From: veritasium
Despite the universe’s general tendency towards disorder as stated by the second law of thermodynamics [00:00:00], instances of spontaneous order, or synchronization, can be observed [00:00:11]. These include the coordinated swings of metronomes, the precise orbits of moons, the simultaneous flashes of fireflies, and even the rhythmic beating of the human heart [00:00:18]. The historical development of understanding this phenomenon spans centuries, impacting both engineering and fundamental physics.
Early Engineering Challenges: Bridges and Synchronized Footsteps
The dangers of synchronization in engineering were highlighted by early bridge failures:
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Broughton Suspension Bridge (1831): It was long known that armies should break step when crossing bridges due to an accident in 1831 [00:01:08]. Seventy-four men from the 60th Rifle Corps were marching across the Broughton suspension bridge in northern England when it collapsed under their synchronized footsteps [00:01:13]. Sixty men fell into the river, with 20 suffering injuries like broken bones or concussions [00:01:24]. This incident led the British Army to order all troops to break step when crossing bridges [00:01:34].
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Millennium Bridge (2000): Upon its opening in London in 2000, the Millennium Bridge began to wobble significantly as crowds filled it [00:00:39]. Despite police restricting access, the wobble persisted, leading to the bridge’s full closure two days later for two years [00:00:52]. This phenomenon was eventually attributed to “crowd synchrony” [00:15:16], where the bridge’s lateral resonant frequency of one cycle per second (matching the frequency of a single footfall) caused it to sway [00:16:06]. People instinctively adjusted their gait to the swaying bridge, inadvertently pumping more energy into its motion and worsening the wobble through a positive feedback loop [00:17:46]. Engineers later installed energy-dissipating dampers to fix the problem [00:18:32].
Christiaan Huygens’ Groundbreaking Observation (1665)
The first recorded observation of spontaneous synchronization in inanimate objects occurred in 1665:
- Pendulum Clocks (1656): In 1656, Dutch physicist Christiaan Huygens created the first working pendulum clock, aiming to help sailors determine longitude [00:02:08]. His clocks were accurate to about 10-15 seconds per day, a significant improvement over the 15 minutes per day typical of other clocks at the time [00:02:38].
- Accidental Discovery (1665): While testing an arrangement of two clocks hanging from a shared wooden beam in February 1665, Huygens made a remarkable discovery [00:02:57]. After about half an hour, the two pendulums would spontaneously synchronize, swinging in opposite directions [00:03:10]. He attempted to disrupt this “strange sympathy” by setting them out of sync or placing a board between them, but they always returned to lockstep [00:03:26]. Huygens realized that the synchronization was caused by mechanical vibrations transferred between the clocks via the shared wooden beam, making the two oscillators coupled [00:03:54].
Mathematical Understanding and Theory
While Huygens qualitatively described synchronization, a rigorous theory began to develop much later [00:04:16]:
- The Kuramoto model: Developed decades ago, this mathematical model describes synchronizing behavior [00:06:52]. It proposes that the rate at which an oscillator progresses through its cycle equals its natural frequency plus an amount related to its distance from other oscillators, determined by a “coupling strength” [00:07:03]. Synchronization occurs as a phase transition; past a critical level of coupling, oscillators suddenly lock their phases in time, similar to water freezing into ice at a critical temperature [00:09:01].
Applications and Observations in Nature and Medicine
The universality of synchronization phenomena in nature and technology has led to observations across various scales and applications:
- Tidal Locking: Our own Moon is tidally locked to Earth, meaning it rotates on its axis exactly once for every orbit around Earth, always showing the same side [00:10:32]. This is a common effect in our solar system, with 34 moons tidally locked to their planets [00:10:43].
- Orbital Resonance: Jupiter’s three innermost moons—Io, Europa, and Ganymede—are not only tidally locked but also in a 1:2:4 orbital resonance, meaning for every orbit of Ganymede, Europa orbits twice, and Io orbits four times [00:11:33].
- Belousov-Zhabotinsky (BZ) Reaction (1950s): Russian chemists in the 1950s sought oscillating chemical reactions [00:11:56]. Boris Belousov and later Anatol Zhabotinsky discovered the BZ reaction, which oscillates in color over time, appearing to defy thermodynamics by not monotonically going to equilibrium [00:12:35]. If unstirred, this reaction can produce spectacular spiral waves or expanding target patterns of color in the liquid [00:13:21].
- Application of synchronization concepts in cardiac arrhythmia research: The spiral waves observed in the BZ reaction are remarkably similar to electrical excitation spiral waves seen in the heart [00:14:02]. This similarity inspired Art Winfree, whose work on chemical reaction waves provided insight into cardiac arrhythmias, specifically ventricular fibrillation [00:14:18]. Winfree’s theory, based on these rotating spirals, aims to understand the cause of ventricular fibrillation and design better defibrillators [00:14:40]. A lack of synchronization in a fibrillating heart prevents blood pumping, leading to sudden death [00:14:58].
Synchronization is a universal phenomenon observed across every scale of nature, from subatomic to cosmic, utilizing every known communication channel, be it gravitational, electrical, chemical, or mechanical [00:10:02]. Understanding how the properties of the whole emerge from the properties of its parts is a significant frontier in science, known as the field of complex systems [00:18:58].