Speed of gravity
in [Relativity]
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From: BURT on 1 Mar 2010 15:49 On Mar 1, 9:53 am, carlip-nos...(a)physics.ucdavis.edu wrote: > Art <n...(a)zilch.com> wrote: > > Has this question been settled yet? I've read that Einstein > > assumed gravity travels at c. But I've also read that certain > > orbits are iunstable unless gravity travels >> c. > > It depends what you mean by "settled." > > General relativity predicts that gravity propagates at the speed of > light, in the sense that if you change the matter configuration in > some finite region, the gravitational effects of that change don't > reach distant regions until after the light-travel time to those > regions. I wouldn't say Einstein "assumed" this -- it was not put > into the derivation of the field equations of general relativity, > but is, rather, a conclusion. There's a rigorous proof in Low, > "Speed limits in general relativity," Class. Quant. Grav. 16 (1999) > 543, on line at arxiv.org/abs/gr-qc/9812067. > > It's also true that if you start with *Newtonian* gravity and stick > in a finite propagation speed, orbits become dramatically unstable. > This does *not* happen in general relativity, though; in GR, there > are additional velocity-dependent interactions that almost (but not > quite) cancel the instability. The lack of exact cancellation leads > to slow changes in the orbits of binary neutron stars ("gravitational > radiation reaction"), which are observed and agree very precisely > with prediction. This cancellation was, again, not put into the > derivation of the field equations of general relativity, but comes > out as a conclusion. It's discussed in my paper, "Aberration and the > speed of gravity," published in Phys. Lett. A267 (2000) 81, on line > at arxiv.org/abs/gr-qc/9909087. > > As for the experimental/observational question, we have no direct > evidence. Gravity is too weak an interaction for the difference > between an infinite propagation speed and the GR prediction of a > finite speed plus velocity-dependent interactions. But a Newtonian > theory with infinite propagation speed would give the wrong results > for binary pulsars, unless some additional radiation reaction terms > were stuck in by hand. > > It's also worth noting that the same issue occurs in electromagnetism. > Almost everyone accepts that the electromagnetic force travels at > the speed of light. But if you look at the force exerted by a charge > moving at a constant velocity, it points towards the "instantaneous" > position of the charge, not the retarded (light-travel-delayed) > position. This is discussed in the Feynman Lectures, Vol. II, chap.21 > -- you can see very explicitly that the effects of finite propagation > speed are canceled to lowest order by additional velocity-dependent > interactions that effectively "extrapolate" the position of the moving > charge. > > Steve Carlip The strength of gravity moves with mass. Curvature moves with the motion of the body in question. Mitch Raemsch
From: Art on 1 Mar 2010 16:42 On Mon, 1 Mar 2010 17:53:55 +0000 (UTC), carlip-nospam(a)physics.ucdavis.edu wrote: >Art <null(a)zilch.com> wrote: >> Has this question been settled yet? I've read that Einstein >> assumed gravity travels at c. But I've also read that certain >> orbits are iunstable unless gravity travels >> c. > >It depends what you mean by "settled." Settled by experience, as Einstein used to say. There was controversy over the Kopeiken and Fomelot result back in 2002, for example: http://www.csa.com/discoveryguides/gravity/overview.php >General relativity predicts that gravity propagates at the speed of >light, in the sense that if you change the matter configuration in >some finite region, the gravitational effects of that change don't >reach distant regions until after the light-travel time to those >regions. I wouldn't say Einstein "assumed" this -- it was not put >into the derivation of the field equations of general relativity, >but is, rather, a conclusion. There's a rigorous proof in Low, >"Speed limits in general relativity," Class. Quant. Grav. 16 (1999) >543, on line at arxiv.org/abs/gr-qc/9812067. I doubt that Einstein released his GR without checking some of its predictions ... especially the "speed of gravity" prediction. That's what I meant when I said "Einstein assumed". >It's also true that if you start with *Newtonian* gravity and stick >in a finite propagation speed, orbits become dramatically unstable. >This does *not* happen in general relativity, though; in GR, there >are additional velocity-dependent interactions that almost (but not >quite) cancel the instability. The lack of exact cancellation leads >to slow changes in the orbits of binary neutron stars ("gravitational >radiation reaction"), which are observed and agree very precisely >with prediction. This cancellation was, again, not put into the >derivation of the field equations of general relativity, but comes >out as a conclusion. It's discussed in my paper, "Aberration and the >speed of gravity," published in Phys. Lett. A267 (2000) 81, on line >at arxiv.org/abs/gr-qc/9909087. > >As for the experimental/observational question, we have no direct >evidence. Gravity is too weak an interaction for the difference >between an infinite propagation speed and the GR prediction of a >finite speed plus velocity-dependent interactions. But a Newtonian >theory with infinite propagation speed would give the wrong results >for binary pulsars, unless some additional radiation reaction terms >were stuck in by hand. I had in mind other experimentalists attempting to duplicate the results of the 2002 experiment I mentioned above ... or maybe coming up with other clever methods. >It's also worth noting that the same issue occurs in electromagnetism. >Almost everyone accepts that the electromagnetic force travels at >the speed of light. But if you look at the force exerted by a charge >moving at a constant velocity, it points towards the "instantaneous" >position of the charge, not the retarded (light-travel-delayed) >position. This is discussed in the Feynman Lectures, Vol. II, chap.21 >-- you can see very explicitly that the effects of finite propagation >speed are canceled to lowest order by additional velocity-dependent >interactions that effectively "extrapolate" the position of the moving >charge. > >Steve Carlip Thanks for your input, Steve. Art
From: G. L. Bradford on 2 Mar 2010 01:22 <carlip-nospam(a)physics.ucdavis.edu> wrote in message news:hmguvj$1f6o$1(a)news.telesweet.net... > Art <null(a)zilch.com> wrote: >> Has this question been settled yet? I've read that Einstein >> assumed gravity travels at c. But I've also read that certain >> orbits are iunstable unless gravity travels >> c. > > It depends what you mean by "settled." > > General relativity predicts that gravity propagates at the speed of > light, in the sense that if you change the matter configuration in > some finite region, the gravitational effects of that change don't > reach distant regions until after the light-travel time to those > regions. I wouldn't say Einstein "assumed" this -- it was not put > into the derivation of the field equations of general relativity, > but is, rather, a conclusion. There's a rigorous proof in Low, > "Speed limits in general relativity," Class. Quant. Grav. 16 (1999) > 543, on line at arxiv.org/abs/gr-qc/9812067. > > It's also true that if you start with *Newtonian* gravity and stick > in a finite propagation speed, orbits become dramatically unstable. > This does *not* happen in general relativity, though; in GR, there > are additional velocity-dependent interactions that almost (but not > quite) cancel the instability. The lack of exact cancellation leads > to slow changes in the orbits of binary neutron stars ("gravitational > radiation reaction"), which are observed and agree very precisely > with prediction. This cancellation was, again, not put into the > derivation of the field equations of general relativity, but comes > out as a conclusion. It's discussed in my paper, "Aberration and the > speed of gravity," published in Phys. Lett. A267 (2000) 81, on line > at arxiv.org/abs/gr-qc/9909087. > > As for the experimental/observational question, we have no direct > evidence. Gravity is too weak an interaction for the difference > between an infinite propagation speed and the GR prediction of a > finite speed plus velocity-dependent interactions. But a Newtonian > theory with infinite propagation speed would give the wrong results > for binary pulsars, unless some additional radiation reaction terms > were stuck in by hand. > > It's also worth noting that the same issue occurs in electromagnetism. > Almost everyone accepts that the electromagnetic force travels at > the speed of light. But if you look at the force exerted by a charge > moving at a constant velocity, it points towards the "instantaneous" > position of the charge, not the retarded (light-travel-delayed) > position. This is discussed in the Feynman Lectures, Vol. II, chap.21 > -- you can see very explicitly that the effects of finite propagation > speed are canceled to lowest order by additional velocity-dependent > interactions that effectively "extrapolate" the position of the moving > charge. > > Steve Carlip ========================= Two bodies are in an orbital system, one orbiting the other as they travel through the universe. The speed of light gives one body the relative position of the other to be 8 light minutes. Their fields of gravity fix the position of each relative to other at [8 light minutes plus 8 minutes]. The Earth does not orbit the history 8 light minutes away, it orbits the solar mass 8 light minutes plus 8 minutes away. It is gravitationally aware of the exact actual position of the sun relative to itself, as is the sun gravitationally aware of the exact actual position of the Earth relative to itself. The speed of light versus gravity's [field] being nothing less than gravity's [singularity] by another name. You told Art that the bodies orbit the history, not the mass. The seemingly small difference in the example above might not mean much, if anything, to the lazy but means everything to physics. You separated the sun's field, the sun's well, from the sun's mass 8 minutes worth at the Earth's distance from it. You would separate the field, the well, from the mass even more at Mars' distance from it....and even more at Jupiter's distance from it...and even more at Saturn's distance....and so on. GLB =========================
From: BURT on 2 Mar 2010 01:45 On Mar 1, 10:22 pm, "G. L. Bradford" <glbra...(a)insightbb.com> wrote: > <carlip-nos...(a)physics.ucdavis.edu> wrote in message > > news:hmguvj$1f6o$1(a)news.telesweet.net... > > > > > > > Art <n...(a)zilch.com> wrote: > >> Has this question been settled yet? I've read that Einstein > >> assumed gravity travels at c. But I've also read that certain > >> orbits are iunstable unless gravity travels >> c. > > > It depends what you mean by "settled." > > > General relativity predicts that gravity propagates at the speed of > > light, in the sense that if you change the matter configuration in > > some finite region, the gravitational effects of that change don't > > reach distant regions until after the light-travel time to those > > regions. I wouldn't say Einstein "assumed" this -- it was not put > > into the derivation of the field equations of general relativity, > > but is, rather, a conclusion. There's a rigorous proof in Low, > > "Speed limits in general relativity," Class. Quant. Grav. 16 (1999) > > 543, on line at arxiv.org/abs/gr-qc/9812067. > > > It's also true that if you start with *Newtonian* gravity and stick > > in a finite propagation speed, orbits become dramatically unstable. > > This does *not* happen in general relativity, though; in GR, there > > are additional velocity-dependent interactions that almost (but not > > quite) cancel the instability. The lack of exact cancellation leads > > to slow changes in the orbits of binary neutron stars ("gravitational > > radiation reaction"), which are observed and agree very precisely > > with prediction. This cancellation was, again, not put into the > > derivation of the field equations of general relativity, but comes > > out as a conclusion. It's discussed in my paper, "Aberration and the > > speed of gravity," published in Phys. Lett. A267 (2000) 81, on line > > at arxiv.org/abs/gr-qc/9909087. > > > As for the experimental/observational question, we have no direct > > evidence. Gravity is too weak an interaction for the difference > > between an infinite propagation speed and the GR prediction of a > > finite speed plus velocity-dependent interactions. But a Newtonian > > theory with infinite propagation speed would give the wrong results > > for binary pulsars, unless some additional radiation reaction terms > > were stuck in by hand. > > > It's also worth noting that the same issue occurs in electromagnetism. > > Almost everyone accepts that the electromagnetic force travels at > > the speed of light. But if you look at the force exerted by a charge > > moving at a constant velocity, it points towards the "instantaneous" > > position of the charge, not the retarded (light-travel-delayed) > > position. This is discussed in the Feynman Lectures, Vol. II, chap.21 > > -- you can see very explicitly that the effects of finite propagation > > speed are canceled to lowest order by additional velocity-dependent > > interactions that effectively "extrapolate" the position of the moving > > charge. > > > Steve Carlip > > ========================= > > Two bodies are in an orbital system, one orbiting the other as they travel > through the universe. The speed of light gives one body the relative > position of the other to be 8 light minutes. Their fields of gravity fix the > position of each relative to other at [8 light minutes plus 8 minutes]. The > Earth does not orbit the history 8 light minutes away, it orbits the solar > mass 8 light minutes plus 8 minutes away. It is gravitationally aware of the > exact actual position of the sun relative to itself, as is the sun > gravitationally aware of the exact actual position of the Earth relative to > itself. The speed of light versus gravity's [field] being nothing less than > gravity's [singularity] by another name. > > You told Art that the bodies orbit the history, not the mass. The > seemingly small difference in the example above might not mean much, if > anything, to the lazy but means everything to physics. You separated the > sun's field, the sun's well, from the sun's mass 8 minutes worth at the > Earth's distance from it. You would separate the field, the well, from the > mass even more at Mars' distance from it....and even more at Jupiter's > distance from it...and even more at Saturn's distance....and so on. > > GLB > > =========================- Hide quoted text - > > - Show quoted text - The gravity field moves slower than lght as it's mass moves. The field moves nonlocally. Mitch Raemsch
From: G. L. Bradford on 2 Mar 2010 04:04
"BURT" <macromitch(a)yahoo.com> wrote in message news:7f7539e6-ae95-4eff-bc8e-101469ac2464(a)u19g2000prh.googlegroups.com... On Mar 1, 10:22 pm, "G. L. Bradford" <glbra...(a)insightbb.com> wrote: > <carlip-nos...(a)physics.ucdavis.edu> wrote in message > > news:hmguvj$1f6o$1(a)news.telesweet.net... > > > > > > > Art <n...(a)zilch.com> wrote: > >> Has this question been settled yet? I've read that Einstein > >> assumed gravity travels at c. But I've also read that certain > >> orbits are iunstable unless gravity travels >> c. > > > It depends what you mean by "settled." > > > General relativity predicts that gravity propagates at the speed of > > light, in the sense that if you change the matter configuration in > > some finite region, the gravitational effects of that change don't > > reach distant regions until after the light-travel time to those > > regions. I wouldn't say Einstein "assumed" this -- it was not put > > into the derivation of the field equations of general relativity, > > but is, rather, a conclusion. There's a rigorous proof in Low, > > "Speed limits in general relativity," Class. Quant. Grav. 16 (1999) > > 543, on line at arxiv.org/abs/gr-qc/9812067. > > > It's also true that if you start with *Newtonian* gravity and stick > > in a finite propagation speed, orbits become dramatically unstable. > > This does *not* happen in general relativity, though; in GR, there > > are additional velocity-dependent interactions that almost (but not > > quite) cancel the instability. The lack of exact cancellation leads > > to slow changes in the orbits of binary neutron stars ("gravitational > > radiation reaction"), which are observed and agree very precisely > > with prediction. This cancellation was, again, not put into the > > derivation of the field equations of general relativity, but comes > > out as a conclusion. It's discussed in my paper, "Aberration and the > > speed of gravity," published in Phys. Lett. A267 (2000) 81, on line > > at arxiv.org/abs/gr-qc/9909087. > > > As for the experimental/observational question, we have no direct > > evidence. Gravity is too weak an interaction for the difference > > between an infinite propagation speed and the GR prediction of a > > finite speed plus velocity-dependent interactions. But a Newtonian > > theory with infinite propagation speed would give the wrong results > > for binary pulsars, unless some additional radiation reaction terms > > were stuck in by hand. > > > It's also worth noting that the same issue occurs in electromagnetism. > > Almost everyone accepts that the electromagnetic force travels at > > the speed of light. But if you look at the force exerted by a charge > > moving at a constant velocity, it points towards the "instantaneous" > > position of the charge, not the retarded (light-travel-delayed) > > position. This is discussed in the Feynman Lectures, Vol. II, chap.21 > > -- you can see very explicitly that the effects of finite propagation > > speed are canceled to lowest order by additional velocity-dependent > > interactions that effectively "extrapolate" the position of the moving > > charge. > > > Steve Carlip > > ========================= > > Two bodies are in an orbital system, one orbiting the other as they travel > through the universe. The speed of light gives one body the relative > position of the other to be 8 light minutes. Their fields of gravity fix > the > position of each relative to other at [8 light minutes plus 8 minutes]. > The > Earth does not orbit the history 8 light minutes away, it orbits the solar > mass 8 light minutes plus 8 minutes away. It is gravitationally aware of > the > exact actual position of the sun relative to itself, as is the sun > gravitationally aware of the exact actual position of the Earth relative > to > itself. The speed of light versus gravity's [field] being nothing less > than > gravity's [singularity] by another name. > > You told Art that the bodies orbit the history, not the mass. The > seemingly small difference in the example above might not mean much, if > anything, to the lazy but means everything to physics. You separated the > sun's field, the sun's well, from the sun's mass 8 minutes worth at the > Earth's distance from it. You would separate the field, the well, from the > mass even more at Mars' distance from it....and even more at Jupiter's > distance from it...and even more at Saturn's distance....and so on. > > GLB > > =========================- Hide quoted text - > > - Show quoted text - The gravity field moves slower than lght as it's mass moves. The field moves nonlocally. Mitch Raemsch ========================= A gravity field is singular in space and time, in space-time, not plural! At one light second's distance from the body it exists at one light second plus one second in the universe. At ten light seconds distance from the body it exists at ten light seconds plus ten seconds in the universe. At one thousand light seconds from the body it exists at one thousand light seconds plus one thousand seconds in the universe. All of the [plus] unobservable and singularly advanced in space and time, in space-time, over the speed of light. Gravity's field (gravity's singularity), integrally, has nothing to do with speed at all, no matter if it's 'c' or even infinity. It's singular, damn it! The gravity field is singular, always existing and in motion as singular, a singularity in space and time, in space-time. It does not exist one light second from the body, which is to say it does not exist -1 second to the body's 0. It exists that -1 second + 1 second to equal 0 (0 = 0). At one light second's distance (-) from any gravitational body (0), it has already (+) beaten the speed of light by one second (0 = 0). At ten light seconds distance (-), it has already (+) beaten 'c' by ten seconds (0 continues to equal 0). And the inequality between the speed of light and gravity's field (gravity's singularity) only continues to grow with all accumulating distance the speed of light has to contend with (that gravity's singular field does not have to contend with). Any so-called gravitational wave (made out to be the material entity of the gravity field) doing the speed of light would be tied to an -observable- massless history [behind] rather than the -unobservable- massive body [ahead]. Any body at any distance trying to orbit a massless history rather than a massive body is certainly going to have an "unstable" orbit. GLB ========================= |