Event horizon
Black Holes are the most extreme objects in our universe, tilting space time to its utmost. Their defining characteristic is the `event horizon’ – a one-way membrane, separating the inside from the outside. What goes in, never comes back – not even light can escape its grip. Will we ever be able to see this enigmatic region in space with our telescopes? Well, we may …
Black holes are in fact no holes, but extremely dense concentrations of matter. The typical size scale for a black hole is the Schwarzschild radius, Rs=2GM/c^2, where G is the Gravitational constant and c is the speed of light. For the sun the Schwarzschildradius would be 1.5 km, the Earth would have to be compressed to a diameter of less than 2 cm to turn it into a black hole. The biggest black hole in our ``neighbourhood’’, however, resides in the center of the Milky Way, some 27.000 light years away and weighs 4 Million times the weight of the sun (a trillion times the Earth mass). It is called Sgr A* (Sagittarius A star).
How would such a black hole look like? Bardeen (1977) showed that, if placed in front of a luminous sun, a stellar mass black hole would really look like a dark disk. The problem is that we never find a black hole that sits right in front of a star that we could resolve with optical telescopes – these black holes are too small. So, can we use Sgr A*, the biggest black hole in the Milky Way? In 1998 we measured the radio spectrum of this source, concluded that the radiation emitted at THz frequencies stems from within a few Schwarzschildradii, and proposed that it could be used to image the event horizon (see Falcke et al. 2000). Similarly our `jet-model’ of the radio emission in Sgr A* suggested that the radio emission would come from progressively closer regions to the black hole as one goes to higher frequencies.
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