Peter, Thanks for the reply. I have been reading around the subject to try to understand the principles ... but it is taking a lot longer than I thought it would! Oh well!

I appreciate what you are saying from the astronauts perspective (finite time ... growth of the event horizon ... etc.) and uderstand the basics, but I'm trying to understand the experience from the distant observers perspective. I appreciate that the astronaut, and his / her clock, will see time pass at the *normal* rate, but from the distant observers perspective I was under the impression that the rate at which the at which the astronaut is seen moving toward the event horizon (and the red shift of his signal) will approach infinity and so never actually (be seen to) cross the event horizon.

But my reading of the previous posts is that from the perspective of the distant observer, irrespective of the astronaut falling-in or the event horizon growing, the astronaut / signals will drop behind the event horizon and disappear. But ... I'm still not certain so on with my reading!

Regards,

Noel.

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 Quote by Lino I'm trying to understand the experience from the distant observers perspective.
The first thing to understand is that the distant observer does not experience what is happening to the astronaut. He only experiences the light signals coming from the astronaut; and those light signals have to pass through the intervening spacetime. See further comments below.

 Quote by Lino I was under the impression that the rate at which the at which the astronaut is seen moving toward the event horizon (and the red shift of his signal) will approach infinity and so never actually (be seen to) cross the event horizon.
A quick correction: the redshift approaches infinity, but this corresponds to the apparent rate of the astronaut's fall approaching zero. Note, however, that this is only the *apparent* rate: the infinite redshift means that the spacetime in between the astronaut and the distant observer is distorting what the distant observer sees--the distortion grows larger and larger as the astronaut gets closer to the horizon, until it becomes an infinite distortion when the astronaut is *at* the horizon.

 Quote by Lino But my reading of the previous posts is that from the perspective of the distant observer, irrespective of the astronaut falling-in or the event horizon growing, the astronaut / signals will drop behind the event horizon and disappear.
This is saying the same thing as the above, just in different words. The astronaut "dropping behind the horizon and disappearing" is equivalent to the astronaut's light signals redshifting to infinity.
 Thanks Peter. The Usenet Physics FAQ was great, & it references MTW's Gravitation that I'm trying to work through at the moment - so good to know that my reading is on the right track! The really interesting (& obvious when you think about it) thing that I've read so faris that the astronaut gets dimmer and fades to nothing as he approaches the event horizon, so I never actually see him *freeze*. I hadn't connected those dots! Regards, Noel.
 I'm afraid that I need to come back to ask a further question. I seem to be reading "around" the subject without actually getting to the nub of the matter! In relation to my previous scenario (the difference between an astronaut / signal free falling toward an event horizon versus the event horizon expanding to encompass astronaut / signal, from the perspective of a distant observer), what I envisaged was that for the free falling astronaut would appear to slowdown / fade but never stop or disappear, but for the expanding event horizon the astronaut would disappear completely. I can appreciate that the speed of the expanding event horizon might slow as it approaches its new resting place, inline with the approach of the in falling shell, and thus if the astronaut were at the location of the new horizon the effect (infalling astronaut or expanding horizon) would be the same. But if the astronaut is inside the new horizon, then he will be completely emcompassed and disappear. Thus the perspective of the distant observer would be different for each situation, and that doesn't seem correct! Am I mis-representing something? Regards, Noel.

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 Quote by Lino what I envisaged was that for the free falling astronaut would appear to slowdown / fade but never stop or disappear, but for the expanding event horizon the astronaut would disappear completely.
Not quite. As the expanding horizon gets closer and closer to the radius where the astronaut is, light signals emitted to the distant observer from that astronaut will get more and more redshifted, and the time between them will get longer and longer. At the instant when the expanding horizon engulfs the astronaut, his outgoing light signals, from the standpoint of the distant observer, become infinitely redshifted and take an infinite time to get out. So there's not really a difference.
 Much appreciated again Peter. I'm still stuck comparing infinite redshift and "froozen" time. When I transpose this analogy into "fading to invisibility" it makes sense for the free falling astronaut, but if the rate at which the horizon expands slows and "freezes" (as it approaches the astronaut) how does it catch up with the smooth trajectory to the enlarged horizon? Regards, Noel.

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 Quote by Lino if the rate at which the horizon expands slows and "freezes" (as it approaches the astronaut)
It doesn't. Light signals emitted outward by the astronaut get more and more redshifted and take longer and longer to get out to the distant observer, but that's not the same as saying the expansion of the horizon itself slows and freezes. The astronaut will find himself at the new horizon in a finite length of time by his own clock; it just takes an infinite amount of time by the distant observer's clock for that information to get out to him.

 Quote by PeterDonis It doesn't. Light signals emitted outward by the astronaut get more and more redshifted and take longer and longer to get out to the distant observer, but that's not the same as saying the expansion of the horizon itself slows and freezes. The astronaut will find himself at the new horizon in a finite length of time by his own clock; it just takes an infinite amount of time by the distant observer's clock for that information to get out to him.
I agree with what you are saying here and (mostly) understand it.

 Quote by Usenet Physics FAQ Now, this led early on to an image of a black hole as a strange sort of suspended-animation object, a "frozen star" with immobilized falling debris and gedankenexperiment astronauts hanging above it in eternally slowing precipitation. This is, however, not what you'd see. The reason is that as things get closer to the event horizon, they also get dimmer. Light from them is redshifted and dimmed, and if one considers that light is actually made up of discrete photons, the time of escape of the last photon is actually finite, and not very large. So things would wink out as they got close, including the dying star, and the name "black hole" is justified.
This paragraph is describing what the distant observer would see as the astronaut free falls toward the event horizon, and I understand the logic of the astronaut dimming but not why it would “wink out” (I don’t think that he “runs-out” of photons). However, if you consider the scenario where the event horizon expands (ahead of a shell of infalling material) from A to B – it does seem to make sense (to me).

Consider we have super sensitive equipment and a line of astronauts stretching for A to B. In order for the event horizon to move from A to B in a smooth / timely fashion, it will encompass the individual astronauts in turn. As it does (from the perspective of the distant observer), the signal / view of the initial astronaut will greatly redshift and then (as the FAQ says) wink out. But this means that the perspective of the distant observer is different if the astronaut is free infalling or the event horizon is expanding – which does not make sense (to me).

Which brings me back to the FAQ quote, maybe both do “wink out”, but why?

Regards,

Noel.

 Quote by BitWiz Much of what people say about the vicinity of black holes doesn't seem to make sense. For instance, it seems to be impossible for a black hole to grow by "ingestion" by scooping up matter around it or in its path, at least in the traditional sense. Gravitational time dilation takes care of that -- no particle having mass will ever reach the event horizon, much less travel through it, and because of the asymptotic partitioning of space-time at the horizon, I don't think that even a photon can penetrate a black hole as it would have to raise itself to an infinite frequency. So an event horizon seems to be impenetrable -- from either direction. However, it seems that a black hole can ingest matter by growing. If a massive object approaches a black hole, and comes close enough such that the two combined masses (or portions of a mass) now fit within their paired Schwarzschild radius, a new shell-like event horizon will form behind the intruding mass, and in the process, any other matter around the original black hole is now engulfed within the new expanded radius. For instance, if a neutron star of about two solar masses approaches a black hole containing about 60 million solar masses such that its entirety is within about 6 kilometers of the event horizon, a new event horizon will form behind it, and in the process engulf enough space to contain the volume of the Sun (if my math is correct). [..] Comments please? Chris
The black hole growth paradox has been discussed somewhat in earlier threads. In particular with reference to mathpages which also brings it up (click on the links for more):

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 Quote by Lino I understand the logic of the astronaut dimming but not why it would “wink out” (I don’t think that he “runs-out” of photons).
He doesn't run out of photons, but once he is at or inside the horizon, the photons he emits can't escape back out to the distant observer. This is true whether he falls to the horizon or the horizon expands to him.

The "wink out" description assumes that there is a finite limit to how low a frequency (or how long a wavelength) the distant observer can detect; when the astronaut's photons are redshifted beyond that limit, he "winks out". This happens *before* the astronaut actually reaches the horizon (or the horizon reaches him). If the distant observer could detect photons of any finite frequency, however low, he would continue to detect them (at lower and lower frequency, and coming further and further apart) forever by his clock; but all the photons he detects would have been emitted by the astronaut from some point above the horizon.

 Quote by Lino Which brings me back to the FAQ quote, maybe both do “wink out”, but why?
Yes, they both do. See above.

 Quote by Lino I can appreciate that the speed of the expanding event horizon might slow as it approaches its new resting place
Notice that whether the horizon "slows" depends on your choice of coordinates. In a local inertial frame, the horizon expands at c! Even a "static" horizon expands at c from the point of view on an observer who is crossing it.
If you are not familiar with the Rindler horizon, I suggest you have a look at it. It's a simple case, involving no gravity and no spacetime curvature, of an event horizon with many features of a black hole's horizon, including redshift, objects falling past the horizon but never appearing to do so to a distant observer, causally disconnected regions of spacetime etc. It's like the horizon of a very large black hole, with negligible tidal forces. See for example this.

 Quote by someGorilla Notice that whether the horizon "slows" depends on your choice of coordinates. In a local inertial frame, the horizon expands at c! Even a "static" horizon expands at c from the point of view on an observer who is crossing it. If you are not familiar with the Rindler horizon, I suggest you have a look at it. It's a simple case, involving no gravity and no spacetime curvature, of an event horizon with many features of a black hole's horizon, including redshift, objects falling past the horizon but never appearing to do so to a distant observer, causally disconnected regions of spacetime etc. It's like the horizon of a very large black hole, with negligible tidal forces. See for example this.
Thanks someGorilla. I do (mostly) appreciate this but will be checking out the referenced link. Thanks again.

Regards,

Noel.

Peter, First off, thanks for the time you have spent responding on this topic, I really appreciate it. I also recognise that I might just have a "mental block" and need to read (alot) more on / around the subject. If I am frustrating you, please consurve your patience and feel free to ignore this post.

I am mostly in general agreement with what you are saying:

 Quote by PeterDonis ... all the photons he detects would have been emitted by the astronaut from some point above the horizon. ...
Accepted. I understand how this comment is consistent in both scenarios.

 Quote by PeterDonis ... If the distant observer could detect photons of any finite frequency, however low, he would continue to detect them (at lower and lower frequency, and coming further and further apart) forever by his clock ...
I understand how this comment will be correct for the scenario of the infalling astronaut. I also understand that for the expanding horizon scenario the distant observer could detect photons at lower and lower frequency ... but I would have assumed for a finite period of time.

(I hope that this doesn't add confusion, but it strikes me like a "radioactive half life" problem: in that (for the distant observer) the time for the horizon to move to the new location appears infinite, but the time taken for the horizon to reach half way (i.e. the location of the astronaut) can be measured specifically.)

Regards,

Noel.

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