What would actually happen in the black hole?

Since astronomers released the first image of a black hole on Wednesday, these exotic objects have come into the spotlight of public attention. High time to answer a few questions about her bizarre nature - and the art of photographing her.

what is a black hole?

If a burned-out star has enough mass, about 25 solar masses or more, then at the end of its life it collapses under its own gravity until its mass is concentrated on a single point. In the process, space-time bends so much that it virtually tears: a so-called singularity is created in which space and time lose their meaning.

What is the event horizon?

The event horizon is a mathematical shell that surrounds the black hole: Beyond this limit, the pull of gravity is so strong that nothing can escape it, not even light - that is why the hole is literally "black". Outside the horizon, however, the black hole behaves well - basically like a "normal" mass, only a very large one. It is also a common misconception that it would inevitably suck in matter from its surroundings like a cosmic vacuum cleaner: it doesn't. Anything that accidentally falls into it, however, has the bad luck of being lost forever.

Why do we speak of spacetime?

Before Albert Einstein, science believed that the universe was a rigid structure in which all objects such as stars and galaxies hang like in front of a theater backdrop. Albert Einstein's general theory of relativity reversed subject and object: space and time are therefore not fixed, but a malleable tissue that is bent and bulged by the heavenly bodies. You can imagine it like marbles lying on a rubber mat. For example, where there is a heavy star, a dent forms in space and time, and objects that come close slide into this indentation. They are apparently attracted. That's gravity.

What happens if you fall into a black hole?

This is very difficult to say, and it depends on the point of view of the beholder. The only sure thing is that you don't hit the backs of bookshelves like in the film "Interstellar", which is otherwise scientifically astonishingly correct over long stretches. Above all, however, a fellow traveler outside the black hole would get old and gray while watching: In his view, time would almost stop at the edge of the black hole because of the strong gravity, and his unhappy friend would fall more and more slowly (while his body at least at a smaller black hole would be stretched spaghetti-like due to the gravitational gradient). On the other hand, especially in the case of a supermassive black hole, one would seemingly pass the horizon without any problems and fall faster and faster, with the past in front of you in the form of everything that previously fell into the black hole; the future behind. But one could never report on the experience.

Why can black holes hurl radiation and matter into space?

It sounds paradoxical that black holes are among the strongest sources of radiation and matter in the universe. Shouldn't they rather swallow things? So do they - but as gas and cosmic dust whirl around the black hole, they are accelerated to high speeds; this is how the so-called accretion disk is created, a disk of matter around the black hole. Part of it eventually falls into the black hole, increasing its mass. But in a mechanism that has not yet been fully clarified, hot gas is also thrown into space perpendicular to the accretion disk. In the case of the now photographed black hole in the galaxy M87, this even happens over the enormous distance of around 5000 light years. However, it is matter that has never crossed the event horizon. So it is accelerated through the black hole, but does not come out of it. The situation is different with the Hawking radiation, named after the physicist Stephen Hawking who died last year: It is based on a bizarre quantum effect and should actually cause a black hole to evaporate without new food at some point.

How big are black holes?

The "hole" itself, the singularity of spacetime, has no extension. The radius of the event horizon, on the other hand, that is the dark shadow that can be seen in the first picture, depends on the mass: it is roughly three kilometers for each solar mass. With the 6.5 billion solar masses of the center of M87, that makes almost 20 billion kilometers. For comparison: The outermost planet Neptune is around 4.5 billion kilometers from the Sun, the outermost Voyager probe around 17 billion kilometers. Thus, the solar system would have relatively loose space within the event horizon - even if that would not be particularly good for it.

Does a black hole really look like the one in the picture?

Not directly. If you could look at the black hole with an incredibly good optical telescope, you would not be able to see the characteristic structures because it is surrounded by opaque clouds of matter. That is why the recording was made with radio waves that can easily penetrate such walls. The colors are also artificial and represent different intensities. But otherwise - provided the physicists have done everything right with the data analysis - the picture shows reality.