The physicist community mourns the death of Stephen Hawking, one of its greatest minds. A life that began in the midst of the deepest crisis of the 20th century and is unlikely to be surpassed in intellectual heights and at the same time physical suffering has come to an end. Stephen Hawking was the pop star of contemporary physics. In this role, he followed in the footsteps of the two greatest scientific pop stars in history, Albert Einstein and Isaac Newton. At the latest with the publication of his popular science book A Short History of Time in 1988, he became the most famous scientist of our time, to which even the honor of two films was given. His book sold more than 10 million copies and is still one of the best-selling popular science books ever. It is sometimes referred to as the „most popular book never read“.
Hawking’s scientific achievements built heavily on Albert Einstein’s theories. Since the early years of his scientific career hehad beenintrigued by the arguably most remarkable prediction of general relativity: black holes. Shortly after Albert Einstein formulated the basic equations of his new theory of gravitation in November 1915, physicists had realized that his new theory seemed to make a rather extraordinaryprediction. If one considers the solution of the Einstein equations for the gravitational field of a point-like mass, the gravitational force or the curvature of space-time in the immediate vicinity of this mass is so great that even light can no longer escape from it. The space-time structure possesses what physicists call a „singularity“ at this point. That means nothing less than that space and time stop existing at this point. However, the density of matter required for such a structure is so large (it corresponds to the entire mass of the earth be contained in a sphere with a radius of 9 mm) that Einstein and his colleagues did not know what to do with such a solution. The term „black hole“ for these structures thus came up only about 50 years later.
The young Stephan Hawking belonged to a small group of scientists who in the mid-1960s turned their mind to the properties and details of these exotic solutions of the Einstein equationswith renewed vigor. In the late 1960s, together with his mentor and colleague Roger Penrose, he proved that the singularities are not artefacts resulting from false theoretical assumptions in solving the Einstein equations, as suggested by many physicists, but a direct consequence of the attractive nature of gravitation itself (this is the so-called singularity theorem). At sufficiently high mass concentrations black holes should therefore actually exist. Shortly thereafter, Hawking and Penrose provided mathematical evidence that the universe was born from a big bang. Their reasoning: the mathematics of the singularity in a black hole and the one at the beginning of the universe is the same. Hawking and Penrose thus provided some significant mathematical support to today’s widely accepted big bang theory. This also constituted the starting signal for cosmology as a science.
In 1974 Hawking’s most famous work followed, which earned him the reputation of a genius. To the surprise of his physicist colleagues, he was able to show that black holes do not irretrievably guzzle up matter and energy, but that they in turn emit radiation. Thus a black hole evaporates slowly until it eventually disappears, including everything that has fallen into it. This also applies to any information it has ever swallowed. However, this contradicts the laws of quantum theory, according to which information cannot be irretrievably lost. Hawking was so convinced of his conclusion that he demanded that established quantum field theory, which had been experimentally confirmed in every detail so far, should be modified accordingly.
The matter takes us deep into the problem structure of today’s theoretical physics. Hawking realized that in order to describe black holes, we need to consider the third essential theory in today’s physics besides relativity theory and quantum theory: thermodynamics. According to it, every physical system has a so-called entropy, a measure of the disorder contained therein, or equivalently its information. According to Hawking we need to assign such a measure equally to a black hole. But if a black hole can be assigned an entropy, it ought to obey the second law of thermodynamics, according to which entropy in a (closed) system can never decline, not to speak disappear. According to the general theory of relativity, however, a black hole would be an entropy (or analogue information) annihilator. The dilemma faced by theoretical physicists can thus be summed up as follows: Either they allow for the loss of information (or entropy) and must modify quantum theory and thermodynamics accordingly, or they allow information to escape from black holes, which requires new features to the general theory of relativity.
In honor of Stephen Hawking physicists named the radiation of black holesHawking radiation. What made his insight so special was that he obtained it by referring to quantum theory. He was the first to set up an astrophysical theory that combined quantum theory and the general theory of relativity. To this day, it remains the dream of physicists to combine these two fundamental but incompatible theories of nature, quantum field theory and the general theory of relativity, into a theory of quantum gravity. Like Einstein 50 years before Hawking sought all his life for a path towards such a unifying theory, a so-called „theory of everything“. But like his great predecessor, he remained unsuccessful.
In the 1980s, his body already marked by grave illness, Hawking pursued a special path to a theory of quantum gravity and general cosmology. For this he applied a complex mathematical method from quantum field theory to the theory of relativity, the so-called Euclidean path integral formulation. As he explains in his book A Short History of Timewithin this approach one has to introduce an imaginary time variable, a trick in quantum theory and thermodynamics called “wick rotation”. This allows to mathematically assessthe beginning of the universe. The method can be illustrated with a spherelike the Earth’s surface, the „beginning“ of which (in the dimension of time) is the North Pole. A sphere has no edge, i.e. there is no natural boundary on it. Paths on it are closed or endless. Applied to the dimension of time this means that there is no beginning and no end. Analogously, the universe is closed in itself. According to this „no-boundary“ hypothesis it started spontaneously from nowhere. In Hawking’s words: „The question of what was before the Big Bang is the same as asking what is a north of the North Pole“ (this statement goes back to Albert Einstein). He was convinced that his concept of „no-boundary conditions“ is the key to creation, the answer to the question why we are here. Hawking’s concept would have important consequences for our world view and religious believes. In his own words:
„So long as the universe had a beginning, we could suppose it had a creator. But if the universe is really completely self-contained, having no boundary or edge, it would have neither beginning nor end: it would simply be. What place, then, for a creator?“
In 2004, when he was no longer able to speak, Hawking recognized a characteristic of black hole that contradicted his work from 30 years earlier: No information is ever lost in them. Any information about objects swallowed by a black hole will eventually be recovered in an altered form. This insight was based on properties of a theoretical framework that had emerged since: string theory. And here he proved himself a fair loser: In 1997 he had agreed on a bet with his colleague John Preskill that there is no way in a quantum gravity theory that in a black hole information is preserved. With his change of opinion, he lived up his debts: an encyclopedia of the winner’s choice from which information “can be recovered at will“.
Even outside his area of expertise Hawking did not hesitate to make his opinion public. He talked about the possible risks that the search for extraterrestrial life might have for humanity, the need to populate space, or that artificial intelligence and robots will replace mankind as a whole. Referring to the later he said:
„Success in creating effective AI, could be the biggest event in the history of our civilization […] Unless we learn how to prepare for, and avoid, the potential risks, AI could be the worst event in the history of our civilization. It brings dangers, like powerful autonomous weapons, or new ways for the few to oppress the many.”
Through Hawking’s public engagement and popular books, a broader audienceobtained some idea of how crazy modern theoretical physics appears and how superb and beautiful it is at the same time. Stephen Hawking, like very few people (among them Albert Einstein), has shown that the highly abstract and mathematical insights of modern astrophysics and cosmology are among the finest products of the human mind. Despite their abstractness which prevents most people from assessing them they shaped our modern world view like barely any other idea. And on just one more level is Stephen Hawking related toEinstein: Is it really a coincidence that his death falls exactly on the birthday of the later: Einstein would have turned 139 years old on March 14th (furthermore, Hawking was born on Galilei’s death day).With Stephen Hawking, a truly beautiful mind has gone from us that has never stopped moving to the highest spheres of thought even when the body hadlong lost almost anyability to move.