Gary Gibbons, Professor of Theoretical Physics at the Department of Applied Mathematics and Theoretical Physics, was awarded the prize for his part in "turning black holes into windows onto the deepest laws of nature". He shares the Medal with Gary Horowitz, University of California Santa Barbara, Roy Kerr, University of Canterbury in New Zealand, and Robert Wald, University of Chicago.
The Dirac Medal is awarded every year by the Abdus Salam International Centre for Theoretical Physics. It's named after Paul Dirac, one of the founders of quantum mechanics and one of the twentieth century's greatest physicists. Dirac was Lucasian Professor of Mathematics here at Cambridge from 1932 to 1969. Winners of the Medal are announced every year on Dirac's birthday, August 8, to "scientists who have made significant contributions to theoretical physics". With 362 published papers and 40,000 citations, Gibbons' contributions certainly tick the box.
"Professor Gary Gibbons has made many seminal contributions to theoretical physics, especially with regards to mathematical descriptions of the mysterious and fascinating objects called black holes," says Colm-cille Caulfield, Head of DAMTP. "Throughout his career, he has been an outstanding exemplar of the DAMTP approach: to apply mathematics to describe the fundamental aspects of not only the world, but the Universe around us. The award of this Dirac Medal is a fitting tribute to a truly outstanding research career, and naturally brings reflected honour and inspiration to members of the department. Bravo!"
Black holes: From fiction to fact
Gibbons' contributions concern the force of nature we're most familiar with — gravity. Our best description of gravity was provided by Albert Einstein in 1915 in the shape of the general theory of relativity. Einstein's theory is extremely successful (you are using it whenever you consult a GPS device to find your way around) but it's far from easy — working with it requires serious mathematics. What it tells us about the gravitational pull of a single object, though, is easy to understand: the smaller and more massive the object is, the stronger its gravitational pull will be. If you could crush a celestial body, such as our Sun, down to a smaller size while keeping its mass the same, you'd increase its gravitational pull.
This raises an intriguing possibility that was already mooted by Newton's theory of gravity: if an object is small and massive enough, could its gravitational pull be so strong that nothing, not even light, can escape its vicinity? While many physicists were unnerved by the possibility that such black holes might exist (Einstein himself attempted to disprove their existence in 1939) the idea gradually gained traction through the work of many physicists. By the late 1960s theoretical doubts were completely wiped out by the work of Roger Penrose, who won the 2020 Nobel Prize in Physics, and Stephen Hawking, former Lucasian Professor of Mathematics here at DAMTP. Decades later, in April 2019, the first direct image of a black hole and its surrounding was revealed, captured by the Event Horizon Telescope (shown above).
Gibbons entered the fray as a PhD student at DAMTP in 1969, under the supervision of Dennis Sciama and later Stephen Hawking. This was at a time when the "golden era" of black hole research appeared to be coming to an end. "I remember Stephen coming back [from a conference in 1973] and saying the subject is finished, we should look for another problem," Gibbons told us on Stephen Hawking's death. "So we started looking at a suggestion that he had made earlier: that there should be very small black holes formed in the early Universe. You should think of them as fossils of the Big Bang. If you could find them they would provide an important clue to the origin of the Universe."
Little did they know that these fossils' failure to exist would lead to Hawking's most celebrated result in black hole theory. While working with Gibbons, Hawking realised that primordial black holes would lose mass and would have evaporated long before our time. This amounted to a revolutionary statement. It said that black holes aren't completely black, but can emit thermal radiation (now known as Hawking radiation)— that's how they evaporate. Building on work by Jacob Bekenstein (and others), Hawking went on to formulate a thermodynamic theory of black holes, the central equation of which is the famous Bekenstein-Hawking entropy formula. At his 60th birthday address Hawking said that he would like this formula on his tombstone.
Black holes turned inside out
When the Bekenstein-Hawking entropy formula was published in 1974, Gibbons had only just completed his PhD. "Gary dropped right in at the beginning," says David Tong, Professor of Theoretical Physics at DAMTP. "A lot of the famous work that Hawking did during the next decade was largely with Gary — he became one of the best, if not the best, relativists of our time. Someone who understands the many, many subtleties of the theory."
One intriguing result flips ideas regarding black holes inside out and applies them to our entire Universe. Black holes come with a boundary of no return called an event horizon. The horizon stops us from seeing inside the black hole. Were we to cross it, we would never get back out again.
Our Universe comes with a similar shield. "We have known since [work of Edwin Hubble in 1929] that the Universe is expanding," says Tong. "Since [1998] we also know that the expansion is accelerating — it's getting faster and faster all the time. The further things are from each other the faster they are moving apart. There are objects roughly 14 billion light years away that are moving away from us faster than the speed of light."
Since the Universe is roughly 14 billion years old, light from these objects has not yet had time to reach us. And since the objects are moving away from us at a speed that is faster than light, we will never be able to see them in the future either. "This means that surrounding us is something like an event horizon we can't see beyond, exactly like you can't see inside a black hole," says Tong. "Gary and Stephen realised that this [horizon] also has an entropy, and that there is radiation [analogous to Hawking radiation] which permeates the whole Universe."
The view of the Universe as a black hole turned inside out helped grapple with some major mysteries. One of them is how the large-scale structures we see today — planets, stars, galaxies — might have come into being. The theory of inflation, first proposed by the physicist Alan Guth, suggests that such structures could have been seeded by quantum fluctuations that occurred at a time when a very young Universe was undergoing a period of extremely rapid expansion (inflation). The ideas of Gibbons and Hawking helped make the theory of inflation precise. Fanciful as it may sound, there now is excellent observational evidence that inflation did indeed occur.
The mystery of quantum gravity
While general relativity describes the world at the scale of planets and stars, Gibbons and Hawking's work also involved quantum physics, which describes the world at the scale of atoms and subatomic particles. But what happens when gravitational effects come into play at small scales, as was the case at the Big Bang, when the Universe was very small and very dense, and as happens inside black holes? The question presents a problem for physicists because the mathematical formulation of general relativity is at odds with quantum physics. So far nobody has found a unifying theory of quantum gravity that is supported by meaningful observational evidence. The Big Bang, and the goings-on inside black holes, remain uncertain.
In this area too Gibbons made important contributions. "In the 1970s Gibbons and Hawking did [what was] the obvious thing: they took Einstein's equations for general relativity and put them into [something called] the Feynman path integral [from quantum physics]," says Tong. The result was called Euclidean quantum gravity. "It was obvious that Euclidean quantum gravity can't work, things have to be more subtle than that, but it was also obvious that it was the right first guess."
Euclidean quantum gravity ended up on the backburner for many decades. "People thought that they had wrung everything they could out of it," says Tong. "But it has really taken off again in the last few years. Somehow [the theory] is not fully right, but it's more right than it has any reason to be." Researchers here at DAMTP are now treading in Gibbons' and Hawking's footsteps, actively pursuing this lead. Aside from formulating Euclidean quantum gravity, Gibbons has also been a leader in the development of string theory, another candidate for a theory of quantum gravity.
These contributions, as well as many others, made Gibbons into a towering figure in the field — Tong says his own PhD thesis was littered with terms named after Gibbons, and that the Medal was long overdue. But like any brilliant physicist Gibbons has not just provided answers, but also opened up a range of new questions. The concept of entropy, originally from thermodynamics, can also be understood as a measure of information. What can the Gibbons-Hawking entropy tell us about the information contained in our Universe? Is that information contained in the Gibbons-Hawking radiation that fills our Universe? And why does the seemingly naive approach of Euclidean quantum gravity suddenly appear to be the way forward?
Answers to these questions will ultimately help us unravel the big mysteries that fascinate everyone who is curious about the world we live in: where did it come from, what is it made of, and where is it going? Future generations of physicists here at DAMTP will probably help work out the answers. They will be standing on the shoulders of a number of giants, including those of Gary Gibbons.
The image above shows the first ever picture of a black hole and its vicinity. Credit: EHT Collaboration.