Wikipedia:Reference desk/Archives/Science/2021 September 17

= September 17 =

Position of black hole event horizon
Is the event horizon (i.e. absolute horizon) of a black hole in the same place for all objects, or does it vary according to their mass or some other factor(s)? PaleCloudedWhite (talk) 07:17, 17 September 2021 (UTC)
 * The event horizon depends on the mass, charge, and angular momentum of the black hole. Dja1979 (talk) 17:09, 17 September 2021 (UTC)
 * But is it in the same position relative to all other objects? i.e. if a large planet and a tiny asteroid approach the black hole, would they both cross the event horizon at the same place? PaleCloudedWhite (talk) 17:45, 17 September 2021 (UTC)
 * As the two objects involved (the black hole and the approaching object) warp spacetime conjointly, the definition of the spatial metric required for a notion of being "in the same place" is tricky. Quoting from : "As two black holes approach each other, a 'duckbill' shape protrudes from each of the two event horizons towards the other one." (This is observed in numerical simulations based on the GR equations, but there is no reason to assume this mathematical prediction is not faithful.) If the approaching object has a mass that is almost enough to make it collapse into a black hole, continuity implies that the receiving black hole also extends somewhat of an expecting duckbill for its kiss of death. For a planetary mass like that of Jupiter, there must then also be a bump in the shape, but (I suspect) so tiny that it is negligible. --Lambiam 07:04, 18 September 2021 (UTC)
 * Thanks. So if your suspicion is correct, any difference would be in terms of degrees of negligibilty. I admit that whenever I try to get my head around the concept of the warping of spacetime, my head starts warping as well....... PaleCloudedWhite (talk) 11:59, 18 September 2021 (UTC)
 * Getting your head around anything tends to warp it. It stretches the imagination and is a good aid to grokking the Einstein field equations, but take care not to warp your brain into a singularity. --Lambiam 21:08, 18 September 2021 (UTC)