Third force in general relativity


by YangMills
Tags: general, relativity
YangMills
YangMills is offline
#1
Jun29-10, 10:02 AM
P: 14
1. The problem statement, all variables and given/known data

I was researching relativity, and stumbled across this:

"General relativity introduces a third force that attracts the particle slightly more strongly than Newtonian gravity, especially at small radii. This third force causes the particle's elliptical orbit to precess..."

Is this accurate? If so, how does it work?
Phys.Org News Partner Science news on Phys.org
NASA's space station Robonaut finally getting legs
Free the seed: OSSI nurtures growing plants without patent barriers
Going nuts? Turkey looks to pistachios to heat new eco-city
latentcorpse
latentcorpse is offline
#2
Jul1-10, 03:07 PM
P: 1,431
it introduces a correction term to the Newtonian calculation which I guess you could view as a "third force" and this is responsible for the precession of the perihelion of Mercury etc.
Dick
Dick is offline
#3
Jul1-10, 11:02 PM
Sci Advisor
HW Helper
Thanks
P: 25,170
Now I'm kind of curious what the first two 'forces' are. Doing gravitation in GR doesn't even usually involve talking about 'forces' except in a perturbative sense. It's all geodesics, isn't it?

YangMills
YangMills is offline
#4
Jul7-10, 10:51 PM
P: 14

Third force in general relativity


Yes, that is exactly what perplexes me. In the context of general relativity, gravity is not a force, in the same way that there is no centrifugal force: it is a consequence of the coordinate system. Of course, this quote was from Wikipedia, but I've been following it for quite some time and have not seen it amended.
Dick
Dick is offline
#5
Jul7-10, 11:03 PM
Sci Advisor
HW Helper
Thanks
P: 25,170
It's probably what latentcorpse said. You can cast the GR solution as a 'correcting' force to the force of gravity. But even in Newtonian theory, there's only one 'force', that's gravity. Centrifugal 'force' is not a force, it's an acceleration. I think you understand the issue. Probably best not to be overly concerned with what Wikipedia says.
dulrich
dulrich is offline
#6
Jul9-10, 06:48 PM
P: 135
I am not sure where the quote is coming from, but the effective potential of an object orbiting a point source in GR does have three terms. Differentiate and you get a total force with three contributions:

1. Inverse square (Newtonian gravity)
2. Inverse cube (Centrifugal force)
3. Inverse fourth power (GR "correction")

The thread has implied it, but I think this is where the talk of a "third" force is coming from. The first two produce the standard elliptical orbit and the third is a kind of perturbation causing precession. This is a poor-man's way to calculate the precession of Mercury, etc.
YangMills
YangMills is offline
#7
Jul9-10, 07:53 PM
P: 14
[QUOTE=dulrich;2794186]
3. Inverse fourth power (GR "correction")

Ah. That explains the dominance of GR at small radii.


Register to reply

Related Discussions
General Relativity, is gravity a force? Special & General Relativity 29
Gravitational force and acceleration in General Relativity. Special & General Relativity 371
General Relativity Revisited (What if gravity actully is a force?) Special & General Relativity 53
General Relativity vs. gravitons/force Special & General Relativity 1
The upward force in General Relativity Special & General Relativity 8