The Einstein telescope

Albert Einstein postulated that gravitational waves (GWs) were waves in the curvature of space-time in his famous Theory of General Relativity. Einstein wrote that a GW´s origin is from objects that undergo a change of their mass quadrupole moment. The distance between free falling test masses will be altered if a GW passes through. The spectral relative length (or distance) change between the test masses h = ΔL/L is the strength of the GW, where L is the distance between the test masses, and ΔL is the absolute spectral length change measured in m/√Hz. Even for rare cosmic events with huge masses (e.g., binary systems with solar mass objects and periods in the millisecond range), an h of only 10^-21/√Hz is expected. We present the operating principles of the modern GW interferometric detectors and the second generation of the detectors. Beyond the era of advanced detectors, we discuss novel instruments that could allow routine GW astronomy. Within a European-wide collaboration, a possible design of such a GW observatory-the Einstein Telescope (ET)-has been developed. It aims for a ten times increase in sensitivity compared to the second generation throughout the frequency range from a few hertz up to a few kilohertz, as Fig. 1 illustrates.