Lecture 22 - Black Holes I

There are two kinds of black holes. The first is formed in supernovae, as the evolutionary end products of the most massive stars. The stellar black holes can be as heavy as a few tens of solar masses. Then there are the super-massive black holes, such as the one sitting at the center of the Milky Way and most other galaxies, which can be more than a million times more massive than the Sun.

On 10 April 2019, eighty years after Oppenheimer & Snyder predicted the existence of black holes, scientists of the ‘Event Horizon Telescope (EHT) released the first-ever image of a black hole - the super-massive object at the heart of the M87 galaxy. This impressive image is a rather remarkable indication of our technological prowess as well as the capability of our scientists to form and work in gigantic collaborations.

The idea that light may not be able to escape from sufficiently dense bodies dates back to 1783 when the English Pastor John Michell first advanced this notion. This found a prominent mention in the great treatise by the French mathematician Laplace (1798). This conclusion was within the premise of Newton's theory of gravity. Einstein published his new theory of gravity in 1915. Within a couple of months, the great German Polymath obtained an exact solution to Einstein's equations for the gravitational field. This solution, describing the geometry of space-time around a non-rotating star, clearly showed that when a star contracts to a critical radius, no signal would be able to escape from it. Many decades later, the New Zealand mathematician Roy Kerr obtained an exact solution to Einstein's equations describing the geometry of space-time outside a rotating star. This lecture is a historical account of these monumental discoveries and explains several of the spectacular consequences of Einstein's theory of gravity.