Lecture 23 - Black Holes II

Chandrasekhar once remarked - The black holes of nature are the most perfect macroscopic objects there are in the universe: the only elements in their construction are our concepts of space and time.''

Very appropriately, one of the most remarkable developments in physics in the later part of the twentieth century, the formulation of the theory of black holes, has depended entirely upon considerations of theoretical physics and abstract mathematics, without hardly any observational corroboration. This lecture describes the path to the most important milestones of this theory in simple terms.

As we saw in Lecture 21, in 1939 Oppenheimer and his student, Snyder, made the spectacular discovery that if a star contracts to the critical radius, and becomes BLACK, it will continue to collapse till it disappears - all that will be left is a Black HOLE! This raised the following fundamental question, ``What is inside a black hole?"" This question was answered emphatically by Sir Roger Penrose in 1965. He showed in a mathematically exact manner that if a black hole forms, there must be a space-time singularity inside it. He was awarded the Nobel Prize in 2020 for this profound discovery. This was followed by a series of spectacular theorems - the ``No Hair Theorems"" - concerning the attributes of black holes. And then came, in 1974, the great discovery by Stephen Hawking that black holes will emit radiation, after all! This lecture will attempt to describe these great discoveries in simple terms.