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Ten Fascinating Facts About Black Holes

Artist's rendition of a black hole | Image: Ute Kraus

A black hole in space is a location in which the pull of gravity is so forceful that even light can not get out. Matter compacted into a small space renders gravity's pull so strong. One circumstance in which this can occur is when a star is dying. Without light, black holes are invisible to the eye; scientists locate them with specially equipped telescopes. 

The linked article lists some interesting facts about this phenomenon in space. For example,

7. Time Slows Down Around It

"If you’ve seen Interstellar, you’ll know what happens when you travel near a black hole; time slows down. What is incorrect about the film is that the time dilation would not be quite that extreme.

Time dilation is ultimately affected by gravity, the stronger the gravity, the stronger the time dilation. Also, time only slows down once you get near the black hole, once you pass the event horizon, time would stop."

Read nine more interesting bits of information about black holes in this article.


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As pointed out above, the slowing down of time has a lot more qualifiers on it. An observer very far away would see a falling body appear to slow down in time as it approached the event horizon. They would also see it red shift, quite rapidly, so it would still disappear from being visible, but not because it appears to fall through the event horizon.

For an observer falling into a black hole, as for any other observer, their own time would appear the same as always. Nothing special happens to the observer at the event horizon. If you were in a small ship with no windows, and the black hole was large enough that there wouldn't be a large difference in pull from the front and back of the ship, you would have no way of knowing the exact moment you cross the event horizon when falling through.

If you did have a window, you wouldn't see all time pass either. This seems to be more blatant wrong than just unqualified. Assuming GR works within the event horizon, the observer would hit the center in a finite (and actually quite short) time, the whole time still seeing stuff in front and behind them, with some time dilation. The only way to get infinite blue shifting of the rest of the universe, so that you could watch all time pass by, would be to stop yourself from falling right at the event horizon, requiring infinite force to stop your fall (and this all assume that quantum mechanics doesn't do something weird right at the event horizon).

We might actually get to see some of this more directly confirmed or proven wrong in the next couple years, as the Event Horizon Telescope, a network of radio telescopes, might be able to resolve the event horizon of the black hole at the center of the Milky Way.

(Oops, this was meant as a reply to the article, not the particular comment.)
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Oy--let's all please do our best to stamp out DDSG (Dumbed-Down Science Garbage) in our lifetime! According to current understanding, time does NOT stop at the event horizon of a black hole; indeed, time can never "stop" because "time" is not a "thing that flows" (contrary to popular conception), nor could it be "stopped," whatever that might mean! The facts: an observer watching a clock fall into a black hole (through a telescope, say) would see the clock slowing down as it fell to the center--yes. But this slowing down would never end as it asymptotically approached the event horizon (never reaching it). An observer who decided to accompany the clock would not notice anything special about the event horizon at all, and could pass through without even knowing it! (of course, this observer might have other problems looming, but these would have nothing to do with an event horizon)
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