In 1796, Simon Pierre LaPlace predicted that there are objects in the universe which are so massive that light cannot escape them. As a result, they would be “invisible” to us. These objects have become known as black holes. More than 100 years later, Einstein published one of the most successful theories of modern science: General Relativity. A year later, Karl Schwartzchild used Einstein’s equation to define a black hole and calculate its radius, which is now known as the Schwarzschild radius. While black holes captured the imagination of many scientists, Einstein himself did not like them. In fact, in 1939, he published a paper that attempted to show they cannot exist.
Over the years, however, several lines of indirect evidence have supported the existence of black holes. For example, astronomers can measure the speed of objects in orbit around other objects. The speed of the orbiting object indicates the mass of the object being orbited. In the center of a nearby galaxy charmingly named “M87,” there is a disc of hot gas that is orbiting so quickly that the mass of what is being orbited must be three billion times the mass of our sun. However, the size of the object is, at most, the size of our solar system. Those measurements are consistent with Schwartzchild’s description of a black hole.
Of course, it’s always possible that the speed measurements are wrong, or that there is a very massive object that is consistent with what we think a black hole might be but isn’t actually a black hole. Thus, we need some other means by which to analyze the object. That’s where the Event Horizon Telescope (EHT) comes in. Despite it’s name, it is not a single telescope. It is a combination of eight different telescopes that are found in different geographic locations. Those telescopes examined the center of M87 for several days, and their data were combined together to produce the images seen at the top of this post. They are exactly what one expects those telescopes to detect if the center of M87 is a black hole. Thus, as the title of this post indicates, they represent the best evidence to date for the existence of a black hole!
Now while these images are excellent evidence for a black hole at the center of M87, it is important to know what they are and what they are not.