A misconception concerning event horizons, especially black hole event horizons, is that they represent an immutable surface that destroys objects that approach them.
In practice, all event horizons appear to be some distance away from any observer, and objects sent towards an event horizon never appear to cross it from the sending observer's point of view as the horizon-crossing event's light cone never intersects the observer's world line. Attempting to make an object near the horizon remain stationary with respect to an observer requires applying a force whose magnitude increases unboundedly becoming infinite the closer it gets. In the case of a horizon perceived by a uniformly accelerating observer in empty space, the horizon seems to remain a fixed distance from the observer no matter how its surroundings move.
Varying the observer's acceleration may cause the horizon to appear to move over time, or may prevent an event horizon from existing, depending on the acceleration function chosen. The observer never touches the horizon and never passes a location where it appeared to be.
In the case of a horizon perceived by an occupant of a de Sitter universe , the horizon always appears to be a fixed distance away for a non-accelerating observer. It is never contacted, even by an accelerating observer. In the case of the horizon around a black hole, observers stationary with respect to a distant object will all agree on where the horizon is. While this seems to allow an observer lowered towards the hole on a rope or rod to contact the horizon, in practice this cannot be done. The proper distance to the horizon is finite,  so the length of rope needed would be finite as well, but if the rope were lowered slowly so that each point on the rope was approximately at rest in Schwarzschild coordinates , the proper acceleration G-force experienced by points on the rope closer and closer to the horizon would approach infinity, so the rope would be torn apart.
Nothing can escape from a black hole… but could another black hole pull something out?
If the rope is lowered quickly perhaps even in freefall , then indeed the observer at the bottom of the rope can touch and even cross the event horizon. But once this happens it is impossible to pull the bottom of rope back out of the event horizon, since if the rope is pulled taut, the forces along the rope increase without bound as they approach the event horizon and at some point the rope must break.
Furthermore, the break must occur not at the event horizon, but at a point where the second observer can observe it. Observers crossing a black hole event horizon can calculate the moment they have crossed it, but will not actually see or feel anything special happen at that moment.
In terms of visual appearance, observers who fall into the hole perceive the black region constituting the horizon as lying at some apparent distance below them, and never experience crossing this visual horizon.
Tidal forces are a function of the mass of the black hole. In realistic stellar black holes , spaghettification occurs early: tidal forces tear materials apart well before the event horizon. However, in supermassive black holes , which are found in centers of galaxies, spaghettification occurs inside the event horizon. A human astronaut would survive the fall through an event horizon only in a black hole with a mass of approximately 10, solar masses or greater. The description of event horizons given by general relativity is thought to be incomplete. When the conditions under which event horizons occur are modeled using a more comprehensive picture of the way the Universe works, that includes both relativity and quantum mechanics , event horizons are expected to have properties that are different from those predicted using general relativity alone.
At present, it is expected by the Hawking radiation mechanism that the primary impact of quantum effects is for event horizons to possess a temperature and so emit radiation. For black holes , this manifests as Hawking radiation , and the larger question of how the black hole possesses a temperature is part of the topic of black hole thermodynamics. For accelerating particles, this manifests as the Unruh effect , which causes space around the particle to appear to be filled with matter and radiation.
According to the controversial black hole firewall hypothesis, matter falling into a black hole would be burned to a crisp by a high energy "firewall" at the event horizon. An alternative is provided by the complementarity principle , according to which, in the chart of the far observer, infalling matter is thermalized at the horizon and reemitted as Hawking radiation, while in the chart of an infalling observer matter continues undisturbed through the inner region and is destroyed at the singularity.
This hypothesis does not violate the no-cloning theorem as there is a single copy of the information according to any given observer. Black hole complementarity is actually suggested by the scaling laws of strings approaching the event horizon, suggesting that in the Schwarzschild chart they stretch to cover the horizon and thermalize into a Planck length -thick membrane. A complete description of event horizons is expected to, at minimum, require a theory of quantum gravity.
One such candidate theory is M-theory. Another such candidate theory is loop quantum gravity. From Wikipedia, the free encyclopedia.
Crossing the event horizon of a black hole | Physics Forums
A region in spacetime from which nothing can escape. For other uses, see Event horizon disambiguation and Horizon general relativity. Introduction History. Fundamental concepts.
Principle of relativity Theory of relativity Frame of reference Inertial frame of reference Rest frame Center-of-momentum frame Equivalence principle Mass—energy equivalence Special relativity Doubly special relativity de Sitter invariant special relativity World line Riemannian geometry. Equations Formalisms. Main article: Black hole. Main article: Cosmological horizon. See also: Hyperbolic motion relativity.
What Would You See As You Fell Into A Black Hole?
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Pearson Education. Nature Astronomy. If you were to watch something fall into a black hole, you would see the object fall slower and slower, and you'd never see them reach the event horizon. This is because the light that the object is emitting is having a tougher and tougher time escaping the black hole's gravity. And since light cannot escape from inside an event horizon, you'd never be able to see a signal that the object fell past it. But if you were to fall into a black hole, and were looking out, you'd see the opposite. Outside light from the rest of the universe would be falling in towards the black hole faster and faster, from your perspective, and you would see all of the universe play out in front of you.
This is a peculiarity of the warping of space and time from the black hole. The moment you cross the event horizon wouldn't appear to be anything special to you, but you'd never be able to escape back into the rest of the universe to describe what you saw. This is what the mathematics tells us happens, but even if someone were curious enough to want to see this for themselves, they wouldn't survive very long As for what's actually beyond the event horizon, at the center of the black hole would be a singularity.
This is the point were all of the matter and energy that falls into the black hole falls into.
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Since it's infinitesimally small, you wouldn't be able to see it, even if light were able to escape. The mass of a black hole is directly proportional to its radius, but volume is proportional to radius cubed. In many collision scenarios in astrophysics, there are ejecta, where matter from inside an object escapes during a cataclysmic event. But in the case of merging black holes, everything from inside remains inside; most of what was outside gets sucked in; and only a little bit of what was outside could conceivably escape.
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06.03 – The Singularity
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