For some "average sized" black hole, how long would it take for a distant (spacesuit-wearing!) astronaut to:

  • be able to see the gravitational lensing with the naked eye
  • clearly feel the gravity gradient
  • reach a lethal gravity gradient
  • cross the event horizon

Ultimately I'm interested in the subjective experience of a human falling into a black hole. Would it be drawn out and unpleasant? Or is it that by the time the gradient would be painful, there'd be barely any time at all before annihilation?

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    $\begingroup$ Actually, this is a problem I remember doing in my cosmology class. There are some really good questions here having to do with proper time versus coordinate time, the relationship between the mass of the black hole and the gradient of its gravitational field, and a few other things. If I find some quantitative calculations I remember doing I'll try to answer the question. $\endgroup$ – astromax Dec 18 '13 at 4:26
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    $\begingroup$ Thanks @astromax, would appreciate an attempt at an answer, and cheers for a counter opinion on this question's appropriateness. $\endgroup$ – aaaidan Dec 18 '13 at 4:35
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    $\begingroup$ Yes - the conclusion I remember coming to is that the larger the black hole mass the closer you could come to the event horizon without feeling this differential tug between your head and your feet. The smaller mass black holes are actually the more lethal ones in terms of tidal forces as you approach the event horizon. However, the amount of time it takes you to get to the event horizon as seen from an "outsider" if I'm not mistaken is infinite. I'll have to double check to see if that's actually the case. $\endgroup$ – astromax Dec 18 '13 at 4:39
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    $\begingroup$ I'm actually teaching a cosmology class next term. One of the homework sets assigned I believe has this particular question. I'll post the answer when I figure it out. $\endgroup$ – astromax Dec 18 '13 at 4:42
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    $\begingroup$ I calculated questions 2/3 for a super-massive black hole once; it's still up on physics: physics.stackexchange.com/questions/38837/… - maybe that helps. (tidal forces can be painful, ouch) $\endgroup$ – Alexander Janssen Dec 18 '13 at 16:01

Interestingly enough, Neil deGrasse Tyson, an american astrophysicist, wrote a book titled Death by black hole.

In this video he goes on to explain the details of what you asked, the subjective experience of the astronaut being sucked into the black hole. It is definitely painful!

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  • $\begingroup$ I disagree that it would be painful. You'd lose consciousness long before you'd feel pain due to gravity gradient stretching your body, because you'd lose intracranial fluids, lose consciousness and consequently die (pretty fast). $\endgroup$ – TildalWave Dec 19 '13 at 8:58
  • $\begingroup$ Yeah, it seems reasonable that if you can feel a gravity gradient, blood would be pooling in your feet (assuming "head-up"), to some degree, eventually knocking you out. Whether the gradient is painful before then is up for debate perhaps? But how long a delay would the taller experience between these events is what I'm particularly interested in. $\endgroup$ – aaaidan Dec 19 '13 at 9:33
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    $\begingroup$ Neil seems to think the gradient would be strong enough to snap a human spine before death, but I am dubious that he's not showboating a little... $\endgroup$ – aaaidan Dec 19 '13 at 9:37
  • $\begingroup$ I wouldn't be so quick to say he's wrong, after all, he wrote a book on the subject and did a thorough investigation followed by an extensive analysis. I bet he know a lot more about black holes than we do. $\endgroup$ – Eduardo Serra Dec 19 '13 at 13:58
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    $\begingroup$ Thanks for the information, but in the context of this site, it would be better to provide an answer here. It's okay to add links for further reading, but this site was created to build a definitive collection of answers to astronomy questions. When someone comes here looking for answers through search, the last thing we should be doing is sending them elsewhere to find that information. Link-only answers do little more than add another barrier between future readers and the actual information they are looking for. That's not making the Internet better. $\endgroup$ – Robert Cartaino Dec 19 '13 at 15:19

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