Jacob Bekenstein

Jacob Bekenstein, who lived from 1947 to 2015, was an Israeli-American theoretical physicist. Though he and Hawking are remembered as much for their early disagreement as for any formal collaboration, their exchange of ideas produced one of the deepest results in modern physics, and a quantity that carries both their names.

His own work

A student of the legendary physicist John Wheeler, Bekenstein spent much of his career on black holes and the limits that physics places on information. He is best known for the bold proposal, made while still a graduate student in the early 1970s, that black holes must have entropy, and for the "Bekenstein bound," a fundamental limit on how much information can be contained in any region of space. These ideas helped lay the groundwork for thinking of physics in terms of information, a theme that now runs throughout the field.

His connection to Hawking

Bekenstein noticed that Hawking's own area theorem, which showed that a black hole's event horizon can never shrink, behaved suspiciously like the second law of thermodynamics, in which entropy never decreases. He proposed that a black hole's entropy is proportional to the area of its horizon.

Hawking initially thought the idea was wrong. If a black hole had entropy, it must have a temperature, and anything with a temperature must radiate, yet black holes were supposed to swallow everything and emit nothing. In trying to prove Bekenstein mistaken, Hawking discovered the opposite: black holes do radiate. That discovery, Hawking radiation, confirmed Bekenstein's intuition and pinned down the exact relationship, now known as the Bekenstein-Hawking entropy. It is a perfect example of how a disagreement, pursued rigorously, can lead to a breakthrough that vindicates both sides.