QUANTUM LEAP

The 2017 Nobel for Physics was the least surprising of awards. Half went to Rainer Weiss. The other half was shared by Barry C Barish and Kip S Thorne. All three were pioneers of the LIGO-Virgo collaboration that first captured gravitational waves in September 2015.

The award presented a peculiar problem. The prize can’t be shared by more than three people and well over a thousand scientists from 20 countries had been important participants in the LIGO (Laser Interferometer GravitationalWave Observatory), designing apparatus, capturing and deciphering data.

Weiss designed the basic detector, a laser-based interferometer. Thorne helped scale up the concept and turn it into reality. Barish led the LIGO team. An interferometer has two L-shaped, vacuum tunnels. Mirrors are suspended at the corner and ends of the L. A laser beam is bounced between the mirrors. If the arm lengths are unvarying, the beams will cancel out at the corner.

A gravitational wave will stretch one arm and compress the other. Then the light changes since the beams travel different distances and don’t cancel out. The LIGO’s huge laser interferometers are sensitive enough to measure little changes thousands of times smaller than an atomic nucleus. By using two or more centres, false signals can be cancelled out and better location data derived.

In September 2015, there were two LIGO observatories, both in the USA. Since then, Italy’s Virgo facility has become operational. India is building its own LIGO and Japan is also building a grav-wave observatory.

The first capture in September 2015 came from a merger of two black holes. Subsequently, three more mergers have