At the heart of each LISA satellite, there are two test masses freely floating in space: Cubes made of a gold-platinum-alloy, with sides four centimetres long. On the whole, the way LISA works is similar to the way that a Michelson interferometer works, such as the one you can see in front of you on the left. Each satellite corresponds to a beam-splitter, sending laser beams to each of the other satellites. These laser beams are not reflected back, as not enough light for that reaches the satellites at the far end, but these satellites send back laser signals of their own in an exact response to the incoming light. In comparing the incoming light from the different satellites, the distances between the floating test masses can be monitored, and any changes due to gravitational waves detected.