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According to https://www.nationalgeographic.com/magazine/article/black-holes-einstein-star-eaters', when something crosses the event horizon of a black hole "A person watching from the outside will not see you fall in. You will appear frozen at the hole’s edge. Frozen for an infinite amount of time. Though technically not infinite. Nothing lasts forever, not even black holes." If so, why do we not see objects that are being sucked in?

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  • $\begingroup$ Did you read the article? It does explain firstly why "sucked in" is wrong. and secondly why "you will appear frozen at the hole's edge". $\endgroup$
    – James K
    Sep 10, 2023 at 21:55
  • $\begingroup$ It explains it but that was not my question. I'm asking why do we, people from Earth, can not observe objects that appear frozen at the event horizon. Why do we need to observe rays emitted by the black holes to detect them and not these frozen objects? $\endgroup$
    – Ph1ra
    Sep 10, 2023 at 22:36
  • $\begingroup$ I see your questino. I've added a paragraph to my answer But pretty soon you can't see the clock. This is because the light emitted by the clock gets more and more stretched. Soon the wavelength of the light grows from nanometres to millimeters, to kilometres to light years to billions of light years. The energy carried by each photon becomes less and less and less, until nothing more can be seen. $\endgroup$
    – James K
    Sep 11, 2023 at 17:56

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From the point of view of a person far from the black hole. Suppose you can see a clock that is falling into the black hole. As it gets closer you notice two things. Firstly the light from the clock is "red-shifted" So if the clock is one of those digital ones with blue glowing numbers, the numbers will appear to become green, then yellow, then red, then infra-red. The light is getting stretched by gravity.

Secondly, you will notice that the clock is running slower than before. Each second takes longer. Time is also getting stretched by gravity. Both of these "stretching" effects get stronger as the clock gets nearer to the event horizon, and that last "second" gets stretched out indefinitely, while the light gets shifted to longer and longer wavelengths.

All sources of gravity have this effect, even things like the Earth, and it needs to be compensated for when you need very accurate clocks in orbit, such as those on GPS satellites.

From the point of view of the person far from the black hole, that last second never ends. And so you don't see the clock reach or cross the event horizon.

But pretty soon you can't see the clock. This is because the light emitted by the clock gets more and more stretched. Soon the wavelength of the light grows from nanometres to millimeters, to kilometres to light years to billions of light years. The energy carried by each photon becomes less and less and less, until nothing more can be seen.

But from the point of view of the clock (or the astronaut carrying the clock) there is no time stretching. They see the clock ticking absolutely normally. They cross the event horizon and continue falling. What happens next is unclear. In the theory of relativity there is a "singularity" which means roughly "the theory is not valid". But whatever it is, two facts are certain. The astronaut can't leave the black hole, and the very atoms of their body would be ripped up by the extreme gravity. There's no coming back.

The point about "technically not infinte" relates to Hawking radiation. The event horizon gives off heat, at a very low rate, this causes the black hole to get smaller over a period of time much much longer than the age of the universe, eventually evaporating. So technically, that last second gets stretched out over the trillions of trillions of years.

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