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Venn Diagram A ∩ B and Black Hole

I wish I knew where I could ask question like these to:

What happens if you put two equal sized black holes close to each other such that thier event horizons overlap. If you sit equidistant from both singularities, then you wouldn't be able to move either direction, right? So, if you just sat there, unmoving, would the event horizon eventually just evaporate away around you, leaving you a jillion years in the future, but the first and only person to see and escape from the inside of a black hole?

—Preceding unsigned comment added by 66.159.227.60 (talk • contribs) (DSL Extreme, San Jose, CA)

I imagine a more likely outcome would be that the two "overlaping" black holes would immediately merge into one bigger black hole. Bryan 06:46, 17 January 2006 (UTC)

A more interesting question is how fast would two decently sized black holes have to be orbiting each other, to stay within several diameters distance of each other? My estimate: Damn fast.

Then if you really want to get technical, how would special relativity affect such near-C movement? (what with all the extra mass and frame dragging and gravity distortions) Tzarius 03:03, 18 January 2006 (UTC)

I think you mean general relativity. Special relativity ignores the effect of gravity, so it's pretty meaningless when applied to black holes. —Keenan Pepper 04:44, 18 January 2006 (UTC)

Dunno how fast they'd have to go, but the situation wouldn't last very long. They'd be putting out gravitational radiation like crazy and the orbit would decay really quickly. Bryan 05:47, 18 January 2006 (UTC)

That's not a very stable place to be. If you are just a nanometer off from being directly between the blackholes, you will be pulled slightly towards one. As you get slightly closer, you are pulled slightly more. You'll get closer and closer to one of the black holes and will fairly quickly be sucked in. To stay in the middle you'd need to be placed with infinite precision, and that's impossible. It's worse than trying to balance a pencil on its tip.130.195.2.100 00:45, 10 March 2006 (UTC)

I was kinda wondering the same thing (what happens when the horizons overlap, but the singularity of each is outside the horizon of the other?), and had just come to the talk page to ask the same question. I'd assume that each would make the horizon of the other closer on the near side and further on the far side, so that they'd end up with some kind of kidney shape instead of being spherical. You could avoid the gravitational radiation question by having them pass each other at such a ludicrous speed (>.9999999999c) that they wouldn't orbit (escape trajectory). The overlap situation wouldn't last long, but could still be interesting. But I'm no physicist so all of this is guesswork. Anyone have enough knowledge to work the numbers? 12.37.33.3 19:53, 17 March 2006 (UTC)

Nobody on earth has enough knowledge to work those numbers. Computer simulations of black holes colliding fail periodically because it becomes necessary to switch coordinate systems, and as a result such simulations have only been done (as far as I know) with approximations. I assume that black holes passing through each other would pose a similar issues. -- SCZenz 23:51, 17 March 2006 (UTC)

To answer the questions from San Jose:

1. "What happens if you put two equal sized black holes close to each other such that thier event horizons overlap?" Event horizons cannot interpenetrate; indeed they are not even physical membranes which can be pictured as existing in a spatial hyperslice, at least not without knowing the entire future history of the spacetime model. Technically speaking, we say they are inherently a global concept in Lorentzian manifolds. So ask rather, "what happens when two black holes run into each other?" Short answer: they coalesce in a violent event which produces a burst of intense gravitational radiation, followed by a characteristic "ringdown". Eventually, you have one black hole. During the coalescence phase, there is a sense in which one can picture the two horizons as becoming distorted, reaching out toward one another, touching and very rapidly merging to form a single larger and roughly spherical horizon (possibly the horizon might briefly be toroidal in topology), but because of the inherently global nature of event horizons, this is somewhat misleading. If you've seen pictures from a numerical simulation, these probably show estimates of the event horizon's location based upon evolving the code as far as it will go before breaking due to numerical instability, but it is not completely clear that these are good approximations. By the way, black holes do not "suck in" anything which comes near; they behave pretty much like ordinary objects with the same mass, save only that they are much, much more compact than anything but a neutron star, which are of course less compact that stellar mass black holes, but not by many orders of magnitude.
2. "If you sit equidistant from both singularities, then you wouldn't be able to move either direction, right?" Like 130.195.2.100, I think this is really a question about Newtonian gravitation: if we suspend two isolated massive objects of mass m and put a test particle between them, will it stay put? The answer is that this test particle is an unstable equilibrium, so it will fall onto one or the other massive object as soon as it is displaced by even a tiny amount. In other words, if you plot the gravitational potential, our test particle is sitting at a saddle point at the center of a figure-eight shaped equipotential; inside the two "lobes" are concentric rings forming two bowls, and outside are distorted circles forming higher elevations. Something similar happens in any reasonable theory of gravitation, including gtr (that is, your test particle will be sitting on an unstable equilibrium), although the simple gravitational potential picture is inadequate to describe gtr. (Pedantic note: in the Bach-Weyl vacuum, a special case of the Weyl family of all static axisymmetric vacuum solutions, one can add a positive mass line mass on the symmetry axis to cancel a clearly unphysical strut between the two objects in the original solution, but upon closer analysis the modified solution remains objectionable. The addition is possible despite nonlinearity due to special symmetry properties of the system of partial differential equations whose solutions define the Weyl family. This does not contradict the assertion that a reasonable gtr model exists, it just shows that gtr, as a more accurate theory than Newtonian gravitation, demands more physically reasonable boundary conditions, which tends to rule out certain kinds of idealizations. Choose your poison.)
3. "So, if you just sat there, unmoving, would the event horizon eventually just evaporate away around you, leaving you a jillion years in the future, but the first and only person to see and escape from the inside of a black hole?" It turns out that in gtr, two rapidly spinning holes with aligned spin axes will tend to experience a spin-spin force which is attractive or repulsive depending on the relative sense of their spin. This is a physical force but normally too weak to be measurable, but a long-standing conjecture holds that it just might be possible for two extreme holes to form a stable equilibrium (related to the Bach-Weyl model but involving rotating objects rather than static objects) in which spin-spin repulsion exactly balances gravitational attraction. You can find some eprints on this topic on the arXiv (keyword: double-Kerr solution, a special case of the Ernst family of all axisymmetric stationary vacuum solutions, but which is not neccessarily physically acceptable for all values of its parameters).

By the way, it is Venn diagram, not "Vin". HTH ---CH 03:44, 22 April 2006 (UTC)

Speed of light (was: Inside a black hole)

"A black hole's gravity as described by the Theory of Relativity causes a number of peculiar effects. An object approaching a simple causes a number of peculiar effects. An object approaching a simple Schwarzschild-type (non-rotating) black hole's center will appear to distant observers as having an increasingly slow descent as the object approaches the event horizon. This is because photons emitted as the object approaches the horizon take an increasingly long time to escape from the pull of the black hole to allow the distant observer to gain information on the object's fate."

I think this is plain nonsense. Photons travel at the speed of light which is independant of the observer.

Could someone with a firmer knowlege than me please clean that up?

84.160.214.121 19:02, 3 January 2006 (UTC)

There are a couple of ways of looking at this. One is to consider different regions of space as moving with respect to each other, and light, while travelling at C, taking a longer than expected amount of time to traverse a gap because either new space is added in the gap or "treadmilling" occurs (these two descriptions are equivalent). A more accurate, but less intuitive, way of looking at this is to try to imagine a curved surface representing the spacetime on which the photon's worldline is being drawn. Because the surface isn't flat, getting from point "a" to point "b" along the photon's path involves a longer distance than the distance between the observer and the emitter measured along a "space-like" axis at the time of emission (from the observer's frame) would indicate. Light emitted inside the horizon has its path warped such that it always folds back in the direction of the singularity (or, if using a "moving space" viewpoint, either extra space is being inserted between the photon and the observer fast enough that the distance is growing faster than the speed of light, or the space through which the photon is moving is being pulled into the hole faster than the speed of light relative to the external observer). You get similar effect at long distances in a universe with accelerating expansion (light from events a large but finite distance away can never propagate to the observer, as the distance between the emitter and the observer is increasing at a rate greater than C). I was thinking about putting together a curved surface figure when making the other two, but it's something that would be difficult to do well in xfig.--Christopher Thomas 19:36, 3 January 2006 (UTC)

Thank you for your fast answer. Alas, I am not able to clearly and unambiguosly understand what you are telling, and none of my interpretations is very convincing. Would it be possible for you to put it into some mathematical formulas so we can diminuish the inherent ambiguity of natural language? The original statement of the article seems to suggest that the photons were pulled back by decresing their velocity, which is impossible within the theory of relativity as any lightlike vector in spacetime is an eigenvector of the Lorentz transformation. Your statement indicates that it is not because of the photons being deaccelerated but because of the distance increasing between successive emissions of photons. 84.160.255.215 21:38, 3 January 2006 (UTC)

I've asked for more capable people than me to add information about this to the article (on Wikipedia:Pages needing attention/Physics), but nobody's stepped forward yet. User:Salsb and User:Pjacobi are people I can name offhand as potentially being able to answer this in detail for you (others on PNA/Physics should also qualify). --Christopher Thomas 21:50, 3 January 2006 (UTC)

Thanks a lot. Now we should wait in patience for the experts :-) 84.160.255.215 22:11, 3 January 2006 (UTC)

The simplest way to think of this is as follows. Light always travels at the speed of light, but this is a local statement: if you have a photon, and it goes through your apparatus, you will always measure its speed to be c. However, if you try to infer the speed of light at some distant place, you may confuse yourself. This is because of the time dilation: light near the event horizon still moves at c, but since clocks near the event horizon are running much slower than clocks at infinity, the light can appear to be moving slower. Mathematically, the physical velocity at r is c, but measuring it in terms of time t at infinity gives

$v=c{\sqrt {1-{\frac {2Gm}{c^{2}r}}}},$

which is much slower. I hope this helps. This little calculation isn't really all that meaningful, because it's fairly meaningless to measure velocity using standards of distance and time taken at two different places: otherwise you end up with absurd results like this, in which light moves slower than... light. –Joke 23:56, 3 January 2006 (UTC)

Hmmm, so what you are saying is that near a black hole (at the outside) photons are traveling with a velocity less than c. If this is realy true (I still think it's not) then it should be worth an own section in the article, explaining why and under which circumstances.

The reason why I still think that a photon (in vacuum), even near a black hole cannot travel but at the speed c is the following:

Think of a photon emitted by something falling into the black hole, travelling radially away and beeing absorbed (detected) by an observer at some distance. For simplicity skip the spacial coordinates perpendicular to the (bent?) line of motion of the photon. There are two events in spacetime, emission at, say, (t,x) = (-t0, -x0) and detection at (0, 0), both expressed in the coordinate system of the observer. Now what you are telling ist that x0 < c*t0.

• If not by the path of light, how do you determine the shortest way between two points (events) in spacetime?
• And if not by the shortest way, how do you determine the distance between two points?
• If you do determine the shortest way between two points by the path of light, and the distance by the shortest way, then either all light travels at c (saying c to avoid "the speed of light"), or there is another way for light in vacuum to travel between the events. If the later was true, what would cause the light to go (exclusively?) along the longer path?

or am I missing some facts that open other possibilities? 84.160.236.244 18:59, 5 January 2006 (UTC)

As User:Joke137 explained above, the interpretation you get depends on the coordinate systems you use (this is why we have Penrose coordinates, Kruskal-Szekeres coordinates, Eddington-Finkelstein coordinates, and others). The speed of light is always locally measured to be C, but what you observe it to be from a distant location depends on how you define the distances involved (you can think of space and time warping in strange ways and the photon always travelling at C, or you can pretend the space round the hole is Euclidean and think of the photon as travelling more slowly). My favourite layman's tutorial on black holes covers a bit of this (http://casa.colorado.edu/~ajsh/schw.shtml). --Christopher Thomas 19:34, 5 January 2006 (UTC)

So which of the points stated above is wrong when viewed nonloacally?

The coordinate systems you mentioned are equivalent, inside a mutually common domain.

Coordinate systems may be different when relative to a different observer. In the above premisses both coordinate points are in the same coordinate system of the same observer.

The speed (of light) is always measured locally, but the overall speed can be calculated by segmenting the actual path and adding up (loacal speed along the path) times (distance in time) using Riemann sums and taking the limit (that is, integrating the (locally constant!) speed over the time passing along the path. This gives the lenght of the path which you can divide by the time passed. The result is c.

If the lenght of this path would be longer than any other path between the two events, ... see questions above.

84.160.227.124 20:04, 5 January 2006 (UTC)

I'm not sure what you're asking, here. If you trace out the path as you describe, you get a distance that doesn't correspond to what you'd expect if the spacetime the hole was in was Euclidean (which is correct, as it actually has very strong negative curvature). Trying to define a coordinate system that separates out "space" and "time" axes gives you a result that makes it look like the amount of space between the outbound photon and the distant observer is growing, as described above (this is the "define light as travelling at C as measured by distant observers" scenario). If you want a more visceral description, trying to define a coordinate system like that is like trying to draw a grid on a donut while keeping all squares the same size. You end up having to cheat (or more accurately, to relax some of your original assumptions). If instead you choose not to do this, you have to deal with tracing out geodesics on the actual four-dimensional non-Euclidean spacetime surface, which gives the most accurate view but is very hard to draw (even for the two-dimensional surface that tracks only radius and time for a nonrotating black hole). On that surface, the path from the photon emitter to the observer is indeed the shortest path. --Christopher Thomas 05:33, 6 January 2006 (UTC)

Of corse, distances in a bent space can, in general, not be calculated by the simple eucledian metric (square root of the sum of squares of the coordinate entries of the vector that results by taking the difference of the vectors of the start and endpoint).

Instead, you can calculate the lenght of a specific path between the two points by segmenting the path into small pieces, calculate the lenght of these locally as an approximation, and add them up to get an approximation of the total lengh. Then take the limit by taking smaller and smaller pieces (see Riemann sum) to get the precise lenght of this path.

Now to determine the distance between the two points, take the minimum of the length of all paths. To be in the picture of the donut, within the embedding space there is a path between opposing points that is shorter than any path on the surface. But to measure distance as the crow flies you first have to find a crow able to travel this way. The standard crow in relativity is light. It doesn't make sense to measure distance using a path outside spacetime where nothing can travel. 84.160.227.124 10:12, 6 January 2006 (UTC)

Again, I'm having trouble seeing where you're going with this. None of my statements have referred to an embedding space. Are you disagreeing with any of my statements, and if so, where? Do my statements, and those of User:Joke137, fail to address your questions, and if so, what are you still asking about? Spacetime is typically analyzed using Riemann geometry, if you're wondering about the tools used. --Christopher Thomas 19:02, 6 January 2006 (UTC)

Your donut example (2-dim) is very well embeded in 3-dim space.

There must be a sound misunderstanding somewhere, so let's dig into it.

I understand that you are saying that, under the special circumstances near a black hole, light emitted near that hole and detected by an observer in a distance, moves with a speed less than c. Is that right?

You say this is so because the space is bent. Correct?

Now I say no matter how space is bent there is one way tho determine the distance between two points, that is taking the length of the shortest path between the points. Do you know any otherwise?

Next I say if there is a path between two points, bent or not, I don't see any reason why light should not take that path, traveling at c everywhere locally and thus, because this path measures the distance, the light travels at c on the total path.

This contradicts your statement that light is traveling with a speed less than c.

84.160.227.124 19:44, 6 January 2006 (UTC)

The donut example's conclusions do _not_ require the surface to be embedded in 3-space. I picked that shape because it was one that just about everyone is familiar with and can visualize. Consider only its surface.

At no point did I say that light travels at a speed less than C. I said that it's always _locally_ measured at C, and that performing integration in the way you describe gives you a coordinate system that makes it look like 1) the light always travels at C, and 2) the amount of space between the emitter and receiver changes over time in such a way that the time required for the light to propagate takes longer than you'd expect it to given the initial distance.

The curvature of spacetime does not alter the speed of light (speed is, rather, a number you get when you define space and time as coordinates on the surface). What happens on the curved surface is that any ray (one-ended line) that follows a geodesic that's "light-like", emitted "forward" in a direction that's "time-like" in any frame inside the hole's horizon, is warped such that it does not ever intersect the worldline of an outside observer. Rays on "light-like" paths emitted _near_ the hole's horizon can eventually intersect the observer's worldline, but are longer than you'd expect if the emitter and receiver were on a Euclidean surface instead of a negatively-curved one.

You see my "space and time are coordinates defined on the surface" comment above? That's what the donut exmple was about, and also what User:Joke137 was trying to tell you. Defining coordinates on a plane can give you a grid where all squares are equal-sized. Doing it on a curved surface doesn't allow this. What you end up doing instead is warping the coordinate system. How you warp it is arbitrary. If you pick a coordinate system distorted such that light is observed non-locally to travel at C, you get distances changing over time in the way I describe above. If you pick a coordinate system distorted such that distances are constant, you get the time coordinate messed with in such a way that photons that aren't nearby can appear to travel at a velocity other than C. The coordinate systems I cited above represent different options for this. Does this make things clearer for you?

The terms "light-like", "space-like", and "time-like" refer to directions that can be concretely defined given the spacetime manifold (it has certain constraints on its shape in each of these directions or classes of direction). --Christopher Thomas 16:49, 7 January 2006 (UTC)

Thank you for your patience, we are getting closer now. Could it be that our mutually talent for misunderstanding has its origin in that I try to look at two events in spacetime (emission and detection of a photon, both as coordinate points in spacetime described in the coordinate system of the observer) whereas you are looking at two objects moving in spacetime, the emitter and the observer?

Sort of. "Movement" is only something you can really talk about after you define a coordinate system with "space" and "time" axes. Along any given worldline, you can define a _local_ coordinate system (each observer sees themself at rest, moving forwards in time, and interacting with things in an arbitrarily small swatch of Euclidean space next to themselves). Things get tricky when you try to figure out how things not near you are moving, so the most basic explanation doesn't talk about it at all (just draws worldlines and indicates which direction on each is "forward" in time, for timelike (STL) worldlines. --Christopher Thomas 00:02, 9 January 2006 (UTC)

So in the special case of Schwarzschild metric, why would it be incorrect to use loacale coordinates to describe every point (in spacetime and outside of the Schwarzschild radius) ? The formula of the metric is defined everywhere except at r=0 and r=r_s. So, at least outside r_s, every classical coordinate system stays valid, although we can't mesure distances (in time or space) by simply taking the Euclidean (or Minkowsi) distance of the coordinate tuples. So what's your way of mesuring distance in space, and in time, for two distant points in spacetime? 84.160.221.40 19:44, 10 January 2006 (UTC)

Further, I don't realy understand what you mean with that "...a geodesic that's "light-like", emitted "forward" in a direction that's "time-like"..." you mentioned above. Is it simply a typo in that you omitted the word "or" before the "emitted forward"? Otherwise I don't understand this, because a geodesic in spacetime is fixed and does not move in neither direction, not if your spacetime coordinate system stays fixed.

It's not a typo, but it's badly-phrased. My intent was to distinguish between the forward (emitted) and reverse (received) light-cones. If you're part-way inside the hole, you can receive light from objects outside the hole (with certain caveats - the photon has to have been pretty close to you before you entered), but nothing you send will get out (the world-lines of all rays on the forward light-cone get warped to converge on the singularity). --Christopher Thomas 00:02, 9 January 2006 (UTC)

Just to check: in the donut example, the surface represents spacetime, not only space alone?

84.160.220.238 09:57, 8 January 2006 (UTC)

Correct. --Christopher Thomas 00:02, 9 January 2006 (UTC)

Oy vey, this discussion looks like a lot to read! I've only read the first comment, and already I have a headache :-/ But fear not, 84.160.214.121, except for faulty grammar, the stuff you quoted sounds fine to me, except that distant observers should be distant static observers and hole's center should be hole's event horizon. If you can find it, Robert Geroch, General relativity from A to B is the perfect (non-technical) book to help you see what this really means without any (well, hardly any) math.

The key point you need to grok is the distinction between coordinate speed and velocity of a particle passing through some event with respect to some observer passing through (almost) the same event. The latter involves the Minkowski analogue of euclidean "angle" between two curves, and is a observable and physically meaningful concept, and also a geometrical concept since it does not depend on the coordinate chart, just the geometry. The former does depend on the coordinate chart. Nevertheless, a Schwarzschild coordinate chart has the unusual property that the coordinates do, in a sense, have a physical interpretion in terms of static observers (relative to some isolated massive object whose exterior gravitational field is measured by this exact vacuum solution). The reason why radio signals from a static observer hovering near the field are strongly redshifted when they are receieved by static observers much farther away is that outgoing null geodesics diverge due to the spacetime curvature, which in general relativity is identified with the gravitational field of our massive object.

This is much easier to explain with pictures which I am far too lazy to draw. Geroch, was not so lazy, so try to find his book if you want to understand this! ---CH 04:02, 22 April 2006 (UTC)

I'd drawn a couple of crude figures for some of the (now-archived) threads preceding this one. Fastest way to find them is via my user page. I don't have a fancy enough art program to draw better ones for the time being. --Christopher Thomas 04:44, 22 April 2006 (UTC)

Yeah, these are similar to the nice figures in Geroch. I myself like to draw light cone at various events with the top cut off parallel to the hyperplane element of a suitable frame field. This is a vivid and geometrically informative way to discuss the physical experience of a particular family of observers. BTW, the article frame fields in general relativity happens to include some simple explicit examples which are relevant here. However, I dare not edit the article itself because of problems with vandals and misinformed but abusive cranks (trolls?) like User:Zordrac, aka User:Internodeuser, aka 203.122.209.179 (see troll alert below). ---CH 05:40, 22 April 2006 (UTC)

Sorry, but

The article states: "A black hole's gravity as described by the Theory of Relativity causes a number of peculiar effects. An object approaching a simple causes a number of peculiar effects. An object approaching a simple Schwarzschild-type (non-rotating) black hole's centre will appear to distant observers as having an increasingly slow descent as the object approaches the event horizon. This is because photons emitted as the object approaches the horizon take an increasingly long time to escape from the pull of the black hole to allow the distant observer to gain information on the object's fate."

Sorry, but I still think the reason given ("This is because photons emitted as the object approaches the horizon take an increasingly long time to escape from the pull of the black hole to allow the distant observer to gain information on the object's fate.") is plain nonsense, as it gives the impression that "the pull of the black hole" pulls the photon back just as a bullet fired into the sky is pulled back by earth's gravity. The Infidel 17:39, 21 January 2006 (UTC)

Actually (according to my physics teacher/guru) the time for the photon leaving from inside the event horizon slows down relative to the outside universe; thus instead it would take an eternity for the photon to get outside the event horizon, rather than it being pulled back. It simply comes to a virtual standstill. Not a real standstill, but slower than any actual speed. Think of the lowest non-negative non-zero speed, and it's even smaller. But it's not zero. --AK7 22:07, 22 January 2006 (UTC)

Yea but the photon in question is outside the Schwarzschild radius. The Infidel 19:13, 23 January 2006 (UTC)

Does anyone support the "This is because photons emitted as the object approaches the horizon take an increasingly long time to escape from the pull of the black hole to allow the distant observer to gain information on the object's fate."? Is it ok if I delete this? The Infidel 19:06, 26 January 2006 (UTC)

I'd reword it to something like "Because of the peculiar effects of Relativity, local space in the vicinity of the black hole is "stretched", constantly increasing the distance required to escape the gravitational field, thus photons take much longer to exit the closer they are created to the event horizon." Tzarius 22:21, 26 January 2006 (UTC)

I think it's correct the way it is. -- SCZenz 23:23, 26 January 2006 (UTC)

Yes, the reason given is correct. The photons do take an increasingly long time, and the effect is due to time dilation. –Joke 00:06, 27 January 2006 (UTC)

One might also mention that the emerging photons are red-shifted -lethe ^talk 00:59, 27 January 2006 (UTC)

Infidel is probably right to complain, because it is grossly misleading to speak of "time slowing down" or such nonsense. At least some of the above comments fail to distinguish between coordinate time and proper time, or between coordinate speeds and velocity measured in some frame at some event. It is correct to speak of gravitational time dilation, but only if one describes the observers one has in mind (as ought to be clear from thinking about relative motion in flat spacetime, using a special relativistic analysis)! And the geometric cause of the gravitational time dilation is easy to spot: the spacetime curvature causes the congruence of radially outgoing null geodesics to diverge. (The very same spacetime curvature can cause other congruences of null geodesics to converge, of course, which is one reason why we need a tensor to define curvature in Lorentzian manifolds of dimensions exceeding two.) I'd recommend Geroch, General Relativity from A to B to everyone who doesn't (literally) see what I mean. Enjoy: it's a great book!---CH 04:56, 22 April 2006 (UTC)

Dave's question

Question: If our galaxy has a black hole at its center, does time dilation from the perspective of someone outside the galaxy increase everywhere as the galaxy swirls closer to the event horizon and how does this impact our galaxy bound observations of the rest of the universe?- Dave

Answer: Very very little. From far outside, a black hole creates the same gravitation as any other object of the same mass. (For example, if our sun were replaced by a black hole of the same mass, nothing would change except that it would be very dark.) Time dialation and other effects of general relativity are very small except when there's very strong gravity, i.e. very near black holes. -- SCZenz 13:17, 27 January 2006 (UTC)

Universe is a black hole?

It is a fact that the Schwarzschild radius of the mass the entire universe is assumed to have (incuding dark matter), is larger than the diameter of the universe. If the definition of a "black hole" is a mass inside its own Schwarzschild radius, then the universe is a black hole. I have searched a little on this matter, starting with this Google search [1]. Accordning to these articles this does not necessarily mean that the universe is a black hole, depending on the definition of "black hole". I have no astronomics education, just a reader of scientific magazines. Please write about this in Wikipedia. /BIL 20:11, 31 January 2006 (UTC)

"Sound"

In 2003, astronomers detected the deepest note ever generated in the cosmos, a B-flat, which was been emitted from a black hole. No human will actually hear the note, because it is 57 octaves below the keys in the middle of a piano. After 53 hours of Chandra observations, it was revealed that the detected note is more than a 1,000,000,000,000,000 times deeper than what humans are able to hear. The reason the sound was detected was because the sound waves from the black hole were heating gas in the Perseus galaxy cluster, some 250 million light-years away.

For one thing, this needs a source. For another, if this "sound" is 57 octaves below A440 it only vibrates once every ten million years. It's interesting to think of that as a sound, but such a description doesn't belong in an encyclopedia. —Keenan Pepper 04:02, 15 February 2006 (UTC)

Sound cannot travel across galactic distance. The section in question must talk about extrapolated sound being heard in a gas region near the black hole. Since direct sound measurement cannot be made, verification might be difficult. -MegaHasher 17:58, 15 February 2006 (UTC)

Perhaps sound with such a long wavelength could propogate through the space, as it's not a total vacuum. This could be pased on a rough layperson's description of real research, maybe—but in any case we need a citation. -- SCZenz 18:13, 15 February 2006 (UTC)

It's just a pop-sci factoid that made the news a couple years back. The calculation was based on density waves propagating outward from a supermassive black hole. You can always take a wavelength (even one thousands of light years long) and convert it into a frequency and then into a musical note. The imaging and wavelength study are real science, but the music note nonsense purposefully describes it in a way to pander to a lay audience. See, for example, [2]. Personally, I think this kind of press-release science has no place in an encyclopedia, but Wikipedia is packed with it. -- Xerxes 19:03, 15 February 2006 (UTC)

Well said. Maybe we should start a project to hunt down this "press-release science" and fix it. —Keenan Pepper 23:12, 15 February 2006 (UTC)

Here is the reference. It was in the external links section.[3]

Temiree 21:31, 17 February 2006 (UTC)

At a guess, the press release is munging the idea that a sound wave propagating through an accretion disk (or maybe a torus of dust) near a black hole heats the dust, causing EM radiation which is detected on Earth; the existence of the sound wave is presumably inferred from optical observations. Just a guess. Someone should read the press release and hunt down the arXiv reference to check this guess, and fix the article accordingly. ---CH 05:31, 22 April 2006 (UTC)

On second thought, I now think Xerxes was right, and this press release was even more misleading than I initially guessed! Sheesh...---CH 00:25, 25 April 2006 (UTC)

Scientific American - cite please

Recently this was added:

According to Scientific American, a black hole can only be formed from a star less than 25 solar masses. If a star is greater than 25 solar masses, when it undergoes supernova it will become a neutron star.

Can someone provide a citation? Kaimiddleton 04:47, 25 February 2006 (UTC)

I'm backing this out, both because we don't (yet) have a link to the SciAm article, and because I think it's backwards (larger stars have a substantial enough implosion to form black holes, while smaller don't, and ones above about 40 solar masses go hypernova, collapsing directly to a black hole). --Christopher Thomas 06:26, 25 February 2006 (UTC)

Escape velocity vs. nothing can escape

The escape velocity has to do with the minimum speed at which an object has to be "thrown" away from the "surface" of a body to escape the gravity well. It only applies to objects that do not use their own power while trying to escape. Escape velocity is irrelevant when objects have their own propulsion. For instance, an astronaut climbing a very long ladder from Earth's surface can easily escape Earth's gravity well while his speed will always be much, much, much lower than Earth's escape velocity. Saying that nothing can escape BECAUSE of the high escape velocity is incorrect. By the same reasoning, the astronaut can never leave Earth's gravity well because his velocity is much less than Earth's escape velocity. However, the astronaut can "easily" climb that ladder and escape.

While it is correct that the escape velocity for a black hole is more than the speed of light, the ONLY reason that nothing can escape (not even a "very strong" astronaut on a "very strong" ladder) is that near a black hole, space bends back on itself. It is like a trapdoor. You can go in, but you can never come out because the space inside is bent in such a way that whichever direction you go, you will remain inside. The astronaut cannot keep climbing away from the singularity, because there IS no way away from it. It has nothing to do with not being able to generate enough speed or having enough power or surviving long enough. Even if you could travel FTL, could withstand the force of gravity, and lived long enough, you would not get out simply because no direction you can take is a direction out of it.

In short: What is mentioned at the beginning at the article as well as in the section "Event Horizon" is incorrect.

—Preceding unsigned comment added by Ben Jos (talk • contribs)

Sorry, didn't mean to leave it unsigned. Ben Jos 05:20, 26 February 2006 (UTC)

The short answer is that while you can apply this type of reasoning to situations where the escape velocity is much lower than C (and gravity is more or less Newtonian), things end up working differently when the effects of relativity become important. The Newtonian view of the problem ends up giving the right answer for not-entirely-right reasons. The short version of why you end up not being able to escape a black hole is that within the event horizon, all "time-like" directions point inward, making further progress into the hole (or away from a distant observer) as inevitable as (and equivalent to) moving forwards in time. The way you show this mathematically involves tracing the light-cones emitted from all points on some boundary surface and showing that this boundary contracts no matter what directions the light gets emitted in. More detailed descriptions, and other ways of looking at it, are given in many of the threads above, and in the archived talk page threads. --Christopher Thomas 08:18, 26 February 2006 (UTC)

OK... maybe I should have said that spacetime is bent, instead of saying space is bent. I still think the argument made in the article is wrong. It says "escape velocity is c, nothing with mass can go at speed c, ergo nothing can escape". I think you are actually saying the same thing I am. That the reason nothing can escape is because there is no way out. Spacetime is bent, no directions point outward. I cannot climb out because there is no way out, not because I can't produce enough speed. Ben Jos 09:03, 26 February 2006 (UTC)

A logical impossibility

The theory that a black hole can be explained as an object who steals time and matter, and sucks it in to nothingness is logically impossible. You cannot create something from nothing, nor can you change something in to nothing. You can only change things from one thing in to one other thing. Thus a black hole in this theory is not possible. Of course, we can never prove this theory to be true, which is another aspect of the evidence criteria, since nobody has ever seen a black hole, and indeed nobody ever will, since they supposedly steal light and thus as soon as you saw one you would be dead. All that we have seen are phenomena which we cannot explain. A black hole, in the concept of something which steals time and matter is not the only explanation, and indeed there are a dozen or more other theories to explain what really happens. Whilst this particular theory is pushed a lot in non-scientific communities, and has been popularised in the media, within the scientific community other theories are well and truly discussed, and as I understand it this is not the most popular theory. The use of "Occam's Razor" to ignore other far more logical and likely explanations is nonsensical and biased, and it should not be used in this way. All of the explanations for what a black hole is or might be should be given equal credit, and it should be made very clear that this is a theory.

The most logical explanation for what is happening is, of course, that things are not destroyed, that they just change shape, or else are moved elsewhere. Since things seem to disappear, therefore it is most likely that the "black hole" is not a portal to nothingness but rather a portal to somewhere else (see white hole). Where this other place is is another matter entirely, but the balloon theory for the universe has been used to explain that it could very easily be a portal from one part of the cosmos to another. This concept is used almost exclusively in Science Fiction genre with examples like Star Trek using some version of a wormhole to move quickly from different parts of space. So why is this concept not even mentioned in the article? Even Albert Einstein thought that white hole theory was a more logical explanation for the theory of black holes, and indeed if I am not mistaken he was the person who postulated that theory.

There are other explanations of equal validity, however, such as the changing shape concept. Just because we cannot see what they change in to does not mean that they disappear. It could be that they just become very black and actually exist in some other form.

Treating this as a fact, however, is a very wrong thing to do. It is a theory, not a fact. It is a theory that is taught in schools, but it is not widely accepted in the scientific community. There are many paradoxes over many elements of this theory that make it a logic impossibility. To accept it, we must make a leap of faith, and presume that the laws of physics no longer apply. But if the laws of physics do not apply for Black Holes, then how can they apply elsewhere? If this exception is true, then the only logical conclusion is that we are misunderstanding the laws and must rewrite them.

But of course before we do this, we must get over the concept that we can have something that is then turned in to nothing. Logical impossibility. Cannot be true. This theory is nonsense. It is an explanation that is only accepted because it has been discussed in more detail than the other more believable theories. And the main reason why it has been discussed is because of the "magical" element of it. Because it could change the laws of physics. Because it could mean so many exciting things. But often the more mundane explanation is the answer. The more mundane white hole theory with or without balloon theory is far more obvious and makes a lot more sense than the black hole nothingness theory. Why must we be always accepting things that take such a stretch of the imagination to believe? This is what Occam's Razor was all about. It was to stop this kind of nonsense theory from being given credibility. Its nice for kids, but for serious scholars they should rely on something a bit more believable. User:Zordrac 20:31, 27 February 2006 (UTC)

Black holes are objects predicted by general relativity; they're not a theory on their own. And General Relativity passes every test we've put it to with flying colors, so we tend to think of its predictions as credible. We've seen objects in the cosmos we're pretty sure are black holes. That being said, we've never tested any details, and certainly nobody knows what exactly happens at the center. But physicists take black holes very seriously; in order to compare them to another theory you'd have to say what the alternate theory is trying to explain, which you're really not doing. Can you clarify your quesetions, please? -- SCZenz 20:47, 27 February 2006 (UTC)

Black holes do not turn "something" into "nothing". When a black hole absorbs light, the mass of the black hole increases. So the mass of the light is still around, it hasn't disappeared. Now, if that light were carrying a signal, some information, then a classical black hole would erase that information. Therefore, classical black holes violate your "something into nothing" rule; they keep the mass-energy, but lose the information (this is known as the black hole information paradox. Physicists find turning "something into nothing" just as objectionable as you do). It's a good thing that real black holes are not classical, and quantum mechanics governs the universe: when you model black holes with quantum mechanics taken into account, then even the information stays. Real world black holes do not turn "something into nothing" (where here, real world means the universe according to string theory. Snicker). Hope that helps you get to sleep at night, finally knowing that black holes don't turn something into nothing. -lethe ^{talk +} 21:06, 27 February 2006 (UTC)

You should read some Einstein for a change. I find him to be quite a bright spark on the topic of black holes, and his theorem for what black holes really are is something that I, along with most scientists, think is the correct way of looking at things. Do a poll on physicists before declaring that most physicists believe in the ludicruous notion of black holes as explained in this article. I am sure that you will have no difficulty at all finding all about Einstein's theory, which was taught in schools until the late 1970's, when this new theory was presented. Why they changed theories, I cannot imagine, but current thinking is that schools should revert back to Einstein's theory. Go do a search, and stop spreading ignorance. —Preceding unsigned comment added by 203.122.209.179 (talk • contribs) on 09:52, 3 April 2006

Watch out, guys! IP address 203.122.209.179 is apparently geolocated in Gosford, New South Wales, AU and registered to Internode Systems Pty Ltd, which is headquartered in Adelaide, AU. This anon is in fact a suspected sock of User:Zordrac, who is in turn a confessed sock of User:Internodeuser. The latter two users have apparently been blocked for vandalism and violating WP:NPA. ---CH 05:22, 22 April 2006 (UTC)

Topic trimming

I am starting an effort to trim this topic a bit. -MegaHasher 20:55, 1 March 2006 (UTC)

You might want to stick a {{inuse}} template on it while you edit, as it seems to be edited fairly frequently. Also, please be very careful to fact-check changes made, to avoid removal of correct information. There are plenty of lurkers on the talk page who will be happy to address any concerns you want to raise about the present content. --Christopher Thomas 21:30, 1 March 2006 (UTC)

Theory vs. observation vs. logic vs. physics

Once more, I'd like to state that the statement "because the escape velocity is (at least) c, nothing, not even a photon, can escape and therefore nothing can escape" is wrong. A photon is "thrown" and after having been thrown/ejected, it proceeds without any internal propulsion. Escape velocity only means "the velocity something (particle/object/etc.) has to have when thrown/ejected/cast/kicked to escape the gravity well of a given "body", without having any propulsion power source of its own". A photon does not have any power source of its own and thus cannot "fight" to escape the gravity well of a black hole. A rocket with enough power, an astronaut, or a very strong snail for that matter, can produce their own power. Escape velocity is MEANINGLESS when talking about objects that have their own propulsion power. Escape velocity for Earth is 11.186 km/s (something like 40,270 km per hour or 25,022 miles per hour)... who cares? I can't walk that fast, let alone climb a ladder that fast. But I can climb a ladder (bean stalk, or some such) out of Earth's gravity well at a snail's pace and thus ESCAPE from Earth's gravity well. I can climb such a ladder at a speed of a few km per year (MUCH less than 40,270 km per HOUR) and still escape Earth's gravity well. Escape velocity is defined as "the minimum velocity something has to be 'thrown' from the 'surface' of an object's 'body' in order to escape that body's gravity well". It says NOTHING about rockets and other objects (including humans) who have their own power of propulsion. A photon cannot escape a black hole because it is THROWN. Therefore, it has to "obey" the "law" of escape velocity. Any other object with its own power source is NOT subject to the "law" of escape velocity. Once more, the ONLY reason nothing can escape from a black hole is because there IS NO WAY OUT... it has NOTHING to do with escape velocity. Damn... aren't there any Physics Majors (or above) here who can correct that fallacy in the Black Hole article? This has nothing to do with Newtonian or even Einsteinian physics going out the window near a black hole... this simply has to do with our CURRENT understanding of physics and the DEFINITION of "escape velocity". Can anybody with some authority on the subject please tell me why "escape velocity is c (or higher) and THEREFORE nothing can escape" is correct? Or - alternatively - someone please confirm that it has nothing to do with escape velocity but that the only reason nothing can escape (even a snail with a VERY STRONG shell trying to climb out at its own pace) is because the singularity bends spacetime in such a way that there IS no way out? Come on, don't mention Hawking and other physicists when they do NOT contradict this. Has Hawking ever said that "because the escape velocity from a black hole is at least c, nothing can escape"? Damn, even Hawking Radiation escapes, but never mind... . I guess what I am really fighting is that a featured wiki article states that nothing can escape BECAUSE of escape velocity, while the only reason nothing can escape has nothing (hmm... what about Hawking Radiation?) to do with escape velocity, but ALL with spacetime being bent in such a way inside the event horizon that there IS no way out. "All roads lead to Rome." OK... maybe it is not the way it "really" is, but as the experts in Physics see things today, isn't it that it's because of spacetime bending, and isn't it that it has nothing to do with escape velocity? wjmt 04:07, 6 March 2006 (UTC)

I'd agree with you that the concept of escape velocity doesn't apply here. But without going into mathematical detail is there a better way of explaining this to a lay audience? This is something I've thought about in the past but haven't thought of a definite way of explaining.

Technically inside the event horizon all future directed timelike vectors (i.e. directions you are allowed to travel in as your proper time increases) move you closer towards the singularity. The event horizon itself is something we call a null (or lightlike) surface, in some sense the event horizon is actually moving at exactly the speed of light, which is why light can appear to be stationary on the surface. --Jpowell 11:51, 6 March 2006 (UTC)

Lay audience: Hmm... a very simple way to see it might be that we are living on Earth. We can walk in all directions, but (without any ladders or rockets), we cannot really escape from Earth. Yes, we can jump, but we return to Earth a few seconds later. We can move in any direction we want, but we (without any "help") are still limited to Earth. Maybe it would help to create an analogy with a two-dimensional world? They move left, right, forwards, backwards, but when I (a "superior" three-dimensional being) make a circle (or, rather, a sphere) from their two-dimensional "piece of paper", those two-dimensional creatures can keep walking in a straight line forever, walking in a direction "outwards" from their point of view, but will always walk in circles, never escape, will find that "all roads lead to Rome".

Once more, and, yes, I know I keep repeating myself and I know that there may be no easy way to explain this to a "lay audience", but, at the same time, the "lay audience" still believes in Newtonian Physics. Einsteinian Relativity is so much contrary to common sense. How can one keep accellerating but never reach the speed of light, much less exceed it? How can it be that we measure something as "light years away", but light itself travels that distance in "no time flat"? Maybe "lay people" will never understand, but enough examples can be given that the current understanding of Physics really supports Einstein and that Newton no longer applies at relativistic events? One example I can think of (and, sorry, but I am too tired right now to look up the details) is those particles that in a Newtonian sense can never reach Earth, not living long enough to reach Earth, but because of Einsteinian Physics taking over at relativistic speeds (benefiting from length contraction), reach Earth anyway? Or, closer to home for "laymen", the experiments with very precise clocks used on high-speed aircraft, showing the time dilation that Einstein predicted/formulated?

Or make it even more simple... imagine yourself trapped in a tunnel where the walls are infinitely strong. No matter how much power you produce, you cannot break through the walls and therefore, you will always stay within the tunnel. Maybe it is not the best way to explain how spacetime is bent within the event horizon, but it will give some kind of idea to "lay people" that there really is no way out.

Actually, the concept I mentioned before about a trapdoor I think may be best. You can go through the trapdoor easily enough, but once you're in, you can never go out again. A very simple example of this would be the "teeth" used when you rent an automobile from certain places – you can easily move into the lot owned by the rental company, but there is a barrier that you "cannot" cross on your way out.

"... which is why light can appear to be stationary on the surface...". Yes... but that is only to an outside observer. Again, this brings me back to "common sense" vs. "Einstein" – it is all about relativity... about how one observer sees things vs. how another observer sees those same things. Outside the black hole, I can watch someone stay there forever, but that same someone does definitely not experience that the same way. That someone will be "sucked" into the singularity at enormous speeds and will only last for a very short period of time, while the outside observer will see that person "live forever".

What constitutes a "featured article" on wiki? I do not think an article explaining things in laymen's terms that are contradictory to current expert knowledge would qualify. Rather, I'd think an article that is able to explain that knowledge and make it accessible and acceptable in laymen's terms WOULD. What I really am fighting against, and have been complaining about, is that the "Black Hole" article was a featured article while most people who knew more about the subject than laymen saw that it was completely wrong in some of its statements. And, yes, I admit I probably would not have been nearly as adamant about this if this article had not been a "featured article".

Now let us do something constructive with this? Can we cooperate and make this article not state any falsehoods, while still (maybe by using one, two, lots of analogies) adhering to our current understanding of Physics? Can we? wjmt

Wjmt (did you know your user name can be interpreted as an ancient Egyptian name?), I agree with the others that you have a point, and in fact I try to avoid talking about escape velocity in this context. Nonetheless it is traditional that a semi-popular encyclopedia article on such a subtle and technical topic as the notion of a black hole in modern astrophysics must begin with tired shiboleths, continue with half-truths, proceed to dazzle (or annoy) baffled readers with some verifiable facts expressed in mathematical language, and hopefully finish with citations to a few good textbooks. The last at least is useful to diligent and well prepared students. (To repeat something I recently wrote somewhere else in a similar context.)

I myself do try to avoid half truths even when attempting to address a general audience, but I have little illusion that this illuminates any more than a traditional approach; it simply deceives a little less. I dunno about Rome, but certainly there is no royal road to gtr.

Uhm... could you try to introduce several paragraph breaks in future comments of similar length? :-/ TIA---CH 05:14, 22 April 2006 (UTC)

About Q9 in the faq

The answer to question 9 says -- "... The earth wouldn't get sucked up because it has the same amount of mass and the same amount of gravitational attraction." It has same amount of mass and gravitational attraction as compared to what ? The sun ? (not possible, of course)

Read it again; I tried to clarify it. —Keenan Pepper 15:41, 8 March 2006 (UTC)

BTW, I'm kinda waiting for someone else to propose deleting that section or changing it into something else, because it really doesn't belong. But in the meantime we might as well improve it. —Keenan Pepper 15:44, 8 March 2006 (UTC)

Yes, it is much more clear now. Thank you. Rohan.

Vandalism in "Formation"?

In the formation section, second paragraph, the last sentence reads: "This is because there is a limit to the strength of materials due to the fact that the speed of sound, related to the materials stiffness, cannot be greater than the speed of light."

I was unable to verify if this sentence is correct or if it is the result of vandalism. Enrique Vargas 16:01, 13 March 2006 (UTC)

This is not vandalism. I wrote that sentence... Basically, any force that could stop the collapse would act like a "pressure". With a pressure, you can calculate a speed of sound, which must necessarily be less than the speed of light. This places an upper bound on the the hypothetical collapse-preventing force. Sfuerst 19:08, 16 March 2006 (UTC)

I agree with all of this. In fact, this sounds like the dominant energy condition to me. But ought it be in the article? –Joke 20:21, 16 March 2006 (UTC)

Parallel Universes

As a child in year nine (13 years) I find all of this facinating, but please forgive me if this is a stupid question: what happens to the matter that gets sucked into a black hole? Were does it go? I am also intrigued by parallel universes. How does this theory co-incide with black holes?

(unsigned comment apparently added by "Brandon")

You'll probably be disappointed to hear this, but the current theories predict that things that fall into a black hole just stay there. They continue to be crushed smaller and smaller, but that's about it. No "parallel universes". —Keenan Pepper 23:23, 13 March 2006 (UTC)

I just want to make sure that I understand this correctly. So matter that falls into a black hole stays as matter. Light that falls into the black hole gets turned to mass (e=mc^2), increasing the mass of the black hole. Where does the Hawking radiation come from? Is it matter turned into thermal energy by the same formula, or does it come from a different source? - Brandon

I think you have a basic misunderstanding of special relativity. Light doesn't get "turned to mass", but the energy it adds is equivalent to mass. It may seem a subtle distinction, but consider this: there are two identical objects, except one of them is spinning very fast. The spinning one takes more force to accelerate and has stronger gravity, because it has what happens to the matter that gets sucked into a black hole? Were does it go? I am also intrigued by parallel universes. How does this theory co-incide with black holes?

You'll probably be disappointed to hear this, but the current theories predict that things that fall into a black hole just stay there. They continue to be crushed smaller and smaller, but that's about it. No "parallel universes". —Keenan Pepper 23:23, 13 March 2006 (UTC)

I just want to make sure that I understand this correctly. So matter that falls into a black hole stays as matter. Light that falls into the black hole gets turned to mass (e=mc^2), increasing the mass of the black hole. Where does the Hawking radiation come from? Is it matter turned into thermal energy by the same formula, or does it come from a different source? - Brandon

I think you have a basic misunderstanding of special relativity. Light doesn't get "turned to mass", but the energy it adds is equivalent to mass. It may seem a subtle distinction, but consider this: there are two identical objects, except one of them is spinning very fast. The spinning one takes more force to accelerate and has stronger gravity, because it has extra rotational kinetic energy. It doesn't make sense to say the rotation is "converted into mass", but it acts like mass. It's the same with any kind of energy. A charged battery weighs a tiny bit more than an identical uncharged battery, because of the chemical energy. You could say mass is really just frozen energy.

Hawking radiation is really weird, and I confess I don't fully understand it, but the usual explanation is that one particle of a virtual particle-antiparticle pair falls into the black hole and the other escapes. For example, just outside the event horizon, an electron and a positron (anti-electron) are created out of nothing, the positron falls in, and the electron flies off as Hawking radiation. This by itself is not enough to explain it though. If a real positron fell in, the net flow of energy would still be into the black hole, not out. The key is that some energy manages to get out of the black hole by quantum tunnelling. In other words, the virtual particle and antiparticle have an energy debt back to the vacuum, which they would repay if they annihilated, but instead one of them falls into the black hole so they go bankrupt and the black hole ends up having to pay for it. Wow, what a terrible metaphor... Well, I hope some of that made sense to you! =P —Keenan Pepper 05:24, 21 March 2006 (UTC)

Thank you!!

To try to answer the first of Brandon's questions: "what happens to the matter that gets sucked into a black hole? Were does it go?" The short answer is that right now no-one knows. Longer answer (sure to go way over your head, but it might be fun reading anyway): according to general relativity, any particle which falls into a (non-rotating) black hole will experience increasing tidal forces which tend to pull it apart radially and also to squeeze it in perpendicular directions (see spaghettification) and eventually its world line will strike a curvature singularity, where the tidal forces become infinite. After this the theory cannot make further predictions, but it does seem that any kind of earthly object, such as person, would be pulled apart into a long thread of subatomic particles as it nears the singularity. Physicists suspect that very near such a curvature singularity, as the spacetime curvature approaches L^-2 where L is the Planck length, gtr should break down, and accurate predictions will require a successful quantum theory of gravity. Ignoring this issue, more sophisticated classical models which attempt to study what might happen inside astrophysical black holes (which are rotating and which generally have radiation and perhaps other stuff falling into them) suggest that the world lines of some particles which fall into a hole might strike a Cauchy horizon, and again gtr cannot predict what happens after that. However, Cauchy horizons are not associated with diverging curvatures and it is possible that such a particle can continue to exist in some new region of spacetime. This sort of scenario is not to be confused with certain features of the interior of the Kerr vacuum and Reissner-Nordström electrovacuum solutions, which are thought to be highly accurate models of rotating uncharged black holes and charged nonrotating black holes respectively outside their horizons, but which are thought not to be accurate everywhere inside their horizons, and which in fact have some features which may be quite misleading about the interiors of astrophysical black holes. ---CH 04:46, 22 April 2006 (UTC)