In 1971, Stephen Hawking showed under general conditions
that the total area of the event horizons of any collection of
classical black holes can never decrease, even if they collide and
merge. This result, now known as the second law of black hole mechanics, is remarkably similar to the second law of thermodynamics, which states that the total entropy of a system can never decrease. As with classical objects at absolute zero
temperature, it was assumed that black holes had zero entropy. If this
were the case, the second law of thermodynamics would be violated by
entropy-laden matter entering the black hole, resulting in a decrease
of the total entropy of the universe. Therefore, Jacob Bekenstein proposed that a black hole should have an entropy, and that it should be proportional to its horizon area.