### Jacob Bekenstein (1947-2015)

Tuesday, August 18th, 2015Today I learned the sad news that Jacob Bekenstein, one of the great theoretical physicists of our time, passed away at the too-early age of 68.

Everyone knows what a big deal it was when Stephen Hawking discovered in 1975 that black holes radiate. Bekenstein was the guy who, as a grad student in Princeton in the early 1970s, was already raving about black holes having nonzero entropy and temperature, and satisfying the Second Law of Thermodynamics—something just about everyone, including Hawking, considered nuts at the time. It was, as I understand it, Hawking’s failed attempt to prove Bekenstein wrong that led to Hawking’s discovery of the Hawking radiation, and thence to the modern picture of black holes.

In the decades since, Bekenstein continued to prove ingenious physical inequalities, often using thought experiments involving black holes. The most famous of these, the Bekenstein bound, says that the number of bits that can be stored in any bounded physical system is finite, and is upper-bounded by ~2.6×10^{43} MR, where M is the system’s mass in kilograms and R is its radius in meters. (This bound is saturated by black holes, and only by black holes, which therefore emerge as the most compact possible storage medium—though probably not the best for retrieval!) Bekenstein’s lectures were models of clarity and rigor: at conferences full of audacious speculations, he stood out to my non-expert eyes as someone who was simply trying to follow chains of logic from accepted physical principles, however mind-bogglingly far those chains led but no further.

I first met Bekenstein in 2003, when I was a grad student spending a semester at Hebrew University in Jerusalem. I was struck by the kindness he showed a 21-year-old nobody, who wasn’t even a *physics* student, coming to bother him. Not only did he listen patiently to my blather about applying computational complexity to physics, he said that *of course* physics should ultimately aim to understand everything as the output of some computer program, that he too was thinking in terms of computation when he studied black-hole entropy. I remember pondering the fact that the greatest reductionist I’d ever met was wearing a yarmulke—and then scolding myself for wasting precious brain-cycles on such a trivial thought when there was science to discuss*.* I met Bekenstein maybe four or five more times on visits to Israel, most recently a year and a half ago, when we shared walks to and from the hotel at a firewall workshop at the Weizmann Institute. He was unfailingly warm, modest, and generous—totally devoid of the egotism that I’ve *heard* can occasionally afflict people of his stature. Now, much like with the qubits hitting the event horizon, the information that comprised Jacob Bekenstein might seem to be gone, but it remains woven into the cosmos.