From Sundials to Optical Clocks: The Evolution of Timekeeping and the Quest to Redefine the Second

Overview

Timekeeping has transformed navigation, science, and daily life, evolving from solar time to precise atomic and optical clocks. This summary highlights the voyage from ancient sundials to modern optical clocks and the profound questions about the nature of time that emerge along the way.

Key moments include Greenwich mean time, the marine chronometer, time signals for the railways and BBC, the shift from mechanical to electronic and atomic clocks, and the ongoing quest to redefine the second based on optical frequencies. The video also surveys contemporary ideas in physics that challenge our everyday sense of time, from relativity to quantum gravity, and the social and emotional dimensions of time in human experience.

Introduction to Timekeeping

The discussion opens with a look at how humans learned to measure time, from compact sundials to the precision of modern clocks, and how time became embedded in everyday life and science. Greenwich and the Royal Observatory are highlighted as pivotal to standardizing time and geography.

Early Timekeeping and Greenwich Mean Time

The sundial, equation of time, and the concept of mean solar time laid the groundwork for Greenwich Mean Time (GMT). John Flamsteed and the Astronomer Royal connected solar time, star time, and daily clock readings, enabling accurate celestial observations and longitude measurements. The octagonal rooms, long pendulums, and heavy driving weights illustrate the design principles of 17th and 18th century clocks.

Longitude, Chronometers, and Global Standardization

The challenge of determining longitude at sea spurred John Harrison to develop marine timekeepers, culminating in H4, a successful marine chronometer. Greenwich time and the Greenwich Meridian underpinned nautical charts and, by the 1880s, the international adoption of Greenwich as the prime meridian aligned global navigation and timekeeping.

From Telegraphs to Radio and Beyond

Time distribution evolved from visual signals such as time balls to telegraph and then radio signals, including the six pips broadcast by the BBC. The rise of quartz clocks and atomic clocks followed, transforming accuracy from seconds per day to seconds per millions of years and beyond.

Optical Clocks and Redefining the Second

Optical clocks, based on optical transitions such as in caesium and ytterbium, offer dramatic improvements in precision. Terbium ion optical clocks illustrate how high-frequency optical standards enable new tests of fundamental physics and geodesy, and they motivate redefining the second in terms of optical frequencies for greater accuracy and stability.

Time as a Physical and Philosophical Concept

The video surveys deeper questions about time, including relativity, the illusion of a universal present, entropy and the arrow of time, and the prospect that time may be emergent or absent at the most fundamental levels. It also examines how our brains construct time through memory and anticipation, weaving physics with neuroscience and philosophy.

Quantum Gravity and New Theories

The discussion closes with frontier ideas that space-time may possess memory through information, potentially observable with the Event Horizon Telescope and quantum computing experiments. These proposals aim to connect gravity, quantum physics, and information in new ways that could reshape our understanding of time itself.