Jacob Bekenstein

Jacob David Bekenstein was born in Mexico City in 1947 to Polish-Jewish immigrants and moved to the United States as a teenager. He completed his doctorate at Princeton in 1972 under John Archibald Wheeler, the physicist who coined the term 'black hole.' It was a question Wheeler put to him in conversation — what happens to the entropy of a cup of hot tea poured into a black hole? — that set the course of his career and, eventually, of an entire field.

Bekenstein's answer, published in his 1972 and 1973 papers, was audacious for a student to make against the prevailing view: a black hole must carry a real, physical entropy proportional to the area of its event horizon, measured in Planck units. His reasoning rested on an information argument — the minimum entropy gained when a black hole swallows a single particle of irreducible quantum uncertainty corresponds to about one Planck area of horizon growth — combined with Stephen Hawking's 1971 theorem that horizon area never decreases. The proposal was widely doubted, not least by Hawking himself, because a classical black hole emits nothing and so could not have a temperature.

Vindication came in 1974, when Hawking's calculation of black-hole radiation supplied the exact temperature the analogy required and fixed the coefficient in Bekenstein's entropy at one quarter. The resulting Bekenstein–Hawking entropy, S = k_B A / 4ℓ_P², is engraved on Bekenstein's tombstone in Jerusalem. From it Bekenstein derived a universal bound on the entropy — and hence the information — that any region of space can contain, a result that became a cornerstone of the holographic principle.

Bekenstein spent most of his career at the Hebrew University of Jerusalem, where he also developed TeVeS, a relativistic version of modified Newtonian dynamics offered as an alternative to dark matter. He received the Wolf Prize in Physics in 2012 and died suddenly in Helsinki in 2015 at the age of 68. He is remembered above all for refusing to treat the thermodynamic analogy as mere metaphor — for being the first to say, and to defend, that a black hole is a thermodynamic object.