tire.however the magnets maintain their force with no energy expended and do so for infinite time...no source.As you know perpetual motion cannot be probable because of the law of cons/energy.

The magnets are doing no work and expending no energy. Just as my table, in holding up my lamp against the pull of gravity, is expending no energy. Or is my table an infinite energy source?

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how can you debate when you tie your own hands.Im wrong,your right.We dont know something exists but this answer is yes this is no because at this moment in time physics says so.you are imoveable until the flavour of the month suits you."black holes....oh no nothing can escape a black hole..we ran the calculations using our formula and no impossible...."well what is the stance now in physics regarding blackholes??

Times change clever people devise formula and we learn more,whilst potential,relativistic,virtual,have there place its a thin line to be challenged tommorrow.

one more statement energy they say is equivalent to mass E=mc2,you can interpret it to mean were there is energy there is mass,were there is mass there is energy.also momentum is related to mass,however a photon has energy and momentum yet no mass.i agree but you could say it has relativistic mass because if we trap it in a container with perfect mirrors so the photons continuelly bounce back and forth.then in the boxes frame of reference the momentum is zero but the energy is not.so light would add to the mass of the box.You could measure this in principle by an increase in inertia when the box is accelerated or by gravitational pull.But we define mass today as invarient mass,so in this case mass wouldnt be conserved so all we can now say is the mass of a box of light is more than the mass of the box and the sum of the masses of the photons (the latter being zero).we now see energy doesnt have to have mass.mass is not equivalent to energy.So does light have mass,we say no but its open to how you interpret it.

The main problem you wont accept is i believe and i hope its soon,someone will provide the formula and do the math showing gravity does travel FTL.Physics cannot measure accurately wether it does or doesnt,you can only calculate its effects which isnt the same thing.Can you not agree to disagree like gentelmen and not lead to my posts being removed.

I just hope when im proved right your all still here, you will all jump on the bandwagon "i didnt say it couldnt travel FTL"

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not a logical argument swansont,can your table exert a force and levitate metal objects (if so your in the wrong forum try ghosts and poltegeists)or are you saying the earth is comparable to your table and the moon should sit nicely on its shoulder..

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how can you debate when you tie your own hands.Im wrong' date='your right.We dont know something exists but this answer is yes this is no because at this moment in time physics says so.you are imoveable until the flavour of the month suits you."black holes....oh no nothing can escape a black hole..we ran the calculations using our formula and no impossible...."well what is the stance now in physics regarding blackholes??

Times change clever people devise formula and we learn more,whilst potential,relativistic,virtual,have there place its a thin line to be challenged tommorrow.

one more statement energy they say is equivalent to mass E=mc2,you can interpret it to mean were there is energy there is mass,were there is mass there is energy.also momentum is related to mass,however a photon has energy and momentum yet no mass.i agree but you could say it has relativistic mass because if we trap it in a container with perfect mirrors so the photons continuelly bounce back and forth.then in the boxes frame of reference the momentum is zero but the energy is not.so light would add to the mass of the box.You could measure this in principle by an increase in inertia when the box is accelerated or by gravitational pull.But we define mass today as invarient mass,so in this case mass wouldnt be conserved so all we can now say is the mass of a box of light is more than the mass of the box and the sum of the masses of the photons (the latter being zero).we now see energy doesnt have to have mass.mass is not equivalent to energy.So does light have mass,we say no but its open to how you interpret it.

The main problem you wont accept is i believe and i hope its soon,someone will provide the formula and do the math showing gravity does travel FTL.Physics cannot measure accurately wether it does or doesnt,you can only calculate its effects which isnt the same thing.Can you not agree to disagree like gentelmen and not lead to my posts being removed.

I just hope when im proved right your all still here, you will all jump on the bandwagon "i didnt say it couldnt travel FTL"[/quote']

The p[roblem is philbo this board is about science, peopel here want to know what science says, not uninformed opinion. Science ius sunject to change but don't expect people to give the time of day to random unsupported hypothesises.

Most of your post is irrelvenat to the subjecct at hand, but I'lll answer the relevenat point. There is no reason whatsovere to believe that some future theory willl produce FTL gravity; our best theory of gravity (genetal relativity currently says gravity acts at c and it would gnereally be expected that any new htoery presevres Lorrentz invaraince which would limit the speed of gravity to c or less.

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Actually, it is exactly the same thing. Philbo's plates are also not interacting at a distance. They are emitting photons, which transfer momentum between the plates - this is just electromagnetism. In Swansont's table, the atoms are likewise emitting photons which transfer momentum between them, keeping the structure of the table, and the table emits photons to hold the lamp in place. So they are both manifestations of electromagnetism, which is well known to be mediated by photon exchange.

Similarly, gravity is mediated by the exchange of a graviton, and since gravitons are also massless, they travel at the speed of light. Therefore the effects of gravity travel at the speed of light too.

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not a logical argument swansont,can your table exert a force and levitate metal objects (if so your in the wrong forum try ghosts and poltegeists)or are you saying the earth is comparable to your table and the moon should sit nicely on its shoulder..

My table "levitates" metal objects all the time. Just not very heavy ones. It's levitating some small copper & zinc objects right now. Not very high above the wood, however.

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thankyou Aeschylus for your first paragraph reply."why didnt someone just say that in the first place".

your not here to discuss breakthroughs or new conceptions in science just to accept the orthadox opinion at the time.from now on i will just ask questions for information,however i dont sit comfortably with presumptions,probability,virtual as solid science it seems to be "our best guess is its this"and everyone just goes along with it kind of like sheep.Well thank goodness for scientists who question and say this is silly theirs got to be a better explanation,otherwise you lot would indeed believe the earth was flat." blackholes"...

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We are all interested in seeing new ideas, but you seem to have no concept about how well tested the theories you oppose are. Unless you can come up with something which explains some of the problems with the theories as they stand, why should we even listen to you? There are big holes in our understanding of physics - why not try to fill them, rather than just disagreeing with our understanding of things which have been very well tested already?

For example, if your theory can explain the quark mass hierarchy, or the uniformity of the universe, or the baryon asymmetry etc etc, I will be very happy to discuss it.

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My table "levitates" metal objects all the time. Just not very heavy ones. It's levitating some small copper & zinc objects right now. Not very high above the wood, however.

thats either a really cryptic deffinition of something quite ordinary, or something Fascinating!

can you explain a little more clearly what you mean please? )

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thats either a really cryptic deffinition of something quite ordinary' date=' or something Fascinating!

can you explain a little more clearly what you mean please? )[/quote']

I think Swansont is being ironic. The table is 'levitating' some coins(?)in that they have not fallen to the floor. For my part I am maintaining the Earth in its present orbit.

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