Re: the existance of black holes

Ofer

As you say the greater the Gravitational Field the greater the effects
of time dilation for a signal being transmitted from within the field. 
The reason why Black Holes are allowed in Relativity is that time
dilation depends on the `Frame of Reference' of the observor. 

When a Black Hole forms it does so in about 1 second, in its frame of 
reference. Once formed, light signals emitted from within the event horizon[1]
will not be able to escape as you say. Light signals emitted from very close
to the event horizon will be able to escape but will be more and more 
red shifted the closer they are to the event horizon. 

To illustrate this there is a classic `gedankenexperiment'. An Astronaut
pilots his space crasft too near a Black Hole and falls into the event horizon.
From his point of view he disappears very swiftly into the hole. As he does
so, light from outside the hole is increasingly time dilated by the gravitational
field making it appear as though the Universe is slowing down. To an outside
observor the opposite happens. As the Astronauts ship gets closer to the event
horizon his signal is increasingly time dilated and the ship never appears to 
cross the event horizon. 

The problem this is that time appears to be normal to the source
of the signal, the Black Hole. How we, in another frame of reference, measure 
things depends on the Gravitational Field surrounding the Black Hole. A third
observor, in another frame, may measure things totally differently. This is
why Special and General Relativity are called that, all things are Relative.
The rules of Relativity only allow us to describe how events appear in other 
frames of reference.

I hope this is an adequate answer for you. I would reccomend reading any 
good book on Relativity such as the Feynman series of Lectures or for a 
good on line source, 

Eric Bairds home page

Please E-Mail me if you need any further clarifications.

Dave Barlow

[1]The event Horizon being defined as the point at which the escape velocity
exceeds the speed of light.