The wave that made history snuck up on them. David Shoemaker will never forget the date—September 14, 2015—when he woke up to a message alerting him that an underground detector had spotted a 1.3-billion-year-old ripple in the fabric of space-time.

A gravitational wave—predicted to exist a century ago by Albert Einstein—had been glimpsed directly for the first time by a pair of US-based detectors.

"It is seared in my brain," said Shoemaker, a top scientist at the Massachusetts Institute of Technology (MIT) and head of the Advanced LIGO Project, an international effort to uncover evidence of gravitational waves.

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