In the works of science fiction writers, you can find many versions that black holes can serve as "gates" to other universes. A black hole is a very massive object, and its gravity is not uniform. As you approach the black hole, you will feel more and more the force of gravity with each step.
Simply put, if the astronaut falls there feet first, then his head will be attracted less than his legs. As a result, a huge force will begin to stretch the person, turn him into pasta, into a thread. The same will happen with the spacecraft. However, this effect is weakened if the black hole has a large mass, such as the one in the center of our Galaxy. As a result, you can whisk there without turning into macaroni.
And then science fiction will begin, because we do not know exactly how physics works inside black holes. It would be very interesting if there was a special substance hidden inside that has an anti-gravity effect. We call it dark energy.
If this substance is actually there, then the space inside the black hole can have amazing properties.
For example, it can permeate our Universe with some kind of tunnels, they are often called wormholes. That is, the space is deformed, if you want, folds like a sheet of paper. As a result, we can instantly fly from one point to another. But for this it is necessary that two black holes, distant from each other, concordantly form this space-time tunnel. Scientists managed to construct such a situation on paper. That is, purely theoretically, it is possible.
In the vicinity of the black hole, not only space changes, but also time. It really behaves very differently there. This is such a common effect that physicists don't even talk about it today. By the way, time goes on differently on Earth.
Back in the 50s of the last century, scientists conducted such an experiment: they took a very accurate, nuclear clock and measured the time on the first floor of the building and on the tenth. And we were convinced that the course of time depends on the gravitational field in which the clock is placed: the closer to the earth, where the field is more powerful, the slower the hands go. Since such an effect exists on Earth with its weak gravity, then what can we say about black holes!
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If, suppose, an outside observer looks from afar at an astronaut approaching a black hole through a telescope, then it will seem to him that the astronaut is moving, blinking, opening his mouth more and more slowly. And when it is at the event horizon, it will forever freeze for us, and we will never see how it crosses the border of the black hole. But for the cosmonaut himself, nothing will change: his clock ticked and ticked.
But since time changes its course in the vicinity of the black hole, it means, theoretically, time travel is possible! Moreover, our Russian scientist, an expert on the theory of gravity, Corresponding Member of the Russian Academy of Sciences Igor Novikov, together with his American colleague, constructed a time machine based on black holes. That is, they showed how you can travel into the past using those very wormholes.
It is possible that in the future such a time machine has ALREADY been invented. It turns out that our descendants may well visit us in the 21st century. All this is possible.
To travel freely in time and space, you need to understand the "structure" of the black hole. New discoveries should be expected in 5 years, when the new Millimetron radio telescope will be launched into space. The ideological inspirer of this project is Academician Nikolai Kardashev. Some parts of the radio telescope are already being produced at Russian and foreign plants. "Millimetron" will look into the Universe at short radio waves, which will provide high-definition images. And there is hope to see the black hole in great detail. Moreover, "Millimetron" will observe not only the black hole, which is located in the center of our Galaxy, but also in the neighboring ones.