Published on 21/02/2013
Why can't scientists detect a black hole with a telescope?
You actually can detect black holes with a telescope, but we will come to that. First, you cannot see black holes directly because they do not emit light: light is sucked into them after all. So, unlike stars, they are not directly visible.
However, there are a couple of ways you can spot them. The way we usually study black holes is by looking at matter falling into them. This often forms a disc about the black hole before spiralling in. This disc gets very hot and so emits X-rays. We can detect these with an X-ray telescope (not a conventional telescope) and infer the presence of a black hole. Another way is to watch a large number of stars for a long time. Eventually, if you are lucky, a black hole will drift between you and a particular star and you will notice a charge in the image. As light passes by the black hole its path gets bent, so the image is distorted. We call these microlensing events. This can be done with more conventional telescopes, but you have to watch many, many stars in order to find one of these rare alignments.