Three embarrassing black hole photography questions

Astronomers (and all of humanity) have a holiday: The first shot of a black hole is presented. It was created using Event Horizon Telescope (EHT), a virtual telescope consisting of several radio telescopes around the world. The image shows material around a supermassive black hole in the center of a galaxy at a distance of 55 million light years. And yes, the black hole is concentrated physics, crazy gravitational phenomena on the verge of possible and impossible, extreme conditions. But there are a few questions.

Is a black hole hard to see because it is black?

Not. That is, yes. It's true: black holes are black. Usually we see all sorts of stars and all that, because the light they emit comes to our telescopes (or directly into our eyes), and we register it. Black holes are really black. They do not emit visible light (due to complex gravitational foci), so they are not visible.

But this is not a big problem. If we had a black hole in the solar system, you would see it. You would see the curvature of space by its presence and you would see a substance that revolves around this funnel. If you saw the film Interstellar, the visualization of a black hole is approximately exactly shown - it was done with the help of astrophysics Kip Thorn.

The black hole is hard to see because ittiny Well, well, not as tiny as an ant, for example. She is tiny in the sense that she is tiny, if you look at him from a distance of a kilometer. The best term is angular size. If you turn your head in a circle, you will get a 360 degree circular view (but do not forget to turn the body, or you will turn the neck). If you hold your thumb at arm's length, it's about half a degree angular. The moon has about the same angular size, so you can cover it with your thumb.

What about the size of a black hole? Yes, it is huge. And she is at a distance of 55 million light years. This means that in order for the light to get this far, it will take 55 million years. This is incredibly far. But in fact, we are hampered by the angular size. A black hole (at least its visible part) has an angular size of about 40 microarseconds.

What is microarsecond? As you know, the circle is divided into degrees (and for a long time). Each degree can be divided into 60 angular minutes, and each minute is 60 arc seconds. If you break an arcsecond into a million parts, you get a microarsecond. Remember that the angular size of the moon - 0.5 degrees (when viewed from Earth)? This means that the angular size of the moon is 45 million times larger than the size of a black hole. The black hole is tiny in terms of angular size.

But that is not all. Due to diffraction, we cannot see things of tiny angular dimensions. When light passes through a hole (for example, it enters a telescope or eye), it is scattered. It bends in such a way that it interferes with the rest of the light passing through the opening. In the case of the eye, this means that people can make out objects with an angular size of about 1 arcmin.

And it also means that something so tiny angular sizes, like a black hole, is difficult to catch in the photo.

How to overcome the diffraction limit?

Let's say. Things of tiny angular sizes are really hard to see - how can we then see the material around the black hole? The angular resolution of the telescope actually depends only on two things: the size of the hole and the wavelength of the light. Using shorter wavelengths (such as ultraviolet or x-rays) gives better resolution. But in this case, the telescope uses the wavelength of light in the millimeter range. This is a fairly long wavelength compared to visible light, which is in the range of 500 nanometers.

And that means the only wayto overcome the diffraction limit - to make a larger telescope. That is, what they did with Event Horizon Telescope. In fact, this is a telescope the size of Earth. Madness, but true. By obtaining data from several telescopes in different parts of the world, you can combine the data to turn them into data from one GIANT telescope. True, have to try. But with this method there are problems. With only a few telescopes, the EHT group uses a number of analytical methods to create the most likely image from the collected data. So they managed to "draw" the material around the black hole.

Is this a real black hole photo?

If you look through a telescope and see Jupiter, youactually see Jupiter. Note: If you have not done this yet, be sure to try. That's cool. Sunlight reflects off the surface of Jupiter and then passes through a telescope into your eye. Boom. Jupiter. He is real.

But with a black hole, everything is a bit wrong. The image you see is not even in the visible range. This is a radio image created from the wavelengths of light. What is the difference between radio waves and ordinary visible light? In fact, the difference is only in the wavelength.

Light and radio waves are electromagneticthe waves. This is the propagation of a varying electric field along with a varying magnetic field (simultaneously). These waves move at the speed of light - because they are light. However, since radio and visible light have different wavelengths, they interact differently with matter. If you turn on the radio at home, you will receive a signal from the nearest radio station. These radio waves pass right through the walls. And visible - do not pass.

The same applies to images. If you have visible light from an object, you can see it with your own eye and record this image on film or with a digital recorder. This image can then be displayed on a computer screen and, in fact, viewed. Something like this you can see a picture of the moon.

As for the material around the black hole, this is notvisible image. This is a radio image. Each pixel in the picture represents a certain wavelength, but radio waves. The orange parts are a false color representation of a wavelength of 1 millimeter. The same thing happens when we want to “see” an image in the infrared or ultraviolet range. We have to transform these wavelengths into what we can see.

So this shot of a black hole is not an ordinary photo. You cannot see it if you look through a telescope. But still great. True? Let's discuss in our chat in Telegram.