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Old 03-12-2004, 12:55 PM   #1
Druid Bremen
 
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Black holes

So... What do you think? From an outsider, what does a ship coasting into a black hole look like? Will the ship get mashed up into potatoes at a normal time-frame, or will it coast in, but more and more slowly to an outsider, until it seems like it will never reach the singularity (middle of black hole)? Interesting... Once again, what do you all think?



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Old 03-12-2004, 02:18 PM   #2
Hiroki
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Ah. Now heres something I actually enjoy talking about, space. Well, I'm not really sure how to awnser you there. A black hole pulls in even light, correct? Would it slowly down time aswell? Its a good question, but sadly one I do not really have the awnser too. The result of such an incounter would be the same eather way though - The ship would eventually exist no more in our universe.
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Old 03-12-2004, 03:04 PM   #3
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I believe there is a miniature black hole within my desk's drawers, seeing as how anything I put in there goes missing almost right away.


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Old 03-12-2004, 08:27 PM   #4
MennoniteHobbit
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Quote:
Originally posted by Uber_Saber
I believe there is a miniature black hole within my desk's drawers, seeing as how anything I put in there goes missing almost right away.
lol, i think everyone in one period of time has had that kinds of black hole, losing things everywhere...

I think... that how would a ship get to a black hole in the first place????

and, what about the space-time continuum? neither of those stop, do they?

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Old 03-13-2004, 05:38 AM   #5
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Well, here's a fairly simple guess. A black hole is so powerful that it even sucks up light, right? Well, if the center of gravity is so huge, everything in it gets compressed to the center, and I'm talking near-big bang kind of compression. As for the ships, it fades into visual nothingness once it gets close enough to where the the black hole sucks the light from it.



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Old 03-17-2004, 03:34 AM   #6
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Interesting... I found this website about black holes.. Read on..

My friend Penelope is sitting still at a safe distance, watching me fall into the black hole. What does she see?
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Penelope sees things quite differently from you. As you get closer and closer to the horizon, she sees you move more and more slowly. In fact, no matter how long she waits, she will never quite see you reach the horizon.

In fact, more or less the same thing can be said about the material that formed the black hole in the first place. Suppose that the black hole formed from a collapsing star. As the material that is to form the black hole collapses, Penelope sees it get smaller and smaller, approaching but never quite reaching its Schwarzschild radius. This is why black holes were originally called frozen stars: because they seem to 'freeze' at a size just slightly bigger than the Schwarzschild radius.

Why does she see things this way? The best way to think about it is that it's really just an optical illusion. It doesn't really take an infinite amount of time for the black hole to form, and it doesn't really take an infinite amount of time for you to cross the horizon. (If you don't believe me, just try jumping in! You'll be across the horizon in eight minutes, and crushed to death mere seconds later.) As you get closer and closer to the horizon, the light that you're emitting takes longer and longer to climb back out to reach Penelope. In fact, the radiation you emit right as you cross the horizon will hover right there at the horizon forever and never reach her. You've long since passed through the horizon, but the light signal telling her that won't reach her for an infinitely long time.

There is another way to look at this whole business. In a sense, time really does pass more slowly near the horizon than it does far away. Suppose you take your spaceship and ride down to a point just outside the horizon, and then just hover there for a while (burning enormous amounts of fuel to keep yourself from falling in). Then you fly back out and rejoin Penelope. You will find that she has aged much more than you during the whole process; time passed more slowly for you than it did for her.

So which of these two explanation (the optical-illusion one or the time-slowing-down one) is really right? The answer depends on what system of coordinates you use to describe the black hole. According to the usual system of coordinates, called "Schwarzschild coordinates," you cross the horizon when the time coordinate t is infinity. So in these coordinates it really does take you infinite time to cross the horizon. But the reason for that is that Schwarzschild coordinates provide a highly distorted view of what's going on near the horizon. In fact, right at the horizon the coordinates are infinitely distorted (or, to use the standard terminology, "singular"). If you choose to use coordinates that are not singular near the horizon, then you find that the time when you cross the horizon is indeed finite, but the time when Penelope sees you cross the horizon is infinite. It took the radiation an infinite amount of time to reach her. In fact, though, you're allowed to use either coordinate system, and so both explanations are valid. They're just different ways of saying the same thing.

In practice, you will actually become invisible to Penelope before too much time has passed. For one thing, light is "redshifted" to longer wavelengths as it rises away from the black hole. So if you are emitting visible light at some particular wavelength, Penelope will see light at some longer wavelength. The wavelengths get longer and longer as you get closer and closer to the horizon. Eventually, it won't be visible light at all: it will be infrared radiation, then radio waves. At some point the wavelengths will be so long that she'll be unable to observe them. Furthermore, remember that light is emitted in individual packets called photons. Suppose you are emitting photons as you fall past the horizon. At some point, you will emit your last photon before you cross the horizon. That photon will reach Penelope at some finite time -- typically less than an hour for that million-solar-mass black hole -- and after that she'll never be able to see you again. (After all, none of the photons you emit *after* you cross the horizon will ever get to her.)

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How do black holes evaporate?
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This is a tough one. Back in the 1970's, Stephen Hawking came up with theoretical arguments showing that black holes are not really entirely black: due to quantum-mechanical effects, they emit radiation. The energy that produces the radiation comes from the mass of the black hole. Consequently, the black hole gradually shrinks. It turns out that the rate of radiation increases as the mass decreases, so the black hole continues to radiate more and more intensely and to shrink more and more rapidly until it presumably vanishes entirely.

Actually, nobody is really sure what happens at the last stages of black hole evaporation: some researchers think that a tiny, stable remnant is left behind. Our current theories simply aren't good enough to let us tell for sure one way or the other. As long as I'm disclaiming, let me add that the entire subject of black hole evaporation is extremely speculative. It involves figuring out how to perform quantum-mechanical (or rather quantum-field-theoretic) calculations in curved spacetime, which is a very difficult task, and which gives results that are essentially impossible to test with experiments. Physicists *think* that we have the correct theories to make predictions about black hole evaporation, but without experimental tests it's impossible to be sure.

Now why do black holes evaporate? Here's one way to look at it, which is only moderately inaccurate. (I don't think it's possible to do much better than this, unless you want to spend a few years learning about quantum field theory in curved space.) One of the consequences of the uncertainty principle of quantum mechanics is that it's possible for the law of energy conservation to be violated, but only for very short durations. The Universe is able to produce mass and energy out of nowhere, but only if that mass and energy disappear again very quickly. One particular way in which this strange phenomenon manifests itself goes by the name of vacuum fluctuations. Pairs consisting of a particle and antiparticle can appear out of nowhere, exist for a very short time, and then annihilate each other. Energy conservation is violated when the particles are created, but all of that energy is restored when they annihilate again. As weird as all of this sounds, we have actually confirmed experimentally that these vacuum fluctuations are real.

Now, suppose one of these vacuum fluctuations happens near the horizon of a black hole. It may happen that one of the two particles falls across the horizon, while the other one escapes. The one that escapes carries energy away from the black hole and may be detected by some observer far away. To that observer, it will look like the black hole has just emitted a particle. This process happens repeatedly, and the observer sees a continuous stream of radiation from the black hole.

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Won't the black hole have evaporated out from under me before I reach it?
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We've observed that, from the point of view of your friend Penelope who remains safely outside of the black hole, it takes you an infinite amount of time to cross the horizon. We've also observed that black holes evaporate via Hawking radiation in a finite amount of time. So by the time you reach the horizon, the black hole will be gone, right?

Wrong. When we said that Penelope would see it take forever for you to cross the horizon, we were imagining a non-evaporating black hole. If the black hole is evaporating, that changes things. Your friend will see you cross the horizon at the exact same moment she sees the black hole evaporate. Let me try to describe why this is true.

Remember what we said before: Penelope is the victim of an optical illusion. The light that you emit when you're very near the horizon (but still on the outside) takes a very long time to climb out and reach her. If the black hole lasts forever, then the light may take arbitrarily long to get out, and that's why she doesn't see you cross the horizon for a very long (even an infinite) time. But once the black hole has evaporated, there's nothing to stop the light that carries the news that you're about to cross the horizon from reaching her. In fact, it reaches her at the same moment as that last burst of Hawking radiation. Of course, none of that will matter to you: you've long since crossed the horizon and been crushed at the singularity. Sorry about that, but you should have thought about it before you jumped in.

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Here's the original website: http://cosmology.berkeley.edu/Education/BHfaq.html#q1

Also, I enjoy talking about space. That's why I started this thread. :P



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Old 03-17-2004, 07:09 AM   #7
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I found that one aswell, very ussefull for my A level project on black holes. dunno why I chose black holes, though it was interesting it was very difficult to understand some of the stuff I found. The realy strange stuff is the parrallal universe theory suroundind black holes. it makes the evaporating black holes and falling into them seem very basic.






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Old 03-17-2004, 11:21 AM   #8
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Yeah thats interesting.

*off-topic* A-levels? Where do you live jon?



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Old 03-17-2004, 01:24 PM   #9
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I'm from Sunny England. A levels are taken at 18 before university if you want to know.






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Old 03-18-2004, 02:19 AM   #10
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Oh I see.. I had thought you were from my country. Sorry..



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Old 03-18-2004, 07:27 AM   #11
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"Black holes are the toxic waste of cosmology. Theories, let alone children, are well advised not to touch anything of infinite density."

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dunno why I chose black holes
I think you've stepped in it, mate. You've entered the crossfire zone between quantum mechanics, general relativity, and thermodynamics. Complicated, to say the least.

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Old 03-18-2004, 08:36 AM   #12
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You've crossed the event horizon, and now, are slowly meeting the singularity.



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