Black holes question... Communicating with the outside

Dumbledore said:
Barring this impossible form of communication, I do not see how you could communicate out of a black hole.
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The most basic reason why you cannot communicate out of a black hole with someone outside is not that it would be difficult, but that as far as someone outside is concerned, you can never get inside. How could you possibly send a message from the inside of a black hole while you remain outside of it?
 
69dodge said:
The "connection" between an object's atoms is never instantaneous anyway. What one atom does now, can have an effect on another atom only some time later---specifically, after light has had enough time to pass between them. But an object falling through a black hole's event horizon always falls through it very quickly---exactly at the speed of light, in fact. By the time an atom inside would have affected an atom outside, the outside atom has fallen inside already. So it can be affected as usual, without any signal escaping from the black hole.
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This was nicely done.
 
tracer said:
Now hold on just a cotton-pickin' minute here!

Long long ago, back when I was studying undergraduate physics, I was told that the Event Horizon (a.k.a. the Schwarzchild Radius) was the distance from the center of the black hole at which the escape velocity became equal to c. Farther out than the Schwarzchild Radius, the escape velocity is less than c; inside the Schwarzchild Radius, it's greater than c.

But if that's the case, shouldn't a beam of light (or an object trravelling at nearly the speed of light) shot straight upward from just inside the Schwarzchild Radius be able to get a little ways up past the Radius before it loses all its energy?

If so, couldn't such a signal be captured by a "relay station" just outside the Schwarzchild Radius, and then re-sent at full energy to another relay station higher up, and so on until it reaches open space and escapes?
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As I undestand it, the Schwarzchild Radius is the distance from the singularity at which the escape velocity becomes the speed of light, or put another way, the point at which a photon loses all its energy on creation, so a photon emitted from just inside the Schwarzchild Radius would lose all its energybefore crossing.
 
Mosquito said:
I have a book that theorizes that black holes are baby universes and that they have "evolved to be near optimal in creating new black holes". My BH = dimensionally freaky mini-universe idea was a parasitic one on that, and what Yllanes said (or at least what I got from what Yllanes said).
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This is a nice idea, but it suffers from a complete lack of evidence. The reasoning goes if black holes are new universes and if the new universe has similar, but not identical, properties to its parent universe, then universes with the best conditions for producing black holes will produce lots of children and therefore virtually all universes will have properties that allow black holes to form. This is quite a satisfying hypothesis from some points of view because it explains why the fundamental constants and properties of our universe are as they are - not because there is anything special about us, but simply because having picked a universe at random it is virtually guaranteed that it will have conditions very similar to ours.

Unfortunately there is absolutely no evidence backing up either of the assumptions that it is founded on. It is a hypothesis that sounds nice and is very neat, but in the end doesn't actually explain anything.

Edit : Of course, once you hypothesise that universes can undergo evolution, another hypothesis is pretty much impossible to avoid. The universe is alive.
 
JQH said:
As I undestand it, the Schwarzchild Radius is the distance from the singularity at which the escape velocity becomes the speed of light, or put another way, the point at which a photon loses all its energy on creation, so a photon emitted from just inside the Schwarzchild Radius would lose all its energybefore crossing.
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This is incorrect.

First off, it actually turns out to be incorrect to say that the escape velocity at the event horizon is c, because at the event horizon, you can't escape, even at c. Anything outside the event horizon has an escape velocity less than c, but light emitted "outwards" AT the event horizon does not escape. It doesn't fall in, either: it stays at the event horizon. This does NOT mean it's frozen in time, BTW. And it does NOT lose all its energy at creation: such a photon can be absorbed by another object passing through the event horizon at a later time, and the energy measured by that falling observer will NOT be zero. And for a photon emitted from inside the event horizon, the problem isn't that it would lose all its energy, it's that all directions now point towards the singularity.

neat little bit of trivia: there's a distance outside the event horizon at which light can make circular orbits around a black hole.
 
Oualawouzou said:
Hello everybody,

I know all sorts of weird things happen when you deal with extreme situations such as what goes on beyond the event horizon of a black hole, and it's ever harder as a layman to wrap my head around this phenomenon, so I hope I'll make sense and not annoy our resident physicists... :)

So, I was wondering the other day if it is at all possible to send information from inside the event horizon to outside it. I figured that since even light cannot escape the black hole, communicating through any kind of light or radio wave is out of the question. But what about "physical vibrations", so to speak? Would two cans connected by a string be able to relay information through the event horizon? I figured not, but I'm at a loss to explain why. The only explanation I can come up with is that it'd be too simple. :P

Thanks!
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It is theorized that the gravity in a 'black hole' is so intense that the escape velocity required to escape the gravitational field would have to exceed the speed of light.
A. Einstein determined that the maximum speed limit in the universe is light speed, so, it seems there is no escape from a black hole. S. Hawking theorizes that there is a trickle of energy eminating from a black hole, but that energy is negligible.
How so ever! There is hope! The gravitational field surrounding a black hole is so intense that it compresses (warps) the space around it so powerfully that it is unlikely that an object that seems to be falling into a black hole would, simply, never get 'there'.
I am reminded of Columbus' crew fearing falling off the edge of the earth.
'Black holes,' seem to be the contemporary 'falling off the edge' fear projected into space.
 
JohnChasWebb said:
It is theorized that the gravity in a 'black hole' is so intense that the escape velocity required to escape the gravitational field would have to exceed the speed of light.
A. Einstein determined that the maximum speed limit in the universe is light speed, so, it seems there is no escape from a black hole. S. Hawking theorizes that there is a trickle of energy eminating from a black hole, but that energy is negligible.
How so ever! There is hope! The gravitational field surrounding a black hole is so intense that it compresses (warps) the space around it so powerfully that it is unlikely that an object that seems to be falling into a black hole would, simply, never get 'there'.
I am reminded of Columbus' crew fearing falling off the edge of the earth.
'Black holes,' seem to be the contemporary 'falling off the edge' fear projected into space.
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Sorry, but I must nitpick. It's in my blood.

Columbus' crew knew the Earth was round. They were just afraid Columbus had badly underestimated how big it was, and they would run out of supplies before making it to the Indies.

It turns out they were right, and Columbus was wrong. Fortunately, there was another continent in the way.
 
There seems to be a lot of confusion as to escape from the event horizon. Just realize this, from the moment you cross the event horizon, it is moving away from you at c. The event horizon will always be moving away from you at c. I've never heard it stated this way, so I may be incorrect, but it is the only conclusion I can come to.
 
JohnChasWebb said:
The gravitational field surrounding a black hole is so intense that it compresses (warps) the space around it so powerfully that it is unlikely that an object that seems to be falling into a black hole would, simply, never get 'there'.
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How so? It seems to me that the accretion disks and jets typical of black holes are radiating away more energy and angular momentum than a disk around a star. That would send things in faster. What would act to keep things from "getting there"?
 
RecoveringYuppy said:
How so? It seems to me that the accretion disks and jets typical of black holes are radiating away more energy and angular momentum than a disk around a star. That would send things in faster. What would act to keep things from "getting there"?
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If you use the Scharzchild metric, the object will only cross the event horizon at t=infinity. But the Schwarzchild metric is actually a poor choice for examining the problem. Its only advantage is that it maps easily onto the Minkowski metric in the large-R limit. But the event horizon in the Schwarzchild metric is a coordinate singularity, and so the fact that something funky is happening to the time coordinate for a falling object passing through the event horizon doesn't mean much. Pick some other coordinate system, like Kruskal coordinates, where the event horizon isn't a coordinate singularity, and nothing like that happens. Objects will indeed cross the event horizon. Now, you still have the whole issue of light emitted by the object as it falls taking longer and longer to escape as that object approaches the black hole, but that's not the same issue.
 
Ziggurat said:
neat little bit of trivia: there's a distance outside the event horizon at which light can make circular orbits around a black hole.
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Is this anything like the rings around Uranus?

Anyway, this is a great thread, yay to everyone in it. :)

The question I have is for that ring you posit, Zigg: does it or would it change shape, or dimension, due to the gravitational forces of other masses/bodies than the black hole as the black hole moves through space?

DR
 
Darth Rotor said:
The question I have is for that ring you posit, Zigg: does it or would it change shape, or dimension, due to the gravitational forces of other masses/bodies than the black hole as the black hole moves through space?
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Unlike for massive objects, this circular orbit isn't stable: it doesn't turn into an eliptical orbit if you shoot your photon at a slightly off angle. So my guess is that any perturbation away from azimuthal symmetry for the gravitational field would probably ruin the orbit.

If you put a big telescope at that point, though, you might be able to see the back of your own head. Which would be kind of neat.
 
Ziggurat said:
If you use the Scharzchild metric, the object will only cross the event horizon at t=infinity. But the Schwarzchild metric is actually a poor choice for examining the problem. Its only advantage is that it maps easily onto the Minkowski metric in the large-R limit. But the event horizon in the Schwarzchild metric is a coordinate singularity, and so the fact that something funky is happening to the time coordinate for a falling object passing through the event horizon doesn't mean much. Pick some other coordinate system, like Kruskal coordinates, where the event horizon isn't a coordinate singularity, and nothing like that happens. Objects will indeed cross the event horizon. Now, you still have the whole issue of light emitted by the object as it falls taking longer and longer to escape as that object approaches the black hole, but that's not the same issue.
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Isn't it a more important issue, though?

What's the difference, to someone outside the black hole, between saying "a falling object never crosses the event horizon" and saying "the object does cross the event horizon, but its crossing of it can't ever affect me in any way"?
 
Ben Tilly said:
Actually you do have a black hole available - we're in one!

I'm somewhat serious. The estimated mass of the visible universe is the same as the mass of a black hole the size of the visible universe!

Good luck with your experiments. :D

Cheers,
Ben
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Would anybody address Taffer's assertion? LLanes?Anyone?

How could we tell if it were true?:confused:
 
FWIW I think the available evidence is that we are in a black hole that is radiating away energy in to a larger cosmos. My opinion is based on the observation that an accleration of the expansion is evident.
 
69dodge said:
Isn't it a more important issue, though?

What's the difference, to someone outside the black hole, between saying "a falling object never crosses the event horizon" and saying "the object does cross the event horizon, but its crossing of it can't ever affect me in any way"?
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One thing is whether the falling object crosses the event horizon from its own perspective, and the answer is yes, with ease (that is, it can cross the place where it used to be; by the time it gets there, it will find that the horizon has moved). Another thing is whether the falling object crosses the event horizon from the perspective of an outside observer, and the answer is no, due to gravitational time dilation. The latter prevents communication with the outside observer - to someone outside, anything that the infalling object experiences or does "inside" the horizon has not happened yet (nor will in a million years), as its time grinds to a halt before it even gets there. Whether the object crosses the event horizon or not "absolutely" is a meaningless question, as the event horizon itself is a product of the choice of the reference frame. One always needs to ask, "relative to what". In any reference frame used to define an event horizon, no object can cross it.
 
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Thabiguy said:
In any reference frame used to define an event horizon, no object can cross it.
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Actually, scratch that sentence, that's not true. But the location of the event horizon does depend on the choice of the reference frame, and from the perspective of an outside stationary observer, falling objects will not cross the event horizon of a black hole due to gravitational time dilation.
 
From reading this thread, I've come to the conclusion that I know less than I thought I did about black holes. Can anybody reccomend a decent book on the subject?
 
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