This week saw the announcement of an extraordinary physics result: the first direct detection of gravitational waves by the LIGO Scientific Collaboration, which includes the GEO team, and the Virgo Collaboration, using the twin Laser Interferometer Gravitational-wave Observatory (LIGO) detectors located in Livingston, Louisiana, and Hanford, Washington, USA.
Albert Einstein predicted gravitational waves in a paper published 100 years ago in 1916. They are a natural consequence of the theory of general relativity, which describes the workings of gravity and was published a few months earlier. Until now, they have remained elusive.
Gravitational waves are tiny ripples in space-time produced by violent gravitational phenomena. Because the fractional change in the space-time geometry can be at the level of 10-21 or smaller, extremely sophisticated, high-sensitivity instruments are needed to detect them. Recently, the Advanced LIGO detector increased its sensitivity by almost a factor of four, which was crucial for the reported observation.
The by-now familiar GW150914 signal, recorded on 14 September 2015, is attributed to the merger of two massive black holes - 30-40 solar masses each -occurring at a distance of about 400 Megaparsecs. Since gravitational waves travel at the speed of light, this catastrophic event happened more than 1 billion years ago. This observation represents another crucial milestone in the experimental verification of general relativity, and opens the door to a new phase of exploration of the universe: gravitational wave astronomy.
The importance of this result for physics is huge. Much of what we take for granted in modern society rests on two pillars, theoretical frameworks that emerged at around the same time. General relativity is one. Quantum mechanics is the other. GPS positioning systems would not work without general relativity, while much of the electronics industry is built on quantum mechanics. Yet the two theories seem to be incompatible.
Four years ago at CERN, we dotted the ‘i’s and crossed the ‘t’s of the Standard Model of particle physics with the discovery of the Higgs boson, the messenger of the Brout-Englert-Higgs mechanism. This was the last missing ingredient of the Standard Model: the quantum theory that describes fundamental particles and all of their interactions, with the exception of gravity. This week’s discovery paves the way to significant improvements in our understanding of gravity through future measurements of gravitational waves. Results such as these two come along only very rarely, and it is a privilege to be able to see them. They also spur us on to the greatest challenge of our time in physics: the reconciliation of general relativity and quantum mechanics.
Congratulations to the LIGO and Virgo Collaborations for this extraordinary contribution to fundamental physics!