Detection, of course, depends on what we think the effect of a passing gravitational wave will be. Theory predicts that the wave will change the shape, ever so slightly, of objects it encounters. A basketball will first be stretched into a football in the horizontal direction, then return to being a basketball, then be stretched into a football in the vertical direction, then return to being a basketball, and so on. The challenge for scientists is to detect these tiny changes in shape.
Enter LIGO — the Laser Interferometer Gravitational-wave Observatory. There are two LIGO detectors, one in Livingston, Louisiana, and one in Hanford, Washington. Each consists of an L-shaped apparatus with a vacuum chamber 2.5 miles (4km) long in both arms with mirrors at the ends. A split laser beam is sent down the arms, and when the beams are reflected back to the center, extremely small changes in the relative position of the two mirrors can be detected.
The idea is that you can think of the two LIGO mirrors as being small parts of the surface of the hypothetical basketball described above. As a gravitational wave causes contortions in the shape of this imagined basketball, the laser beams will detect those changes in position. This is not a simple operation, since the expected changes in the mirror’s position from a passing wave are 1,000 times smaller than the nucleus of an atom!
LIGO went into operation in 2002 and collected data for eight years. It detected no waves, but this wasn’t surprising. It would have taken an extraordinary event (something like two black holes colliding nearby) to produce a wave strong enough to trigger the instrument as it was then.
This first run established that the system actually worked. LIGO was recently closed for several years as engineers upgraded to the Advanced LIGO phase. The instrument was re-dedicated in May 2015, and its first observing run took place between September and January 2016. And, just like so many other experiments over the last hundred years, LIGO has proven Einstein correct once again.