ANSWERS: 1
  • The star's effect on the planet is causal. The appearance of a gravitational field around the planet is caused by the appearance of the star. If gravity traveled faster than light, then from special relativity, the order in which the events occur (the appearance of the star and the appearance of the gravitational field near the planet) would be ambiguous. This is because of the lorentz transformation: an observer moving with some velocity v might see the star appear before the gravitational field at the planet, while another observer moving with velocity w could see the gravitational field appear before the star. Therefore, gravity cannot travel faster than light. Meanwhile, if the star's gravity traveled slower than light, then people on the planet would be able to see the star and acknowledge its existence, but would not detect its gravitational field. Moreover, if the star were moving (which all stars do) then we would feel a gravitational force which does not point in the direction of the star. Therefore, gravity must travel at a speed near or equal to the speed of light.

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