# Black holes, can we see them directly?

1. Jan 9, 2007

### Chaos' lil bro Order

I was wondering if its possible in theory to see a black hole without inferring them from the movements of nearby celestial bodies. In the accretion disk of a black hole, further out from the singularity than the event horizon, could we see matter being sucked in? I could be way off base, but wouldn't the matter in the accretion disk undergo great friction and thus give off light? Is it possible for some of this light to then escape the accretion disk and fly outwards and onwards towards Earth where we could observe it?

2. Jan 9, 2007

### Kurdt

Staff Emeritus
Matter in the accretion disks of balck holes does heat up due to friction and releases X-rays and is how we detect black hole candidates such as cygnus X-1. I did a quick search for Cygnus X-1 and found this link you might like.

http://imagine.gsfc.nasa.gov/docs/science/know_l2/black_holes.html

If you search for yourself you will find a wealth of information on black holes connected with Cygnus X-1. This is however still not a direct sighting it is still passive. It is impossible to see one directly as it reflects no light. It does however act as a massive gravitational lens if one has no accretion disk and could be detected by seeing the warping of the background field of stars.

3. Jan 9, 2007

### Chaos' lil bro Order

Excellent. A very fast and perfectly on topic reply Kurdt, many thanks.

You jogged my memory and I recall that Hawkings theorizes that BH's may also emit energy in the form of orphaned anti-particles. For example, a particle pair like a positron/electron pair that is produced on the Event Horizon of a BH, could be split in two, as one gets pulled into the BH and the other narrowly escapes. The escaped 'orphaned' particle is still apart of our visible universe, while the anti-particle becomes part of the BH. What I was never clear on is how this occurs. I mean, does it occur because the BH is expanding (for some reason or the other) and consequently its EH grows and grows outwards, thus consuming and orphaning more and more particle pairs as it grows outwards? Hmmm.

4. Jan 9, 2007

### Kurdt

Staff Emeritus
The basic jist of hawking radiation is that when pair production occurs the two particles usually have some sort of kinetic energy to preserve energy conservation. When a pair of particles is produced near the event horizon one of them has velocity directed toward the black hole and disappears in the event horizon while the other has a velocity directed away and escapes to freedom.

I'm not an expert on Hawking Radiation and really only remember what I was told in undergraduate classes vaguely so perhaps somebody else may wish to contribute.

5. Jan 10, 2007

### Chris Hillman

A bit about strong lensing effects

Looks like you already got some responses dealing with accretion disks, but you might also be interested to know that in principle, if you were near an isolated black hole which lacked an accretion disk, you could literally "see" a dark disk against the background of distant stars and so on. This disk doesn't quite represent the event horizon, though; rather, it is associated with the unstable circular null geodesic trajectories at $r=3m$.

In more detail: consider a static observer who uses his rocket engine to "hover" over a Schwarzschild hole. How does the gravitational field alter the appearance of the "night sky"? Rather drastically! Suppose the hole is in the direction of the "North pole" on the celestial sphere of our observer. Then in the Southern half, he sees primary images of all the stars, then a narrower "anulus" of secondary images, then a much narrower annulus of tertiary images, ... forming a "bright ring" around a dark disk. If stuff is falling the hole (even on the "opposite side" from his location), he can see (multiple images of) these objects as they appear to cross into the dark disk and quite suddenly redshift out of visibility.

The appearance of a Kerr hole is similar, except that the dark disk is not circular but appears a bit squashed on one side. See for example Chandrasekhar, Mathematical Theory of Black Holes for a very detailed discussion.

There is a proposal to try to actually image the "dark disk" for the supermassive black hole (seen against background stars) at the center of the Milky Way.

6. Jan 10, 2007

### Chaos' lil bro Order

Fantastic Chris, ty.

If this proposal takes off and yields a picture, how fascination would that be huh. It would be on the cover of every magazine and maybe Nobel worthy.

7. Jan 10, 2007

### Chris Hillman

I can't seem to find the paper I had in mind right now, but try this http://www.arxiv.org/abs/astro-ph/0505286

Cheers!