Can someone help with this question, please?
3.a Show that the magnitude of the 4-acceleration [a] for a stationary observer at radial coordinate r in a Schwarzschild black hole of mass M is |a| = (1 - 2M/r)^(-1/2).
b Explain the physics of the limits r -> 2M and r -> ∞.
c When r < 2M, the expression becomes imaginary. What does that mean?