Last week’s Physics Friday was about General Relativity, proposed by Albert Einstein a bit over 100 years ago. General Relativity was theoretical as the technology didn’t exist at the time to experimentally confirm the theory.

One key aspect of General Relativity is that it predicted that “gravitational waves” exist. Gravitational waves, according to phys.org are “mysterious ripples in the fabric of space and time that travel across our universe at the speed of light.” Such waves “are caused by violent astrophysical events, involving massive, compact objects like neutron stars and black holes, colliding into each other.” Close to these collisions, the gravitational waves are massive, but when very far away, the gravitational waves are incredibly faint and extremely hard to detect.

The project to detect gravitational waves was begun in the 1970s. To detect gravitational waves, two massive “LIGO” detectors were created, one in Hanford, Washington and the other in Livingston, Louisiana. LIGO stands for “Laser Interferometer Gravitational-Wave Observatory.”  The LIGO detectors are “L” shaped with 4km long arms. Inside the LIGO detectors laser light is sent down the 4km vacuum tube arms and reflected back. When disturbed by gravitational waves, the two legs of the detectors, being at right angles, are affected differently. Gravitational waves result in one leg of the L being pushed apart by space stretching and the other leg being pushed together by space squeezing. The effects the detectors are registering are unbelievably slight. The ripple in spacetime registered by the detectors is about 1/10,000,000^th the diameter of an atom! Amazing!

In 2015, 40 years of effort culminated in the LIGO detectors registering gravitational waves of two colliding black holes about one billion light years away. Since the 2015 gravitational wave detection, the LIGO detectors have discovered three other gravitational wave events.

For leading the work on the LIGO project, Caltech professors Kip Thorne and Barry Barish and MIT professor Rainer Weiss were awarded the 2017 Nobel Prize in Physics.