# Black hole-event horizon

stevebd1
Gold Member
Slightly off subject but it relates to a web page linked into this thread. I'm currently trying to find out what the maximum gravity gradient is that a human can tolerate (and what might be fatal). I've looked extensively on the internet and found only 2 sources of information-

One source states that '..as you approach a black hole, your body would be rotated lengthwise so that your feet or head would be facing towards the black hole; at approx. 3000 miles (4828 km) from a 10 sol mass black hole, all the blood in your body would rush to that end of the body which was closest to the black hole with fatal effect..'. Using GM/r^2 to calculate gravity and 2GM/r^3 to calculate the gravity gradient, the gravity at 4828 km would be 5.6954x10^7 m/s^2 and the gravity gradient would be 23.593 m/s^2/m which multiplied by 2 gives the gradient over 2 meters, the approx. height of a human; 47.186 m/s^2/m (a change of ~5 Earth g's from head to toe).
http://chandra.harvard.edu/resources/faq/black_hole/bhole-48.html

Another source states that '..for an 8 sol mass black hole, the tidal forces become fatal at 400 km..'. Again, using GM/r^2 and 2GM/r^3 the gravity at 400 km works out at 6.6378x10^9 m/s^2 with a gravity gradient of 33189.128 m/s^2/m, which multiplied by 2 = 66378.256 m/s^2/m (a change of 6773.3 Earth g's from head to toe).
http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/fall_in.html

I'm assuming the first scenario could be avoided by using some form of sophisticated g suit to stop blood from flowing to your feet/head but the second figure (which is considerably higher) I'm assuming is the point that ordinary matter is pulled apart. I'm aware that gravity itself isn't the issue here (unless you want to come to a complete stop) as you are basically free-falling and that it is the gravity gradient that is fatal. Is there something more specific on the web that might provide more indepth information (such as a graphs/gradients)? The only other info I found was that a variation of 15% in the gravity gradient from head to toe was considered uncomfortable! (this was regarding space station design).

regards
Steve

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