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FNWI --- IMAPP Department of Astrophysics
Radboud University > Faculty of Science > Department of Astrophysics

Event horizon

Simulated images of the shadow of the black holeBlack 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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