How did they actually take this picture? (Very Long Baseline Interferometry)

This video is sponsored by KiwiCo, more information about them at the end of the program. This is an image of the supermassive black hole at the center of our Milky Way galaxy, known as Sagittarius A*. The black hole itself does not emit light, so what we are seeing is hot plasma swirling around it. This is only the second photograph ever obtained of a black hole. It was

take

n in collaboration with the Event Horizon Telescope, the same people who brought you

this

image of the supermassive black hole at the center of the M87 galaxy. Now, his original plan was to photograph Sagittarius A* first.

Since it is in our own galaxy, it is 2,000 times closer than M87*, but it is also more than 1,000 times smaller, so from Earth it appears only slightly larger than M87*. And there are a number of additional challenges to observing it. First of all, there is a lot of dust and gas between us and the center of our galaxy, so it can't even be seen in visible light. In

this

video from the European Southern Observatory, we get closer to the core of our galaxy. As we get closer and closer, at some point we have to switch to infrared light, which can better penetrate the debris and allow us to see it from Earth.

Over the past three decades, we have been able to peer into the heart of Dairy Mayo and witness something truly amazing. A collection of stars rotating in all kinds of eccentric orbits. They go incredibly fast. One of the stars recorded a speed of 24 million meters per second. That's 8% of the speed of light. All of these stars appear to be orbiting something incredibly massive and compact, but this object does not shine brightly like a star. If you look closely, you can see it flashing from time to time. This is what we think is a supermassive black hole.

From the movement of the stars around it, we can infer that the mass of the black hole is approximately 4 million times that of our Sun, but all concentrated in a small point, the singularity. Anything, including light, that is within a Schwarzschild radius of this point cannot escape and ends up at the singularity. So for us to see any radiation from the black hole, it must come from further away, usually from superheated plasma as it falls. But for its size, Sagittarius A* does not consume much matter. It is unusually quiet and dark. In contrast, the supermassive black hole at the center of M87 is much more active, devouring matter from its accretion disk.

Additionally, since it is more than 1,000 times larger, objects

take

1,000 times

long

er to orbit it. And that means that from Earth, its appearance over time is more consistent in contrast to Sagittarius A*, where things can change on the order of minutes. These visualizations are by Luciano Rezzolla and colleagues at Goethe University Frankfurt. But the biggest challenge of all when creating an image of any of the supermassive black holes is that these objects are so compact and so far from Earth that in the sky

they

appear

very

,

very

small. To get an idea of ​​how small it is, take the entire sky and divide it into 180 degrees.

The Andromeda galaxy extends about three degrees. Then divide one degree into 60 arc minutes and one arc minute into 60 arc seconds. Divide an arc second by 100, again by 100, and again by 100. And this is the size of the black holes in the sky. It's equivalent to taking a photo of a donut on the moon. There is currently no optical telescope on Earth that can produce such an image. So in this video, I want to answer two questions. How

they

did it? And what are we really looking at? So, to begin with, how did they make these images of black holes?

Well, the first thing to know is that they were not made with visible light. They were created using radio waves with a wavelength of 1.3 millimeters. Therefore, all observations were made with radio telescopes, which essentially look like huge satellite dishes. When a source emits radio waves, they travel radially in all directions, but Earth is so far away that when they reach our planet, the wave fronts are almost completely flat and parallel. This is known as a plane wave. A radio telescope works by scanning the sky from one side to the other. When pointed directly at a radio source, it produces a bright spot.

This is because all radio waves travel the same distance, bounce off the dish, and are received at the same time, so they are in phase, meaning peaks line up with peaks and valleys with valleys. . They interfere constructively. As the telescope passes the source, some of the radio waves travel farther than others and are therefore out of phase, interfere destructively, and the signal strength drops to zero. To create a sharp image, you want this drop-off to be as steep as possible so that the telescope produces maximum intensity only when pointed directly at the source and then the intensity drops off rapidly when the dish moves just a little in either direction.

There are two ways to achieve this. One is to observe higher frequency radio waves. Thus, any slight movement of the telescope represents a larger fraction of a wavelength. This causes destructive interference to occur sooner. The other way is to increase the diameter of the telescope, and this increases the difference in path length between radio waves on opposite sides of the telescope for a given angular setting. The precision with which a telescope can identify the source of radio waves is known as angular resolution. You can think of it as the size of the spot on the sky that the telescope is sensitive to.

It is proportional to the wavelength and inversely proportional to the diameter of the telescope. The challenge with taking a photograph of a black hole is that you are trying to see the structure in a small area of ​​the sky. Imagine scanning a radio telescope through the center of a black hole. You will want to see the bright spot as the telescope passes over the left edge and then a dark spot and then another bright spot as it passes over the right edge. The problem is that, for any individual radio telescope on Earth, the angular resolution is too large.

So when it passes over the black hole, it will still receive radio waves from the left side when it starts receiving radio waves from the right side. The resolution is not high enough to know if there is a ring structure there as we would expect from a black hole, or if it is just a mass. Observing at shorter wavelengths isn't really an option because that light is blocked by our atmosphere or by the matter around the black hole. So if you want to improve the resolution, the only way to do it is by increasing the diameter of the telescope.

But if you

actually

do the math, you find that the telescope you would need would have to be the size of the Earth to be able to see the ring of a black hole, which is obviously impossible, but there is a way to do something. That's almost as good. You don't need a whole plate the size of the Earth, just pieces of it. Individual radio telescopes that are separated by distances up to the diameter of the Earth. As

long

as you can properly combine the signals from all these distant telescopes, you will get the constructive and destructive interference needed to achieve the same angular resolution as an Earth-sized dish.

This technique is called very long base

interferometry

. Therefore, the event horizon telescope is not just a telescope, but a global network of radio observatories. All of these telescopes observe Sagittarius A* at the same time. Unlike a single telescope, you can't bounce all the radio waves off a central receiver and add them up in real time. Therefore, each telescope records the signal at its location and time exactly down to the femtosecond. Petabytes of data are generated. But now that data needs to be gathered, and the quickest way to do it was to carry the hard drives as carry-on luggage to centralized locations.

Now, think about the data we have. Electrical signals and precise timing from several radio telescopes around the world, but none of those radio telescopes have enough angular resolution to see the black hole ring. So how do you combine that data and get finer details than any of the inputs? Well, there is additional information in the relative distances between these telescopes and in the time delays between when a wave front hits one telescope relative to the others. Imagine combining the signals from two distant telescopes. Let's say they both received the same wave at the same time, so those waves came in phase.

Well, then the source must have been located directly between them. The radio waves would have traveled the same distance to each telescope to arrive at the same time, except with only two telescopes, which only reduces it to a line in the sky that is equidistant from both telescopes. The source could have been anywhere on that line. And it's

actually

worse than that. It is possible that the source is exactly one wavelength closer to one of the telescopes and that way the radio waves would arrive perfectly in phase. Or the difference could be two, three, or four wavelengths, but you get the point.

So from a couple of telescopes, the information we get about the source is actually a series of bright and dark fringes. Telescopes that are close together produce wide fringes, while those that are far away produce narrow fringes. So to take an image, you need pairs of telescopes in different orientations and different distances from each other. Each pair forms a different interference pattern. And then by combining all these patterns, we get an image of the black hole that created them. But now that we have this image, what exactly does it show us? Well, that's how I explained it when the first image of a black hole was published.

So here is my mock black hole science. And this sphere represents the event horizon. Once you're in here, there's no turning back, not even for the light. The radius of the event horizon is known as the Schwarzschild radius. Now, if we were to look at a black hole with nothing around it, we wouldn't be able to make an image like this because, well, it would just absorb all the electromagnetic radiation that falls on it, but the black hole that we're looking at has matter around it in an accretion disk. In this accretion disk there is dust and gas swirling in a very chaotic manner.

It's incredibly hot. We are talking about millions of degrees. And it goes very fast, a significant fraction of the speed of light. And it is this matter that the black hole feeds on and gets bigger and bigger over time, but you will notice that the accretion disk does not extend to the event horizon. Why is that? Well, that's because there is an innermost stable 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. But for simplicity, I'm only considering the no-spin case.

You can watch my video on rotating black holes if you want to know more about it. So this is the innermost orbit of matter orbiting a black hole. If it enters this orbit, it will very quickly go to the center of the black hole and we will never hear from it again, but there is something that can orbit closer to the black hole, and that is light. Since light is massless, it can actually orbit at 1.5 Schwarzschild radii. Now here I'm representing it with a ring, but really this could be in any orientation. So it's a sphere of photon orbits.

And if you were standing there, of course you could never go there, but if you could, you could look ahead and see the back of your head because the photons could spin around and complete that orbit. Now, the photon sphere is an unstable orbit, which means that eventually the photons have to spiral in towards the singularity or spiral out and head towards infinity. Now, the question I want to answer is: what does this black shadow, in quotes, correspond to in the image of what is really happening around the black hole? Is it the event horizon? Are we just looking at this?

Or is it the photon sphere or the innermost stable circular orbit? Well, things are complicated. And the reason is that this black hole warps the space-time around it, which changes the path of the light rays so that they don't go in a straight line like we normally imagine they do. I mean, they go in a straight line, but spacetime is curved, so yes, they go in curves. So the best way to think about this is perhaps 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.

It will definitely be a dark region, but if a ray of light arrives just above the event horizon, it will also bend and end up crossing the event horizon. Ends up in the black hole. Even a ray of light that reaches the same distance as the photon sphere will end up warping toward the black hole and curving through the event horizon.So to get a parallel ray that doesn't end at the black hole, you actually have to go out 2.6 radii away. If a ray of light reaches 2.6 Schwarzschild radii away, it will simply graze the photon sphere at its closest point and then shoot off to infinity.

And then the resulting shadow we get looks like this. It is 2.6 times larger than the event horizon. And you say, what are we really seeing here? What is this shadow? Well, in the center is the event horizon. It maps quite clearly to the center of the shadow. But if you think about it, light rays going up or down also end up crossing the event horizon, right at the back. In fact, what we get is the entire back part of the event horizon mapped into a ring in this shadow. So, looking from our single point in space at the black hole, we can 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 that's where the dots would actually be located in this shadow. It gets weirder than that because light can come in and go around the back and, say, get absorbed in the front, you get another image of the entire horizon next to that another annular ring and then another one after that and another one after that. that. And you get basically infinite images of the event horizon as you get closer to the edge of this shadow. So what is the first light we can see?

They are those rays of light that enter at such an angle that they graze the photon sphere and then end up in our telescopes, producing a shadow that is 2.6 times the size of the event horizon. So this is roughly what we would see if we were looking perpendicular to the accretion disk, but we would most likely be looking at some kind of random angle relative to the accretion disk. We may even be staring at the edge of the abyss. And in that case, do we see this shadow of the black hole? You might think we wouldn't, but the truth is that because of the way the black hole warps spacetime and bends light rays, we actually see the back of the accretion disk.

The way it works is that the light rays coming out of the accretion disk bend over the top and end up reaching our telescopes. So what we end up seeing is something that looks like that. Similarly, light from the background of the accretion disk passes underneath, bends under the black hole, and comes toward us that way. And this is where we get an image that looks somewhat like the interstellar black hole. (dramatic music) It gets even crazier than this because the light coming out of the top of the accretion disk here can go around the back of the black hole, graze the photon sphere, and come out down here producing a very thin ring. under the shadow.

Similarly, light from beneath the accretion disk at the front can pass under and around the back and exit at 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 is moving very fast, close to the speed of light. And if it comes towards us, it will look much brighter than if it moves away. This is called relativistic emission or Doppler emission. And so one side of this accretion disk will look much brighter than the other, and that's why we'll see a bright spot in our image.

Hopefully this will give you an idea of ​​what we're really seeing when we look at an image of a black hole. (ending sounds) Hey, this video was sponsored by KiwiCo, creator of awesome hands-on projects for kids. You know, I've used KiwiCo with my own kids for years. They have nine different subscription lines aimed at different age groups, up to newborns. The way it works is that every month a box shows up on your doorstep and inside is everything you need to complete the project. That means no extra trips to the store. And when I show the box to my kids, they jump at the opportunity to make it with me and we can spend hours building something, playing with it, and learning about STEAM concepts together.

There's really no substitute for getting your hands dirty and doing something to find out how it really works. Plus, it's a lot of fun. And for me, that's what learning should be like. I want my children to approach learning as a game. And I've seen how this encourages their curiosity and generates new ideas. KiwiCo has been a long-time supporter of the channel. I've visited their offices, which really look like a giant playground for adults like me. And I met their expert project designers and saw how thoroughly they test and iterate their designs. Now, for viewers of this channel, KiwiCo is offering 30% off the first month of any kit.

Simply go to kiwico.com/veritasium30. I'll put that link in the description. So I want to thank KiwiCo for supporting Veritasium and I want to thank you for watching.