Re: Black holes

["Steven T. Ratliff" <STR@nwc.edu>]

Regarding Chris's message:

> Date sent:      Wed, 01 May 1996 12:03:28 +0100
> Send reply to:  phys-l@atlantis.cc.uwf.edu
> From:           chris.jones@ucl.ac.uk (Chris Jones)
> To:             phys-l@atlantis.cc.uwf.edu
> Subject:        Black holes

> Can anybody explain to me why the calculation of the Schwarzschild radius:
> R(sch) = (2GM/c^2)
> is valid? I have trouble explaining why this should form the event horizon 
> because:
> 1. The equation can be derived purely from the one for escape velocity, 
> which is non-relativistic

I think that this is a case where the wrong theory (classical 
mechanics) gives the right answer.  Also, note that even though the 
formula is the same, the interpretation is different.  If you are 
inside the event horizon, then you cannot get out, no matter what 
your speed (that would be the correct, relativistic result).

The classical formula would say that if your speed was 
large (but less than c), you could leave the black hole and cross 
the event horizon--you would eventually be pulled back, but the 
classical result would say that you would be able to cross the event 
horizon, which is wrong.

Hope this helps,

Steven Ratliff




Steven T. Ratliff
Assistant Professor of Physics
Northwestern College
3003 Snelling Av. N.
St. Paul, MN  55113-1598

Internet:  stratliff@nwc.edu