Event horizon(English: event horizon)space-timeThe curved boundary of.No event in the horizon can affect the observer outside the horizon.stayblack holeThe event horizon is around.In the very largegravitationUnder the influence ofEscape speedGreater than the speed of light, making it impossible for any light to escape from the event horizon.according toGeneral relativityIn the eyes of external observers far away from the horizon, any object that approaches the horizon from the outside of the horizon will take an infinite time to reach the horizon, and its influence will experience endless and gradually increasingred shift；However, the object itself will not feel any abnormality and will cross the horizon in a limited time.
Ahmed Almheiri, Donald Marolf, Joseph Polchinski, James Sully's recent research believes that the event horizon will causeBlack hole fire wallThe existence of the firewall contradicts the black hole itself.
Other relevant but different horizons include those that can also be found near black holesAbsolute horizon AndVisual horizon。Other related nouns includeCauchyAndKeeling horizon、kerr metric InErgosphere、cosmologyInCosmological horizonEtc.
Event horizon of black hole
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The most famous example of the event horizon comes from the general theory of relativityblack holeDescription of: A substance or radiation with a mass high enough to nearby cannot escape from itfield of gravityThe celestial body of.Usually, this boundary is defined when theDisengagement speedPosition greater than the speed of light.However, a more accurate description is that in this horizon, allLight coneHave been deformed toward the center of the black hole.Once a particle enters the event horizon, it is just as inevitable to move towards the black hole as to move forward in time, and the two are even the same under some coordinate systems.
Size isSchwarzschild radius The surface of the object is the event horizon of a non rotating black hole（Rotating black holeIs slightly different).The Schwarzschild radius of an object is proportional to its mass.Theoretically, any object with mass can be compressed into a black hole, as long as we compress all the mass into the space of its corresponding Schwarzschild radius.for examplesunlightThe Schwarzschild radius of is about 3 kilometers, while the Schwarzschild radius of the earth is about 9 millimeters.But in fact, neither the earth nor the sun has enough mass to resist the high densityElectronicsAndNeutron degeneracy pressure。In fact, the mass required to overcome such pressure to form a black hole is calledOppenheimer limit About three times the mass of the sun.[1]
However, the concept of event horizon with black holes is often misunderstood.People often mistakenly believe that a black hole will pump matter into a vacuum. In fact, the attraction of a black hole to matter is the same as that of other matter with gravity.Another concept that is often misunderstood is that people can observe the process of matter falling into a black hole, which is impossible.Astronomers can only seeAccretion diskBecause the high-energy radiation generated by the high-speed movement of matter here can be observed by people.In addition, remote observers cannot see matter crossing the event horizon, but can only see matter approaching it at a slower and slower speed.
Cosmological horizon
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staycosmologyThere are also different horizons.The event horizon of cosmology is located atObservable universe inCo moving distanceThe farthest point, that is, the farthest point where the photon emitted by "now" will be seen by future observers.This is similar to another horizon in cosmology,Particle horizon, are different.The event horizon of particles is that after a given observer at a certain time, the particles released in the past can still reach the farthest co moving distance of the observer.Further away from the event horizon of particles, even the photons emitted at the birth of the universe can no longer reach this observer.[2]The relationship between the position of the cosmological horizon and time is based onCosmic expansionDetermined.When the expansion of the universe has a specific nature, no matter how long the observer waits, part of the universe will never be observed.
To the event horizonCo moving distanceThe function for t is:
"It is the key point of the time axis of the space-time coordinate system. If the universe expands forever, this value is infinite.
However, there are also cosmological models that do not exist event horizons. An example isDe Sitter cosmological model。 Specific calculations about the cosmological horizon are recorded in detail in theFLRWIn the paper on cosmological modelsstate equation A model approximating the universe.
Relative horizon of accelerating particles
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If a particle moves at a constant speed in the nonexpansive universe and is not affected by the gravitational field, then any event in that universe will eventually be seen by the particle, because theLight coneWith particlesWorld lineThere are intersections.On the other hand, if the particle is accelerating, in some cases the light cone of the event will not intersect with the world line of the particle.In this case, let's sayRelative horizonAppears in the (acceleration) reference coordinate system of particles, representing a boundary where an event cannot be observed by particles.
For example, aAcceleration motionThe particles of will have a relative event horizon.When a particle accelerates, it will approach the speed of light more and more, but cannot reach the speed of light relative to its own reference coordinate system.His trajectory is ahyperbola, while hyperbolicAsymptoteIt's a 45 degree straight line.If the edge of the light cone of an event is within the asymptotic line, it cannot be observed by the particle.From the perspective of particles, there is a boundary where no signal can escape, which is called the relative horizon.
In daily life, the horizon does not appear, because it requires infinitely accelerating particles, which means that we need infinite energy and infinite equipment.
Interaction with event horizon
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People often think of the event horizon, especiallyblack holeThe event horizon of is an immutable interface that will destroy substances close to it.In fact, all the concepts of event horizons are for observers far away from them. Objects close to the event horizon will never feel that they have crossed the event horizon (that is, the event that crosses the event horizonLight coneAnd observer'sWorld lineNever intersect).
For an observer moving at constant acceleration in a wide space, no matter how the surrounding matter moves, the horizon seems to be a fixed distance away.If the observer's acceleration is changed, the position of the horizon may change.In addition, observers will never touch or cross the horizon.If the observer lives inDesit universeOne ofInertial coordinateThen the position of the event horizon will remain fixed.
When the event horizon of a black hole is discussed, the remote and relatively static observer will see the event horizon at the same location.This seems to allow the observer to lower a rope so that he can touch the event horizon, becausedistanceIt is limited, so it can be achieved only with a limited length of rope.In fact, it can't be done if the rope is lowered slowly (that is, every point on the ropeSchwarzschild coordinatesIs relatively static), the closer to the horizonacceleration(G force) will approach infinity, and the rope will be torn.If the rope is lowered quickly, the bottom of the rope can indeed touch or even cross the horizon.But once this happens, the observer will not be able to pull the rope back, because pulling the rope back will certainly tension the rope. However, if the rope is tensioned, then the tension on the rope will increase infinitely with the distance close to the event horizon and pull the rope off somewhere.Moreover, the place where the rope breaks will not occur in the event horizon but in a place that can be seen by the observer.
