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Post by petergriffin on Jan 27, 2011 13:27:00 GMT 1
I have been wondering about this for sometime, and with my limited physics can't resolve in my mind would happen.
Given a black hole of large mass, 1000+ solar mass, at the event horizion the tidal gravition effect would be very mininal, thus if you hovered just out side the event horizion and lowered down a video camera on a cable attached to a monitor, would you
a) see anything as camera crossed the event horizon?
b) could you move away from event horizion and take camera with you?
c) or to escape would you have to cut the cable?
I am not sure if these can be answered but I am sure there more informed out there who know the answer,
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Post by speakertoanimals on Jan 27, 2011 13:52:10 GMT 1
Okay, first point to note is that to hover just above the event horizon would take a heck of a rocket!
To try and hover AT the event horizon would need an infinite acceleration.
SO, think about the piece of line that you were lowering down. The piece that appraoched the event horizon would need the tension in the cable to increase and increase (the equivalent of an infinitely powerful rocket), in order to support it in position. Hence, since special relativity prohibits infinitely-strong or totally rigid materials, you can predict that the tension in the cable would increase without limit as the camera approached the horizon -- hence it would snap before you ever got the camera to dangle through the event horizon.
Even if you could, you could never see a picture because any electrical signal is limited by lightspeed, and just as light can't get out, the electrical signal never could either.
Mind you, if you could dangle it, you'd have invented an infinitely strong material anyway, hence you could get vibration signals to travel down that at greater than lightspeed (which is why special relativity prohibits such materials!)........................
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Post by speakertoanimals on Jan 27, 2011 13:56:35 GMT 1
Oh, and the point about the tidal effect is slightly misleading.
If I were a point person free-falling into a black hole I feel nothing.
If I am not a point, then even though I freefall, my feet are slightly closer, hence experience a greater gravitational pull than my head -- I then fall at average acceleration, and my feet get pulled down, and my head thinks it is being pulled down too fast, hence I feel like I am being stretched.
But we weren't talking about free-falling, but trying to hover, when the WHOLE of me needs an ever-increasing shove from a rocket not to fall in, which goes to infinity whether I'm a point person or not as I approach the event horizon and try to hover.
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Post by eamonnshute on Jan 27, 2011 14:19:11 GMT 1
Please Miss!
What if you were in orbit, just outside the event horizon? Then you would not need a rocket to avoid falling in.
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Post by speakertoanimals on Jan 27, 2011 14:42:54 GMT 1
Please Miss! What if you were in orbit, just outside the event horizon? Then you would not need a rocket to avoid falling in. Except freefall circular orbits only exist at 1.5 times Schwarzschild radius (event horizon radius is 1 Schwarzschild radius), and for the closest-in one, the object has to be orbiting at light speed. you could orbit at less than 1.5, but ONLY by constantlt firing your rockets. Hence again, you can't use orbiting to keep you just above the event horizon. |
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Post by carnyx on Jan 27, 2011 17:14:32 GMT 1
A few more questions on black holes ...
Does Hawking radiation imply that black holes are visible?
And what about the stuff that falls in; won't it release huge amounts of radiation as it falls?
Could Black holes in fact be bright?
And what about angular momentum? As I understand it, a neutron star on its way to collapsing down to being a black hole, must spin faster and faster as its radius gets smaller. Surely it must pass through the stage where light could not escape from the polar regions, but could escape from the equatorial regions where the net attractive force is less? In that case we should be able to see them ...
Re the Swartschild radius; I understand this is tiny, so are black holes very small?
And is there direct and conclusive evidence that they actually exist, or are they still conjectural; and 'proof' is still in the mathematical domain?
Personally, I remain to be convinced that they are more than artefacts; fantasias if you like; creatures of the human imagination.
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Post by speakertoanimals on Jan 27, 2011 17:36:19 GMT 1
Well, you don't know that much, but you already seem convinced they don't exist!
No, because the larger the hole, the SMALLER the black-body temperature. Hence only REALLY small black holes would be hot enough to see, or even hotter than the CMB background.
That's the accretion disk, which is what we detect. But we can't SEE the dense object at the centre of the accretion disk, be it a neutron star or a black hole. The hole isn't bright, the disk around it is.
Black holes spin as well, and the effects of spin are included in more complicated models of rotating black holes. But the simplest case is a non-rotating balck hole. You can do exactly same computations for rotating ones, just much harder to describe the results in words. And since the thing that most people are attracted by is the event horizon and light not being able to escape, that concept is easiest-explained for the non-rotating case.
Black hole event horizons are small, that's the point! But the stuff around the black hole, like the accretion disk, need not be either small or dark. In fact, totally the opposite!
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Post by carnyx on Jan 27, 2011 18:21:47 GMT 1
So, black holes could be found at the centre of swirling bright matter?
If that includes spiral galaxies, then they must form remarkably stable systems, judging by the Hubble photos of very distant spiral galaxies.
Anyway, I cannot get my head around how exactly you could get to the event horizon. Seems to me that if you had any kind of trajectory other than dead toward the middle of the black hole, angular momentum would cause you to take up an orbit of some kind ..( even a hyperbolic one ) . And as the event horizon sphere is so small, then surely your space-ship would have to have very powerful engines to decelerate down to an orbit of this radius? In other words, once in an orbit, you are pretty-well stuck there, like a particle in Saturn's rings.
Is this why black holes and their associated accretion discs are so stable .. because it is nigh-impossible to 'fall in' to one?
It would be interesting to look at the orbital mechanics cases, to find the various permissible 'windows'. Who knows, it might actually be impossible to get to the event horizon .. which would at least limit the size of 'black holes' if nothing else can fall in.
( A potential Nobel nomination? Go for it , STA!)
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Post by speakertoanimals on Jan 27, 2011 18:32:52 GMT 1
Uh, galaxies are not accretion disks, accretion disks are not stable, in that matter is continuallt falling in, and being replaced.
AND we have supermassive black holes thought to exist at the centre of galaxies anyway.
accretion disks ARE matter in orbit, just that turbulence, friction etc causes loss of angular momentum, which is why matter ends up falling in to the black hole. But heating before it does so is so energetic that it radiates x-rays.
No more mysterious than the fact that a cloud of gas and dust collapsed to form the solar system, but you are right the matter has to shed angular momentum to collapse.
As regards stability (if a thing that continually feeds matter into a black hole can be called stable!) -- if you have a companion star, then that is a source of gas which joins the accretion disk around the black hole.
Google on accretion disks, jets, black holes, quasars, any of that.
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