What is Event horizon: Definition and 302 Discussions

In astrophysics, an event horizon is a boundary beyond which events cannot affect an observer. The term was coined by Wolfgang Rindler.In 1784, John Michell proposed that in the vicinity of compact massive objects, gravity can be strong enough that even light cannot escape. At that time, the Newtonian theory of gravitation and the so-called corpuscular theory of light were dominant. In these theories, if the escape velocity of an object exceeds the speed of light, then light originating inside or from it can escape temporarily but will return. In 1958, David Finkelstein used General Relativity to introduce a stricter definition of a local black hole event horizon as a boundary beyond which events of any kind cannot affect an outside observer. This led to information and firewall paradoxes, which encouraged the re-examination of the concept of local event horizons and the notion of black holes. Several theories were subsequently developed, some with, and some without, event horizons. Stephen Hawking, who was one of the leading developers of theories to describe black holes, suggested that an apparent horizon should be used instead of an event horizon, saying "gravitational collapse produces apparent horizons but no event horizons". He eventually concluded that "the absence of event horizons means that there are no black holes – in the sense of regimes from which light can't escape to infinity."Any object that approaches the horizon from the observer's side appears to slow down and never quite crosses the horizon. Due to gravitational redshift, its image reddens over time as the object moves away from the observer.In an expanding universe the speed of expansion reaches and even exceeds the speed of light, which prevents signals from travelling to some regions. A cosmic event horizon is a real event horizon because it affects all kinds of signals, including gravitational waves which travel at the speed of light.
More specific types of horizon include the related but distinct absolute and apparent horizons found around a black hole. Other distinct types include the Cauchy and Killing horizons; the photon spheres and ergospheres of the Kerr solution; particle and cosmological horizons relevant to cosmology; and isolated and dynamical horizons important in current black hole research.

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  1. G

    B 83 quasars right up against the Event Horizon

    A few years ago I became intrigued by articles reporting the discovery of stars very close to the purported Big Bang; 400 million years seems an awful short time for a star to evolve. Then more recently the discovery of 2nd generation - hydrogen, carbon stars - in the same proximity, supposedly...
  2. S

    A Penrose diagram of black hole with a changing event horizon

    Dear all, I have a question on Penrose diagrams. Consider a collapsing star that forms a black hole with a Schwarzschild radius normalized to 1. What happens in the Penrose diagram when additional matter falls in? I suspect the diagram then has to look like this : When the outer shell (second...
  3. D

    B Can event horizons cross for passing black holes at high speed

    Scenario: You have two black holes approaching, one from the left (A), one from the right (B), each at speed S. They are offset vertically. S is sufficiently high that they will deflect passed each other without merging. Question: Suppose the speed S is high enough so that the event...
  4. S

    B Exploring the Physics of a Black Hole's Event Horizon

    At the event horizon of a black hole, the curvature of space is infinite. Matter falling in therefore becomes accelerated to the speed of light. General relativity says infinite energy need to accelerate mass to the speed of light. Comments please
  5. Arman777

    I Event Horizon and Particle Horizon

    The furthest distance that we can see is defined by the Radius of the Particle Horizon which its nearly 46 Gly. However, the cosmic event horizon is nearly 16 Gly. Is this means the galaxies that further than the 16 Gly are just will stay the same in the sky? Since their light can never reach...
  6. M

    I If time doesn't pass at the event horizon, how is it crossed?

    So when an object is falling towards a black hole, it's clock relative to us, outside observers, is slowing down. Until said object reaches the Schwarzschild radius and it stops completely. So how can that object ever cross the event horizon, if it is frozen in time?
  7. Ibix

    I Detecting Crossing Event Horizon w/ "Closed Box" Experiment

    @Grinkle recently asked a question about detecting crossing the event horizon, which got me thinking. I think that, at least in principle, I can deduce when I cross an event horizon with a "closed box" experiment, basically by measuring tidal forces. I'm planning to see if the maths works, but...
  8. Grinkle

    B Conundrum thinking about crossing an Event Horizon

    Folks usually point out that tidal forces make questions like the one I am asking below hypothetical at best, I understand that. I am taking as an axiom that a free-falling observer observes nothing unusual when crossing an event horizon. More strongly, the free-falling observer cannot detect...
  9. Milsomonk

    I Lorentz Symmetry at Black Hole Event Horizon

    Hello people, I have been thinking about a concept that I was taught whilst learning GR, If I understand correctly it is that Lorentz symmetry becomes local when we consider GR. This makes sense to me as then the metric is generally speaking not Minkowski, only for a...
  10. N

    B Gravitational Event Horizon: Two Interpretations

    I would like to know which of the following interpretations of what happens when a local observer with a non-zero mass (i.e. not a photon) crosses the event horizon of a black hole: 1. Not only does the falling observer not *notice* anything strange (because his/her clocks run proportionally...
  11. M

    I Black hole event horizon confusion

    Leonard Susskind said "everything that ever fell in, to make the black hole, [..] [is] all contained in [...] progressively thinner and thinner shells that approach the horizon asymptotically, never quite getting there" and from the perspective of someone outside the black hole "a shell, called...
  12. anubodh

    I Separating Overlapping Event Horizons: Theory

    If 2 black holes have event horizons slightly overlapping,can they ever be separated "theoretically" into 2 separate event horizons given we can apply extremely high forces to pull them apart or will it keep stretching and overlapping even if they are pulled apart?
  13. P

    Exploring the Surface of a Black Hole's Event Horizon

    Does the event horizon of a black hole really represents the surface of the "star" (or mass) itself? What I mean to say is: That the event horizon is the (let's say it this way) sphere surface where the scape velocity is => than the speed of light. So it is not necessary for event horizon to be...
  14. F

    I Alice Observes Bob at Event Horizon: DarkLink's Question

    Hello physics forum. I am not very well versed in physics, so this question could be a misfire, but I just wanted to clear this up. I watched one of Susskinds holographic principle lectures. So I get that Bob would see Alice turn into a hot mush of energy as she approaches an event horizon...
  15. nomadreid

    I Survival Time in Black Hole: Myth Debunked

    In a thread a decade ago https://www.physicsforums.com/threads/how-to-survive-in-a-black-hole-myth-debunked.170829/, there was a discussion about the paper https://arxiv.org/abs/0705.1029v1, in which the authors discuss the way to maximize one's survival (proper) time after passing the event...
  16. S

    B People-Safe Event Horizon: Can Alice & Bob Experiment?

    Is is possible for Alice and Bob to find themselves on opposite sides of an event horizon and go about their experiments, without the fear that one of them might be mangled to death by tidal / differential gravitation effects?
  17. D

    I Minkowski metric beyond the event horizon

    My question is regarding how spacetime looks like beyond the event horizon of a black hole, in particular how distances behave. In the Minkowski diagram of a black hole, all paths leads to the singularity. But what is the magnitude of the distances involved here? Let's say a neutron star is...
  18. M

    B How is light influenced really close to the event horizon?

    I was just wondering as it seems pretty counter intuitive that there is a really defined horizon where light can't escape from a black hole. It would make more sense to me if light gradually curved into one. Or does it do this? Please enlighten me ;)!
  19. C

    A Neutrino flux at event horizon

    Hi, Assuming an average Neutrino mass of 2.0 Electron volts how much mass will a black hole with an event horizon the size of the one at the center of our galaxy accrete in a year? I've seen estimates of how many Neutrinos pass through our bodies each second and it seems like the mas would...
  20. mpolo

    B Condensed matter in a Black Hole

    I am wondering if there is some type of matter in the core of the Black Hole. Is it possible to compute the distance from the surface of the Black Hole Core to the Event Horizon? Oh that would be fun to calculate.
  21. C

    I Particle at event horizon as black hole evaporates

    If you are observing a particle enter a black hole, you watch its proper time go to zero at the event horizon as it is 'frozen' there from your frame of reference. What happens in your reference frame as the black hole evaporates? While you can't illuminate where the particle is from your frame...
  22. A

    I Black hole - event horizon and Hawking radiation

    I have come across the following multi-explanations of how Hawking radiation/evaporation of a black hole happens: Particle/anti-particle story: particle/antiparticle pair creation from vacuum near the event get torn apart - one going into black hole, the other away; in some of these...
  23. A

    I Black hole - between horizon and singularity

    What occupies the space between a black hole's event horizon and the singularity? Stuff hurtling to the singularity? Vacuum?
  24. Arman777

    I Event Horizon and the Mass of a Black Hole

    I didnt understand a concept in black holes,So I ll try to make a vısualization to the process to explain my ideas properly. Let's suppose we have a star with mass ##6M_ο##.We know that this star will turn to black hole,So Let's come to the end of the life of the star.It will explode as...
  25. C

    I Rod Near Event Horizon: What Would Happen?

    If I passed within one meter of the event horizon of a supermassive black hole (where the tidal forces are trivial) and stuck out a two meter rod such that one meter of the rod was inside the event horizon, what would I see happen to the rod?
  26. A

    I Area of the event horizon of a rotating black hole

    The Kerr metric for a black hole of mass ##M## and angular momentum ##J = aM## is $$ds^{2} = - \frac{\Delta(r)}{\rho^{2}}(dt-a\sin^{2}\theta d\phi)^{2} + \frac{\rho^{2}}{\Delta(r)}dr^{2} + \rho^{2} d\theta^{2} + \frac{1}{\rho^{2}}\sin^{2}\theta (adt - (r^{2}+a^{2}) d\phi)^{2},$$ where...
  27. T

    I Can a cosmologist cross an event horizon?

    Suppose I am orbiting a black hole (BH) at some distance d outside its event horizon (EH), and with orbital velocity v. I do not like where I am, so I try to increase d by using an amount E of energy to increase my orbital velocity to v', where v' is whatever is necessary to escape the BH and...
  28. A

    A Schwarzschild coordinates beyond the event horizon

    We can write down the metric of the Schwarzschild black hole in Schwarzschild coordinates. Which aspect of the metric in Schwarzschild coordinates indicates that the coordinates are only valid outside the event horizon?
  29. F

    A The CC and cosmological event horizon radiation

    There is Hawking radiation associated with black hole event horizons. And there is Unruh radiation associated with horizons produced by acceleration. I've also heard some suggest that there is radiation associated with the cosmological event horizon due to space itself accelerating in its...
  30. S

    Polar plot for the event horizon of a Kerr BH

    I have made graph of event horizon of Kerr black hole by giving simple command of polar plot. The problem is that the point where the event horizon and static limit meets should be along y-axis but instead its on x-axis. I have tried everything but not getting it right. What mistake I am...
  31. J

    A Compute Induced Metric on Kerr Event Horizon

    Hello there, Suppose ## \Delta = r^2 + 2GMr + a^2## and ## \rho^2 = r^2 + a^2 \cos ^2 \theta ##. The Kerr metric is $$ ds^2 = - (1 - \frac{2GMr}{\rho^2})dt^2 - \frac{4GMar\sin^2 \theta}{\rho^2} d t d \phi + \frac{\rho^2}{\Delta} dr^2 + \rho^2 d \theta^2 + \frac{\sin^2 \theta}{\rho^2} \left[...
  32. H

    A The formation of the event horizon.

    Assume a spherical region of breathable air just under the density needed to form a low density supermassive black hole. Two people float 20 feet apart from each other exchanging small talk. As gravity does its work, the spherical region holding the person closer to the center reaches the...
  33. Chris G

    A Q: Black hole event horizon vs distortion effects

    Hello! I'm having a hard time finding realistic black hole simulations, but I saw one recently (black hole size comparison on youtube) that showed the following 3 black holes (attached). I noticed that the larger the black hole, the smaller the "distortion zone" was relative to the radius of...
  34. FallenApple

    I Is Causality Relative in Black Holes?

    So in GR, for a classical black hole, if A is approaching the event horizon, to an observer far away, let say B, B would never observe A crossing the event horizon as B would observe A's time slow down in the limit to 0 and A's length contract in the limit to 0. In fact, according to B, A never...
  35. T

    I Do laws of physics apply below the event horizon?

    Do laws of physics apply below the event horizon? It appears as if black holes had such gravity as to have an escapr velocity higher than c, which means that things are pulled inwards at higher speeds than the speed of light. Or am I overlooking something?
  36. FallenApple

    I Strange Spacetime behavior inside Event Horizon

    This question is regarding classical black holes. So inside the event horizon, spacetime behaves strangely. Space is now one dimensional and only in the forward direction(that is, into the singularity) and time of events is preceived as forward and backwards since light from a source comes...
  37. D

    I Is it possible to distort a black hole's event horizon

    I know this is (probably) not going to work.. But I can't figure out why not :-) So here is the theoretical situation... Lets say we have 2 black holes that, somehow, we can perfectly control (velocity, position, rotation etc. etc.). Now, one of my probes accidentally falls into one of the...
  38. J

    B Black Hole Collision: What Happens When a Large Swallows Smaller?

    Its thought that nothing can escape a black hole (correct me if I'm wrong) but what happens when a larger black hole eats a smaller one? Could there be a instance in time where matter was torn out of the smaller black hole, past its event horizon and into the bigger one? Thanks!
  39. Will K

    B Unitarity and Entanglement: Examining the Fate of Information in Black Holes

    What happens to the information about the objects falling into the black hole? Are their states somehow contained in the Hawking Radiation, and if so, how? Or is the information scrambled as it passes the EH?
  40. BenAS

    B Confused about how long it takes to cross an event horizon

    If there were 2 observers, 1 at rest with CMB, and the other falling into a black hole, how much time passes for the observer at rest with CMB while the other falls past the event horizon? I ask because I keep reading that time dilation (due to gravity) will cause the object falling into appear...
  41. K

    B Black Holes: Questions on Event Horizon Observer

    I have a doubt about black holes. (I'm sorrying for my poor English.): 1 - Does an observer outside a black hole see forever (i.e. his lifetime) a object stationary when it reaches the event horizon? 2 - Or photons emitted by the object have its wavelength so red-shifted that the observer...
  42. S

    B Schwarzschild Radius: Compression Inside Black Holes

    Learning about Schwarzschild radius from Wikipedia: Is it accurate to say any object of mass crossing the event horizon of a black hole is compressed sufficiently to have its own Schwarzschild radius, becoming a black hole itside of a black hole?
  43. Petr Matas

    I Does time dilation prevent event horizon formation?

    I read that time dilation near a black hole's event horizon causes the infalling matter to "freeze" just above the event horizon and never cross it (in a distant observer's frame of reference). Doesn't the same phenomenon prevent the event horizon and singularity from being formed in the first...
  44. T

    I Rod falling through the event horizon of a black hole

    Having discussed recently https://www.physicsforums.com/threads/rod-falling-radially-towards-the-center-of-a-mass.871169/ I'm now puzzled by the question, what happens to the rod during his radial fall through the event horizon and what would the hypothetical observer at ##r=2M## measure...
  45. Stephanus

    B Exploring What Happens Inside Event Horizon

    Dear PF Forum, What happens inside Event Horizon? 1. Will clock stop inside EH? An object crosses the EH of a black hole around 1 billions solar mass which its Schwarzschild Radius is 3 billions km. It takes light to cross that distance (in 'normal' space) 30 thousands seconds. The time for an...
  46. A

    A Exploring the Pauli Exclusion Principle and Black Holes

    The event horizon of a black hole appears to be plastered with 'afterimages' of everything that ever fell into it. (Because gravitational time dilation makes every such object appear to stop at the event horizon.) Now, suppose an event horizon is 'full' as defined by the Pauli exclusion...
  47. GiantSheeps

    B What does a black hole look like from the inside?

    According to this video, , if a black hole is large enough you could actually travel for some time within the event horizon without dying because the event horizon is so far from the actual singularity. So, assuming that's true, what would you see while you were inside the black hole? Here's...
  48. A

    I Can the Size of the Event Horizon change faster than c?

    Basically, if the size of the event horizon(EH) of a black hole is R (radius) can the rate of change of this R or dR/dt be faster than the speed of light(c)? When it comes to expansion of an EH of a black hole: Does the event horizon immediately expand? For example, if one black hole consumes...
  49. G

    I Alice Sees Sudden Acceleration at Event Horizon

    Scenario 1: Alice drops into the event horizon at free fall and notices nothing as she crosses it. Bob looks on from the outside and sees her flattened against the event horizon. Scenario 2: Alice drops into the event horizon from the outside and notices nothing. But as she nears it, she...
  50. K

    A Can an event horizon telescope observe quantum gravity effects

    its commonly stated that no particle accelerator will ever be built that can probe the Planck scale. what about an can an event horizon telescope observe quantum gravity effects near a black hole, such as Sagittarius A* do the various candidates of QG such as string theory LQG asf gravity...
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