Thank You, Stephen

Stephen Hawking is no longer with us. We’re bereft of his sly smile and his youthful irreverence, which he kept even when afflicted by old age and illness. And what an illness …

Only three months have passed since we lost him, but we can already try to ask ourselves, calmly and beyond our immediate reactions, what his legacy really consists of – in physics and beyond. I’m going to attempt an answer, tiptoeing as it were, out of friendship and the great admiration that I have for him.

Stephen was first and foremost an excellent physicist, one of the very best of his generation; not the greatest scientist of the century, the new Einstein or the new Newton, as he was sometimes called, with an exaggeration that he himself did not for a moment hesitate to impishly, playfully, nourish. So, I will begin with his most important scientific results.

His major discovery, the one that will be linked to his name for ever, was the demonstration that black holes behave as if they were hot: they irradiate heat like a stove. He arrived at this conclusion in 1974, with a complex and delicate calculation that skilfully combined techniques of general relativity and the theory of elementary particles. The temperature that he calculated is today known as ‘Hawking temperature’, and it depends on the size of the black hole. The larger the black hole, the colder it is. Hot black holes are therefore the smallest. This result provoked a big surprise in the seventies, bringing Stephen, who was barely thirty years old, renown among theoretical physicists. Up until then, no one had expected that a black hole could have a temperature. Not even Stephen himself, until he had completed the calculation.

The heat that irradiates from black holes is today called ‘Hawking radiation’. It has never been observed, and it will be difficult to observe it any time soon because it is so weak. But its existence has been vouchsafed in many different ways, and it is accepted as plausible by the vast majority of scientists.

Why is ‘Hawking radiation’ important? Because it is a phenomenon that involves both the structure of spacetime and quantum mechanics. This makes it an important indicator with regard to one of the great open questions in contemporary physics: the search for a theory of ‘quantum gravity’, which is to say a theory that describes all the ‘quantum’ aspects of space and time. Hence much contemporary research uses Hawking’s result or seeks to develop it. The research group in which I work, for example, is currently trying to use a possible theory of quantum gravity to calculate what happens to a black hole after being consumed by irradiating Hawking radiation.

There is a beautiful formula that summarizes Hawking’s result. It is the formula that gives the temperature, as a function of the mass M of its surface. This is the extremely simple formula given above: T = ħc³/8πGkM.

The beauty of this formula lies in its simplicity, but above all in the fact that it combines the four fundamental pillars of physics: the Boltzmann constant k, which is the root of thermodynamics, the speed of light c that characterizes relativity, the Newton constant G that characterizes gravity, which is to say the structure of spacetime, and the Planck constant ħ on which quantum mechanics is based. There is no other formula that puts together so elegantly all the fundamental pillars of our physics. No wonder Stephen asked for this formula to be carved on his headstone.

Among Stephen’s minor results, there are two that stand out as particularly relevant. As a young man, in collaboration with the great English mathematician Roger Penrose, he demonstrated that Einstein’s theory predicts that the universe emerged from a ‘Big Bang’: a singularity where the theory no longer works. This conclusion had been reached previously only by assuming, not very realistically, that the universe is completely homogeneous. Penrose and Hawking’s theorem showed that this simplification was not necessary – a result that helped to make the Big Bang much more plausible.

Stephen returned to the question of the Big Bang in the eighties, seeking to show how a quantum theory can effectively describe the birth of the universe. He constructed a fascinating intuitive model of quantum gravity and applied it to the inception of the universe. The model still inspires current research in quantum gravity.

There is a thread that connects these results. As a young man, Stephen had become passionately devoted to Einstein’s great theory. At the time, the applications of it were few, and the research mathematical. The most spectacular predictions of physics, such as black holes and the Big Bang, were still thought to be esoteric and unsound. Penrose had bolstered them by showing that black holes undoubtedly form when sufficient amounts of matter become concentrated, and Stephen had the idea of using Penrose’s technique for studying the origin of the universe. The idea was that the birth of the universe was something like the collapse of a black hole seen backwards in time.

Having clarified that inside black holes and in the primordial universe Einstein’s general relativity became insufficient, Stephen was prompted to start to consider quantum effects. In this way he arrived at Hawking radiation. Then, in subsequent years, he attempted to use quantum mechanics fully in order to rethink the beginning of the universe in terms of quanta. All these problems are still unresolved. But in contemporary discussions of them it is not uncommon to hear reference to the name Hawking, or to one of his ideas.

My summary hardly exhausts Hawking’s theoretical activity, but I hope to have given a basic sense of what he contributed to physics.

I believe, however, that his greatness lies elsewhere.

His real greatness was his humanity, his character. Tied to a wheelchair, he progressively lost control of all the muscles in his body. The last time I saw him, in Stockholm, he could barely even move his eyes. He communicated by moving them: an electronic system read his eye movements with a small camera, and thanks to this Stephen was able to control a computer to put letters laboriously in line, to construct words that were then spoken by a vocal synthesizer. It was painful to watch him undergo this exhausting and extremely slow process.

And yet the voice of that synthesizer reached the entire world. That so distinctive metallic voice which Stephen somehow managed to make his own, turning it into the almost natural medium of his brilliant intelligence and of his irony. He never lost heart. He continued to produce physics of quality, even as the condition of his body continued to deteriorate. In seemingly impossible circumstances he managed to write a book that became phenomenally successful. In the thirty years since it was published it has sold more than 10 million copies, and it continues to be read. With this book he spoke to young people throughout the world, amazing and inspiring them to study the universe.

For all the extreme misfortune of his condition and disability, Stephen was also the beneficiary of not a little luck: blessed with exceptional intelligence, he was born into an excellent family of English intellectuals and received an education of the first order. The progress of his illness was also much slower than was initially and drastically predicted. His value as a scientist, and then his fame, allowed him to achieve things that others with similar conditions could only dream of. But even when taking all this into account, Stephen, with his rather insolent air of being an untouchably youthful spirit, has given the world an extraordinary lesson in humanity. A lesson in love for life, in intelligence, and in unquenchable curiosity.

The day after our meeting in Stockholm, where communication with him was so difficult and heartbreaking, Stephen gave a lecture in a huge theatre in the city. He was everywhere surrounded, to an incredible degree, by young people hanging on his every word. He arrived onstage with his legendary smile, his legendary wheelchair, and set the prerecorded lecture running with a movement of his eyes. He talked in it about ultimate attempts to understand the future of black holes, made jokes, gently mocked the French, played with the meaning of life, irreverently, rebelliously, a smile on his lips accompanying every phrase. The enormous audience was spellbound. His last words were still an undaunted declaration of love for life, but as always with a play of ambiguity: you can indeed escape from black holes.

Stephen was certain that life does not continue in any other form after death. Like many scientists, he was fond of using ‘God’ for emphasis and effect, but was a confirmed atheist, without ambiguity or uncertainty, and he said so clearly and without hesitation. It wasn’t any kind of transcendence that he found consolation in or drew his strength from. He was imprisoned by the most debilitating of illnesses, linked to the rest of us by an ever-thinner thread. Yet he continued to live until the end with a burning intensity – to joke, to speak to the whole world, to communicate happiness and joy, moving new generations to follow him with his enthusiasm. Is this not an extraordinary lesson in life for all of us whingers? Is this not the infinitely precious gift that Stephen has left us? The irresistible luminous force of life, of curiosity, of thought, of intelligence.

Now that thinnest of threads has been cut. Before disappearing for ever, as happens to all things, dissolving into the immensity of the boundless cosmos that he loved, Stephen still remains for a while, alive and active in our science, in our memory, in our affections, in our thoughts. Thank you, Stephen.