© LIGO Laboratory.
Physicists use incredibly precise equipment to search for gravitational waves, which can provide indirect evidence for the existence of gravitons. Gravitational waves distort spacetime, changing the distance between the test masses as they pass by the apparatus sketched here. The test masses are mirrors in a laser interferometer that is capable of measuring tiny changes in distance, as we will see in Unit 3. This type of experiment detects the wave nature of gravity—the particle nature (or full quantum nature) of gravity cannot be seen in an experiment like this. At this point, it is not known how to directly test the quantum nature of gravity. (Unit: 2)