If a gravitational wave throbs past, light in one of those beams will travel further than light in the other - by a tiny, tiny fraction of the width of an atom.
Ligo's two stations are 3,000km apart in the states of Louisiana and Washington - allowing them to compare notes on the timing and apparent direction of any such disturbance.
Rumours have been circulating for weeks that both instruments had indeed picked up a signal, and that researchers were working on a paper about the discovery.
It was only in September 2015 that these detectors went back into operation, after a $200m upgrade that saw the facility redubbed Advanced Ligo and dramatically boosted its sensitivity.
Its first run, from 2002-2010, had drawn a blank.