/sci/ - Science & Math

>>5831282
Now we naturally expect that gravity should behave similarly to light. Viewing gravity as a force that propagates from Sun to Earth, the Sun's gravity should appear to emanate from the position the Sun occupied when the gravity now arriving left the Sun. From the Sun's perspective, the Earth should "run into" the gravitational force, making it appear to come from a slightly forward angle equal to the ratio of the Earth's orbital speed to the speed of gravity propagation.

This slightly forward angle will tend to accelerate the Earth, since it is an attractive force that does not depend on the mass of the affected body. Such an effect is observed in the case of the pressure of sunlight, which of course does depend on the mass of the affected body. The slightly forward angle for the arrival of light produces a deceleration of the bodies it impacts, since light pressure is a repulsive force. Bodies small enough to notice, such as dust particles, tend to spiral into the Sun as a consequence of this deceleration, which in turn is caused by the finite speed of light. This whole process is called the Poynting-Robertson effect.

But observations indicate that none of this happens in the case of gravity! There is no detectable delay for the propagation of gravity from Sun to Earth. The direction of the Sun's gravitational force is toward its true, instantaneous position, not toward a retarded position, to the full accuracy of observations. And no perceptible change in the Earth's mean orbital speed has yet been detected, even though the effect of a finite speed of gravity is cumulative over time. Gravity has no perceptible aberration, and no Poynting-Robertson effect -- the primary indicators of its propagation speed. Indeed, Newtonian gravity explicitly assumes that gravity propagates with infinite speed.