From Sundials to Optical Clocks: The Evolution of Timekeeping and the Physics of Time

Overview

New Scientist takes viewers on a journey through the history of timekeeping, showing how humanity moved from sun based time to precise clock time, and how this evolution underpins navigation, finance, and modern physics. The video blends the story of instruments such as sundials, the Greenwich meridian, and marine chronometers with modern breakthroughs in optical clocks and concepts from relativity and quantum gravity. It also touches on how our understanding of time shapes technology and our sense of reality, while inviting readers to engage with the topic.

Section 1: Introduction to Time as a Scientific and Societal Thread

Time has always been a practical concern, from the ancient sundial to today’s digital systems. This exploration frames time as something humans learned to measure and standardize, a process that enabled navigators to chart courses across oceans, empires to coordinate, and scientists to test and refine physical theories. The video emphasizes that time is not merely a backdrop but a central thread in the fabric of civilization, enabling modern computational tools and high precision physics. The narrative is anchored by New Scientist host and Greenwich expert Dr. Louise Devoy, who situates 2025 as a milestone year celebrating 350 years since the Royal Observatory foundation and the rich lineage of timekeeping that followed.

Section 2: Early Timekeeping and the Birth of Greenwich Mean Time

The early chapters show that solar time, tied to the sun’s position, is inherently variable and imperfect for astronomers. To achieve consistent clocks, mean solar time is defined as Greenwich Mean Time, a solar time averaged to 24 hours. The equation of time is discussed as the correction needed to align sundial readings with clock time. The octagonal astro-room at Greenwich houses long pendulums, dial geometry, and heavy driving weights that enable clocks to run for long periods with minimal maintenance. John Flamsteed’s sidereal time measuring Sirius and crossing the meridian established constant Earth rotation as a basis for longitude determination, setting the stage for long-term navigation and mapping.

Section 3: The Marine Chronometer and the Longitude Problem

The drive to solve longitude at sea required a reliable reference time. John Harrison’s innovations—H1 with a spring balance, temperature compensation, and lignum vitae components—addressed motion, temperature, and maintenance constraints. The subsequent H4 watch demonstrated sea-travel viability, overturning the prevailing belief that a portable clock could not achieve marine precision. These breakthroughs transformed navigation and allowed the Royal Navy to produce accurate charts, ultimately leading to standardization of the Greenwich meridian as the prime reference. The Shepherd Gate Clock of 1852 and the use of telegraph and later radio signals illustrate how time distribution evolved from visual signals to synchronized electrical networks, culminating in the BBC six pip time signal, which helped households and institutions stay aligned with GMT.

Section 4: From Mechanical to Electronic Timekeeping: Quartz, Atomic, and Beyond

The narrative traverses the move from mechanical to electronic timekeeping. The quartz crystal oscillator offered a new era in the early 20th century, while postwar developments in microwaves and atomic physics yielded clocks that vibrate at billions of cycles per second. The timing precision of these devices directly impacts digital finance and satellite navigation. The video traces how atomic clocks laid the groundwork for a robust international time scale, and how the search for ever higher stability led to optical clocks, which operate at frequencies five orders of magnitude higher than microwaves and offer a leap in precision by roughly two orders of magnitude.

Section 5: Optical Clocks, Fundamental Physics, and Redefining the Second

Optical clocks are introduced via a Terbium ion system housed in an ultra-high vacuum ion trap. The talk distinguishes optical transitions from the microwaves used in caesium clocks, explaining that higher frequencies enable more precise timekeeping. The discussion centers on how optical clocks could drive a redefinition of the second, facilitating improved absolute frequency measurements and a tighter integration with the international time scale. The narrative also describes how optical clocks enable novel tests of fundamental physics, such as detecting potential variations in the fine structure constant and conducting precise geodesy, where height differences as small as a centimeter can be discerned through gravitational potential changes.

Section 6: The Philosophical and Physical Nature of Time

The program pivots to deeper questions about time. It recounts the central insight that time does not present a universal, single arrow across the cosmos. Time is influenced by motion and gravity; in extreme environments such as near black holes, time can slow dramatically. The presenter offers four key insights: time is locally defined; simultaneity is relative; clocks do not measure a universal now; and thermodynamics imposes a direction to time through entropy. The conversation emphasizes that the apparent flow of time and the past-future distinction arise from macroscopic processes rather than fundamental laws, although the second law of thermodynamics remains a foundational anchor for when and how we perceive time's direction.

Section 7: Time, Memory, and Neuroscience

Time, for humans, is intimately tied to memory and anticipation. Saint Augustine’s reflections on memory and time, as cited in the talk, illustrate how our experience of time emerges from cognitive processes that store memories and predict the future. The brain’s timekeeping capabilities and the subjective sensation of time's passage are contrasted with physical clocks. This section argues that understanding time requires interdisciplinary collaboration across physics, neuroscience, psychology, and even philosophy, since our sense of time is inseparably linked to how we remember and imagine.

Section 8: Time Without a Universal Clock: Quantum Gravity and the Memory of Spacetime

The speaker introduces a more radical view: at the most fundamental level, time may not exist as a universal dimension. Quantum gravity suggests a world where events relate to one another without a single temporal parameter. Time becomes a local, emergent phenomenon, analogous to up and down being meaningful only in relation to a gravitational field. The talk presents a roadmap for describing a timeless world using events and their connections, with physical notions like distance and duration arising in specific limits or approximations.

Section 9: The Memorable Universe: Information, Gravity, and Testing the Theory

The discussion highlights how gravity, spacetime geometry, and information may intertwine. The quantum memory matrix proposes spacetime stores information in memory cells that can be entangled, potentially influencing curvature. The Event Horizon Telescope and gravimeter experiments could, in principle, reveal imprints of this informational content in extreme regimes, offering a route to test ideas that initially seem purely theoretical. The possibility that information could influence large-scale gravitational behavior may have implications for dark matter and dark energy, inviting new lines of inquiry into cosmology and fundamental physics.

Section 10: Quantum Computing, Information, and the Future of Physics

Quantum computing provides a laboratory to explore imprint retrieval and other informational processes that could mirror spacetime memory. The researchers describe experiments using qubits to simulate memory cells and retrieve information under realistic noise, demonstrating how information processing can illuminate the physics of spacetime. This approach promises to unify quantum information science with gravity, electromagnetism, and the weak and strong nuclear forces. The conversation also suggests that this lens may offer fresh perspectives on unresolved topics such as dark matter, dark energy, and primordial black holes, without invoking additional spatial dimensions or holography.

Section 11: Societal Implications and The Human Face of Time

Finally, the talk returns to the human dimension of time. Time is not just a physical parameter but a lived experience shaped by memory, emotion, and culture. Buddhism and literature are invoked to emphasize that accepting impermanence is part of learning to live with time. The speaker argues that the emotional aspect of time is not a distraction from science but an essential part of what time means to humanity. The video closes by underscoring the need for cross-disciplinary collaboration to understand time, and for continuing exploration of both practical timekeeping and the profound questions about spacetime, memory, and reality.