/sci/ - Science & Math

>>5831282
Properties of Gravity

Gravity has some curious properties. One of them is that its effect on a body is apparently completely independent of the mass of the affected body. As a result, heavy and light bodies fall in a gravitational field with equal acceleration. Another is the seemingly infinite range of gravitational force. Truly infinite range is not possible for forces conveyed by carriers of finite size and speed -- a point we will elaborate in Part II.

Another curious property of gravity is its apparently instantaneous action. By way of contrast, light from the Sun requires about 500 seconds to travel to the Earth. So when it arrives, we see the Sun in the sky in the position it actually occupied 500 seconds ago rather than in its present position. (Figure 1.) This difference amounts to about 20 seconds of arc, a large and noticeable amount to astronomers.

From our perspective, the Earth is standing still and the Sun is moving. So it seems natural that we see the Sun where it was 500 seconds ago, when it emitted the light now arriving. From the Sun's perspective, the Earth is moving. It's orbital speed is about 10-4 c, where c is the speed of light. So light from the Sun strikes the Earth from a slightly forward angle because the Earth tends to "run into" the light. The forward angle is 10-4 radians (the ratio of Earth's speed to light speed), which is 20 arc seconds, the same displacement angle as in the first perspective. This displacement angle is called aberration, and it is due entirely to the finite speed of light. Note that aberration is a classical effect, not a relativistic one. Frame contraction and time dilation effects are four orders of magnitude smaller, since they are proportional to the square of the ratio of speeds.