A single observation can make history, but it usually takes a second one to begin a science. The quieter repeat is often what turns a remarkable instrument into a working observatory.

MIT News reported on June 15, 2016 that, for the second time, scientists had directly detected gravitational waves. The two LIGO detectors, in Hanford, Washington and Livingston, Louisiana, had caught a second robust signal from two black holes. The signal had reached both detectors, situated more than 3,000 kilometers apart, on Dec. 26, 2015. LIGO named the event GW151226, nicknamed it the Boxing Day event, and placed its source some 1.4 billion light years away. The merging black holes were 14 and 8 times the mass of the sun.

Why does a second detection matter more than the first?

Because a second gravitational wave detection turns a story into a sample. Salvatore Vitale, a research scientist at MIT, said the new detection "proves that indeed we are observing a population of binary black holes in the universe". LIGO said the confirmed detections let it start estimating how often black holes merge "based not on theory, but on real observations". One extraordinary event invites wonder; two begin to describe a population.

Hearing a quieter voice

The second signal was also harder to hear. MIT described it as "essentially a faint squeak" that was almost buried in the data. To find it, the team assembled a bank of hundreds of thousands of known waveforms to compare against the data. LIGO also releases its data to the public, so that others can check that the LIGO and Virgo collaborations did not miss anything.

That combination matters as much as the black holes themselves. A loud first result proves an instrument can hear. A faint second one, pulled out by a known method and opened to outside checking, shows that it can listen reliably. For any new tool, the milestone worth marking may be less the first surprise than the first unremarkable repetition.

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