Share this thread: 
#19
Oct1707, 01:39 AM

P: 2,251

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. 


#20
Oct1907, 01:54 AM

P: 1,480

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 nondivergent 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. 


#21
Oct1907, 02:44 AM

Emeritus
Sci Advisor
P: 7,661

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 wellposed initial value problem and what this implies about propagation speed. 


#22
Oct1907, 11:45 PM

P: 1,480

I sort of expected Eugene to jump into this thread somewhere as he has written a couple of papers on the subject 


#24
Oct2007, 10:30 PM

P: 2,251




#25
Oct2107, 07:00 AM

Sci Advisor
P: 913

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 ?



#26
Oct2107, 03:26 PM

Emeritus
Sci Advisor
P: 7,661




#27
Oct2207, 04:36 AM

P: 127

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 nullspatial).
Is there possibly some symmetry between gravity (an extremely stable, spatial "force" of matter), and superposition an extremely unstable, nonspatial "force" of some kind?? 


#28
Oct2207, 05:13 PM

Sci Advisor
P: 913




#29
Oct2207, 09:16 PM

Emeritus
Sci Advisor
P: 7,661

Yep, that's the sort of thing. Part of what comprises "well posedness" includes the "domain of dependency" on initial values.



#30
Oct2907, 08:23 PM

P: 127

But it's "broken" if gravity "acts" at the speed of light which is distancedependent, 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... 


#31
Jul210, 07:42 AM

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 ? 


#32
Jul210, 11:39 AM

Emeritus
Sci Advisor
PF Gold
P: 5,598

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.



#33
Jul210, 05:18 PM

P: 95

What was the conclusion of the discussionwhat is the speed at which gravitational waves propagateis it c, the same speed that photon wave propagates ? 


#34
Jul210, 05:59 PM

Emeritus
Sci Advisor
PF Gold
P: 5,598




#35
Jul510, 01:52 AM

P: 17

Here is a thought experiment about a way to perhaps detect gravity waves.
All around the Earth at strategic points, attach transducers to solid bedrock. These transducers would output electrical waves in response to mechanical waves in the Earths crust, much like a seismometer. However, the sensitivity would be greater and the response would be faster. All transducers would be data linked and digitally time phase adjusted back to a common data processing center for simultaneous processing. With very close time coordination, remote detonate about a 100 Megaton HBomb directly against the Moons surface. Could we expect to pick up a "ping" and possibly "ringing" in the Earths crust from gravity wave coupling all the way from the moon? I don't have a clue for how to analyze this. I'm not a physicist, so go easy on me. 


#36
Jul510, 09:08 AM

Sci Advisor
P: 1,910




Register to reply 
Related Discussions  
Lightspeed fast, then slowed, then fast again.  General Physics  3  
How Fast?  Astronomy & Astrophysics  2  
Need Help Fast! Plz!  Introductory Physics Homework  2 