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View Full Version : If we looked at a black hole, what exactly would we be looking at?



WaxRubiks
2010-Feb-06, 10:33 PM
If you take the EM Hawking radiation emitted from just above the event horizon as the source of visual information about the black hole, what would we actually be looking at, considering the curved path of that radiation?

http://farm5.static.flickr.com/4028/4335232013_3755299c6c_o.jpg
This shows the path of light on its way to an observer, from a neutron star.

http://farm3.static.flickr.com/2699/4335272841_5892fca095_o.jpg

This shows the sort of surface map of what you would see from neutron stars' surfaces.

(both taken from this site http://www.spacetimetravel.org/licht/light.pdf )

But with a black hole, you would have even more curvature of light paths.

I just wonder what exactly we would be looking at..

Would we be looking at some sort of disc, rather than a sphere?

speedfreek
2010-Feb-06, 10:57 PM
The site below has some interesting animations of a journey into a black hole

http://jila.colorado.edu/~ajsh/insidebh/schw.html

grant hutchison
2010-Feb-07, 12:13 AM
We're looking at nothing: that's kind of the point of black holes. :lol:

We get no light from anything inside the photon sphere, and around that dark region there will be distorted views of stars and of any accreting matter surrounding the black hole.

Grant Hutchison

tdvance
2010-Feb-07, 02:02 AM
we'd only see the light coming in actually. Kip Thorne, in Black holes, timewarps: Einstein's Outrageous Legacy says as you fall in, you see black ahead of you (the bending of light makes the "hole" look bigger (angularly) than it really would if the light rays were straight) that expands beyond 180 degrees, so even before you reach the event horizon (assuming a really big hole), it looks like a small circular patch of universe behind you, and nothing but black elsewhere. Worse: as you reach, I think it was either 2x or 1.5x the critical radius, you start seeing multiple copies of stars in that patch to the rear, from the light actually circling the black hole.

The patch will shrink to a point when you hit the singularity.

grant hutchison
2010-Feb-07, 02:37 AM
we'd only see the light coming in actually. Kip Thorne, in Black holes, timewarps: Einstein's Outrageous Legacy says as you fall in, you see black ahead of you (the bending of light makes the "hole" look bigger (angularly) than it really would if the light rays were straight) that expands beyond 180 degrees, so even before you reach the event horizon (assuming a really big hole), it looks like a small circular patch of universe behind you, and nothing but black elsewhere.I think Frog march was asking about the viewpoint of a distant stationary observer.

What you describe above isn't the view of someone falling into the black hole, but of someone hovering above it. For that observer, the view of the Universe closes down to a small patch in the outward direction. But for someone falling inwards from infinity, the black hole occupies a patch of darkness only about 84 degrees across as the observer crosses the event horizon. The edge of the patch is marked by a concentrated ring of light, containing all the starlight from the whole sky.

Grant Hutchison

WaxRubiks
2010-Feb-07, 04:13 AM
We get no light from anything inside the photon sphere, and around that dark region there will be distorted views of stars and of any accreting matter surrounding the black hole.




But what about EM Hawking radiation, given that it is greatly red-shifted?
I thought we would "see" that, as a distant observer, and that that came from just above the event horizon, which is inside the photon sphere.

If that radiation doesn't escape, then how would a black hole evaporate?

eburacum45
2010-Feb-07, 07:47 AM
Hawking radiation from a stellar mass black hole is minimal, equivalent to black body radiation from an object at a tiny fraction of a degree above zero kelvin. So you would have to look very hard to see it.

WaxRubiks
2010-Feb-07, 09:05 AM
I'm not really talking about actually seeing it, only as Hawking radiation as mechanism for recognizing the black hole's shape.
HR as a path for the transmission of information about the black hole.

grant hutchison
2010-Feb-07, 01:55 PM
But what about EM Hawking radiation, given that it is greatly red-shifted?Hawking radiation from a stellar-mass black hole has a characteristic wavelength of several kilometres: the peak thermal radiation always occurs at wavelengths a little larger than the Schwarzschild radius.
When you asked about "visual information", I confess I didn't think you were talking about ELF radio waves.
Yes, there will be a an extremely dim patch of radio emission originating from inside the photon sphere, but since it has a wavelength larger than the size of the black hole, I doubt if it will provide any information about shape.

Grant Hutchison

astromark
2010-Feb-07, 06:34 PM
What you would see...

Is all that gravity bound mater orbiting and spiraling in...Like white noise. Detecting the gravity well, and staying away from it would become imperative.

Jeff Root
2010-Feb-08, 01:33 AM
You would detect a black hole primarily by its effect on passing light. In some
cases -- the only actual cases, so far -- you can detect it by its gravitational
pull on nearby visible matter. Or by its gravitational effect on you, if you get
close enough to it. You might detect the Hawking radiation of a very small
black hole, if you look at the right time, but chances are there are no small
black holes to be seen, anywhere.

-- Jeff, in Minneapolis

astromark
2010-Feb-08, 04:55 AM
You would detect a black hole primarily by its effect on passing light. -- Jeff, in Minneapolis

Like the strength of micro lensing...cos we have not a tool for gravity have we ?

Its a obvious enough question... I feel silly for asking it... :o