Professor Stephen Hawking, who was probably the most renowned and recognisable scientist in the world, and famed for his work on black holes, has died aged 76.
Despite suffering from crippling motor neurone disease since being diagnosed at the age of 21, Professor Hawking was one of the most brilliant minds of modern times, as well as a best-selling author, most notably with his book A Brief History of Time.
Hawking first came to prominence in the early 1970s, when a trip to Moscow to meet the eminent Soviet physicist Yakov Zel’dovich inspired him to question the assumption of whether black holes are truly black. Such was his genius that he would run complex equations through his mind, night after night, eventually leading him to the conclusion that black holes emit particles and, in the process, lose mass, shrink and ultimately disappear. It was an insight that would make his name immortal, but at first he didn’t quite believe it himself. His equations showed that, quantum mechanically, black holes are able to radiate away mass in the form of virtual particles – particles that fizz in and out of existence thanks to tiny quantum fluctuations. At first he stayed silent about his discovery, but he was encouraged by the support of his former PhD supervisor, Dennis Sciama, who apparently had far more faith in Hawking’s equations than Hawking himself did. Soon news of the breakthrough spread, though not everyone was as welcoming of the discovery as Sciama; some physicists were actively hostile to the idea. It took a few years for everyone to be convinced that the so-called ‘Hawking radiation’ was real. Today it is part of the furniture of black hole physics, and helped transform Stephen Hawking into a true celebrity of science.
It all comes down to the nature of the fabric of space–time. Quantum mechanics tells us that space–time is filled with quantum fields and that any given point in space has an associated energy, rendering space–time as a frothing foam of quantum energy. In his famous equation E=mc2, Albert Einstein had shown that energy and mass are equivalent, while quantum mechanics is a probabilistic approach to understanding nature. The combination of the two means that the energy of any given point in space can experience a random fluctuation and transform itself into mass, in the guise of ‘virtual’ particles that ‘borrow’ energy from the Universe, surviving for just a fraction of a second before annihilating each other and disappearing from existence as they return their energy to the cosmos.
When virtual particles form just on the inside edge of a black hole’s event horizon (the boundary beyond which not even light can escape), one of the particles moves deeper into the black hole while the other jumps across the event horizon thanks to another quantum mechanical process known as tunnelling. One of the early insights into quantum physics was that particles can also act as waves and have an associated wave-function that describes, among other things, the probabilities associated with their exact position. Some parts of a particle’s wave-function have greater amplitudes, implying that the particle is more likely to be found over here, while other sections of the wave function have lower amplitudes, implying a lower probability that the particle is really over there. When virtual particles form just inside the event horizon there is a chance that one of the particles can appear on the other side of the event horizon – its wave-function has allowed it to tunnel through the barrier, and off it goes. Forever separated, the virtual particles cannot annihilate and so the escaping particle becomes a real particle, boosted in energy by the rotation of the black hole. Because the conservation of energy dictates that the virtual particle falling into the black hole must have ‘negative’ energy, the escaping particle must have positive energy, which it removes from the black hole. Since energy is equivalent to mass, the escaping particle is therefore also removing some of the black hole’s mass. After a very, very long time (say, 10120 years) even the most massive black holes will shrink to nothing thanks to Hawking radiation.
The early years
Hawking’s discovery had more than just cosmological relevance. It was a testament to his determination to succeed no matter the odds, given that in 1963, just after his 21st birthday, he’d been diagnosed with a form of motor neurone disease called amyotrophic lateral sclerosis, which results in the nerves that control the body’s muscles shutting down, and was given just two years to live. It is testament to his determination that he lived well into his 70s while remaining a prominent figure in science and the public consciousness.
Hawking once recalled that when he first visited the clinic where he was diagnosed, he shared a room with a boy suffering from leukaemia and realised that, no matter how bad his condition was, he was fortunate that it was not even worse and he still had something to live for. This sparked renewed enthusiasm in his university studies, while on the personal front he married his first wife, Jane Wilde, in 1965 (their romance is depicted in the 2014 film, ‘Theory of Everything’). His illness failed to progress as quickly as doctors had pronounced and it soon became clear that his impending death was anything but. Although he was forced to abandon his crutches for a wheelchair in 1969, he was still able to take up a position as a visiting professor at the California Institute of Technology (Caltech) in 1970 and became known for his reckless driving of his wheelchair!
While in California the Hawking family would take in a grad student each year to help with both Stephen’s work and his health care. It was during his five years at Caltech that Hawking met Kip Thorne, a theoretical physicist best known for his work on black holes, which in the public’s eyes culminated in the 2014 film ‘Interstellar’. Hawking and Thorne enjoyed a playful friendship over the years, frequently placing wagers with one another, or together versus other scientists, over whose theories were correct regarding various aspects of black holes. Often Hawking would be in agreement with the opposing bet but would still place the wager as an ‘insurance policy’, just in case he was wrong, with all manner of light-hearted prizes for the winner.
One of those wagers was as a direct consequence of Hawking radiation. Although it is not an official law of nature, it had been assumed that ‘information’ is a property that must be conserved and cannot be lost or destroyed (when we refer to information, we mean properties referring to a particle’s specific state, such as its charge or quantum spin). It was not clear how Hawking radiation could contain any information pertaining to what has fallen into a black hole; that information seemingly had to stay in there. Yet when a black hole eventually evaporates by losing mass through Hawking radiation, what happens to the information that it contained?
The notion that information could disappear for good, erased from existence, was disquieting because it clashed with our quantum mechanical understanding of information. If information is lost, then the entropy within the black hole would increase, which the laws of thermodynamics say would cause the black holes to heat up to unfathomable temperatures. Nevertheless, Hawking’s calculations convinced him that information was indeed lost from the Universe once it enters a black hole and the black hole evaporates, but not everyone agreed. By the turn of the millennium, the scientific consensus was against Hawking. Hence another wager – in 1997 Caltech’s John Preskill bet Hawking and Kip Thorne that information was conserved and just seven years later, in a development that made the news headlines, Hawking conceded the bet (Thorne, though, is still holding out), buying Preskill a baseball encyclopaedia – from which information could easily be retrieved! – for winning the wager.
The favoured solution to the paradox rests on the idea that our Universe is holographic. What this means is that physics in a given number of dimensions corresponds to different physics operating in a higher number of dimensions. For example, String Theory (which posits that fundamental particles are made from tiny vibrating strings) predicts the existence of eleven dimensions, and in 1997 the Argentinian physicist Juan Maldacena proposed that the quantum field theory belonging to our four-dimensional (length, breadth, width and time) Universe, with the exception of gravity, corresponds to String Theory physics in five dimensions with gravity, as though our Universe were a projection of a four-dimensional boundary in multi-dimensional space, through which gravity leaks into our Universe (which would explain why gravity is so weak compared to the other fundamental forces of nature). Gerard t’Hooft of Utrecht University in the Netherlands and Leonard Susskind of Stanford University suggested something similar for how black holes operate: that the physics of the three-dimensional volume within a black hole, where gravity plays a significant role, corresponds to the physics of a two-dimensional horizon above the black hole that is described by equations that do not need to invoke gravity. This means that the information within the black hole could be encoded onto this two-dimensional surface.
From this point, there is a lot of conjecture as to how the information becomes encoded into virtual particles or otherwise escapes the black hole’s evaporation. One of Hawking’s last scientific papers on the subject, published in January 2016 and written with Harvard’s Andrew Strominger and Cambridge’s Malcolm Perry, suggests that when an object falls into a black hole it imprints its information on what they call ‘soft particles’, which are particles such as photons or gravitons that possess zero energy and which lurk on the black hole’s two-dimensional event horizon.
Hawking returned to Cambridge in 1975 and four years later took up the chair of the Lucasian Professor of Mathematics at the university. He held this position, of which previous incumbents included such greats as Isaac Newton, Charles Babbage and Paul Dirac, for 30 years before the rules forced him to step down in 2009, after which he became the Director of Research at Cambridge’s Department of Applied Mathematics and Theoretical Physics. During the intervening years he lost his voice for good following a bout of pneumonia in 1985 that resulted in a tracheotomy and subsequent around-the-clock care from nurses. Hawking subsequently adopted a synthesised voice provided by a program called ‘The Equalizer’, which was developed by a Californian software developer Walter Woltosz, whose mother-in-law also suffered from amyotrophic lateral sclerosis. It works by selecting words on a computer screen, arranging them into a sentence and then commanding the computer to speak the words. Initially Hawking was able to control The Equalizer with a hand-held device, but as his illness progressed he gradually lost the limited use he had in his hand. This forced him to control the device with an infrared sensor attached to his glasses that could detect movement in his cheek muscle, allowing him to painstakingly construct on the computer the words that were then spoken by the stilted, robotic voice that made Hawking instantly recognisable when he spoke in the media.
His frequent appearances in the media ranged from guest-starring in ‘Star Trek: The Next Generation’ and ‘The Simpsons’ to presenting documentary series. He also voiced his opinion on many subjects, from climate change and politics to the future of humanity and the search for extraterrestrial intelligence. Hawking frequently stated his belief that the future of human civilisation resides in space, spreading amongst the planets and then the stars. To stay on Earth, he said, may doom humanity, whether it is through climate change or war or disease or something else that finally eradicates us.
A Brief History of Time
In 1988 Hawking published his best-selling book, A Brief History of Time, which has since gone on to sell over 10 million copies. It broached the topics that Hawking was most interested in – black holes, the General Theory of Relativity, quantum mechanics and the origin of the Universe. Although well-written, the topics it attempted to describe for general readers are immensely complex and, despite its tremendous sales success, it’s often said that while most people began reading the book, few ever finished it.
Other books followed, including 2001’s The Universe in a Nutshell and 2005’s A Briefer History of Time, in an attempt to convey the ideas presented inside A Brief History of Time in a simpler way, but it was Hawking’s first book that remained his best known tome. Within its pages, Hawking declared that science is all that will ever be needed to understand the Universe and that a complete, unified theory of physics will replace the notion of God. This was a view he continued to hold throughout his life. For Hawking, science was the beginning and the end.