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hkyriazi
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Is there any consensus among experimental and/or theoretical physicists about Tom Van Flandern's ideas about gravity propagating (and having to propagate) much faster than the speed of light?
hkyriazi said:"the quadrupole nature of gravitational radiation."
robphy said:Why is this thread in this forum?
Well obviously if the speed of gravity would not be c then GR would be incorrect. So I would say that GR implies it instead of that it predicts it.pervect said:This is a prediction of GR, not yet an experimentally established fact.
pervect said:I should probably add that the speed of gravity is predicted to be equal to 'c' only for weak fields in a vacuum.
When you linearize the Einstein field equations around a vacuum solution, you come up with a set of linear differential equations. The solution to these linearized equations is a plane wave, just as it is in the case of Maxwell's equations. This plane wave travels at a speed of 1 in geometric units, i.e. it travels at the speed of light, since c=1 in geometric units.
Only becouse STR is taken as guide in linearization where c=1.pervect said:. This plane wave travels at a speed of 1 in geometric units, i.e. it travels at the speed of light, since c=1 in geometric units.
tehno said:Only becouse STR is taken as guide in linearization where c=1.
But this fact about EM was found both experimentally,and theoretically.
The claim that gravity wave propagate with c,isn't found experimentally or theoretically...
yogi said:Pervect - can you clarify - are you saying that a plane wave arises from the sudden destruction of matter (conversion to another form such as photons) which is other than gravitational radiation.
So there you go...And what constant,if not "electromagnetic" c ,is fixed in a Minkowski metric?:tongue:pervect said:Certain assumptions are made to make this derivation. One assumes that one has a metric n_uv which satisfies Einstein's field equations. G_uv can be written as a compiclated second-order non-linear differential equation of n_uv. It is simplest and usually assumed that n_uv is a Minkowski metric
tehno said:So there you go...And what constant,if not "electromagnetic" c ,is fixed in a Minkowski metric?:tongue:
Impression from the books that "electromagnetic" c sets the "gravitational" c.pervect said:And your point is - what, exactly?
It sounds like we might actually agree if you would restrain what appears to be some anti-relativity sentiment. At least that's the way it's coming across to me.
tehno said:Impression from the books that "electromagnetic" c sets the "gravitational" c.
Quite comfortably,I would rather say that it's the other way round .
However,I don't think this could be the correct standpoint either.
c must be the universal constant,not exclusively reserved for electromagnetism or gravity.
Beside the fact that it doesn't deal with the gravity,Maxwell's theory cannot be considered as the complete theory.
Covariance:Maxwel's eqs. for empty space stay unchanged if we apply to space-time coordinates linear tranformations->Lorentz transforms.Covariance holds for a transformation composed of more such transformations.Mathematically that's the property of a Lorentz group.Accordingly,from Maxwell's eqs. arise the Lorentz group,but Maxwell's eqs. from the Lorentz group don't arise .The group can be defined independently of these eqs. as the group of linear transforms with c=1 kept constant.
In GR things are even more interesting ,nonlinear transformations must be applied,and Lorentz group aren't generally valid .
But the point is :in electromagnetism where charges oscillates,we find c. In the gravity,where masses oscillate,we will probably verify one day the same velocity c of the field disturbance propagation.
Also the curiosity :A propagating EM wave induces a gravitational field,but a propagating gravitational wave does not induce a magnetic field.
tehno said:In Maxwell's theory of EM waves these propagate in vacuum at "speed" determined by two constants,vacuum permeability and permitivity namely.
Why the speed of propagating of a gravity wave,which at first glance has nothing to do with electrical charges,has to be linked with these two electrical constants, in the same manner?
tehno said:Only becouse STR is taken as guide in linearization where c=1.
But this fact about EM was found both experimentally,and theoretically.
The claim that gravity wave propagate with c,isn't found experimentally or theoretically...
I don't know why do you feel to write at length about set/system of units?rbj said:but these constants are manifestations only of our anthropometric units used to measure them. they are not fundamental properties of the universe. but, even though the numerical value of the speed of E&M propagation is still an anthropocentric number (unless we were to use natural units like Planck units), the quantity of such a speed is fundamental and believed to be universal ...
now apply that same thought experiment to two infinite lines of uncharged mass. they will attract each other, but for the "stationary" observer their rate of attraction will be reduced due to the same time dilation with the same c in the time dilation formula. this reduction of attraction can be thought of as a gravito-magnetic effect and, for the "classical" model formula very similar to Maxwell's equations (called the GEM equations) can be constructed with mass replacing charge, mass density replacing charge density, [itex]-G[/itex] replacing [itex]1/(4 \pi \epsilon_0)[/itex] but the same c ! if it were a different speed of propagation for gravity, then the time-dilation formula for this second thought experiement, would need a different c to go into it. so different formulae for time-dilation depending on what it is that is moving past an observer? why?...
those two electrical constants are anthropometric crap. it's the speed of propagation of these ostensibly "instantaneous" effects that is fundamental and is the same for all things instantaneous. then, given your set of units you choose to use, you measure [itex]G[/itex] or [itex]\epsilon_0[/itex] to come out to be whatever numbers they do.
In the paper it's only interpretation of the experiment that speed of gravity is close to the speed of light .But that is far from being the measurment of speed of gravity (gravity waves namely).rbj said:Sergei Kopeikin and Edward Fomalont have experimental data that the speed of gravity is within +/- 20% of the speed of light: http://arxiv.org/abs/astro-ph/0302294 . also, there is good theoretical reason to expect the same speed for both, which, i think GR is supposed to nail. if the Gravity-Probe B ends up consistent with the predictions of GR, i think that's another nail in the coffin. these frame-dragging or gravito-magnetic effects would have a different magnitude if the speed of gravity was not the same c.
tehno said:What is Gravity-Probe B?I don't find it in the paper.
tehno said:Thanks jtbell.
Stupid me:)
Why they need so much time to process the data gathered from the experiment?
tehno said:I don't know why do you feel to write at length about set/system of units?
I don't raise that question up becouse I don't find it an issue at all.
I didn't say permeability,permitivity are fundamental units of the universe.
i guess you qualified that as "in Maxwell's theory", but the implication that i got from what you wrote was that it was curious why these two electrical constants (that combine in some way to be c) would have anything to do with the speed of gravity. isn't that the question you asked?tehno said:In Maxwell's theory of EM waves these propagate in vacuum at "speed" determined by two constants,vacuum permeability and permitivity namely.
Why the speed of propagating of a gravity wave,which at first glance has nothing to do with electrical charges,has to be linked with these two electrical constants, in the same manner?
Matter of fact,I expressed my opinion about [itex]c[/itex] being universal constant not reserved exclusively for electromagnetism or gravity.
pervect said:The first LIGO science run was in 2002. http://www.ligo-wa.caltech.edu/ligo_science/P030045-B.pdf
Because stars rotate, supernova are expected to generate gravitational waves - people are starting to try and predict the details (amplitudes and wave spectrum/wave shape).
The rotation is important because according to GR, non-rotating spherical collapse shouldn't generate any gravity waves.
yogi said:Thanks for clarifying that point pervect. Now, how would you treat a situation where one of two nearby non rotating massive bodies disintegrates - causing an abrupt decrease in the mutual gravitational attraction - or if you choose - in the local spacetime curvature. In either case, the surviving object will be affected - if the change is not communicated by gravitational radiation - what is the nature of the wave is involved?