Talk:Event horizon

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Roberts radius

"Roberts radius" is mentioned at the beginning of the first section. But it links to no article and a quick search on Google returns no results about such radius. Perhaps that's a mistake? That there's no "roberts" radius? If there is, at least a small explanation in the text might be very interessant. Rsreston (talk) 11:54, 12 May 2009 (UTC)

Well spotted. This should obviously be "Schwarzschild radius". This got edited into "Roberts radius" on 19th April. Suspect hidden vandalism. Have restored it. JocK (talk) 01:06, 13 May 2009 (UTC)

Clarification Required

Some please rewrite this sentence to make it more clear: "While this seems to allow an infalling observer to relay information from objects outside their perceived horizon but inside the distant observer's perceived horizon, in practice the horizon recedes by an amount small enough that by the time the infalling observer receives any signal from farther into the hole, they've already crossed what the distant observer perceived to be the horizon, and this reception event (and any retransmission) can't be seen by the distant observer." What is this "any signal from farther into the hole" phrase referring to? This sentence concludes the whole last paragraph but it may be too contracted. —Preceding unsigned comment added by 209.77.137.57 (talk) 17:47, 6 October 2008 (UTC)

Rewrite request: "Interacting with an event horizon" section (April 2008)

Please, someone rewrite this section. It is highly contradictory and just muddled with unclarity. I was looking this up for an assignment and I could not for the life of me figure out if being near an event horizon would tear me apart, since it was so unclear. 72.219.14.103 (talk) 00:26, 4 April 2008 (UTC)

I just came across this article, and the entire second half of the "Interacting with an event horizon" section appears to need to be removed or rewritten. I'm no physicist, but I don't even think it's correct at all. FFLaguna (talk) 02:33, 11 May 2008 (UTC)

pretty sure that entire section is wrong, it literally makes no sense at all. wtf?76.87.249.4 (talk) 00:31, 2 October 2008 (UTC)

Proposed rewritten version

I've finished a draft of a proposed rewritten version of this article. It's at Event horizon/rewrite 200606. Please take a few minutes to read it over and respond (here!) with comments on it. I'm going to ping the WikiProject Physics people about it too, and if it looks like a decent starting point to everyone, we can replace the current article with it. --Christopher Thomas 05:47, 25 June 2006 (UTC)

Trivia: Firstly, secondly, thirdly should be first, second, third.

Make it clearer that black hole event horizons are a prediction of classical physics. If black holes evaporate then there will be no event horizon because all the formerly-trapped regions evaporate along with the black hole. (This may be what Hawking was talking about:I'd keep the Hawking quote) --Michael C. Price ^talk 06:35, 25 June 2006 (UTC)

My math isn't good enough to follow exactly what he's saying. A caveat of some kind is definitely in order (meant to add one to the introduction saying that event horizons as described are strictly a feature of the equations of general relativity). I'll deal with this tomorrow, after more comments come in and I've had a chance to sleep. Thanks for the feedback! --Christopher Thomas 07:14, 25 June 2006 (UTC)

I've (finally) tweaked phrasing of the introduction and the black hole sections to attempt to make this clearer, as well as dealing with the grammar fix you noted. --Christopher Thomas 06:42, 28 June 2006 (UTC)

Finished adding a section to replace "sticking your hand through an event horizon" from the original article. --Christopher Thomas 07:14, 25 June 2006 (UTC)

To make it more readable to the non-physicist, should we add a piece about how the event horizon of a general massive object is the radius at which the escape velocity exceeds c? --0SpinBoson 21:13, 27 June 2006 (UTC)

This is already covered in the first sentence of the section about black holes ("...a celestial object compact enough that no matter or radiation can escape"). Talking about escape velocities ends up being kind of misleading, as then you get people trying to invent relay-type schemes where you can use velocity addition to do it (which doesn't work, for reasons that aren't obvious if you're thinking about it in terms of escape velocities). Is there a part of this that you think should be better-phrased in the proposed rewrite?--Christopher Thomas 01:58, 28 June 2006 (UTC)

To a general reader (non-physicist) the second sentence seems to be gibberish. It's all correct, but it requires knowing what light-like paths are. I thought adding a section in terms of escape velocities would help provide a general understanding. You do raise a good point though, we should add a warning to that to make sure that readers know that Galilean transformations do not work in this context due to the velocities being too close to c (provide link to Lorentz transformations maybe?) --0SpinBoson 13:29, 29 June 2006 (UTC)

A more suitable page would be velocity-addition formula. I'll think carefully about how to address the points you've raised, and poke at this a bit more over the weekend. --Christopher Thomas 07:16, 1 July 2006 (UTC)

I suggest you mention somewhere that all event horizons radiate, described as Hawking radiation when the event horizon is around a black hole or Unruh radiation when seen by an accelerating observer. Possibly called the Hubble temperature when it's from the Hubble horizon.--Michael C. Price ^talk 20:24, 28 June 2006 (UTC)

These are non-GR effects, though. I'll have to think carefully about how to add discussion of them. --Christopher Thomas 21:38, 28 June 2006 (UTC)

I've made additions and changes based on the suggestions above. How does it look now? --Christopher Thomas 18:55, 2 July 2006 (UTC)

Looks good; immediate perfection is not required! -- stick it out there. --Michael C. Price ^talk 22:28, 2 July 2006 (UTC)

Done. I'll archive old threads on the talk page momentarily, to avoid confusion over what version they're talking about.--Christopher Thomas 23:52, 2 July 2006 (UTC)

This discussion of infinite forces at the SR is simply untrue either of total or tidal effects. Only the total energy to maintain the approaching orbit with all of the needed fuel is unlimited, the forces and total impulse on the observer are not (but by relativistic contraction into a short jolt). The tidal forces can not be counted on to disrupt communication by solid state effects. More insight is needed in this version. "Stationary" observers are not really. At different distances into the gravitational well, they are free to avoid perceived violations of relativity by putting the event horizon at different distances below the SR. They are allowed to see photons from the same source of the photons that their adjacent but falling competitor sees. And, an object dropped by an observer near the SR does not suddenly freeze before falling below the SR nor does it infinitely accelerate (but momentarily due to the relativistic foreshortening as seen from the ride on a grazing orbit). This relativity of event horizons is not really a problem in classical physics, only in quantum theory. -- 199.232.230.71 10:01, 3 July 2006 (UTC)

While you're correct about tidal forces being finite (which is already described in the article), your statement that the total force required to hold an object at the horizon at rest with respect to an observer who is far from a black hole does not appear to be correct, as that would allow you to use a rigid rod to connect an observer with points beyond that observer's horizon. The arguments presented in the rewritten article are the standard ones used when teaching students about event horizons, to the best of my knowledge, and were vetted by the other editors reviewing this article.

Regarding the event horizon's apparent location changing, this too is already covered in the article. Its position change is never one that would allow events past any observer's perceived horizon to have effects visible to that observer, by definition.

I'll call in more of the WikiProject Physics crew to review the article so as to ensure that any concerns are addressed. --Christopher Thomas 17:23, 3 July 2006 (UTC)

First draft rewrite complete

The rewritten version of the article has been swapped in under the main title, and can now be edited. For easy reference, the old version of the article is here.

The main thing missing in the new version is a list of references and external links. Unfortunately, none of the old article's external links looked suitable (one Geocities page, one page that talked about black holes exclusively, and one tutorial that mostly talked about things other than event horizons). Useful references would be appreciated.--Christopher Thomas 00:07, 3 July 2006 (UTC)

Perhaps you could mention that a black hole, to an fixed external observer, looks like a "frozen star", so you can't see the event horizon at all. --Michael C. Price ^talk 17:15, 6 July 2006 (UTC)

I'll add this after thinking about where best to put it. It isn't just black holes that have this effect. Light emitted from events near any event horizon can take arbitrarily long to reach the observer with respect to whom the horizon is defined. --Christopher Thomas 19:22, 6 July 2006 (UTC)

Infinite tidal forces simply do not exist at the S R location, neither do infinite total forces, not even in summation over time. Your reasoning is backwards. Relativistic contraction due to the dynamics of a grazing orbit is the reason that a rod can not be put through the horizon by the observer there. The far observer sees this flattening of activity near the S R, rather. The near observer notices paradoxically that the event horizon seems out of reach. -- 199.232.230.38 10:55, 6 July 2006 (UTC)

A rod held at rest relative to a distant observer is not in a "grazing orbit". Completely different type of path. Inifinite _tidal_ forces were never claimed, and I pointed this out to you in my first reply. There are plenty of GR-types in WikiProject Physics, and I'm sure some of them will take a look at the article to assess your concerns eventually (I've already made them aware of your previous message). --Christopher Thomas 15:36, 6 July 2006 (UTC)

Dear anon user, please consider creating a login for yourself, and becoming active in the edit process. What you say sounds correct, although I did not see the offensive text you refer to in the article after a quick skim. The article is a bit muddled in places. WP could use competent, knowledgable writers who have both a formal education in the field, and have good english writing skills. There are articles here that are meant to be broadly accessible, and free of jargon, such as this one, and far more technical articles too, such as those in Category:Riemannian geometry. Visit Wikipedia:WikiProject Physics and the far larger and more active Wikipedia:WikiProject Mathematics to get a taste of what goes on here. linas 14:55, 7 July 2006 (UTC)

Are you sure that the anon's statements sound correct? The "total force is not infinite" one in particular seems wrong, for reasons covered in both of my responses. The part he was objecting to was the "interacting with an event horizon" section. I'd appreciate it if you could take a detailed look at that. --Christopher Thomas 16:20, 7 July 2006 (UTC)

I believe anon is correct, per posts at Wikipedia talk:WikiProject Physics. I believe the confusion was between "infinte" force, and "unbounded force". The amount of force required to hover above a black hole becomes unbounded and arbitrarily large, the closer one gets. But it never becomes "infinite": either your engines become overpowered, or your rope breaks, and you fall in. As you fall, you don't feel any infinite forces before impacting the essential singularity. I modified the article as best I can to state this. The discussion of "where the rope breaks" got hairy, though. linas 18:37, 8 July 2006 (UTC)

This seems to be a question of terminology, then, as "unbounded and arbitrarily large" is exactly the definition of "tends towards (positive) infinity" that I've been hearing. I'm satisfied with the content of your rewrite to the relevant section. --Christopher Thomas 22:16, 10 July 2006 (UTC)

It seems to me the whole rope/rod discussion should be removed. There are a couple points that seem clearly wrong: Regardless of the intended audience I feel it is inappropriate to use the phrases, "lowered very slowly" and "lowered quickly" especially since, I suspect, the variable actually meant is the ratio of rod/rope length to SR and speed isn't actually relevant. The rope/rod can cross the observers event horizon (if it's long enough) just not relative to the end. (but not really, "crossing" really means redshifted to nothing.) "...eventual impact with the hole's gravitational singularity" will never be locally observed. Now is relative 16:40, 26 September 2007 (UTC) (edit) Sorry, I no longer believe I understand -Now is relative 15:54, 28 September 2007 (UTC)

Fishing line redux, and quantum mechanics

There's one more entertaining way to try to intuit the physics behind the above discussion. Suppose one drops a fishing line with hook down towards the black hole, with the hook crossing the Schwarzschild radius. Now one gives the line tug, attempting to slow the fall of the whole thing. As the line slows, the event horizon for much of the line appears to recede off to the distance. As the hook is on the other side of the event horizon, the receding horizon ends up stretching the line. As the line is stretched longer and longer, its perhaps not surprising that it eventually breaks. By pulling the line hard enough to stop the fall, the event horizon recedes to infinity, and the fishing line would need to get stretched to infinity, clearly an impossibility.

This picture is doubly nice because it provides intuition about the microscopic mechanism of breaking: the fishing line breaks because each of the atoms in the fishing line get pulled apart from one-another, as the distances between them increase as the line is slowed down. The unbounded stress that the fishing line feels is now not at all mysterious: the atoms are simply moving away from each other. It would perhaps be entertaining to try to understand quantum mechanics based on this insight, as it is simple, direct, and mechanical. The simplest model must surely be solvable: ask what the 1-D harmonic oscillator does if the one and only spatial dimension is slowly stretched. I'm guessing that a system in the ground state evolves continuously into a superposition of excited states, and thus must decay by a cascade of photons. That is, a slow stretching can be viewed as a perturbation (solvable via perturbation theory) that mixes the ground state with the excited states. Thus, a ground-state oscillator, when stretched, is no longer in the ground state. Perhaps this is also a simple and direct way of understanding Unruh radiation? Perhaps this an entirely equivalent formulation to Unruh radiation? Anyone care to work the problem and post the results somewhere? Interesting ... linas 04:58, 10 July 2006 (UTC)

I can forsee one possible objection to the above description which is with phrase "with the hook crossing the Schwarzschild radius" in that the free-falling hook never crosses the horizon w.r.t. the external observer -- unless perhaps you include the increase in Schwarzschild radius associated with the inclusion of the extra mass. --Michael C. Price ^talk 06:36, 10 July 2006 (UTC)

Well, yes, but no. In the frame of reference of the falling hook, it most definitely crosses the schw. rad. The external observer may not be able to use a telescope to observe the hook crossing the schw. rad. but if the external observer has an IQ, she can certainly compute the position of every atom in the line in any reference frame desired. linas 04:11, 31 July 2006 (UTC)

The stretching (as a source of the Unruh radiation) doesn't require the hook to cross the event horizon, hence there is no need to invoke this to explain the observations of the external observer (their IQ is not relevant). --Michael C. Price ^talk 07:18, 31 July 2006 (UTC)

Types of horizons

I think more work needs to be done to make the distinctions between types of horizons clear. I've added a little bit to the article in an attempt to do this. I can think of nine types of horizon used in the literature, including the catch-all "event horizon":

• Event horizon
• Absolute horizon
• Apparent horizon
• Particle horizon
• Cosmological horizon
• Cauchy horizon
• Killing horizon
• Dynamical horizon
• Isolated horizon

(There may be more.) The section on "Interacting with an event horizon" depends heavily on what type of horizon is being discussed. Unfortunately, this changes from paragraph to paragraph. Maybe the reader would be better served by breaking this discussion out to the pages on the relevant sub-topics. MOBle 22:34, 2 September 2006 (UTC)

I've also noticed that there are lots of "horizon" articles floating around... right now the only place where a number of them are collected together is on the Horizon (disambiguation) page, but the list there is smaller than the list MOBle has here. Perhaps we could use a Horizon (relativity) page that could collect everything in one place? Does anyone know of a similar page that already exists? Wesino 12:40, 24 November 2006 (UTC)

Well, the term "event horizon" really is used to describe every type of horizon. The "event" in this case refers to events as points in spacetime, which is all that is being discussed in each case (even in those cases where coordinates are crucial to the definition). Thus, I would argue that this should actually serve as the relativity horizon disambiguation page, in some sense. (I'll add the blue links to the Horizon disambiguation page.) --MOBle 16:11, 24 November 2006 (UTC)

If that's the case, we might want to change the first sentence in the article as well, which currently reads as follows --

In general relativity, event horizon is a general term for a boundary in spacetime, defined with respect to an observer, beyond which events cannot affect the observer. Light emitted beyond the horizon can never reach the observer, and anything that passes through the horizon from the observer's side is never seen again.

The particle horizon (also discussed in this article) fails to satisfy this definition, for example. Though if the second sentence were gone, it might. Wesino 09:04, 25 November 2006 (UTC)

Comments about the rewrite

What does this part of the rewrite mean?

"First, there are many examples, and these examples are near enough to study. Second, black holes tend to pull in matter from their environment, which provides many examples where matter passing through an event horizon is expected to be observable. Third, the description of black holes given by general relativity is known to be an approximation, with quantum gravity effects and possibly unification of gravity with the other forces expected to become significant near the vicinity of the event horizon. This allows observations of matter in the vicinity of a black hole's event horizon to be used to indirectly study general relativity and proposed extensions to it."

'these examples are near enough to study'? Does this mean we know there are event horizons in the observable vicinity of Earth. I don't think we do, and how near they are isn't that relevant anyway. Something like Cygnus X1 that appears to be a black hole, doesn't necessarily contain an event horizon and the fact that it is relatively near doesn't mean it is necessarily easier to study than say, supermassive black holes in the Virgo cluster.

'matter passing through an event horizon is expected to be observable' Technically, matter passing through an event horizon is never observable by definition of an event horizon. It might satisfy pedants like me more if this were changed to 'matter falling towards the event horizon is expected to be observable'.

'quantum gravity effects and possibly unification of gravity with the other forces expected to become significant near the vicinity of the event horizon'. I think most people would claim that quantum gravity effects will only become appreciable when the curvature is very large. Since a very large black hole can have arbitrarily small curvature at the event horizon it is not generally expected that quantum gravity effects will become significant at the event horizon.

In addition, the semiclassical picture of black hole evaporation still contains an event horizon, since there is a region of the spacetime where one is forced to encounter the singularity and hence one cannot escape to infinity. In the semiclassical picture, quantum effects lead to Hawking evaporation but the singularity remains. Whether full quantum gravity allows us to pass through the region where the classical singularity forms is an open question. —The preceding unsigned comment was added by 132.248.29.194 (talk • contribs) on 19:19, 27 October 2006}.

Event horizon of observable universe

I changed "distance" --> "comoving distance" above the formula to be a little more precise. There are a number of various "distances" cosmologically, and this is the one defined by the integral. Wesino 11:08, 24 November 2006 (UTC)

Slightly unclear sentance

"A black hole is surrounded by an event horizon, for example. This means that an outside observer cannot be affected by anything inside the black hole."

Surely the gravitational field created by the singularity of the black hole affects outside observers? and is this singularity not within the event horizon?

I was just going to bring up that sentence myself. The mass of the black hole beyond the event horizon surely can affect objects outside of it. I'm going to remove the sentence for now, I'll assume it was added without considering gravitational causation. Richard001 06:37, 1 January 2007 (UTC)

Light can never escape?

I'm having trouble with this phrase

"Light emitted from inside the horizon can never reach the observer, and anything that passes through the horizon from the observer's side is never seen again."

If a photon has enough energy, why can't it climb out of the potential well? As far as I know, there is no limit to the redshift a photon undergoes. Wouldn't a photon trying to exit a black hole just be severely red-shifted on its way out? —Preceding unsigned comment added by MaizeAndBlue86 (talk • contribs) 16:38, 25 February 2008 (UTC)

Answer: The singularity causes infinite curvature of space-time. This causes photons to be trapped no matter what their energy level is. However photons can escape in some ways(i.e. hawking radiation, and gravitational waves 'knocking' some out of the event horizon (see botton of gravitational waves page). —Preceding unsigned comment added by 98.240.36.114 (talk) 20:16, 1 June 2008 (UTC)

The above is not correct. Photons are not trapped at the black hole singularity, but at the horizon, where the curvature is perfectly well behaved. One way to think about the trapping is that the red-shift factor at the horizon is infinite, so photons lose all of their energy in trying to escape. Note that event horizons need not be associated with singularities at all: The De Sitter cosmologies have event horizons but no singularities, to choose one example.PhysPhD (talk) 17:15, 8 June 2008 (UTC)

Particles avoiding the event horizon

Quantum electrodynamics states that photons can take all possible paths between two points. It also states that they can travel slightly faster :than (or slower than) the speed of light. It was theorized that a photon traveling from point A to point B near the event horizon of a black hole :could take an indirect path (described by Feynman's quantum electrodynamics) which skips into and out of the event horizon while traveling slightly :above the speed of light. This would enable that particle to 'skip' into the event horizon and come back out again due to the fact that the event :horizon is the barrier beyond which particles and photons would have to travel above the speed of light to escape. (theorized by A.J.B)

I removed this section again. Quantum electrodynamics assumes a Minkowski background and hence arguments based on QED should not be applied to describe physics in highly curved regions such as the vicinity of a black hole. This technical point aside, WP:V and WP:NOR also seem to be an issue here. SwordSmurf (talk) 16:39, 8 June 2008 (UTC)

Origin of the Term

Does anyone have a fix on when the term "event horizon" was first used? I've been going through some older (pre-1980) literature, and I'm seeing all sorts of other names. I haven't looked through Thorne & Wheeler or Ellis & Hawking. Warrickball (talk) 14:52, 22 May 2009 (UTC)

Time dilation and other effects near the black hole event horizon

There is a picture with a comment A blue ray of light approaches the black hole, which is not visible. As it approaches, there is an effect of time dilation. The frequency of the ray of light is going to become lower, which has as an effect of extending its wavelengths. This effect is called redshift - it is wrong.

Light coming along a stationary way has constant frequency all the way - regardless of any black hole in neighborhood of the way (this can be easily proved: a frequency change would mean the number of wave crests reaching some point during some time is different from their number which passed some other point; assuming these crests cannot be created or destroyed - this is valid for EM waves in vacuum - this means their number along the way changes in a time with a constant speed, so at some time the amount was zero - such a result is wrong, proving the assumption about frequency change is wrong). The time dilation is an effect on local time, especially simply described in Schwarzschild metric which has time and space coordinates separated - it has common time coordinate, but the local time is described by $t{\sqrt {1-{\frac {2GM}{c^{2}r}}}}$ (note the 'r' here is a coordinate in the metric, not a vertical distance - although $2\pi r$ is circumference of a circle having the black hole in its centre) - it runs the slower (compared to the 't' coordinate, which is a time of a far observer), the nearer the horizon it is; the horizon is a place where it stops. As a result, the light coming from a distant source to an observer near the horizon is blueshifted; to get a redshift, the light need to be emitted from a place near the horizon and reach a distant observer - this is a case of light emitted from a white dwarf.

Near the event horizon the path of the light, measured in the same units in horizontal and vertical directions, is (approximately) a fragment of a circle crossing the horizon perpendicularly - this means a light emitted near the horizon usually goes along such a circle just to the horizon almost for every direction; the circle has a radius $h/sin(\theta )$, where the $\theta$ in an angle between the direction of the light and the vertical direction, and the $h$ is a distance from the horizon; for small $\theta$ the circle becomes distorted, and for some smaller it is an open line - a light emitted in such a direction will escape; the range of angles available for escaping is like ration of the $h$ to the Schwarzschild radius - at the horizon the angle becomes closed, and there is no direction to escape.

The impossibility to escape from the event horizon can also be described differently - by the time dilation: if an object goes near the horizon, and then back to the "distant world", the local time goes slower when it is near the horizon, and by this time is measured its speed on its path, which cannot exceed or even be equal to the speed of light - coming along a short way near the horizon must take proportional amount of the local time which goes slowly; the time of distant object goeas much faster - as a result, much more time passes in the distant world than on a path near the horizon; it the horizon is touched, the distant world time becomes infinite, so possibly there will be no world to return to.

The time dilation also affects the attempt to reach the event horizon with a rigid rod: as the end of the rod approaches the horizon, it moves very fast (in term of local speeds there) in spite the rod is moved slowly; and its speed is limited by the light speed, which is insufficient to reach the horizon in a finite time of any non-infalling observer; the end of the rod cannot reach it, too.

JerzyTarasiuk (talk) 22:26, 22 October 2009 (UTC)

"the light coming from a distant source to an observer near the horizon is blueshifted; to get a redshift, the light need to be emitted from a place near the horizon and reach a distant observer". I agree. The author seems to have got this all backwards, including the diagram. Another question is: how can we view a beam of light that is travelling away from us? If a half-silvered mirror is placed in the path of the light beam, the reflected light will retrace its path, recovering its original frequency as it goes. If the light started off blue, that's what we'd see.

Editing this section would involve more than just a few minor edits: the whole needs revising and the illustration with it. --Dendropithecus (talk) 23:54, 21 January 2010 (UTC)

Interacting with an Event Horizon section

As I see it, the section regarding one observer lowering a second observer into the black hole does not seem entirely correct. Neither observer will see anything enter the event horizon, regardless of any relative velocity between the observers. Suppose we have two observers connected by an unbreakable rope. Let the distant, stationary (with respect to the black hole) observer allows the infalling observer to fall into the black hole for any large, finite length of time. The distant observer will note that the infalling observer never quite reached the event horizon, but got arbitrarily close to it (though in principle, the infalling observer would be redshifted to the point of unobservability). He will be able to retrieve the infalling observer, but the amount of work required to do so will be arbitrarily large (but for any finite 'fall time,' will be a finite amount). The infalling observer, however, will witness the fall for a much much shorter length of proper time, due to gravitational time dilation, and will never have been falling for a large enough length of time to have reached the event horizon, no matter how much time has passed for the first observer. He can reach the event horizon in a finite length of proper time, but an infinite length of time must pass for the distant observer.

If what I have described is NOT the case, and the article is correct, the end result is a paradox: The distant observer says 'I can never retrieve the infalling observer' while the infalling observer says 'I can, with a finite amount of energy, return to the distant observer'. Physics must be correct in both of these reference frames.

While I have a strong background in physics, I do not have a particularly strong background in general relativity, so if this is patently wrong, please do provide a reference. As I understand it, no object can cross an event horizon in a finite length of time as seen from infinitely far away from the horizon (though certainly can do so in a finite length of proper time). —Preceding unsigned comment added by 74.133.49.129 (talk) 22:09, 28 October 2009 (UTC)

What you say appears to be correct. As long as observers haven't crossed the even horizon, they can always accelerate away. However, talking about acceleration by strong ropes is tricky, because of the finite amount of time for an impulse to propagate down the length of the rope. (as well as questions about forces on the rope). Rocket engines are much better for this. Which text, exactly, is the misleading text in the article? linas (talk) 20:10, 13 November 2010 (UTC)

Oh, I see, the article implies that one of the observers has already fallen in, and thus cannot be fished out. That's correct; once one has fallen in, one cannot be fished out. If one has not fallen in, then, as you point out, one can be fished out. Some care must be taken to understand that the fishing is with respect to the location of the fisher (and the fisher's perception of the horizon), not the fish's perception. Err, and the rope, which really does complicate things.

Oh, and perhaps you are confused by another issue. If you are the infalling observer, you "won't notice" that you've crossed the horizon (after all, you've only fallen for a finite amount of time). However, you will notice another horizon forming "behind you", shutting you off from the rest of the universe. Accelerate as strong as you can, you won't ever be able to reach and cross that horizon. Visually, the formation of that horizon will look like a tunnel that eventually pinches off. If you can accelerate straight into the tunnel, you can escape. Once the tunnel pinches off, you can't. linas (talk) 20:32, 13 November 2010 (UTC)

"The effect of the event horizon on light" error?

I believe the formula for lambda/lambda0 is incorrect. Shouldn't it be lambda/lambda0 = sqrt( 1 - [2GM/c^2r] ), where the square brackets represent missing parens? I think this is the standard Schwarzschild time dilation factor.

From the page in question: \frac{\lambda}{\lambda}_o=\sqrt\frac{1-2GM}{c^2r}.\

71.218.150.115 (talk) 21:02, 11 November 2009 (UTC)jeff1.grove@gmail.com

You're right, I've fixed it. (Though, frankly, that whole section isn't well written.) -- Dr Greg  talk  21:29, 11 November 2009 (UTC)

Event Horizon: technical term or generic?

There is the following text in the article:

The definition of "event horizon" given by Hawking & Ellis,[1] Misner, Thorne & Wheeler,[2] and Wald[3] differs from the one presented here. Their definition rules out the cosmological and particle horizons presented below (as well as the apparent horizon). However, modern usage has brought those ideas under the umbrella of the term "event horizon".[4] To make the distinction clearer, some authors refer to their more specific notion of a horizon as an "absolute horizon".

I have major concerns with just about every sentence there, so I'll go through each concern:

The definition of "event horizon" given by Hawking & Ellis,[1] Misner, Thorne & Wheeler,[2] and Wald[3] differs from the one presented here. Their definition rules out the cosmological and particle horizons presented below (as well as the apparent horizon).

Ignoring the definition of an "Event Horizon" as given by all three of the major texts that define it seems a bit of Original Research to me. Where has this new definition come from? Or did an editor simply make it up? It is suggested that the standard definition rules out cosmological and particle horizons. That could not be further from the truth. The confusion seems to be that the editors of this article at some point decided that *all* horizons are event horizons. Which takes me to the next sentence.

However, modern usage has brought those ideas under the umbrella of the term "event horizon".[4]

I was curious about this, as the umbrella term seems to more generally be "horizon." "Event horizon" is a well defined technical term with a technical definition. However, to be thorough, I looked up the reference that is claimed to show how "event horizon" has become an umbrella term. The only quote that I could find regarding this actually states quite the opposite to what is attributed to it: "A particle horizon is not at all the same thing as an event horizon..." (page 86 of reference 4). Thus it seems that some editor at some point made up this fact about "event horizon" being an umbrella term and then falsely attributed it to a reference that says exactly the opposite.

Since the need for a new, different, un-sourced, wikipedia-only definition of an event horizon is predicated on event horizon being an umbrella term and thus needing to be consistent with the definition of a particle horizon, or any other horizon, it seems that the article is in need of a major re-write. One that takes sourced definitions for things and does not assert facts that are untrue, sourced to texts that actually say the opposite.

To make the distinction clearer, some authors refer to their more specific notion of a horizon as an "absolute horizon."

This really needs to be sourced. The term "absolute horizon" is not found at the gr-qc archive (arXiv.org) upon a search of all abstracts. A google search for the term brings up wikipedia, and a webpage from illinois.edu about the difference between apparent and absolute horizons (where the author uses absolute and event horizons interchangingly). The fact that the terms is not found at all on the archive is quite damning (pretty much all work about event horizons gets put on arxiv.org).

68.48.203.77 (talk) 21:04, 6 February 2010 (UTC)

It seems the only discussion about this occurred some time ago on this talk page: http://en.wikipedia.org/wiki/Talk:Event_horizon#Types_of_horizons 68.48.203.77 (talk) 21:10, 7 February 2010 (UTC)

In case anybody's interested, Roger Penrose has provided two neat (though small) illustrations, with accompanying text, in "The Road To Reality", Jonathan Cape, 2004, Fig 27.18, page 726 that I think provide neat, clear and concise definitions of particle and event horizons in a generalised universe, and also explain their causes and the situations in which they occur. --Dendropithecus (talk) 02:16, 29 April 2010 (UTC)

The concerns seem legit, I removed the entire paragraph. However, I am not an active expert in such matters. linas (talk) 20:46, 13 November 2010 (UTC)

Additional Reference

Ok. The game "the white chamber" from developer Studio Trophis is *clearly* influence from this film. Now, that by itself doesn't warrant it to be on here, I understand that. However, there is a *direct* reference to the film. The line "Where we're going, we won't need eyes to see" is spoken by Weir near the end of the film, and that exact quote is included in the game (in written form). In fact it has the whole "eyeless" theme that is present in "7 Days A Stranger," which seems to be an acceptable reference. Now, another user is contesting its inclusion, what are some other people's thoughts? Eridani (talk) 18:46, 2 July 2010 (UTC)

Huh? linas (talk) 20:39, 13 November 2010 (UTC)

Question about old edit to Event horizon

I found the following comment on my talk page, and I am copying it here, as this seems like a better place for the discussion. It is an interesting point.linas (talk) 20:01, 13 November 2010 (UTC)

Hi Linas, in this edit from 2006 (differences from previous version here) you added the sentence "If the observer is lowered very slowly, then, in the observer's frame of reference, the horizon appears to be very far away, and ever more rope needs to be paid out to reach the horizon" and also the sentence "if the rod is lowered extremely slowly, then it is always too short to touch the event horizon, as the coordinate frames near the tip of the rod are extremely compressed." These sentences have survived basically unchanged to the present...but what was your basis for them? You seem to be saying that in some sense the distance to the horizon is infinite, but p. 824 of the Misner/Thorne/Wheeler textbook Gravitation says:

The divergence of $g_{rr}$ at r=2M does not mean that r=2M is infinitely far from all other regions of spacetime. On the contrary, the proper distance from r=2M to a point with arbitrary r is $\int _{2M}^{r}|g_{rr}|^{1/2}\,dr=[r(r-2M)]^{1/2}+2M\,ln|(r/2M-1)^{1/2}+(r/2M)^{1/2}|$ when r > 2M ... which is finite for all 0 < r < ∞

The "proper distance" found by integrating the metric line element along a spacelike path with constant t coordinate and varying r coordinate (analogous to the "proper time" which is found by integrating the metric line element along a timelike path) can be understood like this: imagine we have a chain of observers who are each hovering at some constant r-coordinate, each with only an infinitesimal distance to their nearest neighbors and with the chain stretching from infinitesimally close to the event horizon out to some distance R. Suppose that at a single t-coordinate each observer measures the distance between themselves and their nearest neighbor closer to the horizon, using a short free-falling ruler which is momentarily at rest relative to themselves. Then if we add up all these little measurements, the total will be the "proper distance" from R to the horizon, and as mentioned above it will be finite. It seems to me this notion of "proper distance" is also the best answer to the question of how long a rope lowered very slowly from R would need to be to reach the horizon, is there some other way of answering the question I'm not thinking of, or were you thinking that the distance as measured by a series of very short rulers (each instantaneously at rest in Schwarzschild coordinates) would actually be infinite? Hypnosifl (talk) 20:47, 8 October 2010 (UTC)

Hi, sorry for the delayed response, I don't get on WP very often any more. The sentences were trying to describe the differences between an adiabatic maneuver to get all of those observers in place, which involves an unbounded amount of acceleration, and free-fall. So, yes, you are right, if one just "uses a short free-falling ruler" to measure the distance between observers, one does indeed get the finite proper distance $\int _{2M}^{r}|g_{rr}|^{1/2}\,dr$, as you point out. Rather, these sentences are an attempt to address the question: "what is the length of an accelerated ruler?" Now, rather than measuring the distance between observers using free-falling rulers, one attempts to perform the measurement with rulers that are stationary with respect to the horizon, i.e. are being more and more strongly accelerated as they approach the horizon (the acceleration being provided by the tug of the rope, or rocket engines, as you wish.). I believe that the right answer is that such rulers get strongly contracted, so much so, that they get "infinitely short" (unboundedly short) as the acceleration becomes unboundedly large. Put another way, this is an attempt to explain why, in one frame of reference (the free-falling, proper-time frame) there is no singularity at the event horizon, while in others, there is a perceived singularity. The amount of acceleration one needs to hover at the event horizon is "infinite" -- more properly speaking, "unbounded" as one approaches the horizon. This is the infinity that the dangling rope is trying to illustrate: the tip of the rope must undergo ever stronger acceleration as it approaches the event horizon, and the accelerated rulers become ever shorter. Unfortunately, I cannot give a direct reference for this, but I believe that most texts will point out that rulers get short when they are accelerated. What remains is to integrate the lengths of these rulers, as the acceleration become infinite.

BTW, there is a similar but different phenomenon for freely-falling observers. Suppose one observer, far away from the horizon, drops a blinking light into the black hole. This external observer would never see the blinker actually cross the event horizon! It would take "forever" to even approach the horizon. Basically, the external observer would perceive the blinking to get slower and slower, and the light to be more and more red-shifted, as the falling blinker approached the horizon. The red-shift, and the blinking become infinite as the horizon is approached; the external observer cannot ever see it being crossed. (Of course, the falling blinker reaches and crosses the horizon in finite proper time). The dangling-rope thing is an attempt to illustrate the spatial analog of this time-based divergence. linas (talk) 20:01, 13 November 2010 (UTC)

Hi Linas, thanks for getting back to me. You said:

"rather than measuring the distance between observers using free-falling rulers, one attempts to perform the measurement with rulers that are stationary with respect to the horizon, i.e. are being more and more strongly accelerated as they approach the horizon (the acceleration being provided by the tug of the rope, or rocket engines, as you wish.)"

But there is no necessary contradiction between "free-falling rulers" and rulers that are "stationary with respect to the horizon". I said you could picture it as the sum of measurements by a series of rulers in free-fall, but the point is that at the moment the measurements are made, each of those rulers is instantaneously 'at rest in Schwarzschild coordinates (and so at rest relative to an observer accelerating at a constant Schwarzschild distance from the horizon--that's why I said in my earlier comment that each of the hovering observers measures the distance to a neighboring hovering observer 'using a short free-falling ruler which is momentarily at rest relative to themselves'). Are you saying that if an observer hovering (accelerating) at constant height measures a short vertical distance with a ruler, the distance he measures will be different than the distance measured be a free-falling ruler which is instantaneously at rest relative to his ruler and right next to it? If so, why?

By the way, I agree that the acceleration you need to hover at a given height goes to infinity as your distance above the horizon approaches zero (and I also agree about the blinking light). But I don't see the connection between this and rope length--the problem is not that a rope slowly lowered down would need to be an infinite length to reach it, the problem is that it would break some time before the bottom reached the horizon due to the stresses becoming larger and larger without bound. That was basically what the page originally said at the time you edited it in 2006. Hypnosifl (talk) 01:06, 14 November 2010 (UTC)

Err, um, don't confuse me :-) Its been a few decades since I actively studied GR, and even then I found it confusing. You're right, I suppose I'm quite wrong; at the moment, I am getting myself confused thinking about simultaneously measuring the positions of both ends of a free-falling ruler. I vaguely remember some divergent calculation, but I must be mis-remembering it. I retract my statements, please edit this passage out. linas (talk) 01:33, 14 November 2010 (UTC)

Stationary vs general event horizons...

Event horizons are not strange and rare animals. We pass through them continuously as our clocks tick along. The future and past light cones for a space-time event also define event horizons, as does the 3-d "hyperplane" of an inertial observers t=0 "the universe right now" or any other null or space-like hypersurface. The event horizon of a black hole is special as a stationary event horizon, and the event horizon of an accelerating observer is a real event horizon which is non-stationary to inertial observers but apparently stationary to the constantly accelerating observer's non inertial frame. I think the article should emphasize this point but I don't off hand see how to say so in a simple way. I'll think on it and post an edit if I think up a simple easy to understand exposition. (and of course suggest anyone else try.) I think this point is important for the typical reader of this article.

Regards, James Baugh (talk) 22:42, 21 February 2012 (UTC)

How about this as an explanation? Past and future light cones are horizons which exist to maintain causality. Here, causality is timelike: you can never travel from the future to the past. For a black hole event horizon (which is what is normally meant), spacetime is so deformed that causality becomes a spacelike phenomenon: you can never travel from the inside to the outside, — Preceding unsigned comment added by 86.26.13.2 (talk) 02:46, 28 November 2012 (UTC)

Just a short note

According to Sixty Symbols, kindly see [1], You would be "Terminated" the moment You pass the event horizon. But, at the moment, it might be considered as "Original research". If one could use the term "Research" for something where experimental evidence is a bit scarce... — Preceding unsigned comment added by 83.249.147.54 (talk) 20:12, 21 July 2012 (UTC)

Interacting with the horizon - some questions

At the beginning it says as follows: "any object approaching the horizon from the observer's side appears to slow down and never quite pass through the horizon"

So, I want to ask those questions:

Think that there are two observers.

"Statinory observer" (SO) is at a stationary point away from the black hole. She is not moving but looking towards the black hole. "Falling observer" (FO) is moving towards (falling into) the black hole. She is going to the black hole. SO is observing FO go through the black hole.

1. Approaching the Horizon

1.1. While FO approaches to the horizon, does the time acclerete for her?
1.2. While FO approaches to the horizon, what does she see while looking through the black hole?
1.3. While FO approaches to the horizon, what does she see if she looks back at SO?
1.4. While FO approaches to the horizon, what does shee see while looking towards the black hole? Will she see just a black circle (sphere, the central body/region of the hole)?
1.5. While FO approaches to the horizon, does she see the "black region" smalling down or does that black thing stay same size?
1.6. While FO approaches to the horizon, does she look slowing down according to the SO?
1.7. While FO approaches to the horizon, does she look "hanged at tome point" according to the SO?
1.8. Does SO observe many objects hanged (suspended, frozen, not moving) around the horizon?

2. Passing the Horizon

2.1. Can FO ever "touch" the horizon spehere/horizon line?
2.2. At the horizon, what does FO see while looking through the black hole? A black
circle/sphere? Other objects falling through a black circle, other objects stopped at some distance from the black circle?
2.2.1. At the horizon, If FO ever sees other objects which are beyond the horizon abd falling thorugh the center, does she see they are falling or suspended/not moving?
2.2.2. At the horizon, IF FO ever sees other objects, does she see they disapper or explode/crush after some point?
2.3. At the horizon, what does FO see if she looks back at SO?
2.4. Just while passing the horizon, what does FO experience (if she still survives)?
2.4.1. Does she feel an "impact", a "hit"?
2.4.2. Does she explode/atomize just after passing the horizon?

3. After passing the Horizon (FO is still moving through the center)

3.1. After passing the horizon, can objects still stay intact (no explosion :)?
3.2. After passing the horizon, does the time acclerete for FO?
3.3. After passing the horizon, what does FO see while looking through the central region of the black hole?
3.3.1. Does FO see she is "in" a black region and there is nothing else besides blackness?
3.3.2. Does FO see a black central spherical region? Does she observe a distance from the center?
3.3.2.1. IF FOs see a "black region" does she see it is smalling down as she approaches or does that black thing stay same size?
3.3.3. Does FO ever see other objects which is closer to the center than her?
3.3.3.1. If FO sees closer objects, does she see they ar moving or hanging/not moving at some point between herself and the central region?
3.3.3.2. If FO sees closer "moving" objects, does she see they disappear/explode/crush at some point(at the center of the hole or smthg)?
3.4. After passing the horizon, what does FO see if she looks back at SO?

I'm sorry for many questions. But I'm just the reader and the current article bugged me. I felt I needed to know. These are the questions that the article should address if it aims at delivering information about interacting with the horizon.

I hope someone can fill it.--85.104.54.249 (talk) 06:46, 30 November 2012 (UTC) minor edit.--85.104.54.249 (talk) 06:53, 30 November 2012 (UTC)

Inner event horizon vs Outer event horizon

What is the difference between Inner and Outer event horizon? There is no mention of it in the article... It is mentioned briefly here: Penrose diagram, Malament–Hogarth spacetime and Ring singularity — Preceding unsigned comment added by 213.151.44.210 (talk) 20:11, 27 February 2013 (UTC)

Minor Change for Clarity

Proposed Minor Change

Stated currently in the article: Light emitted from beyond the event horizon can never reach the outside observer.

Proposed change which I believe improves clarity Light emitted from within the event horizon can never reach the outside observer.

Bio watcher (talk) 19:49, 16 May 2014 (UTC)bio watcher

I'm not a scientist, and I only read the article a while ago. If the event horizon is an area for which there is an inside and and outside, then "from within" makes sense. If the event horizon is a border or line with a "this side" and "the other side", then "from beyond" makes more sense. Just judging from the use of the word "horizon", the second one would seem to make more sense. But I don't know. @Vsmith: and @0x0077BE:, what do you think? CorinneSD (talk) 20:51, 16 May 2014 (UTC)

Altho I visualize an event horizon as a spherical shell around say a black hole - and within would be appropriate for from within the sphere. But, then within sounds like within the horizon itself perhaps - rather than within the sphere or region contained by the horizon. If that makes sense, then from beyond seems more reasonable. But, must the horizon be spherical? Another but ... i've no expertise here - you would need to ask an astrophysicist - or find a WP:RS. Vsmith (talk) 00:51, 17 May 2014 (UTC)

I was asked to comment on this via a message on my talk page. Disclaimer: I'm an astronomer, but not an expert on relativity or black holes. For the event horizon around a black hole, within makes perfect sense and would be clearer, as the horizon is a closed boundary which we as observers are outside. However, for other types of event horizon (e.g. the cosmological horizon mentioned in the article) it would actually be the other way around, because we as observers are already within the horizon. A simple solution which would work in both cases would be to say 'Light emitted from the other side of the event horizon can never reach the outside observer', where outside has been deleted for the same reasons. I hope that's helpful. Modest Genius ^talk 01:57, 22 May 2014 (UTC)

Again, I'm not a scientist, but that solution sounds fine to me. CorinneSD (talk) 15:09, 22 May 2014 (UTC)

The best solution would be to cleanly separate discussion of black hole event horizons from cosmological event horizons. The distinction of particle and event horizon makes only sense in cosmological terms, I suppose. It's probably best to discuss black holes here and push the more subtle discussion of cosmological horizons to the cosmological horizon article.

In any case, this article is quite messed up (see also my comments below) and in serious need of expert attention. --dab (𒁳) 11:39, 2 June 2014 (UTC)

Margalef-Bentabol papers

123Hiperion321 (talk · contribs) is an account with no other history than the addition of these two papers [2][3] to Cosmological horizon, Particle horizon and this article back in December.[4]

The papers are fine, I am sure, but the manner of their discussion is moronic. The papers study the question of the evolution of cosmological horizons over time in an accelerated universe. They are not suitable as a "nice and simple example of event horizon" to be given at the outset of the article. The discussion belongs under cosmological horizon, and there should be place in a special section about the time evolution of such horizons (now under a section called "future horizons"). Clearly, the way to do this is first explaining what a horizon is, then discuss its evolution under an expanding universe, and only then introduce the question of accelerated expansion. --dab (𒁳) 11:00, 2 June 2014 (UTC)

looking into the papers[5][6], I find the editor almost certainly did not understand it, or in any case was unable to coherently summarize it; the parameter w of the Equation of state (cosmology) is transcribed as omega, suggesting that this is only a blind copy of random passages. The point of the papers is to calculate the speeds of particle and event horizons and the condition under which they may be equal. The result "m<2" means "there must be one state equation with w<-1/3" (i.e. (?) in an accelerating universe, here). It is stupid to introduce ad hoc notation from some random paper (the m and N are just introduced for convenience of notation, and in a summary of the results you do not need to mention that). It is embarassing to then tag this with a caveat like This section is based mainly on two relatively new articles (2012 and 2013) and is not yet accepted as part of the standard discussion of Black Holes. It simply means, somebody dumped a garbled section and we didn't bother to even read it. It is not surprising that this isn't "part of the standard discussion of Black Holes" because the papers do not even discuss black holes. The problem isn't with the papers, it's with posting random snippets instead of summarizing the point made. I would like to see these papers incorporated into the article, but I do not have the time to do it (it would cost me time as I am not a cosmologist and it took me some time to even figure out what this "equation of state" is the papers go on about), but frankly atm it would be better to have no mention of this than the garbled nonsense we do have. --dab (𒁳) 11:27, 2 June 2014 (UTC)

Bob Berman says a rocket can escape an event horizon.

For as long as I have known about black holes and event horizons I have heard that direct escape is impossible (Hawking radiation sure wouldn't count and wormholes within are irrelevant to the question.), but in the Oct. 2014 Astronomy Magazine, Bob Berman briefly mentioned that a rocket could, in principle accelerate out of an event horizon:

"A second myth is that once you cross a black hole’s event horizon, you would never be able to escape. This is true if you entered via gravity and had no propulsion method. However, if you cruised in using a powerful rocket and had enough fuel, you could fly yourself out again." [7]

This intrigued me and at least one other reader who wrote in and Berman reiterated the statement in the Jan. 2015 issue, noting that escape velocity does not apply for accelerating objects such as rockets. While the notion of an escape contradicts everything I have ever heard about the nature of event horizons, Berman's explanation does seem to make sense, and I might extend the escape velocity metaphor. For an especially massive neutron star just below the TOV limit (so close let's say tossing a brick into it would cause it to implode into a black hole), escape velocity can be .5c, and in principle a super-duper cheela rocket could lift off, so why should extending the escape velocity to c make this impossible? Or what if we imagine some kind of magical way denser than neutronium matter that would form a body with escape velocity of .99c, but is also one brick's mass away from implosion. Not quite a black hole and in principle still possible to escape. If we raise the escape velocity to c by adding the brick and forming a black hole and event horizon, then how does that last .01c of escape velocity suddenly make escape impossible? Writing this now, I know I sound in denial about simple relativistic physics, but forgive my naiveté and go back to Berman's original statement and reiteration. Does Berman have a point, or did he double down on a bad bet and is simply wrong and we can expect a correction in a future issue of Astronomy? — Preceding unsigned comment added by 70.194.81.218 (talk) 07:56, 14 December 2014 (UTC)

It is, I suppose, theoretically possible that space-time could be so configured so that light (aka zero-mass particle geodesics) cannot cross a boundary, but an accelerated "massive" (non-zero mass) particle could, using a finite amount of energy. In general, though, this would require that space not be simply-connected, or that a singularity exist. If the boundary of the forward light cone from a point doesn't pass through the event horizon, and the trajectory of a particle (time-like curve) does pass through the event horizon, then there is an odd topology at play.

That being said, I haven't read the article, and my course-work in general relatively only includes one advanced course at CalTech.— Arthur Rubin (talk) 01:35, 25 December 2014 (UTC)

I found this talk page while pursuing the same question. Berman is not right. If we were only talking about Newtonian physics then constant acceleration will get you out any gravity well you find yourself in; escape velocity in that sense is all about an initial speed necessary to get you off the planet (etc.). But an event horizon is all about relativistic physics, which bends space time so badly that even "up" is toward the singularity - there really is no "out" to speak of. There is a really nice discussion here: http://physics.stackexchange.com/questions/25369/why-cant-you-escape-a-black-hole. 75.69.240.177 (talk) 03:21, 14 January 2015 (UTC)

Since posting the above, I've more or less convinced myself that Berman is wrong, the reason being that escape velocity is indeed only part of it, and the geometry within makes exit impossible. My cute little thought experiment above where I try to baby-step up to c is flawed as an escape velocity of c is also the threshold at which all possible destinations turn inward to the singularity. So, this leaves me with the question of how this could have seen print in Astronomy Magazine ...twice. It made it through as part of a cover story and all the editors gave it a pass, and then again after a reader tried to offer a correction. Page 11 [8] — Preceding unsigned comment added by 70.194.65.238 (talk) 04:25, 22 January 2015 (UTC)

Everybody could be barking up the wrong tree

An event horizon only exists if an object is smaller than its Schwarzchild radius. Schwarzchild showed that a point mass can mathematically contain light but that doesn’t mean a point mass or singularity exists in reality. A star smaller than about twice its Schwarzchild radius would also contain light and makes more sense than a star smaller than its Schwarzchild radius. I'm suggesting there is no such thing as an event horizon (defined as an object smaller than the Schwarzchild radius). Ultimately what is in a black hole will probably be determined by comparing observations of accreting stellar size black holes with accreting neutron stars: Accretion and radiation stop at the surface of a neutron star. If there's a star larger than the Schwarzchild radius in a black hole, accretion and radiation will also stop at the surface. If there's a singularity or something smaller than the Schwarzchild radius, radiation will cease at the Schwarzchild radius. Something might be learned in only a few years by observing the large black hole in the Milky Way's center. Time will tell. 72.69.11.171 (talk) 14:11, 18 February 2015 (UTC)BG

Article error.

On the picture on the right up the equation for radius should be: R=2*G*M/(c*c). — Preceding unsigned comment added by 130.117.142.50 (talk) 19:52, 8 March 2015 (UTC)

No event horizon and singularity possible in the universe

According to the equations it takes infinite time for matter to cross the event horizon. This means that the collapse of the star stops directly before the horizon can be formed. I mean it is so simple if you just follow the equations and compare the times. Formation takes infinite time, but Hawking radiation wastes energy in finite time :D Conseqeuntly, black holes are just extreme red shifted stars on the border to possess a real event horizon and they will not radiate hawking radiation. My theory would also exclude all the silly theories about worm holes and singularities, which violate physics.

Best, Daniel — Preceding unsigned comment added by 109.193.96.59 (talk) 13:21, 29 March 2015 (UTC)

Interacting with an event horizon (2015)

The last paragraph of this section seems entirely wrong, but I'm having trouble finding any sources for or against that are reliable. Most statements like those in the first sentence are off-the-cuff remarks by physicists in interviews, and not from actual papers. My "problem" with these statements is that once anything has crossed the horizon, it's not simply that it can never escape again, but that no signal inside the horizon can travel away from the singularity.

This means statements like "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." are false, or at least very misleading. A person crossing the horizon feet first would surely "feel" that they can no longer feel their feet, for example, as no signal from their feet could ever reach their brain. Once entirely inside the horizon, no signal from any part of your body could reach any other part that is more distant. This applies as much to electrical signals in the nervous system as it does to light from your feet (you could not see them) and even to blood trying to pump back up from your feet. — Preceding unsigned comment added by 69.161.127.120 (talk) 21:25, 5 June 2015 (UTC)

It's still possible for signals to travel "outward" relative to your body, i.e. from foot to head. Your body is travelling very rapidly "inward", faster than the signal, so the overall direction of the signal is still "inward" relative to the horizon. -- Dr Greg  talk  00:49, 8 June 2015 (UTC)

This raises a further question. What if a very long wire (in freefall) was inserted through the event horizon. Would, say, a TV signal or telephone call be able to travel back up the wire from inside the black hole and be received at the other end of the wire still outside the event horizon? Cassandra — Preceding unsigned comment added by 2.99.253.65 (talk) 11:45, 17 March 2016 (UTC)

Hawking radiation

if black holes exist only for a finite time from a perspective of an outside observer and eventually completely evaporate away emitting Hawking radiation, passing through the event horizon is impossible as it would take infinite amount of outside observer's time to do so, meanwhile the lifetime of a black hole would be finite. — Preceding unsigned comment added by 188.167.67.127 (talk) 13:47, 27 August 2015 (UTC)

Possible Contradiction

I think there is contradiction

Article about black hole: "it is impossible to determine the location of the event horizon from local observation"

Article about event horizon: "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" — Preceding unsigned comment added by 78.56.153.73 (talk) 17:23, 2 August 2016 (UTC)

No contradiction. "Local observation" means "what you actually see or feel happening at that moment". The calculation isn't a "local observation". -- Dr Greg  talk  18:42, 2 August 2016 (UTC)

classic mistake: mixing the lightspeed front of particle collapse potentiality with the wavefunctional formalism (wavefunction)

The collapse (wavefunctional particlization) is one accepable solution of the wavefunction. If you mix these two notions you create crap physics like most of the main article.

in the two slit experiment we know that if we open the detector after the time the "lightspeed front of potential collapse" passes the two slits, but before the final screen, then we destroy the interference pattern. This occurs because the wavefunction is the formalism of accepted solutions, not the solution which travel at luminal speeds. The wavefunction might change instantaneously!

The visible horizon isn't a wavefunctional absolute truth, thus it cannot be fundamental!

— Preceding unsigned comment added by 2A02:587:4105:1800:4C57:1430:31C8:E49C (talk) 22:38, 29 August 2016 (UTC) 

Well written article

Overall I think the way this article remains now is actually very good. Although there may be some subsections that can use more information at some point this article is very well written. I am no astrophysicist myself but I will likely be actively tweaking this article as I see fit. With some added information this could be an ever better article. AjBongiorno (talk) 16:40, 7 November 2016 (UTC)

No passing ever

Due to the time dilation, no object can really move across the event horizon in any finite time, so there is no way ever to pass the event horizon.
Besides, there is no way to know what is inside the event horizon, so any statement made about inside the event horizon can’t be falsified, and is therefore not part of science. A claim stating that inside the event horizon our laws of physics also apply doesn’t differ much from one stating inside the event horizon lives a God.
So please some in the future remove the sections that describes the picture inside or of crossing, or at least leave a note there to notify any reader that the description is merely some imagination based on certain assumptions (that will never be verified). — Preceding unsigned comment added by ExistAX (talk • contribs) 08:38, 25 June 2018 (UTC)

Alternative horizons

The earliest original notions of event horizon (EH) are based on escape velocity of light, meaning that a photon originating from EH could escape, while a photon originating inside EH could cross EH but would return back.

Later more strict definition of EH was used, as a boundary from where no information could escape outside at all, or as a "past causal boundary of future null infinity". The strict definition caused information and firewall paradoxes, therefore Hawking suggested in his writing ”Information Preservation and Weather Forecasting for Black Holes” that apparent horizons should be used instead to avoid those paradoxes.

Event horizons can, in principle, arise and evolve in exactly fat regions of spacetime, not having any black hole inside, if a hollow spherically symmetric thin shell of matter collapsing in a vacuum spacetime. The exterior of the shell is a portion of Schwarzschild space and the interior of the hollow shell is exactly fat Minkowski space. Bob Geroch has pointed out that if all the stars in the Milky Way gradually aggregate towards the galactic center while keeping their proportionate distances from each other, they will all fall within their joint Schwarzschild radius long before they are forced to collide.

As a compaction, contradictions with respect to the strict definition of event horizon:

• assimilates event horizon to apparent horizon as a photon stopping surface - still those can be very separate
• a surface that emits no photons is totally black, and it could actually be seen by slowly infalling observer. The original, escape velocity boundary of light, would not, because it is not any physical surface, and photons from inside could cross it momentary
• information and firewall paradoxes
• implies the proposition that spacetime itself could not transmit any information - still gravitation anomalies and waves are measured with greatest interest

Still, the original EH would be the boundary where infaller would seem to froze and get infinitely red, as seen by distant observer. EH would be purely mathematical, not physical as apparent horizon is.

So, I think that the article is conservatively based only on EH, uncertainly regarding of horizons should be presented clearly. -Yoxxa (talk) 07:51, 25 April 2019 (UTC)

Restructuring needeed?

Because the intro is quite Black Hole weighted, I'd suppose the all BH related text to be moved to 'Event horizon of a black hole' section, different kind of event horizons introduced and the basic properties of all event horizons shortly presented. So what would you think if new imtro would be shorly:

"In astrophysics, an event horizon is thougth to be a boundary beyond which events cannot affect an observer on the opposite side of it. The notion was originally based to escape speed of light from massive object's gravity predicted already in 1700s, but event horizon is also related to the field of view of moving objects and expanding universe.

Any object approaching the horizon from the observer's side appears to slow down and its image becoming more and more redshifted as time elapses. This means that the wavelength of the light emitted from the object is getting longer as the object moves away from the observer.[2 ]. Event horizon caused by gravity does not stop gravity waves, so events causing gravity waves can affect observer on the opposite side of such event horizon, but a event horizon caused by speed affects gravity waves too."

I am not native English speaker, so proofreading and fine tuning is welcomed. Or, in a fact, there would be so much to rewrite, because section "Interacting with an event horizon" seems to contain mostly facts about black hole's event horizon, so maybe too much to do... Yoxxa (talk) 08:26, 16 August 2019 (UTC)

Move discussion in progress

There is a move discussion in progress on Wikipedia talk:Disambiguation which affects this page. Please participate on that page and not in this talk page section. Thank you. —RMCD bot 20:01, 9 October 2019 (UTC)

light cones distortions under approaching event horizon of black hole

It's correct, that future cone of free fall object (FFO) is narrows and tilts to event horizon, but what happen with past cone? In really this can define causal connections between FFO and universe and trajectory of FFO too, that is: is possible to reach and cross event horizon or not? If past light cone of FFO tilts away from event horizon in an extreme way (we would intuitively expect this), then it also allows (from distant observers frame only?) time like cause events for FFO, even from complete universe - asymptotically... In other words: FFO may witness total history of complete universe under approach to even horizon of an black hole, while black hole evaporates (Hawking radiation) and event horizon disappears with it... — Preceding unsigned comment added by 80.98.183.105 (talk) 17:29, 23 November 2021 (UTC)