<p>It's not the photons caught in the ring that you are seeing in that simulation, but think about all the photons that don't quite get caught by the ring.</p>
<p>They will be significantly diverted around the ring, especially those that come really close to it. This will evidence itself the ring outlined in light, which is what you see. You are not seeing the trapped photons, but those almost trapped.</p>

CC BY-SA 4.0
2024-05-10T12:43:20.507Z
Thank you: if I may ask an additional question (since I am still a bit confused): this seems a bit similar to the Saturn ring: those are particles caught by Saturn's gravity. If the sun reflects off these particles and the reflected sun photons cross my eye retina, I see the Saturn ring. But if those diverted photons circle around along those deflected paths, how do they reach my eye retina for me to see them? I thought the shape is the path of those photons, so they never leave?
Jan Stuller
CC BY-SA 4.0
2024-05-10T12:46:30.257Z
you aren't seeing the trapped photons. This is nothing like Saturn's rings where you see the sun reflected by the ring itself. This is more like the circle of stars you see round any dense stellar body - lensed into a ring
Rory Alsop
CC BY-SA 4.0
2024-05-10T19:41:25.517Z
Photons can't get "caught in a ring". This is the essence of an answer.
ProfRob
CC BY-SA 4.0
2024-05-10T19:59:19.080Z
@ProfRob: could you please elaborate in a full answer? Your insight sounds super interesting!
Jan Stuller
CC BY-SA 4.0
2024-05-10T12:36:00.380Z
2024-05-10T12:36:00.380Z
Rory Alsop
<p>I think you may be confusing two distinct concepts (though I can fully understand why).</p>
<p>Photon <em>rings</em> on a black hole are a visual effect, visible from a distance, caused by inbound photons making (approximately) N+1/2 full circles around the black hole before escaping back the way they came. The ring structure is made up of 'subrings' named for the value of N -- thus, the N=0 ring is light that made a U-turn and came right back, the N=1 ring made one full loop and then escaped, N=2 made two full loops, and so on. This would create what is essentially a retroreflective band around the black hole, called the photon ring.</p>
<p>The photon <em>shell</em> (or photon sphere) is the lowest theoretically possible orbit around the black hole, where the orbital velocity exactly equals the speed of light. In theory, a photon that falls precisely on that line will enter an orbit and remain there eternally. Realistically, the gravity field won't be precisely spherical, so there would be bulges that disrupt the shell and allow the photons to escape it.</p>

CC BY-SA 4.0
2024-05-12T15:32:20.567Z
Those rings are pretty close to the photon sphere, though. Even the 180° U-turn trajectory gets fairly close to the photon sphere, and to the distant observer, its impact parameter is very close to the BH "shadow" at the critical impact parameter (the dotted circle in this diagram of the 180° trajectory) https://i.sstatic.net/LsG0C.png
PM 2Ring
CC BY-SA 4.0
2024-05-12T16:28:59.463Z
+1 See also [...there is no limit on how many "laps" a hyperbolic orbit can make before it returns to infinity.](https://physics.stackexchange.com/a/467204/83380) and references therein.
uhoh
CC BY-SA 4.0
2024-05-12T19:47:52.847Z
Thank you so much for this answer. If the photons do N + 0.5 circles and then escape back the way they came, what would the observes see though? Would he see an image of the original photon-emitting source, or would they see an image of the photons glowing around the black hole as they make N + 0.5 circles?
Jan Stuller
CC BY-SA 4.0
2024-05-13T04:46:14.063Z
@Jan have you seen https://en.wikipedia.org/wiki/File:BlackHole_Lensing.gif & https://en.wikipedia.org/wiki/File:EnsteinRingZoomOptimised.gif from https://en.wikipedia.org/wiki/Einstein_ring ?
PM 2Ring
CC BY-SA 4.0
2024-05-13T12:21:42.097Z
@JanStuller The observer would see a bright ring that varies in brightness depending on how much light is coming from the observer's direction, much like a retroreflective sign on the highway, but like that sign, it's not a mirror. You'd see brightness apparently originating from a ring around the black hole, not an image.
Darth Pseudonym
CC BY-SA 4.0
2024-05-13T12:32:02.807Z
@JanStuller I'm not sure what you mean by 'an image of the photons glowing around the black hole'. Photons don't glow, you can't see them from the side or while they're in transit.  The photons would appear to be emitted by a ring around the black hole, like there's something glowing just outside the event horizon, but that's an illusion because of the warping effect of gravity.
Darth Pseudonym
CC BY-SA 4.0
2024-05-12T15:09:45.650Z
2024-05-12T17:36:02.353Z
2024-05-12T17:36:02.353Z
Darth Pseudonym
<p>Here I delve into the involved mechanics to define what is meant by a photon that's almost trapped.</p>
<p>Imagine a photon approaching a black hole from distant space, but not directly in line with the (apparent) center of the hole so that the photon has some angular momentum relative to the hole. If the photon is too closely aligned (not enough angular momentum), the light gets curved into the event horizon and thence oblivion. With just the right amount of misalignment/angular momentum, the light beam does not get pulled into the hole but is continually curved around it, trapped around the photosphere. This beam does not reach the photosphere in finite time (so it also avoids the event horizon), but it approaches the photosphere asymptotically.</p>
<p>The light you are seeing is from when the incoming beam has just a little more than that minimal misalignment/angular momentum to avoid capture into the hole. Such a beam gets curved around the hole several times, producing rings/subrings, but the hole does not have quite enough pull to trap it forever given the incoming angular momentum. So the beam spirals out again to tell the tale to our eyes/telescopes.</p>

CC BY-SA 4.0
2024-05-14T09:38:22.047Z
To drive-by downvote always sucks. Care to stick around and elaborate?
Oscar Lanzi
CC BY-SA 4.0
2024-05-12T22:46:54.727Z
2024-05-12T22:46:54.727Z
Oscar Lanzi
<p>NASA recently published a video simulation of what it would look like flying into a black hole (video can be seen <a href="https://www.youtube.com/watch?v=dGEIsnBRWGs&amp;t=16s&amp;ab_channel=NASAGoddard" rel="nofollow noreferrer">here</a> ).</p>
<p>What confuses me is that it appears to show the photon ring around the black hole as being visible from a distance. How is that possible?</p>
<p>&quot;Seeing&quot; a star or any visible light from space means that the corresponding star photon has travelled through space and reached the eye retina. However, I thought the photon ring around the black hole consists of photons that have been caught by the massive gravitational pull of the black hole and therefore cannot escape: rather, they keep rotating around the hole. Therefore, they cannot travel &quot;away&quot; and reach the eye retina of a distant observer.</p>
<p>Surely, one would only notice the photon ring when flying <em>through it</em> (i.e. reaching the ring and intersecting it: then the rotating photons could penetrate the eye retina).</p>
<p>Am I wrong in my assumptions?</p>

CC BY-SA 4.0
2024-05-10T12:50:55.827Z
Hi Steve - that question seems to be more articulating that what we see are the bulge stars, including those between us and the black hole. I think this question is more applicable to gravitational lensing.
Rory Alsop
CC BY-SA 4.0
2024-05-10T22:12:56.307Z
I have some photon trajectory diagrams here: https://physics.stackexchange.com/a/810981/123208 that may be helpful. Also see https://physics.stackexchange.com/a/805213 & https://astronomy.stackexchange.com/a/49874/16685
PM 2Ring
CC BY-SA 4.0
2024-05-14T18:08:38.383Z
Much better video here: https://www.youtube.com/watch?v=XgF46YYPplI
IconDaemon
<black-hole>
Black Hole Photon Ring
Jan Stuller
# Black Hole Photon Ring

(Asked by: username_0 on 2024-05-10)

NASA recently published a video simulation of what it would look like flying into a black hole (video can be seen [here](https://www.youtube.com/watch?v=dGEIsnBRWGs&t=16s&ab_channel=NASAGoddard) ).

What confuses me is that it appears to show the photon ring around the black hole as being visible from a distance. How is that possible?

"Seeing" a star or any visible light from space means that the corresponding star photon has travelled through space and reached the eye retina. However, I thought the photon ring around the black hole consists of photons that have been caught by the massive gravitational pull of the black hole and therefore cannot escape: rather, they keep rotating around the hole. Therefore, they cannot travel "away" and reach the eye retina of a distant observer.

Surely, one would only notice the photon ring when flying *through it* (i.e. reaching the ring and intersecting it: then the rotating photons could penetrate the eye retina).

Am I wrong in my assumptions?

**Tags:** black-hole
> **Comments:**
> **username_1**: Hi Steve - that question seems to be more articulating that what we see are the bulge stars, including those between us and the black hole. I think this question is more applicable to gravitational lensing.  
> **username_2**: I have some photon trajectory diagrams here: https://physics.stackexchange.com/a/810981/123208 that may be helpful. Also see https://physics.stackexchange.com/a/805213 & https://astronomy.stackexchange.com/a/49874/16685  
> **username_3**: Much better video here: https://www.youtube.com/watch?v=XgF46YYPplI  

---

## 3 answers

### Answer by username_1 on 2024-05-10. Score: 5

It's not the photons caught in the ring that you are seeing in that simulation, but think about all the photons that don't quite get caught by the ring.

They will be significantly diverted around the ring, especially those that come really close to it. This will evidence itself the ring outlined in light, which is what you see. You are not seeing the trapped photons, but those almost trapped.

> **Comments:**
> **username_0**: Thank you: if I may ask an additional question (since I am still a bit confused): this seems a bit similar to the Saturn ring: those are particles caught by Saturn's gravity. If the sun reflects off these particles and the reflected sun photons cross my eye retina, I see the Saturn ring. But if those diverted photons circle around along those deflected paths, how do they reach my eye retina for me to see them? I thought the shape is the path of those photons, so they never leave?  
> **username_1**: you aren't seeing the trapped photons. This is nothing like Saturn's rings where you see the sun reflected by the ring itself. This is more like the circle of stars you see round any dense stellar body - lensed into a ring  
> **username_4**: Photons can't get "caught in a ring". This is the essence of an answer.  
> **username_0**: @ProfRob: could you please elaborate in a full answer? Your insight sounds super interesting!  

---

### Answer by username_5 on 2024-05-12. Score: 3

I think you may be confusing two distinct concepts (though I can fully understand why).

Photon *rings* on a black hole are a visual effect, visible from a distance, caused by inbound photons making (approximately) N+1/2 full circles around the black hole before escaping back the way they came. The ring structure is made up of 'subrings' named for the value of N -- thus, the N=0 ring is light that made a U-turn and came right back, the N=1 ring made one full loop and then escaped, N=2 made two full loops, and so on. This would create what is essentially a retroreflective band around the black hole, called the photon ring.

The photon *shell* (or photon sphere) is the lowest theoretically possible orbit around the black hole, where the orbital velocity exactly equals the speed of light. In theory, a photon that falls precisely on that line will enter an orbit and remain there eternally. Realistically, the gravity field won't be precisely spherical, so there would be bulges that disrupt the shell and allow the photons to escape it.

> **Comments:**
> **username_2**: Those rings are pretty close to the photon sphere, though. Even the 180° U-turn trajectory gets fairly close to the photon sphere, and to the distant observer, its impact parameter is very close to the BH "shadow" at the critical impact parameter (the dotted circle in this diagram of the 180° trajectory) https://i.sstatic.net/LsG0C.png  
> **username_6**: +1 See also [...there is no limit on how many "laps" a hyperbolic orbit can make before it returns to infinity.](https://physics.stackexchange.com/a/467204/83380) and references therein.  
> **username_0**: Thank you so much for this answer. If the photons do N + 0.5 circles and then escape back the way they came, what would the observes see though? Would he see an image of the original photon-emitting source, or would they see an image of the photons glowing around the black hole as they make N + 0.5 circles?  
> **username_2**: @Jan have you seen https://en.wikipedia.org/wiki/File:BlackHole_Lensing.gif & https://en.wikipedia.org/wiki/File:EnsteinRingZoomOptimised.gif from https://en.wikipedia.org/wiki/Einstein_ring ?  
> **username_5**: @JanStuller The observer would see a bright ring that varies in brightness depending on how much light is coming from the observer's direction, much like a retroreflective sign on the highway, but like that sign, it's not a mirror. You'd see brightness apparently originating from a ring around the black hole, not an image.  
> **username_5**: @JanStuller I'm not sure what you mean by 'an image of the photons glowing around the black hole'. Photons don't glow, you can't see them from the side or while they're in transit.  The photons would appear to be emitted by a ring around the black hole, like there's something glowing just outside the event horizon, but that's an illusion because of the warping effect of gravity.  

---

### Answer by username_7 on 2024-05-12. Score: 0

Here I delve into the involved mechanics to define what is meant by a photon that's almost trapped.

Imagine a photon approaching a black hole from distant space, but not directly in line with the (apparent) center of the hole so that the photon has some angular momentum relative to the hole. If the photon is too closely aligned (not enough angular momentum), the light gets curved into the event horizon and thence oblivion. With just the right amount of misalignment/angular momentum, the light beam does not get pulled into the hole but is continually curved around it, trapped around the photosphere. This beam does not reach the photosphere in finite time (so it also avoids the event horizon), but it approaches the photosphere asymptotically.

The light you are seeing is from when the incoming beam has just a little more than that minimal misalignment/angular momentum to avoid capture into the hole. Such a beam gets curved around the hole several times, producing rings/subrings, but the hole does not have quite enough pull to trap it forever given the incoming angular momentum. So the beam spirals out again to tell the tale to our eyes/telescopes.

> **Comments:**
> **username_7**: To drive-by downvote always sucks. Care to stick around and elaborate?  

---

