# Is there a delay in the effect of the gravitational or electromagnetic force if a secondary body suddenly appears? [closed]

If the sun were to suddenly vanish in our solar system, it would take about 8 minutes for the Earth to exit its orbit. This is because the gravitational field of the sun would vanish at a rate of $c$. I am basing this on a question where the sun suddenly disappeared.

However, suppose the sun was already present for, say, a million years. If I were to suddenly add a massive body, such as a planet, at a distance of 149.60 million kilometres (the current average radius of the Earth's orbit), then shouldn't the planet immediately be attracted to the sun? Would there be a delay? And, if so, why?

This question would also apply to charges and electromagnetic fields since they also travel at the speed of light.

• If by 'suddenly' you mean instantaneously, then that in itself would of course break the speed limit of the Universe. – Gert Jul 28 '16 at 19:09
• Why should there be any difference between the two scenarios? – Lewis Miller Jul 28 '16 at 19:13
• One can play this game with electromagnetic forces, but not with gravity, which is simply yet another indication that gravity is not a force. – CuriousOne Jul 28 '16 at 19:15
• Because @LewisMiller in this case the gravitational filed of the sun isn't disappearing. It already exists and the planet is placed within its boundaries (so the planet interacts with it immediately when it is placed). And I don't know if there will be a difference... that is what I'm trying to understand. – Dieblitzen Jul 28 '16 at 19:16
• @CuriousOne Why not? I am talking about the force of gravitation – Dieblitzen Jul 28 '16 at 19:18

Note: your example isn't quite right, because Einstein's field equation implies $\partial_\mu T^{\mu\nu} = 0$, i.e. conservation of energy. Attempting to consider a situation where a mass suddenly appears 'out of nowhere' is mathematically self-contradictory in GR. But, putting this issue aside...