Stephen Hawking enrolled at the University of Cambridge in 1962 to study cosmology. He wanted to work with the astronomer Fred Hoyle who, along with two of his colleagues, had floated the steady state theory of the universe a few years earlier. The theory proposed that even though it had been experimentally observed that heavenly bodies were moving away from each other, the universe had always remained more or less in the same condition — it neither had a beginning, nor would it ever have an end. Hawking, however, could not get Hoyle as his PhD guide, and instead worked with Dennis Sciama. In his PhD thesis titled ‘Properties of Expanding Universes’, Hawking showed that there were problems with the steady state theory which had been further developed by Hoyle and Jayant Narlikar.
Hawking developed his ideas further, and working on Einstein’s general theory of relativity, showed, in a landmark 1970 mathematical work with another celebrated British mathematical physicist Roger Penrose, that the universe did, in fact, have a beginning — in what is now famously known as the Big Bang — and could possibly have an end as well. The Big Bang is now the most widely accepted theory of the origin of the universe.
Another landmark scientific achievement came four years later, when Hawking revealed something fundamentally new about Black Holes, those unimaginably dense structures which remain one of the most puzzling areas in the outer universe. In doing so, he showed, for the first time, that the two pillars of modern physics — quantum theory and Einstein’s general relativity — could be inconsistent with each other, a possibility that could call for a major revision of physics as we know it.
Quantum theory describes the behaviour of very small particles, those smaller than an atom, like protons or electrons, or the even smaller ones like quarks. General relativity, on the other hand, describes how gravity works around massive objects like planets, stars and Black Holes. Both theories have so far passed every experimental test. In 2015, yet another prediction of general relativity was experimentally verified, when gravitational waves were detected for the first time, exactly 100 years after these were proposed by Einstein. Scientists believe that quantum theory and general relativity can be unified into a single theory that can explain all phenomena, big or small. This unified theory remains elusive still, but these have also not been found to be contradicting each other.
Until Hawking came up with what has since been called the Information Paradox. Until then, Black Holes were believed to be so dense and massive, that once sucked into it, nothing could ever come out. Hawking, while trying to apply quantum mechanical rules to activities happening near a Black Hole — in a way, trying to bring quantum theory in sync with general relativity — showed that rotating Black Holes would in fact emit thermal radiation. Emission is similar to something escaping from Black Holes.
This was entirely new, and a fundamental discovery. He also showed that because of the emission of this thermal radiation, the black hole would lose energy and eventually disappear or “evaporate”. What was even more radical was his suggestion a few years later, in 1981, that along with the “evaporation” of the Black Hole, all “information” that was once contained in the Black Hole would also disappear. No signal would remain that would give any indication of anything that existed in the Black Hole.
Now, quantum theory strongly disapproves of this kind of vanishing act by any matter or radiation. If something disintegrates into nothingness, it must reveal itself in some form of energy somewhere else.
The Information Paradox, or Hawking Paradox, as it is also called, became highly studied and debated, and there were several attempts, including by Hawking himself, to resolve it — with limited success. Some scientists believe that if it could be shown that the thermal radiation emitted from the black hole was more than just thermal, it could be possible to examine whether the “lost information” was actually going out with this radiation, therefore opening up the possibility of resolving the paradox. A complete resolution of the paradox, to the satisfaction of everyone, has still not been found — and the search for a single unified theory of the universe, too, remains elusive.
Apart from these, Hawking contributed immensely to the knowledge of Black Holes. Much of what is today known about these mysterious areas in the outer universe, where established laws of physics seem to become increasingly uncertain, are due to his scientific work. He was the world’s foremost authority on Black Holes. Much of this work is theoretical, and without experimental evidence as of now — which possibly explains why Hawking never received the Nobel Prize.