Experimental chapter describing different experiments allowing us to accurately measure physical parameters, such as very small concentrations of atomic species by intensity monitoring, the observation of gravitational waves using giant interferometers, transverse positioning of light beams, transition frequencies of metrological interest using laser frequency combs, and magnetometry of ultra-small magnetic fields using superconducting quantum interference devices (SQUIDs).

Mass produces spacetime curvature – that is a central lesson of general relativity. The static spherical mass of the Sun produces the Schwarzschild geometry outside it. Mass in (nonspherical, nonuniform) motion is the source of ripples of curved spacetime, which propagate away at the speed of light. These propagating ripples in spacetime curvature are called gravitational waves. Their free propagation will be discussed in this chapter. There are many important sources of gravitational waves in the universe – binary star systems, supernova explosions, collapse to black holes, and the Big Bang are all examples. Gravitational waves provide a window for exploring these astronomical phenomena that is qualitatively different from any band of the electromagnetic spectrum. However, the weakness of the gravitational interaction in everyday circumstances means that gravitational waves are not easily detected.