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Two beams of light race around a closed loop, heading toward each other. Since the speed of light is constant, when the ring turns, one beam has to catch up with a moving finish line, while the other meets it sooner. Their meeting creates a pattern, and from that pattern we can tell how fast the whole system is spinning. That’s the light carousel at the heart of TRIO, a device assembled in the underground Gran Sasso laboratory.

Thanks to a clever design, the carousel barely shakes on its own, and its ring can be enlarged to five meters. This will allow it not only to monitor the planet’s rotation but also to catch gravitational waves — ripples in the very fabric of the universe. Someday the instrument will test how massive objects warp spacetime — a prediction by Einstein. And the largest such rings, already the size of a football field, can detect how distant earthquakes knock the planet off its rhythm.