Jacob Bekenstein, a physicist who prevailed in an argument with Stephen Hawking that revolutionized the study of black holes, and indeed the nature of space-time itself, died on Sunday in Helsinki, Finland, where he was to give a physics lecture. He was 68.

The cause was a heart attack, said the Hebrew University of Jerusalem, where Dr. Bekenstein was the Michael Polak professor emeritus of theoretical physics.

Dr. Bekenstein’s greatest achievement came in the early 1970s, when he was a graduate student at Princeton and got into a feud with Dr. Hawking, the celebrated physicist and expert on black holes.

Black holes are the prima donnas of Einstein’s general theory of relativity, which predicts that space wraps itself completely around some object, causing it to disappear as a black hole. Dr. Bekenstein suggested in his Ph.D. thesis that the black hole’s entropy, a measure of the disorder or wasted energy in a system, was proportional to the area of a black hole’s event horizon, the spherical surface in space from which there is no return. According to accepted physical laws, including Dr. Hawking’s own work, neither entropy nor the area of a black hole could ever decrease.