How to Understand the Black Hole Image

On Wednesday, April 10, 2019, we will probably see the first

image

of a

black

hole

. This is when the Horizon Telescope will publish the results and I haven't seen them yet. I think they will look like this and I can be relatively sure because it will look a bit like a blurry stain from a coffee cup. But if this

image

disappoints you, I think the seriousness of the situation is overlooked. From this picture we should be able to tell whether general relativity predicts exactly what happens in the strong gravity regime. This is what happens around a

black

hole

.

What I want to do here is

understand

what exactly we are seeing in this image, so here is my science. simulacrum of a black hole and this sphere represents the event horizon. This is the place from which even light that is not turned on due to black hole radiation can be detected by an external observer. All world lines end at the center of the black hole, especially once inside there is no turning back even for light. The radius of the event horizon is known as the Schwarzschild ray. Now if we were right to see a black hole with nothing around it we couldn't make an image like this because it would simply absorb all the electromagnetic radiation that falls on it, but the black hole you are looking at specifically the one in the center of Our galaxy, the Milky Way, Sagittarius A* has significance around it in a growing disk.

In this accretion disk there is a lot of dust and gas circulating here chaotically, it is very hot, we are talking about millions of degrees and it goes very fast at a significant fraction of the speed of light and it is this matter that feeds the black hole and becomes getting larger and larger over time, but you will notice that the accretion disk does not extend to the event horizon. Why is she? Well, this is because there is a more stable interior around the circular orbit and for matter around a non-spinning black hole, that orbit is at three Schwarzschild radii, now in all likelihood the black hole at the center of our galaxy will be spinning, if not for simplicity I am only considering the non-spinning case.

You can watch my video on black holes if you want to learn more about that. So this is the innermost orbit because the matter revolves around the black hole, if so it enters this orbit very quickly, it goes to the center of the black hole and we never hear from it again, but there is something that may be orbiting closer . to the black hole and this is easy because light has no mass that can orbit at 1.5 Schwarzschild radii. Now here I'm representing it with a ring, but actually this can have any orientation, so it's a sphere of photon orbits and if you've been there, of course you can't go there, but if you can look ahead and really see the back of the head because the photons can circle around and complete the orbit.

Now the spherical photon is an ultimately unstable orbit or the photon must spiral in towards the singularity or spiral out and go to infinity. Now the question I want to answer is what does this black shadow in my shadow image cited in this photo correspond to? currently happening in the black hole. Is it the event horizon? Are we just seeing this? Or is it that photon sphere? Or a more stable inner circular orbit? Well, things are complicated and the reason is that this black hole drags space-time around it, which changes the path of the light rays so that they do not go in a straight line as we normally imagine, which means that they go in a straight line. straight line, but spacetime is curved into curves, so the best way to think about this is probably to imagine parallel rays of light coming from the observer and hitting this geometry here.

Of course, if parallel light rays cross the event horizon, we will never see them again, so they will disappear, which will definitely be a dark region, but if a light ray arrives just above the Rison event, it will also will tend to complete the event horizon transition ends the black hole. Even a ray of light that reaches the same distance as the photon sphere will end up distorting the black hole and bending along the event horizon, so to get a parallel ray that doesn't wipe out the black hole, you have to go 2.6 radii away, if a ray of light reaches 2.6 Schwarzschild radii, it will simply graze the photon sphere at its closest point and then extend to infinity, so the resulting shadow we get is You will thus see 2.6 times the event horizon.

Shall I tell you what we're really seeing here? What is this shadow? Well, at its center is the event horizon. It is drawn very clearly over the center of this shadow, but if you think about it, light rays going up or down also end up crossing the event horizon right behind it. So in effect, what we get is the entire back portion of the event horizon mapped to a ring in this shader. So, by looking from our single point in space toward the black hole, we actually see the entirety of the black hole's event horizon. I mean, maybe it's silly to talk about seeing it because it's completely black, but really where the point is will be assigned to this shade.

It gets weirder than that because light can come in and go around the back and say, dip in the front and get another image of the entire horizon next to that and another annular ring and then another one after that and another one after that and you get basically infinite images of the event horizon when you get close to this shadow. So what is the first light we can see? They are those rays of light that arrive at such an angle that they graze the photon sphere and then end up in our telescopes. And they produce a shadow that is 2.6 times the size of the event horizon.

So this is roughly what we'll see if we look perpendicular to the growing disk, but most likely we'll look at some kind of random angle in the rising disk. We may also be looking for the edge. And in this case do we see this shadow of the black hole? You may think we won't, but the truth is that because of the way the black hole bends space-time and light rays, we actually see the back of the growth disk. The way it works is light rays coming from the core. The disk rises high and ends up in our telescopes, so we end up seeing something that looks like this.

Similarly, the light from the bottom of the disk passes under the reception located below the black hole and reaches us like this and this is where we have an image that looks like an interstellar black hole. It gets even crazier than this because the light coming from the top of the growth disk here can go around the back of the black hole, graze the photon sphere and end up right here, producing a very thin ring under the shadow . Similarly, light from under the growth disk in the front can pass under and around the back and out over the top, which is why we see this ring of light here.

This is what we could see if we were very close to the black hole, something that looks really spectacular. Another really important effect to consider is that the matter in this accretion disk moves very fast, close to the speed of light, so if it comes towards us it will look much brighter than if it moves away. This is called relativistic radiation or Doppler radiation and therefore one side of this rising disk will appear much brighter than the other and therefore we will see a bright spot in our image. I hope this gives you an idea of ​​what we are really seeing when we look at an image of a black hole.

If you have any questions about this, leave them in the comments below and I'll probably make a video. of the release of the first image of a black hole so I will try to answer them. Until then, I hope you enjoy this as much as I do because this has really been my obsession for the last week. I think it would be exciting to see how that time changes, right? There's a lot of hope that there will be specks moving around and you know, if you see a mass spinning around first and then it's spinning back, but you see it in the back picture, etc., then it's going to be great.