
#1
Dec1012, 07:21 AM

P: 14

I am confused some people are saying Gravity has speed of light and some are talking about infinite speed.
Friends tell me what exact is speed of gravity. If you have any way to demonstrate it please share with me. ok and my new question is why matter has mass?? tHANKS 



#2
Dec1012, 08:08 AM

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P: 11,216

Speed of gravity: How fast do changes in the gravitational field propagate? 



#3
Dec1012, 09:27 AM

P: 68

However, author of that simulator himself said on his forum that's not completely accurate, even though he also claims the simulator stays stable/accurate for thousands if not millions of simulated years. Hmmm. On the other hand I've seen on some Wikipediatalk page some people supposedly made similar simulation but limited speed of gravity to the speed of light and that simulation then became unstable and planets flew away. Response to that was that they failed to account for many other effects given by SR/GR that would counter the error and that if you take all the effects into account they would balance out so that simulation taking into account all the SR/GR effects would produce the same result as if you don't account for any them and just assume the speed of gravity is instantaneous. Go figure! 



#4
Dec1012, 09:34 AM

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P: 10,766

What is the Speed of Gravity?
Finite propagation of gravitational influence does change the orbits a bit, but the dominant effect (emission of gravitational waves) is already negligible for planets (~200 W for earth I think). The modified effective potential is more interesting, as it causes a measurable perihelion shift of mercury  something you won't get in a (proper) simulation with Newtonian gravity.




#5
Dec1012, 10:05 AM

P: 68





#6
Dec1012, 10:14 AM

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P: 10,766

It is a modification to Newtonian gravity which can be calculated in General Relativity. See Perihelion precession of Mercury and relativistic twobody problem at Wikipedia for details, or try to find some book about GR (there are many).




#7
Dec1012, 10:59 AM

P: 68

However other articles on the internet seem to suggest the correction is about LienardWiechert retarded potentials, which I'd think belongs more to Special than General relativity, but in either case it's about finite propagation of gravity rather than curvatures of spacetime, unless curvatures of spacetime somehow imply finite propagation of gravity. 



#8
Dec1012, 11:06 AM

P: 642

Until Newton and Galileo came along, the theories of motion, despite being somewhat wrong, still used the same definition. It remained the same after Newton and Galileo. So, I guess you're asking why some stuff is harder to accelerate? No one really truly knows why (though there would be a few mathematical weirdnesses otherwise,) but the bottom line is that every experiment or observation so far agrees with the current theories of mass. All physical theories are, in a sense, guesses; just because one experiment agrees with it doesn't mean all will. (An interesting parallel would be that the sequence 2, 4, 6, 8, ... doesn't necessarily have to have 10 next, it could be the sequence of even numbers that don't have a 0 as their last digit.) The current theories are just absurdly welltested; no observations thus far contradict them. No one really knows "why" they're true, we just know (or, rather, are really, really, really close to being absolutely sure) they're true. Let's imagine there are two answers to the question you asked. I doubt there would be any experiment that could distinguish between them. 



#9
Dec1012, 12:55 PM

P: 832

It's probably better to ask what the speed of gravity waves are. Asking what the speed of gravity is is like asking what the speed of electromagnetism is.




#10
Dec1012, 01:03 PM

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P: 10,766





#11
Dec1012, 02:01 PM

P: 284





#12
Dec1112, 08:24 AM

P: 139

The tension in the rope is felt by both parties all the time they swing. But if one person tugs or let's go, it will take some amount of time for this to be felt by the person on the other end. Now, for short ropes the changes will seem instant because our perceptions are limited, but if you watched on high speed film, you could actually see the change propagating down the rope. 


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