```Ralf Mler wrote:

> for a small black-hole you need 15 x mass of the sun, which is 15 *
> 1,99*10^30 = 1.76261265e+16 kg. With an average of 75 kg, there
> should be at least 2.3501502e+14 Rubyists (that's what I call "World
> Domination").

It is correct that it needs such a mass if the black hole is the result
of a collapsing star. Smaller masses do not form a black hole but a
neutron star (or something even less dense). IIRC it was the famous
indian theoretical physicist Subramanyan Chandraskhar who showed that
given a mass of more than 15 times that of the sun the pressure becomes
so huge that not even the supermassive neutron matter can stand and
collapses even further.

Nevertheless it is possible that black holes do exist that have a much
lower mass. To understand this one should recall what a black hole is.
The speed you need to escape from a gravitational field essentially
depends on two factors: The distance between you and the source of that
field and the mass of that source. From the mass of an object you can
compute a certain distance that is called the Schwartzschild radius. If
your distance to the object (more precisely: to its center of mass) is
smaller than this radius you would need to fly at superluminal speeds
which to the best of our best knowledge are impossible to reach.

There is one *extremely* important limitation to this: The statement is
only true if the Schwartzschild radius is larger than the actual size of
the physical body. If the Schwarzschild radius is smaller than the
physical size of the object (which is by far the most common case) the
Schwarzschild radius has no real-life meaning.

But what if the physical size of the object is smaller than its
Schwarzschild radius? In that case there is a region around the object
from wich nothing can escape. This is a black hole. The boundary of that
region is called the 'event horizon' of the black hole which means that
nothing beyond that horizion is an event that has any effect on the
region outside the black hole.

In principle the condition that the Schwarzschild radius is larger than
the physical size of the object can be met for *any* mass. Now enters
empirics. It is an unsolved question if black holes with small masses do
exist. Theoretical physics cannot decide, only nature knows. Up to now
no way of asking nature has been found.

Josef 'Jupp' Schugt
--
An often unanswered question in SciFi: Why does the Schwarzschild radius
mean a point of no return to space vessels that can fly faster than light?

```