05 Dec '14 12:29>
Originally posted by SuzianneThe black hole would have to be huge. This is a hack's argument - this isn't rigorous as I'm liberally mixing results from Newtonian gravity with Einstein's theory. The formula for gravitational tidal acceleration is:
I always thought with a sufficiently large black hole that one may not realize when one has passed the event horizon. Not immediately, anyway.
a = 2GML/R^3
This is the difference in acceleration between the top or bottom of the body and the centre (L is the half length).
M is the mass of the gravitating body - black hole in this case.
G is Newton's constant.
R is the radial distance outward.
L is the length of the tidally stretched body in the direction radially outward from the hole.
this is from Newton's theory. The Schartzschild radius is r_s = 2*G*M/c² from Einstein's theory.
We can replace 2GM with c²r_s to give:
a = c²*L*r_s/R^3
We want to know this at the event horizon so set R = r_s
a = c²*L/r_s²
r_s = c*sqrt(L/a)
Suppose L = 1 metre (about right for a human), and a = 1 Newton/kg = 1 metre per second squared ('cos it's convenient). (1/10th the gravitational acceleration at the Earth's surface)
then r_s = 3*10^8 metres or 1 light second.
This corresponds to a black hole mass of 100,000 times the mass of the sun. Saggitarius A*, the radio source at the centre of the galaxy is believe to have a mass of 4.31 million solar masses. This corresponds to a Schwartzschild radius of 40 light seconds, and the tidal acceleration would be 0.024 N/kg, so yes barely noticeable.
A stellar mass black hole on the other hand would produce a tidal acceleration of 100,000 metres per seconds squared.