The Einstein Telescope (ET) is a design concept for a third-generation gravitational-wave (GW) detector, which will be 10 times more sensitive than the current advanced instruments of the second generation.
Like the first two generations of GW detectors, the concept for the Einstein Telescope is based on the measurement of tiny changes (far less than the size of an atomic nucleus) in the lengths of two connected arms several kilometres long, caused by a passing gravitational wave. Laser beams passing down the arms record their periodic stretching and shrinking as interference patterns in a central photo-detector.
The first generation of these interferometric detectors built a few years ago (GEO600, LIGO, Virgo and TAMA) successfully demonstrated the proof-of-principle and constrained the gravitational wave emission from several sources. The next generation (Advanced LIGO and Advanced Virgo), which were constructed until late 2015, have made the first direct detection of gravitational waves from a pair of black holes spiralling into each other. This discovery heralded the new field of GW astronomy. However, these detectors will not be sensitive enough for very precise astronomical studies of the GW sources.
The strategy behind the ET project is to build an observatory that overcomes the limitations of current detector sites by hosting more than one GW detector. It will consist of three nested detectors, each composed of two interferometers with arms 10 kilometres long. One interferometer will detect low-frequency gravitational wave signals (2 to 40 Hz), while the other will detect the high-frequency components. The configuration is designed to allow the observatory to evolve by accommodating successive upgrades or replacement components that can take advantage of future developments in interferometry and also respond to a variety of science objectives.