Few great scientists are famous to the general public, but Stephen Hawking achieved rock star status, thanks to his best selling ‘Brief History of Time’, and to the epic saga of his struggle not merely to survive but to master the disease that struck him – his inspirational refusal to give in to the unfair twists of fate. But the man or woman in the street would claim no knowledge of his scientific achievements: what it was that made his reputation in the scientific community, enabling him to write and speak to the public with such authority.
Although there was a lot of solid first rate work that gave him a reputation in the specialised community of cosmologists, the big item was undoubtedly his development of the theory of Hawking Radiation (as we now call it). This was a complete paradigm shift, revolutionising our ideas of black holes, but based on a simple and understandable concept.
Black holes – at the time they were just an oddity predicted by equations, today they are matter of fact items of galactic astronomy – occur when a body is so massive that its escape velocity is more than the velocity of light, so nothing can escape its gravitational pull. Such black hole will accumulate matter as it falls into it, never to re-emerge: it will just get more and more massive forever, or until the end of the universe.
Hawking took this description from General Relativity, and linked it to the picture of the vacuum that comes from particle physics. At the quantum level the vacuum is not empty but abuzz with virtual particle-antiparticle pairs (predominantly electrons and positrons) being created out of nothing and then re-merging into nothing. Hawking pointed out that in a very strong gravitational field, such as you get just outside black holes, one of these particles could fall down into the gravity well, giving the other enough energy to become real. The surface of a black hole will radiate electrons and positrons (“Hawking Radiation”), losing energy and mass as it does so. It will not exist forever, but will evaporate. Our whole concept of black holes changed: they are not just gobblers of everything, but dynamic objects that can be formed and also destroyed.
As well as its own intrinsic importance, his proposal showed how cosmology and particle physics could be fruitfully combined. Previously the former, which involves physics at the largest scale, had been very separate from the smallest-scale physics of the latter. Theoretical physicists might specialise in General Relativity or Quantum Field Theory, but very few worked on both, and never at the same time. Today that’s all changed, and our attempts to understand the earliest stages of the big bang, and the most fundamental laws of nature, are framed using combinations of microscopic and megascopic physics.
So he wasn’t just the author of ‘that book by that wheelchair guy’, as Homer Simpson put it, or the character that enabled Eddie Redmayne to win his Oscar. He revolutionised our ideas about black holes and their role in the universe, and he showed the way to a combination of the physics of the very small and the very large which is being carried forward today. That is his gift to us and future generations.