<p>There isn't really a minimum size. All black holes have an event horizon from which nothing, not even light can escape. The (Schwarzschild) radius of this event horizon is 3km for a black hole of a solar mass and scales linearly with mass. There is a region just outside the event horizon called the <a href="https://en.m.wikipedia.org/wiki/Photon_sphere" rel="noreferrer">photon sphere</a>, within which light can briefly orbit the black hole, but not in a stable fashion. It will either fly off in some other direction or spiral into the black hole.</p>

<p>You can check out <a href="http://physics.stackexchange.com/questions/52315/what-is-the-capture-cross-section-of-a-black-hole-region-for-ultra-relativistic">this Physics SE</a> question to see a derivation of the impact parameter (the effective radius at which light will be absorbed) and you will see that it is a small numerical factor ($\sqrt{27/4}$ for a non-rotating black hole) times the Schwarzschild radius (the event horizon of a non-rotating black hole), which is indeed proportional to the black hole mass.</p>

CC BY-SA 4.0
2019-12-08T01:51:13.980Z
What happens if the impact parameter is less than the wavelength of the photon?
Steve Linton
CC BY-SA 3.0
2018-01-21T22:54:43.853Z
2018-01-21T23:00:55.333Z
2018-01-21T23:00:55.333Z
ProfRob
<p>If a photon is moving past a black-hole, what is the minimum size of the black-hole that will capture the photon and prevent the photon from going onto orbit around the mass? Do black-holes exist that have a stream of photons orbiting them?   </p>

CC BY-SA 3.0
2018-01-21T22:17:33.380Z
Your question is a little bit unclear, I suggest to give it a more descriptive title.
peterh
CC BY-SA 3.0
2018-01-21T23:55:47.707Z
I'm a little weak on the physics, but photons can't orbit black holes in the traditional sense of an orbit, they will either bend but escape or spiral in towards the black hole.   A true orbit of a photon around a black hole is essentially impossible, unless it's nearly perfectly lined up and that kind of perfect-precision is very rare.
userLTK
<black-hole><light><photons>
minimum size for a black hole to capture a photon
tnt-rox
# minimum size for a black hole to capture a photon

(Asked by: username_0 on 2018-01-21)

If a photon is moving past a black-hole, what is the minimum size of the black-hole that will capture the photon and prevent the photon from going onto orbit around the mass? Do black-holes exist that have a stream of photons orbiting them?

**Tags:** black-hole, light, photons
> **Comments:**
> **username_1**: Your question is a little bit unclear, I suggest to give it a more descriptive title.  
> **username_2**: I'm a little weak on the physics, but photons can't orbit black holes in the traditional sense of an orbit, they will either bend but escape or spiral in towards the black hole.   A true orbit of a photon around a black hole is essentially impossible, unless it's nearly perfectly lined up and that kind of perfect-precision is very rare.  

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## 1 answers

### Answer by username_3 on 2018-01-21. Score: 5 | Accepted answer

There isn't really a minimum size. All black holes have an event horizon from which nothing, not even light can escape. The (Schwarzschild) radius of this event horizon is 3km for a black hole of a solar mass and scales linearly with mass. There is a region just outside the event horizon called the [photon sphere](https://en.m.wikipedia.org/wiki/Photon_sphere), within which light can briefly orbit the black hole, but not in a stable fashion. It will either fly off in some other direction or spiral into the black hole.

You can check out [this Physics SE](http://physics.stackexchange.com/questions/52315/what-is-the-capture-cross-section-of-a-black-hole-region-for-ultra-relativistic) question to see a derivation of the impact parameter (the effective radius at which light will be absorbed) and you will see that it is a small numerical factor ($\sqrt{27/4}$ for a non-rotating black hole) times the Schwarzschild radius (the event horizon of a non-rotating black hole), which is indeed proportional to the black hole mass.

> **Comments:**
> **username_4**: What happens if the impact parameter is less than the wavelength of the photon?  

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