Let there be a solar system without Earth. Now what happen if we place the Earth suddenly on its actual position does the gravitational force of sun acts on it immediately because of its pre established field or the force will act after 8 minutes (the force reaches with the speed of light) I just want to know that does field have an emergent nature like light.

Edit: can some body please tell me how can i collapse my question.I just want to know that does pre existing field cause a force instantly or not? But it's put on hold because it's off topic. (Sorry for bad english)


1 Answer 1


The force of the sun will act immediately on earth but the force of earth on the sun will take 8 minutes to reach sun.

Remember: Sun and earth orbit around a common center because earth is acting on the sun, too. Only that for the sun it’s rather a wobbly motion. This wobbling will not start more than 8 minutes after earth’s appearance. However, I’m not sure if and how earth’s orbit is affected a tiny bit by the missing of the wobbling until then.

Edit: However, these kind of gedanken experiments, the one from your question and my answer, should be taken with a grain of salt (a big one). Earths sudden "appearance" is unrealistic and it is unclear to me if any physically sensible conclusion can be drawn from it. But at least it is well-proven tradition in science to ask questions in that way even when they later turn out to lead to inconsistent situations.

  • $\begingroup$ Obviously your answer does not conserve momentum so it is internally inconsistent, as is the question. $\endgroup$
    – my2cts
    Aug 5, 2019 at 14:57
  • $\begingroup$ @my2cts I’m aware of the problems in my answer. I read from your comment that you are in favour of not answering at all, or at least not in this inconsistent way. In the frame of the question, with earth suddenly appearing (which is unrealistic in itself) my answer seemed correct to me. But you are right: if no valid physical insight can be drawn from it then I should have not answered in the first place. $\endgroup$ Aug 5, 2019 at 15:33
  • $\begingroup$ It is important to point out the inconsistencies. It simply cannot be that the Earth attracts the Sun and not the other way around. Your answer can be used to point out the inconsistencies. $\endgroup$
    – my2cts
    Aug 5, 2019 at 16:27
  • $\begingroup$ @my2cts First..Gravity is a fictitious force under GR, the Earth is traveling along the geodesic at constant speed and without acceleration. Second there is no generic way to define conservation of momentum or conservation of mass-energy outside of some very specific cases in GR. “Attract” is really “curve space time” if you are using GR as your model. $\endgroup$
    – gdahlm
    Aug 5, 2019 at 21:45
  • $\begingroup$ @gdahlm But ... the physics of Earth moving in the gravitational field of the Sun is very well described by Newton gravity so action is reaction. $\endgroup$
    – my2cts
    Aug 5, 2019 at 22:09

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