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Author Topic: Why doesn't a black hole look like a star?  (Read 2672 times)

Offline Peter Telford

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Why doesn't a black hole look like a star?
« on: 01/10/2008 20:01:20 »
Peter Telford asked the Naked Scientists:

An observer watching an object fall toward a black hole would never see the object cross the event horizon due to time dilation effects. So, why doesn't a black hole look like the star just prior to it shrinking to the Schwartzschild radius?

Obviously it can't because that would mean photons being emitted by something that isn't there in perpetuity but I can't square a "black" hole with the time dilation.

newbielink:http://www.thenakedscientists.com/HTML/podcasts/ [nonactive], thanks.

What do you think?


 

Offline Soul Surfer

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Why doesn't a black hole look like a star?
« Reply #1 on: 01/10/2008 23:51:55 »
This is a very bad case of what I call "gee whizz" science writing and is the sort of thing that confuses people and does a lot of harm to the reputation of science.  Whilst it is technically accurate that because of the time dilation effect there could in theory always be an image of something falling across an event horizon because the exponential decay of the image never actually reaches zero in a mathematical sense,  actually the image would rapidly red shift and fade to become undetectable.  It's a just like saying when I strike a bell the note goes on for ever because it decays exponentially, while we all know very well that it fades to become undetectable in the general noise after a few seconds.  I sincerely wish people would stop using very bad descriptions like this and concentrate on presenting scientific facts cleanly and simply in ways that are understandable in terms of everyday life and stop trying to make them unrealistically wierd or spectacular.  The simple truth is quite adequate.
 

Offline syhprum

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Why doesn't a black hole look like a star?
« Reply #2 on: 02/10/2008 14:03:47 »
There is evidence of the light spreading out from a supernova event illuminating surrounding gas and dust.
See 'New Scientist' 5/5/2007 page 46, the light from the 1987 supernova has been detected in this way and it is hoped that that from earlier events may be detectable.
 

Offline Soul Surfer

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Why doesn't a black hole look like a star?
« Reply #3 on: 02/10/2008 23:18:55 »
This is a very interesting phenomenon which could produce valuable insight into the structure of gases and dust around supernova locations but it has nothing to do with the question.
 

Offline LeeE

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Why doesn't a black hole look like a star?
« Reply #4 on: 03/10/2008 00:35:13 »
The time dilation effects will only be felt, to any significant degree, by the object closely approaching the event horizon - a distant observer won't experience the time dilation and so won't seem to see the object slowing down.  However, if the object were to emit a series of signals back to the observer at regular intervals the time between the signals, as measured by the distant observer, would increase due to the effect of time dilation upon the object.  At the same time, and as Soul Surfer says, the frequency of the signals, as seen by the distant observer, would decrease, reducing their energy, ultimately to zero at the point where the object crosses the EH.
 

Offline Peter Telford

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Why doesn't a black hole look like a star?
« Reply #5 on: 03/10/2008 17:07:42 »
This is a very bad case of what I call "gee whizz" science writing and is the sort of thing that confuses people and does a lot of harm to the reputation of science.
Hi,
Thanks for the reply. Whilst a lot of science is dumbed down I think my particular confusion came from my particularly poor memory :*) Your comments on redshift immediately started to ring bells (no pun intended). I think now the details are more firmly locked into the old grey matter! Thanks to everyone who replied.
PT.
 

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Why doesn't a black hole look like a star?
« Reply #5 on: 03/10/2008 17:07:42 »

 

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