James Hartle

James Hartle, who lived from 1939 to 2023, was an American theoretical physicist based for most of his career at the University of California, Santa Barbara. A specialist in gravitation and the foundations of quantum mechanics, he was the partner behind one of Hawking's most ambitious ideas.

His own work

Hartle worked on applying quantum mechanics to gravity and to the universe as a whole, a notoriously difficult problem. Beyond his work with Hawking, he is known for developing, with the Nobel laureate Murray Gell-Mann, the "decoherent histories" approach to quantum mechanics, an attempt to make sense of how the definite, classical world we experience emerges from underlying quantum uncertainty. He also wrote a respected modern textbook on general relativity.

His connection to Hawking

Hartle and Hawking are jointly remembered for the no-boundary proposal of 1983, sometimes called the Hartle-Hawking state. It is one of the most striking ideas in modern cosmology. Asking what came "before" the Big Bang, they argued, may be as meaningless as asking what lies north of the North Pole.

Using a mathematical device called imaginary time, they proposed that near its origin the universe had no sharp boundary or starting edge, no initial singularity at all. Instead, time near the beginning behaved smoothly, like another direction of space, so the universe is finite but has no first moment, just as the Earth's surface is finite but has no edge. It was a serious attempt to remove the singularity that Hawking's own earlier theorems had insisted upon, by bringing quantum mechanics into the picture. The proposal remains one of the boldest answers anyone has offered to the question of how everything began.