I recently asked a question about the influence of external gravitational fields on the stability of the geometry of a part or all the event horizon of a black hole. I understood the answer in a comment but was shocked by this phenomena that should be counterintuitive as we know that for regular bodies like the Earth and the Moon their tidal forces cause that the gravitational force in a chosen point near both of them caused by each other reduce the gravitational field in that point of both of them. So if that point was let say a point near Earth with an escape velocity of 1km/sec and we call it for example 'the-one-kilometer-per-second' horizon of Earth it should shrink due to the tidal force of the Moon and not expand as the Moon tidal force is acting against and not along wth the Earth's gravitational field. As the answer as that comment suggests for a point near the BH seems counterintuitively different or more precisely the opposite can somebody explain clearly why is that so?
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asked Jan 14, 2023 at 20:23
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1$\begingroup$ Please link to the other question. $\endgroup$– GhosterCommented Jan 14, 2023 at 22:29
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$\begingroup$ Ok. physics.stackexchange.com/questions/742898/… $\endgroup$– Krešimir BradvicaCommented Jan 15, 2023 at 11:37
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$\begingroup$ @Ghoster Link in comment above.... $\endgroup$– Krešimir BradvicaCommented Jan 15, 2023 at 14:29
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An event horizon is not defined in terms of the strength of gravity or the absolute value of gravitational potential. It is the surface from behind which, photons cannot emerge and escape to be seen by a distant observer at any future time.
In particular, the event horizon is not the surface from which the escape velocity is the speed of light using Newtonian gravity. It is coincidentally so, for a static, spherically symmetric, isolated Schwarzschild black hole.
