Do black holes exist?
Black holes are so strange, that for a long time they were thought to be just a theoretical curiosity with no relevance to our world. But in the 1970s a powerful x-ray source, called Cygnus X-1, was discovered lying about 8000 light years from us. This source flickers on time scales of about one hundredth of a second. Observations suggest that every part of the source changes its brightness at the same time. That can only happen if the source is small enough for some influence to travel from one part to the other, to keep the flickering in sync, in about one hundredth of a second. This implies that Cygnus X-1 must be smaller (probably much smaller) than 1/100 th of a light-second across; that is, smaller than the size of the Earth! We have some idea of the mass of the x-ray source because if forms a binary with a blue supergiant star (HDE 226868) whose mass is expected to be about 30 solar masses. Information about the masses in a binary system can be had by using (a more exact form) o
