Friday, 10 April 2020

Schwarzschild Black Holes


We're just looking at the first two pages of section 5.2 here where Carroll shows closing up light cones as one approaches an event horizon and then a beacon on a radial geodesic which purports to show that clock ticks on the beacon appear to get slower and slower according to a stationary observer who maintains a safe distance. 

I slightly improve on the former and show the world lines of in- and out-going photons starting at a given radius. I then try to reproduce the latter, first with an invented geodesic, and then with a properly calculated one. The invented geodesic (shown above) produced the result Carroll suggests but the 'properly calculated ones' did not. The first method produced correct geodesics, but not the desired ones. I believe that the second method failed due to my mathematical inexperience. Possibly there is no exact solution.

The diagram is like Carroll's Fig 5.8. It shows the world lines of a photon and a beacon falling directly towards the centre of the black hole. There is an observer hovering above them at a safe distance. Using the Schwarzschild metric the photon never seems to cross the event horizon at ##r=R_S##! The beacon, also falling directly in, sends signals (flashes of light) back out at intervals ##\Delta\tau_1##. They arrive at the observer separated by longer and longer times. The beacon also appears to take forever to get to the event horizon!

The maths didn't really work. Witness my struggles at Commentary 5.6 Schwarzschild Black Holes.pdf (8 pages)

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