The quest to understand what happens to information in a black hole has transformed fundamental physics and energized a generation of young theorists. At stake is whether Einsteinian gravity, which governs the cosmos, and quantum mechanics, which governs the microcosm, play by the same rules.

“It all started with Hawking’s realization that the total horizon area of black holes can never go down,” Dr. Isi said.

But with no black holes to experiment on, Dr. Hawking’s ideas could not be tested.

## LIGO to the rescue

LIGO would change that. This was the promise Kip Thorne, a theoretical physicist at the California Institute of Technology and one of LIGO’s founders, made to Dr. Hawking in 2003. The new array would be able to sort out the properties of black holes by the time Dr. Hawking turned 70 in 2012.

“Your gift is that our gravitational-wave detectors — LIGO, GEO, Virgo and LISA — will test your Golden Age black-hole predictions, and they will begin to do so well before your 70th birthday,” Dr. Thorne recently recalled telling him.

It took longer than that — until Sept. 14, 2015 — for LIGO to observe its first epochal event: two colliding black holes. By matching the detected wave patterns with computer simulations, the LIGO team concluded that one of the black holes was 36 times as massive as our sun and the other was 29 times as massive — equaling 65 suns total. The collision resulted in a new black hole with a mass of about 62 suns. Three suns worth of energy had disappeared into the gravitational waves that shook the universe.

The observation confirmed not only the existence of gravitational waves, as Einstein had predicted 100 years earlier, but provided the first direct evidence of black holes.

A leaked copy of the discovery paper reached Dr. Hawking a few days before the official announcement of the findings. He was startled to find no mention of the four laws of black hole mechanics, or of the possibility that the discovery might test them. He Skyped Dr. Thorne, an author of the paper.