# Re: This Week's Finds in Mathematical Physics (Week 224)

1. Nov 4, 2006

### Greg Egan

In article <k7l222ijLQvDFwoL@farmeroz.port995.com>, Oz
<oz@farmeroz.port995.com> wrote:

> Greg Egan <gregegan@netspace.net.au> writes
>
> >What is an "s/c radius"?

>
> Ok, damned dyslexia, I'll have to look up the spelling...
>
>
> > All the ways I know of giving a "radius" to a
> >black hole give results that are proportional to the mass: e.g. the
> >circumference of the event horizon divided by 2pi, or taking the square
> >root of the scattering cross-section.

>
> any reasonable measure will do.

The Schwarzschild radius is proportional to the mass, not the square root
of the mass. The Schwarzschild "r" coordinate for the geometry is equal
to the circumference of a circle centred on the point of symmetry divided
by 2pi, and the Schwarzschild radius is the r coordinate of the event
horizon. In units where G=c=1, the Schwarzschild radius is equal to 2M.