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Author Topic: The Nature Of Viewing Light At An Event Horizon ?  (Read 8714 times)

Offline neilep

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Dear Ian (et al)

Here's some artists conception of an event horizon..





Have I understood this bit correctly because it forms the basis of my following questions.

The event horizon is the part of the black hole that is at the cusp between return and no return !......a specator watching someone approach the event horizon will never see them actually enter the blak hole because the light emitted from them is caught. They will in fact witness what appears to be the slowing down and eventual stopping yes ?

What happens when another object also enters the black hole at the exact same position ?...does their image join/merge overlap with the other ?.......does this mean that a collection of light builds up as a circle around the black hole ?....does this light become brighter and brighter as more light gets caught there ?

THANK YOU


Neil
« Last Edit: 18/03/2008 18:33:44 by neilep »


 

Offline DoctorBeaver

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The Nature Of Viewing Light At An Event Horizon ?
« Reply #1 on: 18/03/2008 19:11:49 »
Good question; and I haven't a clue what the answer is  ???
 

Offline neilep

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The Nature Of Viewing Light At An Event Horizon ?
« Reply #2 on: 18/03/2008 19:40:06 »
Good question; and I haven't a clue what the answer is  ???

Gosh !!..i am honoured !..I feel feint....
 

another_someone

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The Nature Of Viewing Light At An Event Horizon ?
« Reply #3 on: 18/03/2008 19:52:46 »
As I understand it, event horizons cannot be absolute, but relative (although I think I may have been down this road before, and Ian said my interpretation was wrong - so not absolutely sure).

Two observers, at different distances from the black hole, I would have thought would see two different event horizons.

As an object approaches the event horizon, yes, they will appear to slow down, but if my understanding is correct, they will only stop at an infinite time in the future.

The light leaving the object approaching the black hole will have two characteristics: firstly, it will be substantially red shifted, so you end up actually seeing radio transmissions rather than light; and secondly, the cone of light that will actually reach you will become ever narrower, so that the last thing you will see is the thing entering the black hole in a direct line between yourself and the black hole, but objects entering at an angle, whose light is coming at you at an acute angle to the event horizon must have that light bent back into the event horizon sooner, so I am not sure how those objects will appear/disappear.
 

Offline DoctorBeaver

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The Nature Of Viewing Light At An Event Horizon ?
« Reply #4 on: 18/03/2008 21:22:49 »
I've been pondering the question and I think this is what would happen.

If a rotating black hole drags spacetime around with it, there could be some interesting results. For instance, you would see the object approaching the event horizon orbit the black hole in the direction spacetime is being dragged.

The object would have an increased red shift as the orbit took it away from you, and increased blue shift after it had passed behind the black hole and re-appeared on the other side moving towards you. The faster spacetime was being dragged, the more shifted the light would be towards both the red & the blue ends of the spectrum.

Of course, the closer to the event horizon the object got, the greater the time dilation you would perceive, which would add an overall red shift to the light. You could get the situation where the object would only be visible either as it approached you or receded from you depending on the total extent of the shifts. Maybe you would only see it when it was in a direct line between you & the black hole as that is where the red & blue shifts due to its orbit would be at their minimum.

You would also see the front of the object as being further from you no matter whether it were travelling away from you in its orbit, or towards you. The longer the object, the greater this effect would be. It would not look as if the object were angled more away from you; each part of it would look the same as normal. In other words, if you were observing the object from an angle of 45o, each part of the object would still look as though you were observing it from that angle, but the front would be further away. The object would look skewed.

But back to the original question about 2 images being superimposed. Time would appear to stop only when the object actually crossed the horizon and, hence, would be out of sight. It would appear to move slower & slower as it approached the event horizon. Were another object approaching the same point on the event horizon, that too would appear to move slower and slower.

Imagine an Earthbound analogy. Two vehicles are travelling along the same road, one some distance behind the other. The rules are that they would apply their brakes at the same point to cause the same rate of deceleration. That rate of deceleration would cause both of them to stop at the same point. However, that point is just over the edge of a cliff. You would see both of them slow down as they approached the cliff edge, but their images would never coincide as their point of convergence is over the edge.

My feeling is that this would be the case no matter how long the objects were as the longer they are, the greater the time dilation difference between the front and the back and, hence, between the back of the first object and the front of the second.

Hmmm, that's a thought - would they appear to shrink in length if the front were subject to greater time dilation than the back?  Then again, the front would be subject to greater gravitational force which would tend to stretch the object. Would the two effects cancel each other out?

Aaarrrgh - my brain hurts again  [xx(]
« Last Edit: 18/03/2008 21:50:07 by DoctorBeaver »
 

Offline neilep

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The Nature Of Viewing Light At An Event Horizon ?
« Reply #5 on: 19/03/2008 11:43:46 »
Gosh !!

THANK YOU, THANK YOU both for these fascinating posts.

I appreciate your thoughts very much.........

*reached for the headache tablets*

Fascinating subject !!
 

Offline turnipsock

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The Nature Of Viewing Light At An Event Horizon ?
« Reply #6 on: 19/03/2008 23:31:29 »
I'm still a wee bit confussed.

Imagine if you will, DoctorBeaver gets sucked into a black hole and I was standing at a safe distance. I would see his horrified face as he gets torn apart by near infinite gravity for ever more. Then, imagine if you will, neilep gets drawn into the same back hole at the same point. Would their two horrified faces get morphed together?

If every such bit of space debrise slowed down on the event horizon the light emitting would become infinte.

I would think that DoctorBeaver could only emit a certain amount of light and would just fade away, followed by neilep.

Anyway, I think we should give it a try to see what happens.
 

Offline Soul Surfer

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The Nature Of Viewing Light At An Event Horizon ?
« Reply #7 on: 19/03/2008 23:44:30 »
I think that a lot of conceptual problems arise because people to not check out the real timescales or physical size of the processes involved.  Also the descriptions used often emphasise certain features for the "gee whizz"  effect.  Watching things fall into black holes is one of them.

Consider a typical stellar mass black hole.  These are a few miles across and the final orbital velocity is pretty close to the velocity of light  this means that the orbital period is in the region 10-100KHZ yes things orbit around the hole around 25thousand times a second or more.  A safe watching distance is probably a few million miles so you would need something about as good as the Hubble telescope to see the event horizon disc let alone watch a specific object say about the size of a person entering it.  However the differential tidal forces would tear any object much bigger that a small dust particle to pieces long before it got to the horizon and the visibility of any object would fade quite quickly even if you could see it.  I am not sure what the fadeout equation is likely to be but its probably likely to be exponential in which case the visibility will fall to half its value after a fixed period and half again after a similar period and so on this never actually reaches zero but after a few time constants its as near zero as makes no difference.  

Watching things fall into blackholes and slowly fade out is entirely hypothetical and completely not worth worrying about.
 

another_someone

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The Nature Of Viewing Light At An Event Horizon ?
« Reply #8 on: 20/03/2008 01:04:02 »
Stellar mass black holes are one thing, but what about the black holes supposedly at the centre of a galaxy?
 

Offline rosalind dna

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The Nature Of Viewing Light At An Event Horizon ?
« Reply #9 on: 20/03/2008 11:42:03 »
Neil, I don't know if this will help but it's interesting and I don't fully understand that except that when a "Black Hole" is formed it's because an old
star implodes because of the gassy matter involved and if anything goes near it then  it's never seen again.

In other words it disappears forever.

http://www.soton.ac.uk/maths/research/applied/subgroups/gr/Res-GR-Maths.html
http://en.wikipedia.org/wiki/Black_hole

I think that the Wiki pictures are scientifically beautiful.

« Last Edit: 20/03/2008 11:45:27 by rosalind dna »
 

Offline neilep

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The Nature Of Viewing Light At An Event Horizon ?
« Reply #10 on: 20/03/2008 15:56:10 »
Thank you all for your continued wonderful responses.

Thank you IAN and Rosalind DNA for the Links.

Thank you DoctorBeaver, George and Turnipsock
 

Offline Gabe2k2

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The Nature Of Viewing Light At An Event Horizon ?
« Reply #11 on: 11/09/2008 21:52:15 »
Event horizons hummm ok Please read the following thread "Is a Black Hole a point "
Now do you understand ?
 

Offline LeeE

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The Nature Of Viewing Light At An Event Horizon ?
« Reply #12 on: 12/09/2008 16:43:40 »
I worked out the orbital velocity at the Schwarzchild radius/Event Horizon to be roughly 2.11x10^8 m/s - about 2/3 'c' and it's irrespective of the BH mass, so although it's relativistic it's quite a long way short of 'c' itself.

This implies that it should be possible to orbit a probe very close to the EH, although I'm not sure if spaghettification would actually be a problem.  If gravity distorts space-time then it seems to me that although anything inside such a zone would appear distorted to a distant observer, it should still be internally consistent within it's space-time frame i.e. the spaghettification wouldn't be apparent to the spaghettified observer (although I'm not going to volunteer to find out).

The time gradient across the probe would be a problem to the instruments aboard though - different sides of the probe would be running at appreciably different time rates.  There might be a problem with getting the data transmitted out to us as well - due to time dilation the frequency, and therefore energy of any data transmitted back to us would be very low, and this I think would also apply to watching something fall in to a BH - as an object moved closer to the EH any radiation reflected/transmitted by it would have lower and lower energy and at the point where it reached the EH (from the distant observers point of view) any energy reflected/radiated would have infinite wavelength, which equals zero energy.
 

Offline Gabe2k2

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The Nature Of Viewing Light At An Event Horizon ?
« Reply #13 on: 12/09/2008 20:37:47 »
If you could work out the gravitational forces being exerted there should be a point at which a probe could be set in an orbital path within the event horizon. As beautiful as it might be looking out, you would see be a brilliant light show of mass being torn to pieces. Looking in well not much really.
« Last Edit: 12/09/2008 20:39:54 by Gabe2k2 »
 

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