To observe gravitational waves, the ET will measure tiny changes in the length of detector tunnels 200-300 metres underground and several kilometres long (see video). Laser beams shone down the tunnels and reflected back to their source point, normally cancel one another out exactly. When a gravitational wave passes however, the tunnels briefly expand and contract. The light peak thus generated is the wave’s fingerprint and contains information about the source of the signal, such as the formation of a black hole or colliding neutron stars.  

The telescope has a design sensitivity of 22 decimal places. This means that it can detect differences in distance ten thousand times smaller than the size of the protons in an atomic nucleus. To achieve that incredible accuracy, the observatory bounces laser beams up and down long tunnels. An ultra-high vacuum and specially designed dampers filter out disruptive vibrations from the immediate environment. The entire technology is quiet, safe and clean: all it does is passively observe and record gravitational waves, which are an existing natural phenomenon. 

ET Pathfinder

To prepare for the high-tech demands made by the ET, scientists and companies from the Netherlands, Belgium and Germany set up a joint R&D facility: the ETpathfinder. This is a bit like a small test version of the eventual ET. See the video below for an impression of the pathfinder project.