# How fast is gravity?

by farmer
Tags: gravity
P: 2,237
 Quote by pervect OK, I see where you're coming from now: if you assume that G and hbar remain constant, the planck length is just $$\sqrt{\frac{G \bar{h}}{c^3}}$$ so that's where your factor of sqrt(8) came from. As far as what I had in mind, if 1 new meter = 2 old meters, then c = 3e8 old meter / second = 1.5e8 new meter / second so doubling the meter halves the "speed of light" from 3e8 "old meters" per second to 1.5e8 "new meters"/ second.
that doesn't quite work for me. i think that, if all of the dimensionless parameters remain constant,

c = 299792458 old_meters/old_second = 299792458 new_meters/new_second

and the new_second cannot be the same as the old_second if the meter had changed.

but i think we (as well as Duff) agree: ain't no operational difference. a change in c (or in G or h or any other sole dimensionful "constant") is not merely impossible, but is functionally meaningless.

i still don't know what to think of this inflationary universe theory where the universe expands faster than c at some time in its past.
 P: 1,441 Several of the comments re the speed of fields are not established by experiment - the speed of light in a vacuum is c, we all know that, but the speed with which a closed non-divergent magnetic field propagates in a loop of magnetic material is not readily explainable in terms of the field starting out at each pole of the energized magnet and meeting itself somewhere in middle - waves go from place to place - we do not know the mechanism by which fields make their forces felt at a distance - It seems when physics needs to explain quantum entanglements and virtual photons the speed barrier is shunted to the side. In the case of gravity, it is usually assumed there is a graviton exchange between attracted particles - but gravity and inertia may be the result of global dynamics - the cosmological constant or, like expansion, an ongoing change that does not happen at one place and travel to another, but rather something that affects spacetime continuously. The curvature of GR may be the result of local mass interaction therewith, in which case it may not be meaningful to assign a propagation velocity to the curvature.
Emeritus
P: 7,204
 Quote by yogi Several of the comments re the speed of fields are not established by experiment
The speeds are certainly established by theory, though. And the theory has survived every experimental test thrown at it, to date.

For instance, if Maxwell's equations were wrong, we'd start to see disagreement with experiment, even if that experiment wasn't directly designed to measure some sort of "speed".

Maxwell's equations certainly give us a good reason to expect that electromagnetism, in general, travels at 'c' in the general sense that if you change something "here", it won't have any effect "there" until after a delay of at least c/distance.

Some care does need to be taken as to what means by speed. Specifically, one has to use the above defintion, and not try and guess the speed from the direction of the coulomb force, a common sorce of confusion that is also often repeated in "speed of gravity" threads.

GR is no different as far as the theoretical aspects go. (However, we don't have any direct measurements of the speed or even the existence of gravity waves, while of course we do have direct observations of light).

The equations are a lot messier than Maxwell's equation, but there is proof that GR is a well posed initial value problem, which implies that the "fields" propagate at less than 'c'. (You can regard the "fields" as changes in the metric, which will also change the Christoffel symbols and the curvature tensor).

The details of the proof that GR is a well posed initial value problem are rather complicated and I'm not especially familiar with them, but you can find the proof in Wald, "General Relativity". I've written a little about this in the past, as to what it means to be a well-posed initial value problem and what this implies about propagation speed.
P: 1,441
 Quote by pervect The speeds are certainly established by theory, though. And the theory has survived every experimental test thrown at it, to date. For instance, if Maxwell's equations were wrong, we'd start to see disagreement with experiment, even if that experiment wasn't directly designed to measure some sort of "speed". Maxwell's equations certainly give us a good reason to expect that electromagnetism, in general, travels at 'c' in the general sense that if you change something "here", it won't have any effect "there" until after a delay of at least c/distance.
Would concur - there is much indirect/consequential evidence of c as the limiting communication velocity - but being the eternal skeptic, I always find myself compelled to comment when absolute assertions are made about propagation rates of fields

I sort of expected Eugene to jump into this thread somewhere as he has written a couple of papers on the subject
P: 15,294
 Quote by keinve gravity's influence is technically finite ... once you take out black holes, gravity is finite.
Do you mean in terms of distance? No it's not. It's infinite.
P: 2,237
 Quote by keinve gravity's influence is technically finite, though not if you count black holes. a singularity is infinintly dense, so it's gravitational influence is infinite. it's jsut the range that the gravity works on that is affected. once you take out black holes, gravity is finite.
 Quote by DaveC426913 Do you mean in terms of distance? No it's not. It's infinite.
i thought he meant in terms of magnitude of field (or the degree of curvature of space-time).
 Sci Advisor P: 869 I haven't read the whole topic, but wat I was wondering, is if there are people who did some calculations about the speed of gravitational waves without the linearization, so for arbitrary large gravitational fields. The calculations for linear fields I understand, but how would one be sure if this speed is the same for arbitrary fields? Why is it still possible to write down a wave equation for the metric field ?
Emeritus
P: 7,204
 Quote by haushofer I haven't read the whole topic, but wat I was wondering, is if there are people who did some calculations about the speed of gravitational waves without the linearization, so for arbitrary large gravitational fields. The calculations for linear fields I understand, but how would one be sure if this speed is the same for arbitrary fields? Why is it still possible to write down a wave equation for the metric field ?
Yes, Wald talks about this in the context of whether or not gravity is "a well posed initial value problem".
 P: 129 Gravity is one if the unexplained "forces", and we know it has a symmetry with EM and "charge". We also know that matter -waves, give off waves -photons from bound electrons (and electrons can do this if they move fast enough); and we know about this other extremely unstable property (superposition) that, unlike the others, seems to ignore space (it's null-spatial). Is there possibly some symmetry between gravity (an extremely stable, spatial "force" of matter), and superposition -an extremely unstable, non-spatial "force" of some kind??
P: 869
 Quote by pervect Yes, Wald talks about this in the context of whether or not gravity is "a well posed initial value problem".
Ok, I can remember such discussions, Carroll also pays attention to it. I will take a look at those texts. If i remember it correctly it was about cutting the space time into slices, and that one tries to describe the evolution at every hypersurface. But how can one proof that one is always able to write down wave equations for the metric, from the field equations? If this question is answered in Wald, I will find out soon :)
 Sci Advisor Emeritus P: 7,204 Yep, that's the sort of thing. Part of what comprises "well posedness" includes the "domain of dependency" on initial values.
P: 129
 some symmetry between gravity (an extremely stable, spatial "force" of matter), and superposition -an extremely unstable, non-spatial "force"
There's a symmetry in our understanding (or lack thereof), of the two...?
But it's "broken" if gravity "acts" at the speed of light which is distance-dependent, and superposition, which is independent of distance, acts instantaneously? A true symmetry would mean both were instantaneous "forces" (independent of spatiality) -as Sir Isaac believed...
 P: 95 Reading this old thread..but..what is the correct answer to the OP question ?: OP Question: How fast is gravity? == Does gravity have a "speed" ? Is it fast or slow ? It does not seem correct to me that we can say "gravity" is fast or slow. Seems to me it would be the object of motion (particle and/or wave) that is fast or slow. Thus, a fast particle/wave is one that moves much in a short period of time, slow particle/wave moves little in a long period of time. Some particles/wave (such as photons) always move the same distance in any period of time and are thus neither fast or slow, they move at c = speed of light. What am I missing in my understanding ?
 PF Patron Sci Advisor Emeritus P: 5,302 Salman2, this thread has been dead since 2007. The speed being referred to by the OP was the speed at which gravitational waves propagate, not the speed of material particles.
P: 95
 Quote by bcrowell Salman2, this thread has been dead since 2007. The speed being referred to by the OP was the speed at which gravitational waves propagate, not the speed of material particles.
OK thanks.

What was the conclusion of the discussion--what is the speed at which gravitational waves propagate--is it c, the same speed that photon wave propagates ?
PF Patron