caveman1917

2010-Jul-29, 03:25 AM

I thought the precession of the planets was explained in relativity by the fact that gravity propagates at finite speed (instead of instantaneous), hence the attraction would be towards where the sun 'was' instead of 'is'.

Now i came across this (http://en.wikipedia.org/wiki/Kepler_problem_in_general_relativity) wiki article (2 body problem in GR).

Laplace had shown {snip} if gravitational influence does propagate at a finite speed, then at all points in time a planet is attracted to a point where the Sun was some time before, and not towards the instanteneous position of the Sun.

Laplace's estimate for the velocity of gravity is not correct, because in a field theory which respects the principle of relativity, the attraction of a point charge which is moving at a constant velocity is towards the extrapolated instantaneous position, not to the apparent position it seems to occupy when looked at

(my bold)

Could someone explain to me why this is so? This seems to suggest there is a fundamental difference between gravitational attraction and EM radation. In the sense that the photons will point towards the apparent position, but the 'gravitons' (i know they're hypothetical - but i didn't know how else to put the question clearly) will point towards the extrapolated position. At this point in the article we're still in Newtonian + SR, not yet in GR.

Gravity {GR by now} is distinct from the fictitious forces centrifugal force and coriolis force in the sense that the curvature of spacetime is regarded as physically real, whereas the fictitious forces are not regarded as forces.

This would seem to suggest we can consider both perspectives, either consider curvature real and free-fall inertial, or consider curvature not real and free-fall as non-inertial, as equal.

I'm having trouble understanding how this distinction would relate to the bolded part in the quote above this one. It would seem one perspective would make the argument made there to be true (inertial), while the other would make the argument untrue (non-inertial). And it seems to be suggested both perspectives are equivalent, so i'm having a little trouble putting it all together.

Now i came across this (http://en.wikipedia.org/wiki/Kepler_problem_in_general_relativity) wiki article (2 body problem in GR).

Laplace had shown {snip} if gravitational influence does propagate at a finite speed, then at all points in time a planet is attracted to a point where the Sun was some time before, and not towards the instanteneous position of the Sun.

Laplace's estimate for the velocity of gravity is not correct, because in a field theory which respects the principle of relativity, the attraction of a point charge which is moving at a constant velocity is towards the extrapolated instantaneous position, not to the apparent position it seems to occupy when looked at

(my bold)

Could someone explain to me why this is so? This seems to suggest there is a fundamental difference between gravitational attraction and EM radation. In the sense that the photons will point towards the apparent position, but the 'gravitons' (i know they're hypothetical - but i didn't know how else to put the question clearly) will point towards the extrapolated position. At this point in the article we're still in Newtonian + SR, not yet in GR.

Gravity {GR by now} is distinct from the fictitious forces centrifugal force and coriolis force in the sense that the curvature of spacetime is regarded as physically real, whereas the fictitious forces are not regarded as forces.

This would seem to suggest we can consider both perspectives, either consider curvature real and free-fall inertial, or consider curvature not real and free-fall as non-inertial, as equal.

I'm having trouble understanding how this distinction would relate to the bolded part in the quote above this one. It would seem one perspective would make the argument made there to be true (inertial), while the other would make the argument untrue (non-inertial). And it seems to be suggested both perspectives are equivalent, so i'm having a little trouble putting it all together.