**Effects of Gravity#2**
Decoding the Extreme Light Warp Phenomenon around Black holes
 |
| Image by AlexAntropov - Pixabay |
Okay, real talk: black holes are weird. Weird on a lot of levels. They violently curve and twist spacetime, gobble up and spew out matter and deadly radiation, make photons orbit them, and to top all of that, they can literally turn anything near its vicinity into nice lil' spaghetti strands that are basically frozen in time!
But one of the most mindblowing (still weird though) features of black holes is their interaction with light itself!
From relativistic effects like Doppler beaming to Gravitational Lensing, these gigantic space monsters show up some truly spectacular visuals in space. Just to give you an idea of what we're dealing with, here's another weird fact about black holes - you could actually see the back of your head right in front of you if you look straight ahead through the photon sphere of a black hole! And that is exactly what we'll be discussing in this post: the mind-boggling phenomenon of light bending and the formation of images at the event horizon of a black hole, and also, a little more about the formation of those distorted layered images of the event horizon and the accretion disk around the entire black hole - similar to that of the majestic Interstellar black hole.
Formation of Photon Sphere in Black holes
It is already a known fact from Einstein's General Theory of Relativity that massive objects create curvatures in spacetime which is manifested as gravity itself. And because it's the spacetime that is being curved, whatever matter or radiation (including light) that passes through these curvatures is bound to follow the curved paths in space, and if it is light that is travelling through such distorted spacetime, it can cause something called gravitational lensing around that massive object. In most cases, gravitational lensing by a massive galaxy or a star cluster can distort the spacetime around it, which can create more than one image of a distant object lying in the same line of sight.
In the case of black holes, however, this light-bending phenomenon is at its extreme peak because not only does it curve spacetime to a baffling extent but it also makes the light go around it to form infinite images of the event horizon itself! To be more clear, a black hole's gravity is so strong that light literally orbits around the event horizon, and this is what we call the photon sphere of that black hole. The photon sphere of a black hole would be closer to the event horizon of the black hole (about 1.5 Schwarzchild radii) than the accretion disk of swirling matter since light is essentially just massless photons and that means them orbiting closer to the event horizon unlike the ordinary matter possessing some rest mass.
That being said, the image formation around a black hole greatly depends on our line of sight, or how the black hole is aligned in space concerning us. For instance, if we observe a black hole perpendicular (head-on) to its axis, we'd see something similar to that of M87's supermassive black hole which was first released by the Event Horizon Telescope back in April 2019. But if we observe the black hole at an angle other than 90°, we could actually catch a glimpse of the bent accretion disk that is being warped around the black hole, just like that of Gargantua in Interstellar.
Light Bending around Black Holes
The phenomenon of distortion of light around the event horizon of a black hole may seem counter-intuitive or complex in some ways, but it is pretty obvious once we understand the warping effects of black holes in spacetime. It all starts from the photon sphere where photons are orbiting the black hole - either they can orbit into the black hole by crossing the event horizon (in which case it'll no longer be observed by us), or they can orbit out of the photon sphere by escaping the pull of the black hole, and eventually reaching our telescopes.
This means that even though a light ray is approaching the black hole, given that it is not too close to the photon sphere (and crosses the event horizon), the light ray can travel all the way around the black hole and can still end up being pulled over to the black hole (as shown in the figure).
 |
| Orange-yellow - the Photon Sphere Purple - the Event Horizon White lines - the incoming light rays |
But, instead of viewing the black hole head-on, what if we observe it laterally at an angle? Well, everything remains almost the same as in the previous case except the accretion disk! Since the entire region of spacetime enclosing the black hole is curved around it, we could actually see the backside (the side that is away from our view) of the accretion disk, and it'd appear as if it warps the entire black hole. So technically speaking, the light from the other side of the accretion disk warps over and under the black hole, giving the observer a glimpse of every bit and side of the black hole in just one glance!
That being said, in both cases, one of the sides of the illuminated accretion disk of the black hole will appear much brighter than the other, due to an effect called Doppler beaming. This is simply because the accretion disk, which constitutes matter that is moving at relativistic speeds (speeds that are closer to that of light), will appear much brighter if its direction is towards us rather than away from us.
Anyways, as we saw earlier in this post, the image of a black hole can greatly depend on how and from where you plan on observing it. So, if you're looking for a cheesy little conclusion to this post, it's just that if you're feeling like you don't understand the whole picture of anything, maybe it's not your perspective, it's the spacetime that's curved enough to distort the reality!
For more info on this topic, check out:
That black hole photo: How event horizons bend time, space, and light | Michelle Thaller | Big think
Wow
ReplyDelete