Reading aloud A completely different world. They burst beyond our imagination like no other. But despite their bizarre features, black holes are the simplest thing in the world for the physicist: they can be fully described with only three physical parameters - mass, charge and angular momentum. They are at the same time the most weighty thing in the world: their density is so great that not even light can escape their gravity field. As Albert Einstein predicted with his General Theory of Relativity, mass writhes space. As a result, the rectilinear path is bent by light rays toward a mass concentration. This effect was first observed in 1919 during a total solar eclipse - a historic event for modern physics and at the same time a masterpiece in the art of exact measurement: the stars are deflected less than a thousandth of a degree. When light rays pass black holes, they are only really put on the wrong path by the heavyweight fellows. Through this gravitational lensing effect, the position of neighboring background stars appears systematically shifted: distant galaxies appear distorted and brighter than they really are. Sometimes their light splits up so that an observer sees several pictures of the same background object or even a ring.

At a certain distance, light rays passing a black hole may or may spiral into orbit around the cosmic finsterling and then disappear forever into the rest of the world. In addition, computer simulations have shown the glow of a heated gas and dust disk around a rotating black hole being emitted behind the gravity monster. The room here is so bent that you can literally look around the corner.

For the astronaut who would boldly embark on a journey into a black hole, this "hell-trip" would be something like this: From the slowing down of his own time - compared to the way he lives on earth - he does not notice. Instead, the clocks of those left behind start racing for him. The environment gains a peculiarly distorted shape, the colors are shimmering and the gravity of the black hole is drawing ever harder on its spaceship. In a black hole that is only a few times as heavy as our sun, the tidal forces are so strong that the astronaut and his ship is stretched like spaghetti and torn a little later.

With large black holes, as they sit in the centers of the galaxies, the density at the event horizon is so small that the daring astronaut can safely cross this "place of no return". All the light of the universe finally shrinks to a small, glittering disc above it. For a few minutes, the astronaut can still explore the interior of the cosmic darkling. For the outside world, however, the discoveries of the death candidate would be of no value, because his radio signals can never leave the gravity field of the black hole. display

Of course, the foolhardy astronaut will no longer know what happens to matter inside a black hole: even its own atoms can not survive the fall into the center. Will their rubble finally be destroyed or will they reappear somewhere else, maybe even in a strange universe? A similar difficulty arises in the question of what happens to the information contained in the collapsing matter. It is possible that fundamental conservation laws of physics are violated in the black hole. All answers to such problems are so far pure speculation.

=== Rüdiger Vaas

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