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michl
2009-Nov-08, 12:23 PM
Hi,

first i'd like to say, that i really like your show!

Now recently, there was this show on bbc about black holes. I think it was called "Who is afraid of a black hole". There was this one scientist, who is about to measure the shadow of a black hole. Now i've never heard about that, and they didn't explain (at least i didn't understand), so i was wondering if somebody could shed some light on this...

THX

ps.: btw, where did all those questions-shows go? Did they evaporate?

Arnold Layne
2009-Nov-08, 02:13 PM
Hi,

first i'd like to say, that i really like your show!

Now recently, there was this show on bbc about black holes. I think it was called "Who is afraid of a black hole". There was this one scientist, who is about to measure the shadow of a black hole. Now i've never heard about that, and they didn't explain (at least i didn't understand), so i was wondering if somebody could shed some light on this...

THX

ps.: btw, where did all those questions-shows go? Did they evaporate?

Well, I didn't see the show, but my first thought is, it would refer to the black hole blocking light from objects behind it. But black holes are not really big, and I don't know of any that are close by, so their "shadows" (if my interpretation is the right one) ought to be tiny.

antoniseb
2009-Nov-08, 02:44 PM
Well, I didn't see the show, ....

I didn't see it either. Sometimes the word shadow is used a bit allegorically. If a black hole were to pass in front of a light source (a star for example), the light as we see it would increase while the black hole was between us and the light source due to gravitational lensing. This is an observed phenomenon, not just a guess.

But perhaps the answer was using the term shadow to refer to observable effects of the black hole that aren't the black hole itself, such as the proper motions of the stars near it, or the gravitational lensing, or even things related to its surrounding magnetic field.

agingjb
2009-Nov-08, 03:05 PM
I think the program was talking about the rather small scale lensing around a galactic black hole. The idea being that such lensing might turn out to be measurably different from that predicted by theory.

The technique was, apparently, to combine observations from many widely separated telescopes.

Irrelevantly, I've often wondered just how a very condensed object was predicted to behave under Newtonian gravity.

grant hutchison
2009-Nov-08, 03:17 PM
I wonder if this was a reference to Doeleman et al.'s plan to image the supermassive black hole at the galaxy's centre using very long baseline interferometry: Imaging an Event Horizon: submm-VLBI of a Super Massive Black Hole (http://arxiv.org/abs/0906.3899).
The resulting images (which are simulated in the above "white paper") will give a view of the accretion disc surround the black hole, strongly distorted by the gravitational bending of light, and featuring a central dark defect caused by the existence of the event horizon.
Personally I'd refer to the central dark spot as a "silhouette" rather than a "shadow": but it's certainly a dark region caused by the obscuration of a more distant source of radiation, and it will tell us interesting things about our understanding of general relativity as applied to supermassive black holes.

Grant Hutchison

slang
2009-Nov-08, 08:02 PM
I wonder if this was a reference to Doeleman et al.'s plan to image the supermassive black hole at the galaxy's centre using very long baseline interferometry: Imaging an Event Horizon: submm-VLBI of a Super Massive Black Hole (http://arxiv.org/abs/0906.3899).

Highly likely, if michl is referring to the recent Horizon episode (http://www.bautforum.com/small-media-large/95997-bbc-horizon-black-holes.html). The only reason I recognize the name Doeleman. :)

Cougar
2009-Nov-08, 10:52 PM
Hey michl, welcome to the board.


There was this one scientist, who is about to measure the shadow of a black hole. Now i've never heard about that, and they didn't explain (at least i didn't understand), so i was wondering if somebody could shed some light on this...

No, michl, shedding light is difficult when discussing black holes. :)

Seriously, though, I'm not real familiar with the term black hole "shadow" either, but it's apparently just the black hole itself. It's certainly a prominent term on google.


As gas is pulled into a black hole by its strong gravitational force, the gas heats up and radiates. That radiation can be used to illuminate the black hole and paint its profile. - article (http://www.cfa.harvard.edu/news/2005/pr200532.html)

As Grant has said, I imagine the 'shadow' term is somewhat specific to the field utilizing telescopic technology like the Very Long Baseline Array (http://en.wikipedia.org/wiki/Very_Long_Baseline_Array), a cross-continental array of submillimeter telescopes that can really pinpoint where that radiating gas is, and hence get a good fix on the location of the black hole's event horizon.

More recent article. (http://www.astroengine.com/?p=5754)

mugaliens
2009-Nov-09, 04:13 AM
Hi,

first i'd like to say, that i really like your show!

Thanks! To quote Fazor (http://www.bautforum.com/members/fazor.html), I'm here every Tuesday! (ba-dumpshh...)

Oh! Are you talking about Fraser's (http://www.bautforum.com/members/fraser.html) Astronomy Cast (http://www.astronomycast.com/)?

Yes - that is a wonderful show - I'm glad he and Dr. Pamela L. Gay produce it.


Now recently, there was this show on bbc about black holes. I think it was called "Who is afraid of a black hole". There was this one scientist, who is about to measure the shadow of a black hole. Now i've never heard about that, and they didn't explain (at least i didn't understand), so i was wondering if somebody could shed some light on this...

Ba-dumpshh... Nice pun!




THX

Back to all seriousness...

Ray-tracing techniques can easily be adapted to depict a black hole's shadow. And they show...

...very little.

There is an effect, just not an outline. What's seen is an overall reduction in the amount of light hitting the surface on which the shadow is being projected. It's fairly well scattered, however, but the amount of reduction is a function of the distance from the center of the line described between the illuminator, the black-hole, and the body on which the shadow is falling, with that centerpoint involving the most reduction.

astromark
2009-Nov-09, 06:20 AM
The term used, 'Shadow of a black hole' Is just wrong... Silhouette, or eclipse. What would you see ? If a distant star or Galaxy was occulted by a Black Hole ? Not a shadow at all unless you think that the occulted objects light that is now not reaching you because of this BH passing in front of it is a shadow... The fact is the opposing might actually happen. That distant light might now be magnified by the gravity well of the BH. Micro lensing.

michl
2009-Nov-09, 10:10 AM
I wonder if this was a reference to Doeleman et al.'s plan to image the supermassive black hole at the galaxy's centre using very long baseline interferometry: Imaging an Event Horizon: submm-VLBI of a Super Massive Black Hole (http://arxiv.org/abs/0906.3899).
The resulting images (which are simulated in the above "white paper") will give a view of the accretion disc surround the black hole, strongly distorted by the gravitational bending of light, and featuring a central dark defect caused by the existence of the event horizon.
Personally I'd refer to the central dark spot as a "silhouette" rather than a "shadow": but it's certainly a dark region caused by the obscuration of a more distant source of radiation, and it will tell us interesting things about our understanding of general relativity as applied to supermassive black holes.

Grant Hutchison

oh, Yes! That's it. Thanks

grant hutchison
2009-Nov-09, 06:51 PM
oh, Yes! That's it. ThanksPleasure. :)

Grant Hutchison