<p>There are no <strong>stable</strong> photon orbits around black holes (<a href="https://astronomy.stackexchange.com/a/29777/7982">see this answer</a>).</p>
<p>Black holes do have an <em>unstable</em> photon orbit though. &quot;Unstable&quot; here means that any tiny deviation from this orbit, will case the photon to be either scattered to infinity or sent into the black hole. It should be emphasized that there such orbits are always circular an can exist at only one radius (1.5 times the Schwarzschild radius for a Schwarzschild black hole). There are no elliptic photon orbits. This orbit is often called the &quot;light ring&quot; (or sometimes &quot;photonsphere&quot;)</p>
<p>As long as we treat the photon as a perfect massless point (test) particle, we can find a highly fine-tuned trajectory that will asymptote to the light-ring. Of course, in reality, a photon always needs to be a wave packet with a finite size, making this fine tuning impossible in practice.</p>

CC BY-SA 4.0
2021-04-08T22:01:39.270Z
Can a photon emitted from a distant location be captured into an orbit if not emitted from a point that intersects with that orbit?
Wesley Adams
CC BY-SA 4.0
2021-04-08T22:12:50.633Z
You don’t have stable photon orbits, so a photon cannot be captured in one.
TimRias
CC BY-SA 4.0
2021-04-08T22:14:01.650Z
But as I said for any unstable orbit you can find a trajectory that approaches it asymptotically.
TimRias
CC BY-SA 4.0
2021-04-10T20:11:29.593Z
Do the photons on such orbits move in the same direction as black hole's spin?
Anixx
CC BY-SA 4.0
2021-04-10T20:14:07.013Z
If a photon shifts inward from such orbit, how many revolution it still can do before being swallowed by the event horizon?
Anixx
CC BY-SA 4.0
2021-04-12T07:12:59.730Z
@Anixx Feel free to ask those questions as separate questions.
TimRias
CC BY-SA 4.0
2021-04-08T15:51:43.647Z
2021-04-08T22:17:26.577Z
2021-04-08T22:17:26.577Z
TimRias
<p>My experience with Kerbal Space Program gives me the impression that it's impossible to transfer from a stable orbit of one body to the stable orbit of another without deceleration. But is this true for a photon? Could light enter a stable orbit of a black hole if fired at the correct angle from a distance?</p>

CC BY-SA 4.0
2021-04-08T03:00:18.760Z
@WesleyAdams Well then how did light enter the photon sphere in the first place? The photons had to be fired into the sphere. Thus answering your question (mostly)
Star Man
CC BY-SA 4.0
2021-04-08T11:07:42.293Z
@WesleyAdams in the mean time, you may enjoy reading answers to [Could a trajectory around a large mass ever deflect by more than 180 degrees due to general relativistic effects?](https://physics.stackexchange.com/q/467144/83380) from what I see there unbound orbits stay unbound and bound orbits stay bound, but those are ballistic trajectories, and you've asked about *photons.*
uhoh
CC BY-SA 4.0
2021-04-08T21:56:24.600Z
@uhoh This information was the basis for my question. Thanks anyway!
Wesley Adams
CC BY-SA 4.0
2022-08-24T18:23:31.407Z
Does this answer your question? [Can light be trapped in orbit?](https://astronomy.stackexchange.com/questions/25552/can-light-be-trapped-in-orbit)
ProfRob
<orbital-mechanics><black-hole><light>
Could photons orbit a black hole if fired at the correct angle?
Wesley Adams
# Could photons orbit a black hole if fired at the correct angle?

(Asked by: username_0 on 2021-04-08)

My experience with Kerbal Space Program gives me the impression that it's impossible to transfer from a stable orbit of one body to the stable orbit of another without deceleration. But is this true for a photon? Could light enter a stable orbit of a black hole if fired at the correct angle from a distance?

**Tags:** orbital-mechanics, black-hole, light

> **Comments:**
> **username_1**: @WesleyAdams Well then how did light enter the photon sphere in the first place? The photons had to be fired into the sphere. Thus answering your question (mostly)  
> **username_2**: @WesleyAdams in the mean time, you may enjoy reading answers to [Could a trajectory around a large mass ever deflect by more than 180 degrees due to general relativistic effects?](https://physics.stackexchange.com/q/467144/83380) from what I see there unbound orbits stay unbound and bound orbits stay bound, but those are ballistic trajectories, and you've asked about *photons.*  
> **username_0**: @username_2 This information was the basis for my question. Thanks anyway!  
> **username_3**: Does this answer your question? [Can light be trapped in orbit?](https://astronomy.stackexchange.com/questions/25552/can-light-be-trapped-in-orbit)  

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## 1 answers

### Answer by username_4 on 2021-04-08. Score: 6

There are no **stable** photon orbits around black holes ([see this answer](https://astronomy.stackexchange.com/a/29777/7982)).

Black holes do have an *unstable* photon orbit though. "Unstable" here means that any tiny deviation from this orbit, will case the photon to be either scattered to infinity or sent into the black hole. It should be emphasized that there such orbits are always circular an can exist at only one radius (1.5 times the Schwarzschild radius for a Schwarzschild black hole). There are no elliptic photon orbits. This orbit is often called the "light ring" (or sometimes "photonsphere")

As long as we treat the photon as a perfect massless point (test) particle, we can find a highly fine-tuned trajectory that will asymptote to the light-ring. Of course, in reality, a photon always needs to be a wave packet with a finite size, making this fine tuning impossible in practice.

> **Comments:**
> **username_0**: Can a photon emitted from a distant location be captured into an orbit if not emitted from a point that intersects with that orbit?  
> **username_4**: You don’t have stable photon orbits, so a photon cannot be captured in one.  
> **username_4**: But as I said for any unstable orbit you can find a trajectory that approaches it asymptotically.  
> **username_5**: Do the photons on such orbits move in the same direction as black hole's spin?  
> **username_5**: If a photon shifts inward from such orbit, how many revolution it still can do before being swallowed by the event horizon?  
> **username_4**: @username_5 Feel free to ask those questions as separate questions.  

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