The legacy of a genius locked in a sick body

Beyond the familiar image of him as a scientist on a wheelchair speaking through a machine, Stephen Hawking (1942-2018) was a theoretical physicist who from Cambridge, the same university city where he died this Wednesday, never stopped researching the nature of cosmos despite his limitations. In 1963 he was diagnosed with ALS and doctors sentenced him to an early death, but, against all odds, he continued working for more than half a century in the field of cosmology.

"When he was just a doctoral student, Hawking was already able to take Einstein's theory of relativity to its limits and show that it failed to describe aspects such as the beginning of the universe or the end of a star that ends up forming a black hole,” Professor Roberto Emparan from the University of Barcelona (UB) points out to Sinc.

With him, black holes turned gray

Black holes have been one of the central objectives of Hawking's research. His first breakthrough came in 1970 when he and Roger Penrose applied mathematics to these dark objects and showed that a singularity, a region of infinite curvature in space-time, also underlies the beginning of everything: the Big Bang.

"The first thing that made Hawking famous was this proof that the expansion of the universe meant that the known laws of physics were 'broken' in the past during that event we call the Big Bang," explains John Ellis, a researcher at the European Particle Physics Lab (CERN) and professor at King's College in London.

Ellis, who considers Hawking one of the best theoretical physicists of the last 50 years, highlights his most famous contribution: "He proved that black holes are not completely black; that is, they emit quantum radiation, which implies that in the end they end up fading".

The astrophysicist Pilar Ruiz Lapuente, of the Institute of Cosmos Sciences (UB), sums it up in one sentence: "The quantum treatment of black holes makes them only gray," and explains it: "A black hole is a region of space where the gravitational field created by an object is so strong that radiation cannot escape it. However, taking into account the quantum processes that occur at its outer limit (called the event horizon), there are particles that can be emitted from there. These constitute the so-called "Hawking radiation," which is now being recorded in laboratories.

Another researcher, Alicia Sintes, professor at the University of the Balearic Islands and leader of the Spanish group of the international LIGO cooperation that has discovered gravitational waves, emphasises "the courage and persistence of Hawking," but points out that with these waves "we’ll be able to extract information about the Big Bang and what the expansion of the universe has been like, but nothing that makes it possible to measure the radiation of black holes".

It was in 1974 when Hawking published the study in which he resorted to quantum theory to claim that black holes could emit radiation in the form of heat and vanish. The times it takes for normal-sized black holes to disappear are as long as the age of the universe. However, the tiniest ones could do so before, releasing heat at a spectacular rate, with the energy of one million hydrogen bombs.

Crisis of principles in physics

The proposal that black holes radiate heat caused one of the most passionate debates in modern cosmology. Hawking argued that if a black hole evaporated, all the information that had fallen into it before would be lost forever. This contradicted one of the most basic laws of quantum mechanics and many physicists did not agree.

As José Luis Fernández Barbón, a researcher at the Institute of Theoretical Physics (UAM-CSIC) stresses, "the process by which black holes disintegrate slowly emitting quantum particles, far from representing an anecdotal detail, triggers a crisis of principles in the foundations of physics, a conflict between the two master pillars that represent the theory of relativity and quantum theory”.

"Whether black holes do or don’t destroy the information trapped inside them is a question that has dominated much of the speculation in fundamental physics during the last forty years," he adds. “Today, hundreds of theoretical physicists are still working on solving this dilemma, which has transformed our way of interpreting the quantum properties of gravitational force".

Emparan agrees: "Hawking explored how to overcome the limits of Einstein's theory by incorporating the effects of quantum, that is, how to combine the physics of the very large (gravity) with that of the very small: quantum. But what he found when combining them in the presence of a black hole was a paradox, a fundamental contradiction between both theories that continues to perplex us”.

Three lost bets

Hawking and his colleague Kip Thorne wagered an encyclopaedia – a baseball encyclopaedia, as it turned out – with the physicist John Preskill on whether information was lost in black holes. Hawking finally accepted in 2004 that he had been wrong, conceding that quantum fluctuations (small variations in the distribution of matter) can occur at the edges of black holes, meaning that information could escape and was therefore not lost. His long-time friend Roger Penrose never agreed with this change of opinion.

This was not the first time that Hawking lost a bet. In 1990 he had to pay Thorne a subscription to Penthouse magazine for mistakenly forecasting that the cosmic X-ray source Cygnus X-1 was not a black hole. More recently, in 2012, he also lost $100 with Professor Gordon Kane for betting that the Higgs boson would not be discovered

Quantum seeds that create galaxies

While a terrible gambler, Hawking continued to offer relevant contributions to cosmology. In 1982 he was one of the first to predict that during the first instants of the universe, when it began to expand by cosmic inflation, quantum fluctuations could act as seeds to create galaxies and, ultimately, stars, planets, life and everything we know today.

Independently, the Russian physicist Viatcheslav Mukhanov came to this same conclusion and the two were awarded a Frontiers of Knowledge Award in 2016 for these works, which have been experimentally confirmed.

In 2013, the Planck satellite of the European Space Agency detected very slight temperature variations in cosmic microwave background radiation, which can be related to the presence of quantum fluctuations and matter since the beginning of the universe.

Giant of dissemination

"Much of what we know about cosmology bear the marks of Hawking´s contributions," says Fernández Barbón, who also stresses his informative facet and his struggle, throughout most of his life, with his disease: "He has become such a recognizable character at a popular level that he will be remembered as one of the historical champions of the human species”.

The Prince of Asturias Award Hawking received in 1989 in Spain also recognized his role when it came to popularising scientific contributions about the origin and destiny of the universe.

"One example is his bestseller A Brief History of Time," recalls Ellis, who is amazed at what he was able to achieve despite his health problems: "The world has lost a scientific giant and an inspiration for all."

A new era of human expansion

"Hawking is the champion of a world in which possibilities have no border if we continue working together," says Professor Ruiz Lapuente, who also highlights some of the revolutionary projects that this visionary embarked on: "One of them is the Breakthrough Starshot initiative for the exploration of extra-solar planets, with nano-spacecrafts that will travel at speeds much greater than those of the ships we know, at 20% of the speed of light. This will mean the beginning of a new era of the expansion of mankind in the universe”.

"Stephen Hawking has not only been one of the most brilliant scientists in the last fifty years: he is one of the most extraordinary people in the history of mankind, someone who has shown that our destiny, however tragic it may seem, is not written," concludes Emparan.