RETROSPECTIVE

StephenHawking(1942–2018):Towardacomplete understanding of the universe RETROSPECTIVE James Hartlea,b,1

Stephen Hawking overcame the limitations of a de- The End of Black Holes bilitating disease to make major contributions to When a massive star runs out science. He did this through remarkable persistence, of thermonuclear fuel to burn determination, conviction, courage, and will. He was for heat to support itself supported in this by his family, his students, and against the force of gravity, it his colleagues. can collapse and make a black Stephen said, “My goal is simple. It is a complete hole—a region of spacetime understanding of the universe.” Stephen’s many con- where, according to classical tributions to science moved us significantly toward physics, the gravitational at- that goal. Much of his work was in two major areas— traction is so strong that noth- cosmology and the physics of black holes. I mention ing inside, not even light, only one contribution in each that stands out—his can get out. work on the beginning of the universe and his work Stephen’s work on black Stephen Hawking. Image courtesy of Alan Fersht (University of Cambridge, Cambridge, on the end of black holes. holes began about 1970 when United Kingdom). in a “eureka moment” he re- The Beginning of the Universe alized that the powerful global Stephen’s PhD advisor Dennis Sciama started him techniques used for the singularity theorems could be working on cosmology. An important question in the applied to understand the properties of black holes in 1960s was, “Did the universe have a beginning?” new levels of generality. What do we mean generally Observations and the Einstein equation suggested by a black hole? What is the most general black hole that the early universe was very hot. But did the uni- allowed by general relativity? Stephen contributed verse have a beginning or was it first big and then essential pieces to the answers of these questions. contracted, heated, and then reexpanded? The sin- In 1974, Stephen revolutionized the field of black gularity theorems worked out by Stephen and Roger hole physics by showing that black holes are not Penrose settled the matter at the level of classical black when quantum mechanics is taken into ac- gravitational physics. The universe did have a begin- count. They have a temperature and radiate (the ning in a big bang of infinite temperature and density Hawking radiation). As he put it, “black holes can be before which we cannot see and at which the classical white hot.” This remarkable result touches on the Einstein equation breaks down. Much of our confi- most fundamental issues in physics because it means dence in the big bang rests on these theorems. that black holes evaporate and disappear. More than Stephen’s singularity theorems meant that the 40 years later many scientists are still working to un- universe could not begin with the three space and one derstand the implications of this for fundamental time dimensions of classical physics. But it could start physics. with four space dimensions that later made a quantum transition to three dimensions of space and one of time. The Unique Stephen A quantum universe is described by a wave func- Stephen had a number of qualities that helped him tion representing its quantum state. Stephen and I achieve what he did. provided one for our universe—the no-boundary There is the unity of his physics. Black holes and the wave function. Its semiclassical approximation does beginning of the universe have several similarities. describe a transition from four space dimensions to First, the key features of both are accessible by the three of space and one of time. Stephen provided global techniques that Stephen and others developed. a new birth for the field of quantum cosmology. Second, classically they both exhibit singularities—the

aDepartment of Physics, University of California, Santa Barbara, Santa Barbara, CA 93106; and bSanta Fe Institute, Santa Fe, NM, 87501 Author contributions: J.H. wrote the paper. The author declares no conflict of interest. Published under the PNAS license. 1Email: [email protected]. Published online May 7, 2018.

www.pnas.org/cgi/doi/10.1073/pnas.1806196115 PNAS | May 22, 2018 | vol. 115 | no. 21 | 5309–5310 Downloaded by guest on September 28, 2021 big bang in the past of our universe and the inevitable the big bang. At the time these ideas were surprising, future singularity inside black holes hidden from us by but Stephen loved to surprise. Today, many of them its event horizon. seem simple, natural, and even inevitable, but that is Stephen had a remarkable ability to focus on the his genius. essentials of a problem and ignore the clutter. He was helped in this by a commitment to making pre- Stephen and Us dictions for observation. Stephen always knew the Stephen’s scientific legacy does not lie only in his right question to ask whose answer would move us papers. It lies also in the inspiration he provided to forward. those of us that followed him. That is most concretely But he also knew the right thing to give up. Ad- exhibited by the graduate students he supervised, vances in physics of course involve new ideas. But many of whom have become distinguished scientists many times they also involve giving up on cherished themselves. It lies also in his outstanding ability to current ideas. Stephen taught us that we have to give communicate science to the general public in books like up on the idea that black holes were black to get to his best-selling A Brief History of Time (1) and in his the Hawking radiation. We have to give up on the idea beautifully crafted public lectures. Finally, it lies also in that spacetime always had three space dimensions the example that he set with his life which has been an and one time dimension to get to a quantum theory of inspiration to many.

1 Hawking S (1988) A Brief History of Time (Bantam Press, London).

5310 | www.pnas.org/cgi/doi/10.1073/pnas.1806196115 Hartle Downloaded by guest on September 28, 2021