User talk:Eequor/Reference/Event horizons

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Event horizons and much else[edit]

What is the relation between accelerating universe, cosmic inflation, Hubble's law, event horizon, and gravitational collapse? In particular, how might an observer inside an event horizon distinguish between inflation and their own collapse? --Eequor 10:12, 9 Aug 2004 (UTC)

I don't really understand the question. Inflation is rapid expansion - so rapid that the different parts of the universe move away from each other faster than the speed of light. This makes the cosmic microwave background radiation look unnaturally even temperatured. I.e different parts of the sky are at the same temperature even though no signal could have passed between them. The heat simple would not have had time to flow if inflation didn't happen. If the observer were collapsing, there would be no reason for the CMBR to be at the same temperature everywhere. As for Hubble's law - the further away a galaxy is from us the faster it is speeding away. This is easily explained by saying the universe is expanding, if the observer were collapsing would we see the same effect? (Let me think on it)theresa knott 15:13, 9 Aug 2004 (UTC)
I'm curious what we might see if we were somewhere between an extremely massive, cooling object and its event horizon which appears (from our perspective) to be about 15 billion light years away. Wouldn't we see something very much like a cosmic microwave background? --Eequor 16:02, 9 Aug 2004 (UTC)
Not an expert, but I don't think the radiation from the massive, cooling object can go outward from it and reach us. Pretty certain that from the event horizon, more or less everything goes in. At least, with conventional theory.--Fangz 00:42, 10 Aug 2004 (UTC)
Light produced in the present will not reach us, but light that had been produced far enough in the past may return to us from orbit, assuming the object has moved relative to its position when the light was emitted. --Eequor 02:54, 10 Aug 2004 (UTC)
I don't see why this would be so. Central postulate of Special Relativity - Light speed constant in all inertial frames of reference. Whether or not the emmiting object has moved from its original position must neccessarily be undetectable, and so it must make no difference. And the problem with the event horizon is still there - if it is impossible to move outwards from the BH at the event horizon, how would it be possible to move outwards from the object within the event horizon, where the gravitational force is even stronger? And how can we account for the perceived recession of galaxies relative to us in all directions, when surely they must be accelerating towards a common centre.
Besides, if the light was from the past... what is the point of there being a massive cooling object right now? I mean, look at what are we left with - photon emmiting event in the past, which created CMB radiation, peculiar spacetime structure to wrap it around... Is this any different from conventional big bang theory? (Aside from introducing a absolute centre of the universe, which opens lots of cans of worms) Did anyone say Occam's Razor?--Fangz 18:10, 10 Aug 2004 (UTC)
Spaghettification

OK I've I've had overnight to think on this, and I'm certainly no expert on general relativity, but I think you'd notice tidal effects. Towards the singularaity and towards the event horizon galaxies would appear to be moving away from us. Ok so far :-) But at right angles to these directions galaxies would appear to be moving towards us :-( Also if there were no big bang there is nothing to explain why the CMBR even existed. And there is the ratio of light elements in the universe. There is a lot of primordial helium and deuterium about, where did it all come from if not the big bang? theresa knott 07:19, 10 Aug 2004 (UTC)

Tidal forces isn't a problem. See Supermassive black hole.--Fangz 18:10, 10 Aug 2004 (UTC)
No I think it is a problem. We wouldn't suffer spagettification because we are so small, but the unverse is huge. I suspect even very weak tidal forces would be detectable over the huge scales involved.theresa knott 23:43, 10 Aug 2004 (UTC)

Ee, I'm not sure what the intent of your question was. The answers above might be appropriate, but such feedback as there is has so veered off from the original question as not to clarify much. So... Here's my input. The concepts you've asked about are in some respects quite different, so I'll take them one at a time. If there was a Big Bang (as all the evidence currently available suggests), then there was a time when everthing was crammed into a much smaller space than it now occupies. At some very early time (for reasons I understand are unclear to all), there was a brief period of Very Rapid expansion. That is, stuff was crammed this close and afterwards it was much less closely crammed, and afterward came very quickly. As tk points out, this had effects which are (just) still observable. We think. The guy who thought it up (Guth) is going to win the Nobel Prize one of these days (if he hasn't already, and I missed it). Well, if things aren't expanding anywhere near so fast now (14 billion years later), they are nevertheless still expanding. Hubble (after whom the telescope is named) noticed that there was a connection between speed of recession (at least from our perspective) and distance from us. It appears to be a constant linear relation. That's Hubble's Law.

Now, is the motion of everything in that expansionary way (now muuuuuch slower than it was during inflation) a constant one? If not, gravity (of everything taken together) might be strong enough to gradually slow everything down eventually, in which case things will finally reverse and start to contract. When it eventually all gets more or less to one place (the Big Crunch), that will be one kind of gravitational collapse. No one thinks that's likely just now as the measurements of how much stuff there is and how fast things are (still) expanding don't seem to leave much chance. But we've only been watching for maybe 100 years and so we may have missed something. and there is all the 'missing' matter (ie, dark matter) which is clearly present (from its observable gravitational effects here and there and mostly everywhere) but not at all well understood. And in recent years there's also been 'dark energy' which seems to be hanging around but which is even less well understood. If there isn't enough total gravity to cause the Big Crunch, will things just keep expanding (slower and slower but never quite stopping)? Or, might there be some previously unknown force which opposes gravity but only works at really long distances (or only adds up enough to be noticeable over really long distances, which is not quite the same thing exactly)? There might be, as some recent observations have suggested such a thing. If so, things might eventually be moving in an expanding way faster and faster and faster.

General Relativity (thus far the most successful of the theoretical accounts of the nature of gravity) predicts many things about how light behaves in a gravitational field. One of them is that if the field is sufficiently intense, light will be bent so strongly it can't escape from the field. When this happens to a distant galaxy's light passing near a closer galaxy we sometimes notice a lens effect, producing two images of the distant galaxy (sort of like a mirage, but with rather different causation). No one looking (from a distance) at a light inside that field will be able to see it. In fact, as a light emitter approaches such a field, there will come a place (sort of an invisible spherical shell, if you will) after which the light being emitted can no longer escape. That's an event horizon. For material objects it's only the limit of the various distances after which the object is trapped. Light, by Special Relativity, moves at the fastest possible speed and material objects can only approach that speed (with greater and greater difficulty the closer they get).

Black holes are exactly this sort of thing. With enough mass (what's left of a really big star, for instance, after it 'runs down', as it were), there will be a sufficiently intense gravitational field to overcome the repulsions of elementary particles and there will be a local Crunch. If a glop of matter isn't quite big enough to do that, it might only reduce all the particles to neutrons producing a neutron star. Anyway, black holes (but not neutron stars) show just this effect. Hence, 'black' hole. Actually there will be some radiation from it due to quantum mechanical effects (Hawking radiation, named after its discoverer) but that's a sort of quibble at this level of discussion.

Inside an event horizon, it is thought that much will 'remain the same'. For example, my candle will still be visible to me as it disappers for others, assuming I survive the various other effects (high intensity radiation, tidal effects, ...). Though there are simultanity issues here that must be considered carefully. But what I will see (modulo my survival) won't matter in some sense as whatever it can never be communicated to anyone outside the field. A one hand clapping or tree falling in the forest making noise sort of thing. Pragmatically, it won't matter as no one anywhere else will ever learn about it. But the odd effects have been the subject of many a science fiction story, some of them quite good.

Maybe this helps? ww 18:52, 11 Aug 2004 (UTC)

Thanks. More specifically:
--Eequor 20:18, 11 Aug 2004 (UTC)
Maybe you like this animations. Simon A. 18:07, 12 Aug 2004 (UTC)
That's very impressive! --Eequor 16:59, 16 Aug 2004 (UTC)
Dah, ultracompact neutron stars win for oddest visual effects. Too bad the series doesn't explore what is observed if one crosses the photon sphere. It seems that gravitational collapse is more or less correct -- infalling light from the photon sphere may make arbitrarily many orbits before reaching the observer, so everything inside the sphere must be visible at once in every direction, with the entire sky outside the sphere superimposed on top in innumerable layers. I'm curious whether the light sources would remain distinct. --Eequor 18:28, 16 Aug 2004 (UTC)

Photon spheres[edit]

In the above-mentioned website, R.J. Nemiroff describes a surface called a photon sphere, which is the locus of orbits with an orbital velocity equal to the velocity of light -- that is, the distance at which photons may orbit a star or black hole indefinitely, neither spiraling inward nor escaping. How does this differ from an event horizon? How closely may a body in an elliptical orbit approach the speed of light? --Eequor 16:59, 16 Aug 2004 (UTC)

Is this similar to the particle horizon? --Eequor 17:38, 19 Aug 2004 (UTC)

Intracommunication[edit]

Within the event horizon of a black hole, is it possible for any signal to reach a receiver less deep in the gravity well? Would infalling observers see themselves pass through the horizon, or would the horizon seem to fall with them? --Eequor 23:16, 16 Aug 2004 (UTC)

The short version: We don't know what happens inside the event horizon, and we can never know.
The long version: By definition, no information can get out. My understanding of black holes (which I'm sure is limited - of course, so no one really understands black holes, but if someone did, it would not be me) is that they're a sort of a one-way information transfer zone. I would take that to mean that nothing (specifically, no information) from within a black hole can get any further from the center of the black hole.
The gravitional field that keeps the photons within the event horizon is stronger the closer you are to the center. If it's strong enough, at the event horizon, to keep a photon from escaping, then it's even stronger further in, so that same poor little photon hasn't got a chance of getting out.
The question of what an observer approaching the event horizon from the outside would see is addressed in the black hole article under section 2.3 ("falling in"). The event horizon is an imaginary boundary. You can't really "see" it. You can only deduce that it is there by the effect of the black hole on its surroundings. There wouldn't be anything spectacular to let you know that you had crossed it. Aranel 22:27, 17 Aug 2004 (UTC)
See Virtual Trips to Black Holes and Neutron Stars. The event horizon is easily visible if one is close enough to it, because light from the entire universe is visible if one looks up from the horizon, along with infinitely many copies of all light sources. Approaching the horizon, the sky becomes increasingly compressed into a circle, so that it would appear that one was looking up at it from the center of a perfectly black bowl. Exactly at the surface of the horizon, the sky shrinks to a point and disappears.
My intuitive sense says the same thing that you do about light escaping upward: it shouldn't be able to, from any location within the horizon. It's hard to think about, though; it seems like (for example) a person ought to receive nerve impulses from the rest of their body, since they're connected (though of course the tide would be tearing them apart, and their body might already be cooked by their own nerve impulses traveling downward). --Eequor 16:26, 18 Aug 2004 (UTC)

To summarize: Since we can never know what happens after an object crosses an event horizon, it is pointless to speculate. Such speculation only leads to non-falsifiable speculation, it can't lead to scientific theory. There's only one exception: the hypothesis that we're already inside a black hole. In that case, speculation could help us confirm or deny the hypothesis.

it seems like (for example) a person ought to receive nerve impulses from the rest of their body, since they're connected

I think you're right. "a hypothetical astronaut travelling towards the black hole center would not experience significant tidal force until very deep into the black hole." -- supermassive black hole. That astronaut would always feel "connected" to all parts of his body while falling through the event horizon, because he is falling *faster* than the nerve impulses are traveling. If you smuggle a watergun into the front seats of an airplane, and continuously spray me (in the back seats) with it, I continously feel "connected" to the stream of water hitting me. I don't feel it let up for an instant -- nothing special happens at the instant the airplane starts going faster forward than the (relative) speed of the water droplets (so the water droplets have a ground speed of zero), or when the airplane is going faster than the speed of sound (but the ground speed of the water droplets is still subsonic). I still don't understand how time dilation affects the astronaut. -- DavidCary 23:32, 9 Nov 2004 (UTC)
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