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Inferno returns
2018-May-07, 09:31 AM
You've just crossed the event horizon of a super massive black hole. I understand from your point of view nothing funny should happen. But what if immediately after crossing that point you stick your hand up and try to reach outside? Obviously you can't. But what happens? Does your hand bend with space so you sort of 'miss' the horizon? What if you were going through the event horizon and somehow stayed there with just your head outside the horizon. Presumably your heart couldn't pump blood to your brain or lungs send oxygen to your brain and you'd die. (Assume otherwise you have the greatest spacesuit of all time).

grant hutchison
2018-May-07, 08:16 PM
You've just crossed the event horizon of a super massive black hole. I understand from your point of view nothing funny should happen. But what if immediately after crossing that point you stick your hand up and try to reach outside? Obviously you can't. But what happens? Does your hand bend with space so you sort of 'miss' the horizon? What if you were going through the event horizon and somehow stayed there with just your head outside the horizon. Presumably your heart couldn't pump blood to your brain or lungs send oxygen to your brain and you'd die. (Assume otherwise you have the greatest spacesuit of all time).You can't stick your hand up fast enough to reach the event horizon - it goes past at light speed in your infalling frame. And, for the same reason, you can't hover at the event horizon and stick your head out (or in).

Grant Hutchison

Inferno returns
2018-May-07, 11:13 PM
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You can't stick your hand up fast enough to reach the event horizon - it goes past at light speed in your infalling frame. And, for the same reason, you can't hover at the event horizon and stick your head out (or in).

Grant Hutchison

Ah, so from the infalling persons perspective the EH comes at them and receeds at the speed of light?

grant hutchison
2018-May-09, 04:15 PM
Ah, so from the infalling persons perspective the EH comes at them and receeds at the speed of light?That's right. The event horizon is a null surface - it passes a at light speed for all local observers. So it doesn't matter whether you fall from infinity or from a short distance away, and it doesn't matter if you're firing your rocket engines or free-falling - the event horizon goes past at c.
One way to realize that this has to be so is to consider that if the event horizon didn't pass at c for all observers, then some observers would be able to send a light signal from just below the horizon, because the light from their torches would be able to overtake the event horizon as it passed.

Grant Hutchison

DaCaptain
2018-May-10, 09:43 AM
So any guesses where the black hole actually physically starts? I mean, there seems there's a gap between the event horizon and the black hole itself. To me the event horizon starts at the distance from the black hole where light can no longer escape. Perhaps this is a bad assumption.

grant hutchison
2018-May-10, 11:51 AM
The black hole starts at the event horizon, conventionally. It's the event horizon that makes it a black hole.

Grant Hutchison