<p>There are two "usual" ways to detect a black hole:</p>

<p>If it is part of a binary system (two stars closely rotating around each other) and dragging matter from the other star into an accretion disk, that disk will heat up. Energy distribution will tell us whether the accreting object is indeed a black hole or a neutron star instead. This is primarily visible in X-ray, so not an answer to your question.</p>

<p>If the black hole has a mass of 100.000 to millions of times that of the sun (supermassive black hole), it will sit in the centre of a galaxy, and sometimes accrete matter. This is mostly visible in Radio, and again in X-ray. Still not an answer.</p>

<p>So, how can we detect a black hole in the optical? Best chance is an indirect detection, like in the images you linked above. The one with the distorted stars in the background is not "just fantasy", but a simulation based on our understanding of gravity. The problem when we try to see this is scale, and hence resolution. The event horizon of a stellar mass black hole is on the order of a few kilometers. </p>

<p>Let's replace our sun by a black hole. At a distance of $1.5\cdot10^8$km, with a black hole diameter of 6km, we'd see it occupying an angle of
\begin{equation}
 \alpha = \tan{\left(\frac{6}{1.5\cdot10^8}\right)} = \frac{6}{1.5\cdot10^8} = 2.3^{\circ}\cdot10^{-6} = 8''.3 \cdot 10^{-3}\,.
\end{equation}
So a solar mass black hole at the position of the sun would be 8 milli-arcseconds (mas) in size. A single 8m Telescope at the  <a href="http://www.eso.org/public/teles-instr/paranal/" rel="nofollow">VLT</a> has a resolution of 50mas. If we consider that the gravitational lensing effect takes place on scales larger than the Schwarzschild radius, it could just manage to see this effect. (The theoretical resolution limit for an 8m telescope at 500nm wavelength is $1.22 \lambda / d = 15$mas.)</p>

<p>For real black holes at distances of 10s or 100s of lightyears, you would need a truly gigantic telescope.</p>

<p>Next option, again with gravitational lensing: The <a href="https://en.wikipedia.org/wiki/Gravitational_microlensing" rel="nofollow">microlensing</a> effect causes a background star to be temporarily magnified - it shines brighter for short period of time. A black hole passing in the foreground can cause this, and it is an optical observation, but it is very difficult to know that the lens was indeed a black hole and not some star or other compact object.</p>

<p>Last option, Hawking radiation: Hawking postulated that black holes might emit radiation inversely proportional to their size. For small enough black holes, this is a runaway process - they emit more as they get smaller, and because the emission causes them to shrink, they emit yet more again. Since Hawking radiation has a black body spectrum, the maximum of this emission will lie in the visible range for a brief period of time before going into UV, X-ray, and gamma radiation. The problem here is that no one has observed it yet, and you'd need a fairly small (but not too small) black hole close by. Also, you might get killed as the radiation gets harder.</p>

<p>To your questions:</p>

<ol>
<li>Possible, but probably not. You would need to get very close, but not close enough to get killed by tidal forces. Or you'd need to be able to identify a microlens as a black hole by some means other than lensing.</li>
<li>Either lensing effects (see above), or a brief flash going from red to blue (probably too short to observe).</li>
<li>These black holes would be mostly visible by the absence of other particles. You'd get less elementary particles than you would expect for the reaction,some energy and some momentum would be missing, and there would be characteristic gamma radiation. However, for CERN to announce this, this would have to have happened often enough to not be a statistical fluke (<a href="http://www.physics.cornell.edu/2016/05/11/may-2016-physicists-abuzz-about-possible-new-particle-as-cern-revs-up/" rel="nofollow">this</a> turned out to not be a detection after all.)</li>
</ol>

CC BY-SA 3.0
2016-08-19T11:31:06.967Z
This is great answer to someone like me without much of the astronomy background. Thank you for making it accessible. I still have to ponder a bit on what you wrote. I'm not giving you the mark right now to not rebuke other people from answering, but you definitely deserve it in a few days.
Olivier Grégoire
CC BY-SA 3.0
2016-08-19T11:08:50.790Z
2016-08-22T05:12:32.247Z
2016-08-22T05:12:32.247Z
Alex
<p>Sorry for my dumb terms. I'm interrested in astronomy but I have next to zero background.</p>

<p>Given that a black hole absorbs everything, including light, could we ever discern one with visible light (and live to pass the information on)?</p>

<p><a href="https://astronomy.stackexchange.com/q/1206/13863">This question</a> contains one generated picture of a black hole and Google image shows visual representations that all look similar (black circle in the centre, light that's distorted around it and the further we go away from the center the background is less and less distorted), but are those actually representative? Is there really a black spot in our vision field at the place where the black hole is? Both the answer to the question and <a href="http://www.techinsider.io/black-hole-photograph-event-horizon-telescope-algorithm-2016-6" rel="nofollow noreferrer">this page</a> discards such images as "fantasy", but they don't really answer the questions I'm asking, which are:</p>

<ol>
<li>Will we ever be able to discern a black hole with visible light and live?</li>
<li>If we can, what is a currently acceptable description of what we'll see with visible light? I don't especially expect a picture, so a textual description is also good, but keep in mind it's in regards to visible light. Other questions on Astronomy SE explain what we can "detect" using radio waves or other means of observation.</li>
<li>In 2014-2015 CERN said they might be creating micropscopic short-lived (10<sup>-27</sup> seconds) black holes with the LHC (and this lead to a global nonsensical fear that it'd destroy the planet). Did they create any? If yes, was there any opportunity to picture them despite their tiny size and short life?</li>
</ol>

<black-hole>
Can we discern black holes with visible light?
Olivier Grégoire
# Can we discern black holes with visible light?

(Asked by: username_0 on 2016-08-19)

Sorry for my dumb terms. I'm interrested in astronomy but I have next to zero background.

Given that a black hole absorbs everything, including light, could we ever discern one with visible light (and live to pass the information on)?

[This question](https://astronomy.stackexchange.com/q/1206/13863) contains one generated picture of a black hole and Google image shows visual representations that all look similar (black circle in the centre, light that's distorted around it and the further we go away from the center the background is less and less distorted), but are those actually representative? Is there really a black spot in our vision field at the place where the black hole is? Both the answer to the question and [this page](http://www.techinsider.io/black-hole-photograph-event-horizon-telescope-algorithm-2016-6) discards such images as "fantasy", but they don't really answer the questions I'm asking, which are:

1. Will we ever be able to discern a black hole with visible light and live?
2. If we can, what is a currently acceptable description of what we'll see with visible light? I don't especially expect a picture, so a textual description is also good, but keep in mind it's in regards to visible light. Other questions on Astronomy SE explain what we can "detect" using radio waves or other means of observation.
3. In 2014-2015 CERN said they might be creating micropscopic short-lived (10-27 seconds) black holes with the LHC (and this lead to a global nonsensical fear that it'd destroy the planet). Did they create any? If yes, was there any opportunity to picture them despite their tiny size and short life?

**Tags:** black-hole


---

## 1 answers

### Answer by username_1 on 2016-08-19. Score: 7 | Accepted answer

There are two "usual" ways to detect a black hole:

If it is part of a binary system (two stars closely rotating around each other) and dragging matter from the other star into an accretion disk, that disk will heat up. Energy distribution will tell us whether the accreting object is indeed a black hole or a neutron star instead. This is primarily visible in X-ray, so not an answer to your question.

If the black hole has a mass of 100.000 to millions of times that of the sun (supermassive black hole), it will sit in the centre of a galaxy, and sometimes accrete matter. This is mostly visible in Radio, and again in X-ray. Still not an answer.

So, how can we detect a black hole in the optical? Best chance is an indirect detection, like in the images you linked above. The one with the distorted stars in the background is not "just fantasy", but a simulation based on our understanding of gravity. The problem when we try to see this is scale, and hence resolution. The event horizon of a stellar mass black hole is on the order of a few kilometers.

Let's replace our sun by a black hole. At a distance of $1.5\cdot10^8$km, with a black hole diameter of 6km, we'd see it occupying an angle of
\begin{equation}
\alpha = \tan{\left(\frac{6}{1.5\cdot10^8}\right)} = \frac{6}{1.5\cdot10^8} = 2.3^{\circ}\cdot10^{-6} = 8''.3 \cdot 10^{-3}\,.
\end{equation}
So a solar mass black hole at the position of the sun would be 8 milli-arcseconds (mas) in size. A single 8m Telescope at the [VLT](http://www.eso.org/public/teles-instr/paranal/) has a resolution of 50mas. If we consider that the gravitational lensing effect takes place on scales larger than the Schwarzschild radius, it could just manage to see this effect. (The theoretical resolution limit for an 8m telescope at 500nm wavelength is $1.22 \lambda / d = 15$mas.)

For real black holes at distances of 10s or 100s of lightyears, you would need a truly gigantic telescope.

Next option, again with gravitational lensing: The [microlensing](https://en.wikipedia.org/wiki/Gravitational_microlensing) effect causes a background star to be temporarily magnified - it shines brighter for short period of time. A black hole passing in the foreground can cause this, and it is an optical observation, but it is very difficult to know that the lens was indeed a black hole and not some star or other compact object.

Last option, Hawking radiation: Hawking postulated that black holes might emit radiation inversely proportional to their size. For small enough black holes, this is a runaway process - they emit more as they get smaller, and because the emission causes them to shrink, they emit yet more again. Since Hawking radiation has a black body spectrum, the maximum of this emission will lie in the visible range for a brief period of time before going into UV, X-ray, and gamma radiation. The problem here is that no one has observed it yet, and you'd need a fairly small (but not too small) black hole close by. Also, you might get killed as the radiation gets harder.

To your questions:

1. Possible, but probably not. You would need to get very close, but not close enough to get killed by tidal forces. Or you'd need to be able to identify a microlens as a black hole by some means other than lensing.
2. Either lensing effects (see above), or a brief flash going from red to blue (probably too short to observe).
3. These black holes would be mostly visible by the absence of other particles. You'd get less elementary particles than you would expect for the reaction,some energy and some momentum would be missing, and there would be characteristic gamma radiation. However, for CERN to announce this, this would have to have happened often enough to not be a statistical fluke ([this](http://www.physics.cornell.edu/2016/05/11/may-2016-physicists-abuzz-about-possible-new-particle-as-cern-revs-up/) turned out to not be a detection after all.)

> **Comments:**
> **username_0**: This is great answer to someone like me without much of the astronomy background. Thank you for making it accessible. I still have to ponder a bit on what you wrote. I'm not giving you the mark right now to not rebuke other people from answering, but you definitely deserve it in a few days.  

---

