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Thread: How Fast is Gravity ??

  1. #1 How Fast is Gravity ?? 
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    Does anyone know at all - or even a rough idea

    We know that all Particles, and even Light cannot travel Faster than the Speed of Light - well except maybe Neutrinos.

    Big Question - Does Gravity travel at the Speed of Light ??

    Faster ??

    Or Slower ??

    If you could create a Gravity Wave - would it propagate through Space -
    at Faster than Light, the same Speed, or Slower.

    Could we use Gravity Waves for long wave Communications - Faster than
    the Speed of Light, maybe ??
    We know they will effectively propagate or "go through"
    almost anything and everything.

    Ideas - anyone.
    I am just a Bod, with a Scientific Interest -
    but not an Advanced Mathematician, Super Physicist
    or a Cosmologist.


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    IIRC, current theories predict that if gravity waves exist, they'll travel at the speed of light.


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    Gravity transmits information about the curvature of space/time to the mass/energy travelling along it. All information is constrained to travel at the speed c .
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    as i see it gravity is the revers imege in time of the prticals that make it push
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    Almost makes you want to give up.

    Some people just post whatever comes into their heads without first bothering to learn any accepted physics by doing some reading or work.

    This is not a pseudoscience or speculation forum, but a physics forum.

    ACCEPTED PHYSICS.
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  7. #6  
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    At least it's constrained to a vocal minority. I think that most people that read this site don't post much, if at all. Of course, it doesn't help that there aren't that many experts around either.
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    Thankfully there are one or two of you guys around. Thank you for sticking around!

    *Downthumbs for the Water Nosfim post*
    Disclaimer: I do not declare myself to be an expert on ANY subject. If I state something as fact that is obviously wrong, please don't hesitate to correct me. I welcome such corrections in an attempt to be as truthful and accurate as possible.

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  9. #8  
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    gravity waves would travel faster than light...and thats what made einstein confused..
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    a small change in mass could be felt instantaneously at anywhere
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    @ajmalazeez007, What makes you so certain?

    @KALSTER, Unfortunately, I'd only consider myself an expert in computer science. I'm not quite sure how I'd rank my knowledge of physics, but at least I know when I don't know.
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  12. #11  
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    Gravity seems to propagate as a geometric effect ,according to general relativity (curved space time),suggesting an instantaneous force.This 'speed' should not to be confused with gravitation radiation
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    because the partical gets infinity small , because time can gets backwerd , because the part of ower universe is round in time , and because space is naked
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    Quote Originally Posted by Water Nosfim View Post
    because the partical gets infinity small , because time can gets backwerd , because the part of ower universe is round in time , and because space is naked
    This is just gibberish.

    Quote Originally Posted by brane wave
    Gravity seems to propagate as a geometric effect ,according to general relativity (curved space time),suggesting an instantaneous force.This 'speed' should not to be confused with gravitation radiation
    The gravity waves that is talked about is precisely that; geometric distortions, which are not thought to be instant. They also still travel at C.

    @MagiMaster: Well, not an expert as such, but you do seem to have a good grasp of the basics. Knowing when you don't know is, of course, a good trait to have.
    Disclaimer: I do not declare myself to be an expert on ANY subject. If I state something as fact that is obviously wrong, please don't hesitate to correct me. I welcome such corrections in an attempt to be as truthful and accurate as possible.

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    Quote Originally Posted by MagiMaster View Post
    ... but at least I know when I don't know.
    Many experts could learn from you.
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    Good Question Karakris! I was wondering about something else and my research with Wikipedia confused me further so I am going to post a question on this forum. I did however see a reference to the speed of gravity under "Speed of light" in Wikipedia. There is a "Speed of Gravity" Wiki entry too. The speed of gravity is currently thought to be c (the speed of light in a vacuum) which is thought to be the fastest rate at which information can propagate through the Universe. Who knows if this will change or be proved in future? Check it out.
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    Quote Originally Posted by Water Nosfim View Post
    because the partical gets infinity small , because time can gets backwerd , because the part of ower universe is round in time , and because space is naked
    A radical new hypothesis, Water Nosfim, but it may have some flaws.
    I am not an expert on gravity, but I certainly feel its effects more than most!
    Corporal Will-your expert on scones and girdles.
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    Quote Originally Posted by KALSTER View Post
    Quote Originally Posted by brane wave
    Gravity seems to propagate as a geometric effect ,according to general relativity (curved space time),suggesting an instantaneous force.This 'speed' should not to be confused with gravitation radiation
    The gravity waves that is talked about is precisely that; geometric distortions, which are not thought to be instant. They also still travel at C.
    I think you might have misunderstood what brane wave is trying to say, and are confusing gravitational radiation with the curvature of spacetime. The curvature of spacetime is a geometric effect and so gravity acts instantaneously, but gravitational radiation (gravity waves) propagate at c.

    As I understand it, the Earth orbits the barycentre of the Solar System as it is "now", not where it was 8 minutes ago.
    Last edited by SpeedFreek; September 5th, 2011 at 02:06 PM.
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    Quote Originally Posted by SpeedFreek View Post
    Quote Originally Posted by KALSTER View Post
    Quote Originally Posted by brane wave
    Gravity seems to propagate as a geometric effect ,according to general relativity (curved space time),suggesting an instantaneous force.This 'speed' should not to be confused with gravitation radiation
    The gravity waves that is talked about is precisely that; geometric distortions, which are not thought to be instant. They also still travel at C.
    I think you might have misunderstood what brane wave is trying to say, and are confusing gravitational radiation with the curvature of spacetime. The curvature of spacetime is a geometric effect and so gravity acts instantaneously, but gravitational radiation (gravity waves) propagate at c.

    As I understand it, the Earth orbits the barycentre of the Solar System as it is "now", not where it was 8 minutes ago.
    I don't think I did misunderstand him.

    As I understand it, if you were to magically disappear Sol, that the earth would merrily continue on its orbit for another 8 minutes. This is according to good old GR as far as I know. As was noted earlier, instantaneous communication of information is not possible.

    It is like if you had a rubber sheet with a frictionless surface, with the sun at the centre making the big indent and the earth rolling along the edge Roulette style. If you were to suddenly remove the sun, it would take a while for the ripple to reach the earth based on the properties of the rubber. As I have it, the fundamental properties of space-time means the ripple would travel at C.

    But, having said this, I am certainly no expert and fully accept that I could be wrong about this.
    Disclaimer: I do not declare myself to be an expert on ANY subject. If I state something as fact that is obviously wrong, please don't hesitate to correct me. I welcome such corrections in an attempt to be as truthful and accurate as possible.

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  20. #19  
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    Quote Originally Posted by KALSTER View Post
    Quote Originally Posted by SpeedFreek View Post
    Quote Originally Posted by KALSTER View Post
    Quote Originally Posted by brane wave
    Gravity seems to propagate as a geometric effect ,according to general relativity (curved space time),suggesting an instantaneous force.This 'speed' should not to be confused with gravitation radiation
    The gravity waves that is talked about is precisely that; geometric distortions, which are not thought to be instant. They also still travel at C.
    I think you might have misunderstood what brane wave is trying to say, and are confusing gravitational radiation with the curvature of spacetime. The curvature of spacetime is a geometric effect and so gravity acts instantaneously, but gravitational radiation (gravity waves) propagate at c.

    As I understand it, the Earth orbits the barycentre of the Solar System as it is "now", not where it was 8 minutes ago.
    I don't think I did misunderstand him.

    As I understand it, if you were to magically disappear Sol, that the earth would merrily continue on its orbit for another 8 minutes. This is according to good old GR as far as I know. As was noted earlier, instantaneous communication of information is not possible.

    It is like if you had a rubber sheet with a frictionless surface, with the sun at the centre making the big indent and the earth rolling along the edge Roulette style. If you were to suddenly remove the sun, it would take a while for the ripple to reach the earth based on the properties of the rubber. As I have it, the fundamental properties of space-time means the ripple would travel at C.

    But, having said this, I am certainly no expert and fully accept that I could be wrong about this.
    You've got it correct. Gravitational radiation carries information about changes in the gravitational field and travels at c.
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


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  21. #20  
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    Heh, we are talking past each other, more than anything! This is complicated.

    You are correct in saying that if we were to magically disappear Sol, the Earth would continue to orbit for 8 minutes before noticing that Sol was gone. This is because changes in the curvature propagate at c. A change in the "gravitational field" itself, propagates at c.

    But the influence of the curvature itself is instantaneous, across any distance. A gravitational field does not propagate in of itself, the only thing that propagates is a change in that field. The field just is.

    Speed of gravity - Wikipedia, the free encyclopedia
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    This time, for a change, you are wrong SpeedFreek.
    The Earth orbits the gravitational centre 'as it was 8 minutes ago', which is essentially its local 'now'. There is no global now according to GR.
    Any effect which moves information or signals is constrained by GR to do so at speed c.
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    Gravity is as fast as the energy infront of it but sometimes as the energy from behind been the main energy slows the gravity (energy) infront needs more fuel, this fuel could be natural air of the earth so therefore as gravity is pulled down from space both forms of gravity speed up and as the air rises back into space again both forms of gravity slow down. Problem here is that both forms of gravity should not be in either space or natural air for a length of time. I guess both forms of gravity can be apart as one drifts into space and the other in earths natural air. Like the air strips that revolve around the earth they are parted and drif into each other at various positions. It's easy to think that the gravity from space is the same but I doubt that because that would mean the distance would be far to easy to reach.

    Citation >>> Dunno


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  24. #23  
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    Quote Originally Posted by MigL View Post
    This time, for a change, you are wrong SpeedFreek.
    The Earth orbits the gravitational centre 'as it was 8 minutes ago', which is essentially its local 'now'. There is no global now according to GR.
    Any effect which moves information or signals is constrained by GR to do so at speed c.
    Does Gravity Travel at the Speed of Light?

    Please read the whole thing, and you might see what both I and Brane Wave were trying to say.

    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 central—it does not point directly towards the source of the gravitational field—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.


    This cancellation may seem less strange if one notes that a similar effect occurs in electromagnetism. If a charged particle is moving at a constant velocity, it exerts a force that points toward its present position, not its retarded position, even though electromagnetic interactions certainly move at the speed of light. Here, as in general relativity, subtleties in the nature of the interaction "conspire" to disguise the effect of propagation delay. It should be emphasized that in both electromagnetism and general relativity, this effect is not put in ad hoc but comes out of the equations. Also, the cancellation is nearly exact only forconstant velocities. If a charged particle or a gravitating mass suddenly accelerates, the change in the electric or gravitational field propagates outward at the speed of light.
    Hence me saying that the Earth orbits the barycentre as it is now (present position), not where it was 8 minutes ago (retarded position).

    Of course, it is always possible I am interpreting things wrongly, in which case feel free to pull me up!
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    This is certainly perplexing,on one hand i can see how the effect of G must be limited by c. On the other hand,it seems independant from these restrictions.
    pedronaut likes this.
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    It is also my understanding that it is only the tiny difference between Newtonian gravity acting instantaneously, and GR acting in a way that almost covers up any propagation delay so it seems almost instaneous, which accounted for the unsolved component in the precession of the perihelion of Mercury.

    But perhaps my understanding is flawed?
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    That tiny difference seems the only clue i can spot.
    "general relativity very nearly reproduces the newtonian result" quote from Does Gravity Travel at the Speed of Light?
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    I guess the proof of GR based gravity,will almost certainly be beyond doubt if the higgs boson is found
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  29. #28  
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    Prof Steve Carlip seems to be the most cited on this subject:

    [gr-qc/9909087] Aberration and the Speed of Gravity

    It is certainly true, although perhaps not widely enough appreciated, that observations are incompatible with Newtonian gravity with a light-speed propagation delay added in. If one begins with a purely central force and puts in a finite propagation speed by hand, the forces in a two-body system no longer point toward the center of mass, and the resulting tangential accelerations make orbits drastically unstable.
    As I said, the situation is complicated! It is not enough to say that gravity propagates at c. It also should be pointed out that this would lead to an aberration that is not observed, due to a number of factors, which means that, except in extreme situations, it acts almost as if it is instantaneous. This makes it more difficult to actually calculate the speed of gravity from observations.
    Last edited by SpeedFreek; September 6th, 2011 at 04:47 PM.
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    Quote Originally Posted by SpeedFreek View Post
    It is also my understanding that it is only the tiny difference between Newtonian gravity acting instantaneously, and GR acting in a way that almost covers up any propagation delay so it seems almost instaneous, which accounted for the unsolved component in the precession of the perihelion of Mercury.

    But perhaps my understanding is flawed?
    I guess we were talking past each other. Interesting thread!

    I think you are right with the above (Edit: the old above). What I gather from the two links, is that the effect is instantaneous, provided both bodies are in the same inertial frame, i.e. their respective velocities are constant. In the case of orbits, the planets' orbits means they are under constant acceleration, but they still very nearly approximate a constant velocity. The small difference is what accounts for the precession of Mercury due to the divergence of GR from Newtonian gravity.

    My brain is going full steam trying to infer the mechanism of inertia from this, with the limited understanding I have.



    I was for a moment starting to think you were allowing for some kind of instant transmission of information with, say, a mass being vibrated in a pattern, with a very sensitive gravitational wave detector being able to pick up the signal at any distance instantaneously.



    [OT]PS. (not sure if this warrants a new thread) Where vacuum fluctuations are concerned, don't the virtual particles exert gravity? As I have it, particles and antiparticles alike would cause gravity, so won't there be like a net gravitational distortion present from these fluctuations that would usually cancel out to a near net zero effect on any chosen point? [/OT]
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    Just a quick thought...Bear in mind that gravitational waves have not yet been detected.
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    Quote Originally Posted by brane wave View Post
    Just a quick thought...Bear in mind that gravitational waves have not yet been detected.
    Not directly, but there is some pretty good indirect evidence of their existence.
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    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


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    Quote Originally Posted by KALSTER View Post
    I was for a moment starting to think you were allowing for some kind of instant transmission of information with, say, a mass being vibrated in a pattern, with a very sensitive gravitational wave detector being able to pick up the signal at any distance instantaneously.
    Actually - when I started this Thread - that was the sort of thing I had in mind.

    Larry Niven wrote of something like this in a Short S.F. Story called "the hole man".

    An Ancient Alien Spacedock is found on the dark side of the Moon - with a massive
    Communications Station, using a Quantum Black Hole to generate gravity waves,
    which can be detected waaaay across space.

    This produces a wave with extremely sharp peaks and troughs, on the Mass Detector
    of the Survey Ship.

    The Quantum Black hole is one of those left over from the times of the Big Bang -
    produced by "micro-vortices" created duing those events.
    With an extremely tiny Mass, Micrograms - but incredibly small radius.

    Just wondering - COULD there be "Quantum Black Holes" in space, somewhere -

    The Author suggests that they could be "hiding" in small mass objects, like Asteroids,
    where they have been "captured".
    If I wasn't so stupid - I might know what I was doing
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  34. #33  
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    Quote Originally Posted by KALSTER View Post
    I was for a moment starting to think you were allowing for some kind of instant transmission of information with, say, a mass being vibrated in a pattern, with a very sensitive gravitational wave detector being able to pick up the signal at any distance instantaneously.
    I realised, too late, that my posts might seem to imply instantaneous transmission of information, which was certainly not my intention. I am not claiming any paradoxes or causality violations or anything that is against the mainstream here, although it might have seemed so (I was worried I was sailing close to the wind there, for a while!).

    It is just that there is more to the influence of gravity than the simple concept that it propagates at c, which, if taken in isolation, can lead to the conclusion that gravity attracts you at a finite speed towards the place the mass was, which would lead to a "lagging behind" effect. This conclusion is false.

    Perhaps I shouldn't have echoed Brane Wave's usage of the curvature of space-time in the analogy, although that is how it was explained to me - that we can think of it (as an analogy in lieu of all the intricacies explained in Carlip's paper) as if mass curves spacetime and other smaller masses would simply follow that curvature. There is no propagation to consider if the mass is already there - the curvature "just is". There is no lagging behind of our gravitational well, for instance, due to our Solar System's motion around the galaxy.

    If a mass is in constant motion (or in a good approximation), the finite propagation speed of gravity does not cause the curvature to "lag behind" that mass - the curvature remains essentially centred on that mass, in a manner that is almost Newtonian.

    So, the Earth does not orbit the place where the barycentre of the Solar System was, 8 minutes ago, due to gravity propagating at c from that place. Perhaps I should not have said we orbit the barycentre as it is "now" (I do understand there is no absolute simultaneity!), but it is a good approximation of the situation, as I understand it.
    Last edited by SpeedFreek; September 6th, 2011 at 06:06 PM. Reason: clarification
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    I agree,and cant see any violations in GR or Newtonian.
    Very interesting posts,and i for one,have learnt quite a lot from this thread so far.
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    I realised, too late, that my posts might seem to imply instantaneous transmission of information, which was certainly not my intention.
    Yeah, my doubt was short lived though. Judging from my limited knowledge base, your posts are usually of a high fidelity. I certainly didn't know all the finer points before checking my facts (after I made the post, which is not good form).



    Just wondering - COULD there be "Quantum Black Holes" in space, somewhere -
    Last time I checked, they are considered a distinct possibility. In fact, Micro black hole - Wikipedia, the free encyclopedia.
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    So. What you are saying is we are orbiting the sun from eight minutes ago due to gravitational "transmission delay" which is fine while everything stays stable as the gravity well is a perfectly spherical "dip" in Kalsters's rubber sheet.
    But if the sun suddenly accelerated (I really hope it doesn't) then Earth would not respond to this change in her gravitational effect for eight minutes while the gravity headed our way. Should this kind of delay be observable around mobile heavy bodies in space as they influence other bodies like exploding stars having delayed effects on their planets or Nebulas having delayed effects on nearby stars?
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    I believe space/time is not required to follow the laws of physical matter. just because its not physical matter.. It has its own laws.. gravity is warped space/time. If the warp is strong enough there is no limit how fast it could travel.
    Last edited by Brandon; September 7th, 2011 at 04:42 AM.
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  39. #38 Good Question 
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    Uhh...This might seem really dumb, but the speed of light is pretty much instantaneous. The speed of gravity doesn't pull you down as quick as that if you jump.
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    Quote Originally Posted by Little Brother View Post
    Uhh...This might seem really dumb, but the speed of light is pretty much instantaneous. The speed of gravity doesn't pull you down as quick as that if you jump.
    The speed of light is just under 300 000 km.s-1, which is far from instantaneous. It still takes light about 8 minutes to reach earth from the surface of the sun. Secondly, you seem to be confusing the force of gravity with the acceleration due to the force of gravity. On earth, the average acceleration due to gravity is about 9.8m.s-2.

    Quote Originally Posted by Brandon View Post
    I believe space/time is not required to follow the laws of physical matter. just because its not physical matter.. It has its own laws.. gravity is warped space/time. If the warp is strong enough there is no limit how fast it could travel.
    Gravity is warped space-time, but there is a limit to how fast a change in gravity can travel, i.e. the speed of light. When enough matter is concentrated in a small enough space, the curvature due to gravity closes in on itself in a manner of speaking, with the radius at which this happens being the event horizon of a black hole. Beyond this event horizon no information can cross back into normal space, so nothing confined inside the event horizon could leave, no matter how fast it was moving. For something inside the event horizon, every direction would point away from the event horizon from an outsider perspective.
    Last edited by KALSTER; September 7th, 2011 at 06:37 AM.
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    this is weird.. the gravity of the sun, the "dip" in space, moves with the sun, if the sun moves 200 miles in 1 second, the earth will feel the effect immediatly, not 8 mins later.
    you didnt really explain how or why laws of physical matter apply to space time. if you know more please inform me. im just curious haha
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    if space can be warped to trap light and information then it has to be following different laws. i wonder if dark matter acts like this in a way trapping light and information?
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    Matter and space-time follow the rules of our universe, i.e. they are tied into each other. The laws of space-time is not separate from that governing matter (whatever this law might be. We don't have a complete theory of everything yet).

    The "dip" of a mass is something that is always there and extends to infinity in theory. Due to the laws of relativity, there is no preferred frame of reference, i.e. no way to absolutely declare a mass to be moving. Under simplification then, the "dip" of a source mass, as measured by a constantly moving mass relative to the source mass, is of the precise value that it would be for where the source mass is at that instant. This is, I presume, because, unlike with the rubber sheet/ball bearing analogy, a mass and it's gravitational distortion are not separate things, but one encompassing phenomenon.

    As an analogy, imagine a ruler that is a light year in length. A body could fly into it along any part of its length, but if a resultant force was applied on one end, the information of this would only be transmitted along the ruler at the speed of light.

    The only properties of dark matter we are sure of (if it exists) is that it has mass and associated gravity. It does not interact with light or any normal matter or anti-matter we know of. It does not therefore trap light, but it's gravitational field does influence light.

    This is if I'm not mistaken, which I might be.
    Last edited by KALSTER; September 7th, 2011 at 07:50 AM.
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    I see. So the unified field theory would shed more light on this subject. Thank you for explaining.
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    I think "the speed of gravity" is the speed of light. Take a look at Science of LIGO and note the bit that says The space-time ripples cause the distance measured by a light beam to change. Also see Gravitational wave - Wikipedia, the free encyclopedia and note the bit that says Polarization of a gravitational wave is just like polarization of a light wave except.... Now take a look at Displacement current - Wikipedia, the free encyclopedia and see the bit that says However it is not an electric current of moving charges, but a time-varying electric field. Now ask yourself what an electromagnetic wave is. It's a time-varying electromagnetic field-variation going past you at c. So displacement current is involved. And LIGO is trying to measure a distance-change, which is a displacement. So gravitational waves and electromagnetic waves have some strong similarities. I think the speed of both is determined by "the nature of space", wherein c = √(1/ε0μ0) and Z0 = √(μ00).
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    Let me see if I can clarify my position SpeedFreek.
    The Earth orbits the barycentre as it is in the position we see it. Unfortunately we see it where it was 8 min. ago according to our local time definition. Light and gravity take the same time to reach us.
    The fact that the barycentre moves means that the field produced is time-varying, ie it transmits information and as such is constrained to move at c. If we were orbiting an infinite mass, such that the barycebtre ( centre of the infinite mass since the earth's mass is then inconsequential ) then there would be a static gravitational field with no time-variant signal. It would always have been there and is therefore instantaneous.
    Any change in the field has to propagate at c. The earth, for example as it moves through space/time, is constantly reconfiguring the space/time curvature around it and this change is transmitted to us as it holds us down at 'only' the speed of light.
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    And my position is based on the following:

    It is certainly true, although perhaps not widely enough appreciated, that observations are incompatible with Newtonian gravity with a light-speed propagation delay added in [3,4]. If one begins with a purely central force and puts in a finite propagation speed by hand, the forces in a two-body system no longer point toward the center of mass, and the resulting tangential accelerations make orbits drastically unstable. A simple derivation is given in problem 12.4 of Ref. [4], where it is shown that Solar System orbits would shift substantially on a time scale on the order of a hundred years. By analyzing the motion of the Moon, Laplace concluded in 1805 that the speed of (Newtonian) gravity must be at least c.
    From [gr-qc/9909087] Aberration and the Speed of Gravity

    Now I am not, of course, saying that gravity propagates faster than c. I agree that gravity propagates at c.

    What I am saying is the opposite of what you are saying however. I am saying that the gravitational force does not point directly towards the centre of mass, with a light speed delay added in, which is what Steve Carlip author of that paper (a respected expert in GR as far as I know, who is debunking a supposed flaw in GR proposed by Tom Van Flandern) seems to confirm.

    It is not as simple as saying the gravitational force points in the same direction we see the barycentre. It is not as simple as saying the Earth orbits the barycentre as it was 8 minutes ago. The gravitational force does not point to the place it was 8 minutes ago, in fact, it closely approximates the "instantaneous" Newtonian solution, cancelling out the propagation delay due to velocity-dependent interactions.

    There is a lot of "woo" surrounding this subject, where people take this perfectly valid observation regarding aberration and propagation delay in GR and assume it to mean that GR is wrong and that gravity must propagate faster than light, but this is not my argument.
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    Here is a few examples of some experiments worth examining

    Kopeikin and the speed of gravity
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    Quote Originally Posted by brane wave View Post
    Here is a few examples of some experiments worth examining

    Kopeikin and the speed of gravity
    Sorry to tell you this, but you have found the very "woo" that Steve Carlip was addressing. That link is to part of Tom Van Flandern's argument that the speed of gravity is not c. The Metaresearch site is not a trustworthy source - Van Flandern was the "Face on Mars" guy, amongst other things.

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    Thanks SpeedFreek...i was beginning to think if the site was trustworthy..sorry about that
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    I realised I should point out that I am not making any claims against Kopeikin - it might well be worth examining his experiments as you suggested. Just beware of Tom Van Flandern's conclusions.

    Here is wiki's summation of the point I am trying to make here, in case I am still being misunderstood.

    The finite speed of gravitational interaction in general relativity does not to lead the sorts of problems with the aberration of gravity that Newton was originally concerned with, because there is no aberration in static field effects. Because the acceleration of the Earth with regard to the Sun is small (meaning, to a good approximation, the two bodies can be regarded as traveling in straight lines past each other with unchanging velocity), the orbital results calculated by general relativity are the same as those of Newtonian gravity with instantaneous action at a distance, because they are modelled by the behavior of a static field with constant-velocity relative motion, and no aberration for the forces involved.[15] Although the calculations are considerably more complicated, one can show that a static field in general relativity does not suffer from aberration problems as seen by an unaccelerated observer (or a weakly accelerated observer, such as the Earth). Analogously, the "static term" in the electromagnetic Liénard–Wiechert potential theory of the fields from a moving charge, does not suffer from either aberration or positional-retardation. (Only the term corresponding to acceleration and electromagnetic emission in the Liénard–Wiechert potential shows a direction toward the time-retarded position of the emitter). It is in fact not very easy to construct a self-consistent gravity theory in which gravitational interaction propagates at a speed other than the speed of light, which complicates discussion of this possibility.
    In retrospect, I should have simply used the Sun, rather than the barycentre. I muddied the waters here, and this is where I think I was wrong.

    The Sun is moving around the barycentre, as well as everything else in the solar system, due to the gravity of all the bodies involved. So, the force would not point to the barycentre anyway. From Earth, the force would point in different directions in different proportions - towards the Sun, and Jupiter, and Venus and everything else. Of course, this sounds all very Newtonian, but that is the point here - forces pointing at masses, with a speed of light delay.

    So, whilst one might expect the gravitational force of the Sun to be reaching Earth directly from where we see the Sun, due to gravity propagating at the same speed as light, it actually comes from a place closer to where the Sun is "now", rather than where the Sun was, 8 minutes ago! Due to various factors, the GR view approximates the Newtonian view of instantaneous action, using a finite speed of gravity but without any of the resultant aberration one might expect.

    And this is also why I say we can consider the influence of this curvature to "just be there" (acting like a static field) for objects in constant motion (acting like instantaneous action at a distance, whilst actually acting at c but conspiring to hide it!) and that the curvature does not need to be considered to propagate, unless something accelerates which would change the curvature, causing gravitational waves. With constant movement through space-time, although the change in curvature propagates at c, other factors cancel out the propagation delay to hide it from other observers in constant motion.

    Strange, eh? I can't quite believe it myself!
    Last edited by SpeedFreek; September 8th, 2011 at 05:07 PM.
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    ah, so a change in the gravity 'curvature' would cause reaction at the limit of c
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    Well that is where it would be easiest to measure the speed of gravity as being c. This is why we look for gravitational waves coming from events in extreme environments, like neutron stars or black holes or where there are pairs of massive objects in close proximity. The gravity waves propagate outwards from the event, across the universe, across the curvature of space-time, at c. This should not be confused with the curvature itself, which does not exhibit the need to constantly update itself.

    Have another look at the wiki page I linked in post #20 - It covers everything I am trying to say here.
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    I totally agree.The gravity waves are the ripple of gravity.
    Btw, the Steve Carlip link is very interesting.Probably a few months to digest.As this is one of my favourite topics,i will study it with gusto.
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    I dont want to sound stupid.. but what if the speed of a photon is limited by the speed of gravity. since photons are particles with a quantium gravity. and the distortion of gravity cannot travel faster than the distortion of gravity. would it be sufficent to say this?

    (I dont know spit about QP yet)
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    Quote Originally Posted by Brandon View Post
    I dont want to sound stupid.. but . . . . (I dont know spit about QP yet)
    I think you should start with (very) basic math, physics, chemistry, and astronomy courses. If you had these basic courses under your belt you wouldn't "sound stupid". Relativity and quantum physics can (must) come later after you have an adequate foundation.
    Last edited by PumaMan; September 9th, 2011 at 09:38 AM. Reason: Clarify
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    maybe im right... probably not.. but maybe i am.. it makes sence to me.. if all matter is a wave/particle. and a photon is the fasted, what stops it from passing c in a vacume? could miniture gravity waves it emits give it a speed limit? therefore gravity or gravity waves would be the speedlimit of energy/waves/particles?

    I love being ignorant because i have the capability of asking new questions other people may have over looked.. im just asking a question.. not proposing a theory.. Its not like you even tried to give an answer, wow my mind is curious so i join a forum to propose ideas and ask questions...

    hey admin or mods, please post a sign that says do not ask questions. thanks.
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    Ok, I see what you mean SpeedFreek.
    Because orbital speeds of the planet and sun are very much smaller than the speed of light, we can consider ( to a very good approximation ) the gravitational field to be static and therefore instantaneous ( nearly ). But lets keep in mind that that's what it is , an approximation.

    If we were to consider a very energetic proton ( say at the LHC ) and observe its gravitational field, we would find that its effects propagate at 'only' the speed of light. In effect two protons travelling parallel paths, would be gravitationally attracted to the position where each proton 'sees' the other ( which because of the finite speed, c, is 'behind' where it actually is ). There was a lively discussion of this effect on one of the other Science Forums ( with input from the good Doctor ) if you wish to look it up.
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    I'm still not sure you are getting what I am saying, MigL.

    Please, I implore you to read the paper I linked earlier, to understand what I am referring to, then you might either be able to see what I am saying, or to put me right as to where I am misinterpreting things. Judging by the authors credentials, he knows what he is talking about - the question is, do I know what he is talking about?

    This cancellation may seem less strange if one notes that a similar effect occurs in electromagnetism. If a charged particle is moving at a constant velocity, it exerts a force that points toward its present position, not its retarded position, even though electromagnetic interactions certainly move at the speed of light. Here, as in general relativity, subtleties in the nature of the interaction "conspire" to disguise the effect of propagation delay. It should be emphasized that in both electromagnetism and general relativity, this effect is not put in ad hoc but comes out of the equations. Also, the cancellation is nearly exact only forconstant velocities. If a charged particle or a gravitating mass suddenly accelerates, thechange in the electric or gravitational field propagates outward at the speed of light.
    If I am correct, your two protons would not be attracted to the retarded position (whilst in constant parallel motion). If I am wrong, I need to know why.

    It has always been my intention to be able to understand General Relativity as "properly" as is possible for a layman, in order to be able to explain it in layman's terms to others. My current understanding includes a hidden propagation delay for the effects of space-time curvature in respect to freely falling or non-accelerating frames and I would hate to be labouring under such a misconception. All the links I have posted so far in this thread seem to agree with my position, as far as I can tell.
    Last edited by SpeedFreek; September 9th, 2011 at 04:26 PM. Reason: clarification
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    Quote Originally Posted by Brandon View Post
    I dont want to sound stupid.. but what if the speed of a photon is limited by the speed of gravity. since photons are particles with a quantium gravity. and the distortion of gravity cannot travel faster than the distortion of gravity. would it be sufficent to say this?

    (I dont know spit about QP yet)
    Good idea. What happens to you,re idea if the gravitywave would be correspondent (with a detection) with the particle part of the photon as accomplishing part in the duality. The particle part is the graviton for the detection but it,s influence comes as gravitywave from a distance even before the detection of/as a photon. The gravity wave or influence attracts the non particle part from a distance perpendicular to the light direction due to a lorentz principle. That seems to bend space.

    That would be a problem for C but there are mostly two C,s in the more crucial equations : C^2 where a variable C would lead to an energyproblem. If one of these C would be exclusive for the particle character and the other for the non-particle it would be Cg*C instead of "squared C" Then assume a perpendicularity (as in components) between both Cg and C.

    Cg limits C then because the produkt Cg*C stays constant for all media and direktions.
    Last edited by Ghrasp; September 9th, 2011 at 05:24 PM.
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    In the earth the acceleration due to gravity is approximately 9.8m/sec^2.
    Last edited by Little Brother; September 9th, 2011 at 05:52 PM. Reason: clarity
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    Basically,as i see it,if there was a lag,as the retarded position would cause, between the 2 photons then there is a leak in that system,causing change.So the direction must be pointing forward
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    Quote Originally Posted by Ghrasp View Post
    Quote Originally Posted by Brandon View Post
    I dont want to sound stupid.. but what if the speed of a photon is limited by the speed of gravity. since photons are particles with a quantium gravity. and the distortion of gravity cannot travel faster than the distortion of gravity. would it be sufficent to say this?

    (I dont know spit about QP yet)
    Good idea. What happens to you,re idea if the gravitywave would be correspondent (with a detection) with the particle part of the photon as accomplishing part in the duality. The particle part is the graviton for the detection but it,s influence comes as gravitywave from a distance even before the detection of/as a photon. The gravity wave or influence attracts the non particle part from a distance perpendicular to the light direction due to a lorentz principle. That seems to bend space.

    That would be a problem for C but there are mostly two C,s in the more crucial equations : C^2 where a variable C would lead to an energyproblem. If one of these C would be exclusive for the particle character and the other for the non-particle it would be Cg*C instead of "squared C" Then assume a perpendicularity (as in components) between both Cg and C.

    Cg limits C then because the produkt Cg*C stays constant for all media and direktions.
    Thank you.. I thought it was a good idea worth speculating on
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    The more I think about it SpeedFreek, things don't get clearer but more confusing.

    Picture the two protons again moving parallel to each other at relativistic speeds. If I was riding on one of the protons and looked across at the other, I would see it trailing behind because of the finite speed of light. Now, as you've been saying all along, if each proton gravitated to its 'retarded' position, then they would mutually slow each other down and lose energy to nothing and for no reason.
    So they should gravitate to their 'present' positions.

    This is still troubling to me because it manages to transmit information at superluminal speed in clear violation of relativity. How does one proton know that the other hasn't changed direction between its 'retarded' position and its 'present' position? How is the information regarding the protons ( mass and energy ) transmitted at superluminal speeds to approaching space/time to tell it how to curve?

    I think I'd better read up on it.
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    Quote Originally Posted by Karakris View Post
    Does anyone know at all - or even a rough idea

    We know that all Particles, and even Light cannot travel Faster than the Speed of Light - well except maybe Neutrinos.

    Big Question - Does Gravity travel at the Speed of Light ??

    Faster ??

    Or Slower ??

    If you could create a Gravity Wave - would it propagate through Space -
    at Faster than Light, the same Speed, or Slower.

    Could we use Gravity Waves for long wave Communications - Faster than
    the Speed of Light, maybe ??
    We know they will effectively propagate or "go through"
    almost anything and everything.

    Ideas - anyone.
    I am just a Bod, with a Scientific Interest -
    but not an Advanced Mathematician, Super Physicist
    or a Cosmologist.
    Dear Karakris,
    I have a fresh concept on gravity which reveals gravity is caused by a push from particular particles and i can prove it. I call those particles G-crones. I can propose the ideal experiments to measure the speed of G-Crones [ gravity waves ]. I would like to discuss the matter with you further and i expect your writing to me in reply.
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    Quote Originally Posted by MagiMaster View Post
    IIRC, current theories predict that if gravity waves exist, they'll travel at the speed of light.
    I can prove that Gravity Waves exist. I have a new concept on gravity and I hope to post it soon.

    I'm Nilan from Sri Lanka.
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    Quote Originally Posted by MigL View Post
    Picture the two protons again moving parallel to each other at relativistic speeds. If I was riding on one of the protons and looked across at the other, I would see it trailing behind because of the finite speed of light. Now, as you've been saying all along, if each proton gravitated to its 'retarded' position, then they would mutually slow each other down and lose energy to nothing and for no reason.
    So they should gravitate to their 'present' positions.
    Thank you for thinking this through MigL! This is exactly what I have been trying to say. And in the case of the motions of objects in the Solar System, if they gravitated towards the retarded positions of the other bodies, the orbits would shift substantially compared to what is observed.

    Quote Originally Posted by MigL View Post
    This is still troubling to me because it manages to transmit information at superluminal speed in clear violation of relativity. How does one proton know that the other hasn't changed direction between its 'retarded' position and its 'present' position? How is the information regarding the protons ( mass and energy ) transmitted at superluminal speeds to approaching space/time to tell it how to curve?
    Well, we know that no information is transmitted superluminally, but other factors in GR "conspire" to cancel out the propagation delay. So, in that situation, I think we should consider that each proton continues, in the absence of any information to the contrary, to "assume" the other has continued on its present path, unless there has been an acceleration that would change it's path. If one proton changed direction, the information would be transmitted to the other at c, with the propagation delay.

    I am glad you finally see what I am saying, and I think there was someone in that other "gravitational lag and motion dampnening" thread who was saying the same thing (username J.C. MacSwell) but his comments were ignored by all the "experts".

    Last edited by SpeedFreek; September 10th, 2011 at 08:24 AM.
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    Quote Originally Posted by nilan View Post
    Quote Originally Posted by MagiMaster View Post
    IIRC, current theories predict that if gravity waves exist, they'll travel at the speed of light.
    I can prove that Gravity Waves exist. I have a new concept on gravity and I hope to post it soon.

    I'm Nilan from Sri Lanka.
    Hi Nilan and welcome. Make sure you post your new concept in the correct forum - "New Hypotheses and Ideas", and not here in the Physics section.
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    I cant see how superluminal speed can solve this either.The scaling up would still present the same result,(if im correct in my understanding)
    Rather bizarrely,The only way i can feasably get accurate results is if there are no particles involved,...strange, i know
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    Quote Originally Posted by MigL View Post
    The more I think about it SpeedFreek, things don't get clearer but more confusing.

    Picture the two protons again moving parallel to each other at relativistic speeds. If I was riding on one of the protons and looked across at the other, I would see it trailing behind because of the finite speed of light.
    No, you wouldn't. There is no "preferred" frame. You cannot tell if you and the other proton are "at rest" or "in motion". This is just another way of looking at the light clock experiment. The person traveling with the light clock sees the pulse traveling straight up and down, just as he would if the he and the clock were "at rest".

    Remember, there is no test that you can perform that will give you your "absolute" motion, and seeing the other proton in a "retarded" position would qualify as such a test. You will see the other proton as being exactly opposite of you.

    The only times that you would see the other proton in another position than its actual location would be if you had a relative velocity with respect to it, or you were accelerating.
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    Quote Originally Posted by MigL View Post
    Picture the two protons again moving parallel to each other at relativistic speeds. If I was riding on one of the protons and looked across at the other, I would see it trailing behind because of the finite speed of light. Now, as you've been saying all along, if each proton gravitated to its 'retarded' position, then they would mutually slow each other down and lose energy to nothing and for no reason.
    So they should gravitate to their 'present' positions.
    I may have not read this correctly but it seems to me that you've switched from the proton's frame to the laboratory frame without accounting for it. If two protons are moving parallel to each other and at relativistic speeds then each proton will be drawn to the other by a gravitating influence (ignoring the obvious electromagnetic field interactions that would take place here) to the position that each proton observes the other to be at. Notice consideration is on the proton's frame of reference as opposed to a laboratory frame. As such changes in the gravitating influence will appear instantaneous in one proton's frame of reference, but to an observer in the laboratory frame there will be a time delay resulting from the finite speed at which information, about the change in a bodies gravitational field, propagates.

    So in the sun-earth analogy, an observer on the sun will observe the entire mass of the sun evaporate into thin air (not that this could happen). However in the Earth's frame the last light emitted from the sun has yet to reach the earth, likewise the information about the sun's mass (gravity) is yet to reach the Earth. It is only when the Earth observes the loss of the sun's gravity, some 8 minutes after it occurs in the sun's frame of reference, that the Earth's trajectory will be freed from the influence of the sun's mass. The effect appears instantaneous in the Earth frame but it in fact occurs 8 minutes after it happens in the sun's frame of reference.
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    Yeah, you guys are right, in my haste last nite ( I've gotta stop doing this at work ) to answer SpeedFreek, I totally disregarded frames of reference in my thinking.

    So I'm back to square one, or my original stand, SpeedFreek, you gravitate towards the point you see because light and gravity travel at c.

    But I'm still thinking about, and trying to reconcile with your point of view.
    You are considering orbits which are not inertial frames since they involve accelerated motions, while my example involved two protons which can be considered to be in the same frame.
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    I notice that Janus is conspicuously not contradicting me.

    They seem to agree with me at both BAUT and physicsforums (they cite Carlip too), places I have always assumed to be rigorous. I will let you do the searches.

    When both in free fall and in a weakly accelerating frame like the Earth, GR pulls a trick on us, which means it seems as if gravity acts instantaneously, rather than at the speed of light. The gravity does not come from the place we see the object to be, it comes from the place it is "now", as any light-speed propagation delay is almost exactly cancelled out. Without that cancellation, there would be an aberration for gravity which would change the orbits in the Solar System and would have been detected observationally.

    If two planets pass each other, having a relative velocity, the gravitational force points towards their instantaneous position, rather than their retarded position.
    Last edited by SpeedFreek; September 10th, 2011 at 08:23 PM.
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    I have often wondered how many scientific minds have pondered this question.
    Newton appears to be the first,maybe the greeks did it earlier ? we will probably never know that.
    Back in my student days this type of problem was debated amongst my circle quite often.
    As i recall those discussions where something related to invariant mass,although now i dont see that specific connection as meaningfull in regards to speed of gravity.
    Tentatively, i cite loss would affect everything,therefore according to the laws of conservation,a violation.This would cause significant abherrations,not least planetary system stability and more fundamentaly universally.
    Maybe this force does have a limiting speed,but it must be at least to the power 8 of c,most likely infinite.Though,logarithmic properties are present in gravity,i wouldnt be surprised to find it having linear properties also.duality? maybe..im not sure how to test that particular theoretical aspect yet.Maybe its fundamentally irrelevant
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    Quote Originally Posted by SpeedFreek View Post
    They seem to agree with me at both BAUT and physicsforums (they cite Carlip too), places I have always assumed to be rigorous. I will let you do the searches.
    I think BAUT is one of the finest (and best moderated) forums on the internet. +1
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    Im particularly interested in how Mr Carlip sees how things seem to indicate a 2 dimensional breakdown.What where those 2 dimensions properties regarding gravity.As i envisage gravity wasnt present then?(2 dimensions).. implying an emergent property.therefore peculiar to 3 dimensions.(excluding time)
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    MigL, I have been doing some research, and have also had a nice chat with someone who knows GR well, and am quoting him here with his explicit permission, in order to provide some sort of formalism for you:

    Quote Originally Posted by publius
    Let me elaborate on why EM behavior is so relevant. You'll also note Carlip started out with EM analogy, too, before he delved into the much more complex GR calculations.

    First of all, EM is simpler. Second, we're all familiar with it, and no one disputes that EM influences propagate at 'c'. I mean, that's what the speed of light is all about. Third, we know that EM forces involve more than just a basic inverse square Coulomb interaction. There's the magnetic side, and what I like to call the "Maxwellian dance", which is the coupling between the two through time variation (or just go to the 4-vector form where it's all one tensor field, not two vector fields).

    Thus, it is no great surprise that when a source charge is in motion, there will components to the field beyond the simple inverse square Coulomb part, and these components are velocity dependent. Indeed, calculating the fields directly is a bit of chore, so we use the simple potentials, and we learn that E = -del(phi) - dA/dt and B = curl A.

    When a point charge is moving, there is an A, and it is the dA/dt part that is responsible for the "correction" from your basic inverse square Coulomb part, which comes from the gradient of the scalar potential part.

    That's what makes the force point ahead of the retarded, light image position of the source.

    Now, that falls right out of the equations, and should surprise no one.

    Now, enter SR. From the spirit of relativity, we know that we should be able to switch to the frame of an inertially moving source charge and get the same invariant results as in a frame where the source is moving. Going to that frame, the source isn't moving, and the field is static, and pure inverse square Coulomb. We know that the test particle force has to point toward the static source.

    Now, whether the force "misses" or not is an invariant. It cannot point at the source in one frame and not point at it in another!

    Thus, we deduce that if EM is going to be compatible with SR, then it must somehow "extrapolate" for inertial motion in frames where a source is moving. It has to.

    So, relatively simple EM does this "extrapolation" and in a way that should surprise no one.

    Thus, the objection to "extrapolation" should be dispensed with. Then we move on to gravity. Because of the additional complexity of gravity, it extrapolates on higher order than EM. It's the same basic behavior at work, just doing it a little better than EM.

    As we learn from EM, because of Lorentz invariance (compatibility with SR), we see that there must be these velocity dependent components of the forces (in the 4-vector form, you see that is nothing but the components of a tensor changing with coordinate transforms -- it's the same tensor, it just has different components in different frames).

    So, if gravity is to be compatible with relativity, then by George, it's going to have to have velocity dependent components as well, as further, it's going to "extrapolate" to at least velocity as EM must.

    But gravity also must obey the Equivalence Principle, which makes it more complex, and also requires it to "extrapolate" to second order, to acceleration, as well as velocity.
    Quote Originally Posted by publius
    The best way to think of it is the GR gravitational field contains information of the position, velocity, *and acceleration* of the sources and the first order of radiation is quadrapole. By contrast, the EM field contains only velocity and position information and the first order is dipole.

    Let r be the light retarded position of the source, and v its velocity, and a its acceleration. That is, that's the position of vector, in your coordinates, of the light delayed image of the source you would see. It's rather straightfoward to show that the E field of a moving source points in the direction:

    r + v(r/c)

    (r/c) is the light travel mean time. If the source is moving at constant velocity, that points exactly at the instantanous position of the source. Hence EM can be said to "extrapolate" the velocity of the source. One can, as shown by Carlip, get that very quickly by the elegant and succinct, but more advanced, 4-vector formulation of EM, but you can get it from the more familiar 3-vector form Maxwell as well, it just takes more groking.

    Now, as Carlip shows, GR gravity (in the weak field, low velocity limit) points along:

    r + v(r/c) + 1/2 a (r/c)^2

    IOW, it further extrapolates for the acceleration of the source in addition to velocity, something not in EM. And the magnitude of that acceleration in that limit is just GM/R^2, where R is that "extrapolated" position magnitude. And that's just instantaneous Newton!

    So, as long as the source doesn't change acceleration, gravity points at the instantaneous position of the source, following Newton exactly (in the limit).
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    Is that publius from the BAUT forum?
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    Yes indeed, although BAUT is not the only forum he posts at.
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    Went away for a few days SpeedFreek.

    Agree with you 100%. Invariance dictates that all inertial frames act the same such that a uniformly moving object acts the same as a 'stationary' object. And I believe that's what Publius was stating in the first quote. And that agrees with my view that we gravitate towards the point we see.

    An orbit, such as we first considered, is not, however,an inertial frame since it involves acceleration, and that complicates matters. You may be right although 'extrapolation' may just be a name for an effect we haven't thought of yet.

    Heven't really looked into it much for the last week or so, but had a great time in Montreal ( Canada ).
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    Quote Originally Posted by MigL View Post
    The more I think about it SpeedFreek, things don't get clearer but more confusing.

    Picture the two protons again moving parallel to each other at relativistic speeds. If I was riding on one of the protons and looked across at the other, I would see it trailing behind because of the finite speed of light. Now, as you've been saying all along, if each proton gravitated to its 'retarded' position, then they would mutually slow each other down and lose energy to nothing and for no reason.
    So they should gravitate to their 'present' positions.

    This is still troubling to me because it manages to transmit information at superluminal speed in clear violation of relativity. How does one proton know that the other hasn't changed direction between its 'retarded' position and its 'present' position? How is the information regarding the protons ( mass and energy ) transmitted at superluminal speeds to approaching space/time to tell it how to curve?

    I think I'd better read up on it.
    I've seen this scenario presented elsewhere. At the time I wondered what, if any, effects would one notice in this experiment if the two protons were replaced by a mirror and a laser pointer mounted opposite each other across the axis of travel of the hull of a very big spaceship (say, 299,792,458 km in diameter). If the laser pointer is pointed directly at the mirror, what happens when you click the pointer on?

    As far as I know, the laser light will be reflected directly back to the laser pointer, whether the spaceship is "standing still" relative to some third object or travelling at 0.9999 c relative to that object. If this is true, then I don't see where there can possibly be any difference between where you "see" the mirror (if you're standing next to the laser pointer) and where the mirror actually "is" relative to you. If I'm wrong, corrections are welcome.

    Now, if you're accelerating (or in a gravitational field), that puts a different slant on things [I just couldn't resist the pun] .

    I've read through this thread and have been properly confused by most of what I've read. I take it that an object carries along with it a static gravitational field that's sort of "stuck" to it. The object and its gravitational field are a package deal - where one goes, so goes the other. Since no object can go faster than the speed of light relative to another object (ignoring cosmological expansion - which is best addressed elsewhere), this seems to say (to me) that an object and the gravitational field of that object move at the same velocity.

    The question of the velocity of gravitational waves is a bit different than the above scenario. I don't see any reason why they shouldn't travel at c. Since gravitational waves haven't been unambiguously detected, there's certainly no experimental evidence to nail down a specific velocity. Are there any theoretical arguments that convincingly show that gravitational waves do travel at c?

    Chris
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    well you already how fast gavity is beuse you deal with it any day of the weeek just think of a fall object and do the math elquiezer on ththe object and the math, if you an imagine an object falling you know exactley it's speed so you can just in a second do the math and get the speed precisely
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    Quote Originally Posted by SoularMatter View Post
    well you already how fast gavity is beuse you deal with it any day of the weeek just think of a fall object and do the math elquiezer on ththe object and the math, if you an imagine an object falling you know exactley it's speed so you can just in a second do the math and get the speed precisely
    That would give you the acceleration due to gravity (and only that for the surface of the Earth). This is not the "speed of gravity" in the sense that it is being discussed here.
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    Hi MigL, I'm glad you had a good time in Montreal!

    Quote Originally Posted by MigL View Post
    Agree with you 100%. Invariance dictates that all inertial frames act the same such that a uniformly moving object acts the same as a 'stationary' object. And I believe that's what Publius was stating in the first quote. And that agrees with my view that we gravitate towards the point we see.
    I don't believe that is what publius was saying.

    For objects in uniform inertial motion, each can consider themselves to be at rest and that the other is moving, and the force points "ahead" of the light retarded image of the moving object. It works both ways. Each sees the force pointing ahead of the other object, so each can consider it is themselves that is at rest and that the other is moving, or they can think of it the other way round, it doesn't matter! It is relative motion that causes this extrapolation, regardless.

    In the second quote he expressly states that the force points to a different place than the light retarded position, be it the EM force, or the gravitational force, doesn't he? This means that, from the Earth, the gravitational force points towards the instantaneous position of the Sun, rather than the light-retarded position, which is where I started from in this thread. I knew I was stirring things up when I posted, but it seems there are a lot of people out there who assume, in error it seems, that gravity attracts you towards the place the source was, due to a propagation delay, rather than the instantaneous position of the source. This is what I have been trying (in my rather inept way) to address all along.

    Here is another little gem from publius that should settle the issue.

    Quote Originally Posted by publius
    That experiment has actually been done, but for the acceleration of the earth toward the sun. I don't know the exact source or papers on it, but I've seen it referenced in discussion about the "speed of gravity" and it involves radar ranging. Eclipses, when the moon is lined up with the sun provide the perfect opportunity.

    The moon is moving across the sky, "chasing" the sun and it aligns up with the retarded *image* of the sun before it lines up with the instantaneous position. The data shows the peak acceleration of the earth occurs after the moment of maximum eclipse, when the moon is aligned with the sun's instantaneous position, and not its retarded position.

    As Carlip demonstrates, that is not due to much faster than light propagation, but at the moment of maximum visual ecclipse the sun's gravitational field, the metric, is giving earth a lot of information about where to accelerate. It's "saying", here is my position, my velocity, and my accleration at the point this information left my location. The earth's accelerates according to that additional information and so does not accelerate towards the image, but towards the extrapolated position.
    I think that says it all, and I trust publius to know what he is talking about.

    In weak fields, space-time conspires to "telegraph" ahead the instantaneous position of a gravitational source. Unless we suddenly accelerate, we can consider the curvature of space-time to be telling us the instantaneous position of the source, rather than the retarded light-delayed position, and it is towards the instantaneous position that we accelerate. Gravity points us towards the place things are "now", not where we see them to be.

    Strange, but true.
    Last edited by SpeedFreek; September 20th, 2011 at 07:25 PM. Reason: clarification
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    If one uses the Newton's force expression to calculate planet motion, F = km1m2/x^2 You will get the correct answer to a remarkable degree of accuracy. Newton made avery strange comment about his universal law of physics. But first, if one uses any model of force retardation meaning you assume the force sent out by the sun to Jupitor at the speed of light and then calculate the reaction and so forth you will not get the correct answer.SO Sorry. Itis inconceivable, that inanimate brute matter should, without the mediation ofsomething else which is not material, operate upon, and affect other matterwithout mutual contact. . . . . And this is one reason why I desired you wouldnot ascribe innate gravity to me. That gravity should be innate, inherent, andessential to matter, so that one body may act upon another, at a distancethrough a vacuum without the mediation of anything else, by and through whichtheir action and force may be conveyedfrom one to another, is to me so great an absurdity, that I believe that no manwho has in philosophical matters a competent faculty of thinking, can everfall into it. Gravity must be caused byan agent acting constantly according to certain laws; but whether this agent bematerial or immaterial, I have left to the consideration of my readers.”
    Newtonwas referring to the age old question of ‘action at a distance’, which is theeasy part of his understanding of the law and its implications. The otherparameter to which Newton addressed his words cautioning calculation of stellartrajectories and position using his new law - to calculate a stellar trajectory or positionrequires the assumption that the force exchange between two stellar bodieslinked with some gravitational mechanism was necessarily instantaneous.

    Thetwo matters took the Newton era cosmologists by surprise, which apparently hasnot lasted with an equivalent level of intensity up to the advanced state ofthird millennium cosmology and cosmologists.
    Whatever the einsteinians say, Newton's law of gravity assumes, from thr force expression, that the force of "gravity" is "instantaneous action at a distance." Using a retarded force model just will not give the correct answer.

    Mother Nature include time in her creation so everything wouldn't happen all at once. Anon
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    geistkiesel, I think SpeedFreek is saying the same thing that you're saying - just in a different way.

    If I understand it correctly, the static gravitational field of an object moves with the object and always points to where the object is "now". Thus, planetary orbits can be calculated using Newton's "instantaneous" action at a distance (even though he found the idea absurd).

    The speed of propogation of a gravitational waves is another matter. They aren't part of the static gravitational field of the object. They radiate away from the object. At what speed? I don't think the answer to that question has been experimentally or observationally verified yet.

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    Quote Originally Posted by geistkiesel View Post

    Whatever the einsteinians say, Newton's law of gravity assumes, from thr force expression, that the force of "gravity" is "instantaneous action at a distance." Using a retarded force model just will not give the correct answer.
    "Einsteinians" say the same thing, but in a different way. The force of gravity propagates at c, but velocity-dependant interactions conspire to cover up any propagation delay, so the the gravitational force points towards the "instantaneous" position of the gravitational source. Nobody is using a "retarded force" model.
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    What do you guys think to this ! see attached photo
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    I have had some further PM discussion with publius on this subject which I think some of you should see (he has been reading this thread, and has given permission for me to repost his PMs here):

    Quote Originally Posted by publius
    I read over that thread, and now again after there's been more activity. I don't think some of them understand what I said where you quoted me. To summarize:

    EM "extapolates" velocity. The field of a moving charge moves as a rigid body so to speak. However, if the source charges *accelerate*, then the forces will "miss". That is the force will not point at the instantaneous position. That causes a loss of energy and momentum. That lost energy and momentum goes to *radiation*. Accelerating charges radiate. Thus, propagation delay can be seen as the cause of radiation.

    GR Gravity goes further and extrapolates acceleration of the source as well as velocity. Thus with gravity, the forces "miss" only if there is a change in acceleration, "jerk", the third time derivative of position. That causes orbits to inspiral, which is a non-Newtonian behavior seen in binary pulsars. The reason gravity must do better is basically because of the Equivalence Principle. Obeying that demands the field equations must extrapolate acceleration as well.

    Thus, the first order of EM radiation is dipole, which corresponds to "extrapolating" to velocity. The first order of gravitational radiation is quadrapole, which corresponds to extrapolating to acceleration.

    So, yes EM and gravity do miss due to the progragation delay, but not as much as if you naively insert a retardation into an inverse square force. GR gravity does much, much better than EM in this regard. In the case of the solar system, the "jerk" terms are so small that gravity does not miss much at all. Indeed, I think the solar system would stable over a period several times the age of the universe or something like that. The earth for example is losing orbital energy to gravitational radiation at rate of only 300W, three light bulbs worth.
    Quote Originally Posted by SpeedFreek
    Firstly, thank you for indulging me! I just hope I haven't made too many misleading statements in that thread. As you will have seen, I have been saying things like we can consider the curvature of spacetime to be "telegraphing" the instantaneous positions of gravitational sources, unless something accelerates (or to be more exact, changes its acceleration), and that the curvature caused by the solar system as it moves around the galaxy doesn't "lag behind", due to a finite propagation speed for gravity (i.e. the field moves as a rigid body, as you so nicely put it in reference to EM).

    So, just to be sure that my basic, simplistic understanding of this whole concept is sound, can you confirm the following for me:

    For objects in uniform motion or uniform acceleration, gravity points towards the extrapolated, "instantaneous" position of the source, rather than the light retarded position, due to "velocity dependent interactions". This means gravity points towards the extrapolated position of the Sun, rather than the place we see it in the sky, as evidenced by measurements made during a lunar eclipse,

    (It is the "gravity doesn't point towards the place we see the Sun" part that seems to be the sticking point in my discussion with MigL, and I made that assertion in my first post in the thread. Well, what I actually started off saying was that the Earth orbits the barycentre as it is now, rather than where it was 8 minutes ago, a statement that may have different issues?)

    Gravity propagates at c, but other velocity dependent factors in GR conspire in such a way that gravity points towards the instantaneous position, rather than the place we see the Sun. Gravity comes to us at c from the place the Sun was, 8 minutes ago, but as it reaches us the resulting curvature of space-time around Earth points us towards where the Sun in "now" (almost exactly).

    GR conspires to cancel out the propagation delay and so extrapolates the instantaneous position when there is both velocity and constant acceleration, but if there is a change of acceleration that extrapolation is not exact, so there is a "miss", which is results in a loss of energy/momentum in the form of gravitational radiation.
    Quote Originally Posted by publius
    Yes, your statements are correct. And yes, it has been confirmed during eclipses, that the earth is "falling" not at the image of the sun in the sky we see, but a point ahead, where the sun will be ~8min from the present.

    Yes, gravity only "misses" due to changes in acceleration ("jerk") during the light travel meantime, and that miss produces gravitational radiation. For the solar system, these terms are so small they are of no consequence. (Note the precession of the orbit of Mercury has nothing to do with the propagation delay, but is due to other non-Newtonian behavior). For orbits, the gravitational radiation term goes ~(v/c)^5, compared to EM which would be (v/c)^3. That demonstates how much better GR gravity "extrapolates" than EM forces.

    For binary pulsars, which are highly relativistic systems where (v/c)^5 is significant, these effects are observed, and agree with the predictions of GR to high accuracy.
    So, at least I can be confident that my understanding of how GR deals with gravity is pretty sound. Phew!

    Although I must admit, until I discussed this with publius, I hadn't realised the extrapolation also holds for uniform acceleration as well as velocity. So hopefully, we have all learned something new through this thread!
    Last edited by SpeedFreek; September 23rd, 2011 at 01:17 PM.
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    Thanks SpeedFreek, appreciate the time and effort you have put in.most helpful
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    That's why he gets the big bucks
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