Talk:Photon sphere

message: If there is a plenitude of black holes in a given volume of space, wouldn't a photon follow an erratic course around them, or between them?
Similarly, why is it assumed that the event horizon of a single black hole will be perfectly circular?  It was my understanding that the event horizon was anything but perfectly circular, but rather vibrating and turbulent due to quantum effects.198.177.27.21 (talk) 07:17, 1 August 2008 (UTC)
thread_title:  Erratic orbits of photons 

message: I believe that if we are going to describe such object as black hole we shall use only GR as mathematical argument for our deductions. Unfortunately there is still "classical gravity" step in derivation of photon sphere radius, where it is found "speed of light on it's orbit". So I'm going to edit it in purely GR way.  —Preceding unsigned comment added by Goykhman (talk • contribs) 09:47, 11 May 2009 (UTC)
thread_title:  Purely GR approach 

message: "In the standard electromagnetic and gravitational resonator approach used the following photon radius:"
Someone know what this is supposed to say?
LokiClock (talk) 17:45, 14 September 2009 (UTC)
thread_title:  Broken Sentence 

message: ... a general circular orbit can have a varying theta, as long as r remains constant. Right?
Suggest merging with the article on gravitational lensing; the photon sphere is really a special case of the general light-bending behaviour around a spherically symmetric mass. 130.56.71.50 (talk) 16:00, 23 September 2009 (UTC)
thread_title:  D-theta should not be zero 

message: Isn't it true that, from the point of view of a photon, the entire universe looks like a singularity? A photon travels EVERYWHERE in the universe in ZERO time from its own point of view. So doesn't that mean that from a photon's point of view, the universe is a singularity? 68.200.98.166 (talk) 01:33, 25 December 2009 (UTC)
replies: text: Well according to Relativity, the photon would not experience time, nor distance (remember that time and distance are connected and is especially apparent when upping velocity). I'm not sure if it can travel everywhere at once because where is it going to travel? How can it when there is no time to do it in? From what I can tell, that is still a great conundrum of physics. You tell clearly, from our perspective of close to 0c that light travels and takes time to do it in. I'm not quite sure, but both are true, the photon is instantaneously created and destroyed at perspective from c but yet travels from stars millions of ly away and hits our telescopes from perspective 0c. Maybe someone more experienced in SR/GR could help us out on getting our heads around both being true? Cody-7 (talk) 03:26, 24 February 2016 (UTC)
thread_title:  Singularity 

message: From the statement:
"There are no stable free fall orbits that exist within or cross the photon sphere. Any free fall orbit that crosses it from the outside spirals into the black hole. Any orbit that crosses it from the inside escapes permanently. "
How can you cross the orbit from the inside? That would require escaping from the event horizon.
Silenceisgod (talk) 23:52, 3 October 2015 (UTC)
replies: text: The photon sphere is outside the event horizon.  A rocket above the event horizon but below the photon sphere could get an impulsive boost and then coast through the photon sphere on an escape orbit. Cloudswrest (talk) 04:42, 4 October 2015 (UTC)
replies: text: Wouldn't that require the rocket to travel faster than the trapped light? --Ayeroxor (talk) 14:19, 16 November 2016 (UTC)
replies: text:  No, as this is only necessary inside the event horizon, which is inside the photon sphere. Absolutelypuremilk (talk) 14:41, 16 November 2016 (UTC)

text:  The light "trapped" on the photon sphere is not really trapped. It is moving parallel/tangential to it. Give it a "nudge" either inward or outward and it's gone. It is in a meta-stable state. Orbits that *cross* the photon sphere are by definition not moving parallel to it. Cloudswrest (talk) 22:40, 19 April 2019 (UTC)
thread_title:  Layman question 

message: The article currently states that photon orbits "are rarely stable in the long term."  Well, no orbit goes on forever, and "long term" can mean different things in different contexts.  I think the average term  or term range, and aspects such as how the mass of the black hole affects these calculations, should be more precise. --Ayeroxor (talk) 14:18, 16 November 2016 (UTC)
replies: text: That means that even the smallest perturbation (or in numerical simulations the smallest numerical rounding error) will lead to the photon either plunging or escaping to infinity. And in nature there will be small perturbations sooner or later. Other orbits are not that unstable, even when there are small perturbations or numerical rounding errors the particle will not necessarily plunge or escape. --Yukterez (talk) 22:35, 30 July 2017 (UTC)

text: Ayeroxor is right. That portion of the article doesn't make much sense as written. I've deleted it along with the two sentences about energy that were totally incorrect (see section below in talk page). In general, this article doesn't seem to have been competently written.--76.169.116.244 (talk) 17:46, 19 April 2019 (UTC)
thread_title:  "Long Term" 

message: The article currently says, "As photons approach the event horizon of a black hole, those with the appropriate energy avoid being pulled into the black hole by traveling in a nearly tangential direction known as an exit cone. A photon on the boundary of this cone does not possess the energy to escape the gravity well of the black hole." This is just wrong. The trajectory of a photon near a black hole has nothing to do with its energy. The geodesic equation has uniquely defined solutions for a given initial position and initial four-velocity. Two photons with different energies will still have the same trajectories if they start at the same position and with the same initial four-velocity. I've deleted the two sentences in question, since they're false, but that is going to disrupt the flow of the article. Maybe the word "energy" just needs to be replaced with "four-velocity," but I can't read the mind of the person who wrote this and tell what they had in mind or whether it made sense.--76.169.116.244 (talk) 17:43, 19 April 2019 (UTC)
thread_title: Incorrect material about energy

message: :Undid vague 3rd cite, returned to original cite, thanks45.49.226.155 (talk) 20:15, 3 July 2020 (UTC)
thread_title:  undid vague cite