Huygens' pendulum clocks (1665)
Christiaan Huygens noticed two pendulum clocks mounted on the same wall swung in perfect anti-phase, with no visible connection between them. He suspected a hidden 'sympathy' between the clocks.
The effect became the founding observation of synchronization science.
Modern physics explains it as weak mechanical coupling through the shared wall. It seeded the entire field of synchronization dynamics.
The semiconductor time crystals sync through an invisible medium, exactly like Huygens' clocks syncing through the wall, but now in a solid-state system.
