blockmemento

// block 374476 · science · 14 September 2015

Gravitational waves detected

The moment this block marks: the signal GW150914 reaches the two LIGO detectors, 14 September 2015, 09:50:45 UTC. The first block mined after it is block 374,476, 14 September 2015, 10:12:50 UTC.

Early on the morning of 14 September 2015, a faint tremor passed through the two detectors of the Laser Interferometer Gravitational-Wave Observatory, LIGO, one in Livingston, Louisiana, the other in Hanford, Washington, some 3,000 kilometres apart. Livingston registered it first, and Hanford seven milliseconds later. The detectors had only just started observing after a major upgrade.

The signal lasted a fraction of a second and rose in frequency as it went, the pattern expected from two black holes spiralling into each other. The analysis found black holes of about 36 and 29 times the mass of the Sun, which had merged 1.3 billion years ago into a single one of 62 solar masses; the equivalent of three Suns had been carried away as gravitational waves. Einstein had predicted such waves a century earlier, and no one had ever detected them directly.

About an hour later, an email titled “Very interesting event on ER8” reached the collaboration. Then came five months of silence: more than a thousand scientists, sworn to secrecy, checked the data, the instruments and every possible false alarm, including a signal faked on purpose, before saying anything in public. The world learned of it on 11 February 2016.

The first block mined after the signal arrived, block 374,476, came twenty-two minutes later, at 10:12 UTC, before that first email had even been sent, when almost no one knew that anything had happened. Its hash, like every hash, can be checked on any node.

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