# What would a black hole look like?

1. May 25, 2007

### MonstersFromTheId

If you were close enough to see one with the naked eye, what would you see?

Would you see a "black hole" in the center of the accretion disk?

Or would any view of the black hole itself be completely obscured by light emitted by matter near the black hole as it's torn to shreds by tidal forces?

Would you be able to see the jets at the poles with the naked eye?

Would intense gravitational lensing distort or color stars in the background?

Does anyone know of any artistic renderings of what a black hole would look like to the naked eye, that lean more toward scientific accuracy and less toward dramatic Hollywood glitz?

2. May 25, 2007

### Chris Hillman

The view from near a black hole

Ah good, you want astrophysical accuracy--- be careful, since many websites which purport to show visualizations are highly misleading in various ways. Sites from respectable physicists are more likely to be accurate--- or rather, to announce their limiting assumptions up front. I tried to collect some examples here:

http://math.ucr.edu/home/baez/RelWWW/HTML/visual.html

This rather depends upon whether you have in mind a specific astrophysical object such as the supermassive black hole near the center of our galaxy, or an idealized isolated black hole, with only "distant stars" providing background light. Even the simplest model, the Schwarzschild vacuum (nonrotating isolated hole) provides spectacular effects close up (strong field gravitational lensing), which are sufficiently complicated to explain that you should probably begin with that.

You'd see a dark disk against whatever background (I'd suggest not worrying about accretion disks at first). This does not represent the horizon but the locus $r=3m$ sometimes called the "photon sphere", because this is the location of unstable circular photon orbits. Just around this dark disk you'd see amplified and smeared images of background galaxies and stars and around that some concentric rings of n-ary, (n-1)-ary, .. 2-ary images of stars whose primary images are seen somewhere else on your celestial sphere.

As I said, the dark disk is the locus $r=3m$, not $r=2m$. But you can see glowing matter falling into the hole--- even the "back face" since light rays this close to a hole wind around it several times before escaping to infinity (if indeed they do escape). But you'd see this falling matter rapidly redshift out of visibility at it nears the hole.

Light bending is independent of frequency, but light from distant stars would tend to be blueshifted as it falls toward you "from infinity".

At this point, you should ask: how does the picture I sketched change for observers orbiting the hole? For observers who fall into it? The details depend sensitively upon the motion of the observer, but some simple special cases are studied in detail in Andrew Hamilton's website (see the link at the above cited web page).

AFAIK, few serious attempts to render scientifically accurate pictures of realistic black holes have been done; the closest I am aware of is an attempt to visualize how any accretion disk would appear warped and how different parts would appear frequency shifted. But there are so many things to consider (not least: human color vision is quite tricky to model!) that nothing I have seen seems sufficiently careful. But I am suprised that (apparently) noone has commissioned a state-of-the-art IMAX simulation with all the bells and whistles.

3. May 29, 2007

### MonstersFromTheId

Thanks so much Chris!
Some of those applets within the links on the first site you mensioned are really cool.

"But I am suprised that (apparently) no one has commissioned a state-of-the-art IMAX simulation with all the bells and whistles."

Ya know?
And the thing is I have a very hard time figuring out why that's the case.

I can see it in regard to things like asteroid fields in entertainment, at least to an extent. To entertain you need a bit of drama, thus "Star Wars" style asteroid fields so thick with debris it's a hair raising ride racing through one.

But with black holes, (not to mention a whole host of other astronomical features) it seems to me that, reality is likely to provide a whole heck of a lot more in the way of dramatic spectacle than the wildest imaginings of the very best concept artists.

I've got high hopes for THE UNIVERSE which premiers tonight. Guess we'll see if they get most of the details right (in fact I should post a thread to remind people that it's on tonight). But as far as I've been able to find on the show's official website, the only rendering they've got is of a supermassive black hole in the center of a galaxy. Cool, but a bit of a far off shot considering it's done from the point of view of someone well outside the galaxy rendered.

4. May 29, 2007

### chroot

Staff Emeritus
Excellent post, Chris.

I'd actually suspect that a ray-tracing program like Povray could be shoehorned into doing all the complex math for you. After all, the point of a ray-tracer is to literally trace the path of photons leaving a source and arriving at your eye.

A little Googling indicates this has already been done, to various degrees of accuracy. You'd be a much better judge of accuracy than me, Chris.

http://astro.ago.uni-lj.si/bh/pmwiki.php?n=Main.Lensing

http://www.tat.physik.uni-tuebingen.de/~tmueller/visual.html

- Warren

5. May 29, 2007

### cesiumfrog

The tubingen work is pretty good in this field (they've also got the advantage of great resources).

Regards "all the bells and whistles", I don't think jets are well enough understood yet, but maybe matter accretion is something that could be done soon. Hmm...

6. May 30, 2007

### Chris Hillman

Be very careful!

We are talking about ray tracing in curved spacetime, of course. If you try to go through this I think you'll find unexpected issues, particularly if you try to model the visual experience of an observer falling through the horizon. And there are other issues like frequency shifting, not to mention a reasonable simulation of an accretion disk and toroidal cloud dust if you want to include such things.

Thanks Warren--- the first site is new to me. And goody, it is about strong field lensing! (The sites I list at the current version of RelWW are concerned with the much more tractable but still very valuable, nontrivial and interesting weak-field lensing theory.) Check out the movie depicting the view of a rotating accretion disk from a vantage point near a Kerr hole at http://astro.ago.uni-lj.si/bh/pmwiki.php?n=Main.Accretion

It seems Cadez has published with de Felice, which is a good sign that this is based on careful work. (If that's not an oxymoron concerning anything run under Windows, an inherently unsafe/unstable OS--- good grief!) Still, I wouldn't trust anything unless I'd gone over it with a fine toothed comb. My experience is that whenever things are really complex, very very few authors can remember to keep all the relevant issues in play, and thus, unstated assumptions or even outright errors creep in.

Last edited: May 30, 2007
7. May 31, 2007