PHOTOGRAPH COURTESY OF BILL UNRUH. l Canadian theoretical physicist Bill Unruh. Black holes became a theoretical reality with the Schwarzschild solution of general relativity (that describes how a star and a black hole warps space and time), and they became a physical possibility with the understanding of stellar evolution. In 1958 (around the same time that Leon Cooper found his solutions to superconductivity), one of my heroes in physics, David Finkelstein, discovered something truly remarkable to make the black hole story more interesting yet. 16 David Finkelstein is quiet, sage-like, and beaming with genius as if the entire cosmos were contained in his head. So inspiring is he that it’s not altogether surprising that pioneers of the two main competing theories of the unification of quantum mechanics with gravity, Lee Smolin and Lenny Susskind, were both mentored by David. It is interesting that both of them became pioneers in Loop Quantum Gravity and String Theory respectively. David was the chair of Yeshiva University in New York City in the 1960’s, when it had humble beginnings. He suddenly left New York for Tougaloo College in Jackson, Mississippi, for three years to “participate in the fight for human rights for all, regardless of race” — to help African Americans attain civil rights. I became such a fan of David’s that, in 2014, I hosted a symposium at Dartmouth to celebrate his lifetime achievement. What David wanted to understand was how a beam of light moved in the warped space-time around a black hole. After all, it was the observation of the bending of light from a distant star around our sun that confirmed Einstein’s idea that gravity was, in fact, the warping of space-time around a massive object. But, as David found out, the movement of light around a black hole was even more bizarre. By an ingenious reshuffling of the equations that govern space-time, David found that there was a spherical bubble-like region surrounding the singularity in Schwarzschild’s solution such that if anything entered this region, including light itself, it could never escape. That’s why John Wheeler coined the term black hole to describe these things, in fact. If no light could escape the Schwarzschild region surrounding the singularity, you’d never be able to see it. Anything entering this region would essentially disappear into blackness. What David had discovered was a one-way invisible spherical surface, which he called a horizon. It was a horizon no one could see beyond, not completely dissimilar to our visual horizon into the universe’s past, making its study all the more intriguing. When David made his calculations, black holes were still a subject of sci-fi novels, a playground for the imagination, but they were beginning to be understood, as well. While some physicists, like Lee Smolin, speculated that black holes spawned baby universes at their singularities, we also learned that black holes can grow in mass by consuming matter and that they can radiate, due to quantum effects near the event horizon of the black hole. David’s work made the study of black hole physics concrete. The event horizon was a definitive, albeit intangible, mathematical element to work with, one that might even shed light on the structure of our universe and the ancient cosmic horizon. To better understand how, we need to look at sound — specifically, how sound moves in water. Canadian physicist Bill Unruh found this brilliant analogy in terms of sound that captures a great deal of the physics of black holes. Bill is one of Canada’s and the world’s most revered theoretical physicists. I spent a half year at his home institution, the University of British Columbia in Vancouver, to work on my PhD dissertation. Bill is a big man with a full beard and usually wears overalls. He has a tendency to intimidate other physicists and is quick to pounce on any inaccuracies that may present themselves, but he was always kind to me even when I said dumb things. His mastery of finding analogies for physics concepts spoke loudly and clearly to me one day at the University of British Columbia, when he found a mistake in the first seminar I ever gave and proceeded to suggest a correction. A year later his proposal worked impeccably.