Time
Overview
Time is a fundamental concept representing the order and duration of events, and it is defined in different ways across physics, philosophy, biology, and culture. Newton posited an absolute and uniformly flowing time, but Einstein's theory of relativity showed that time is a relative dimension that varies with inertial frames and gravitational fields. In modern science, time has a directionality tied to entropy increase while also being a measurable physical quantity precisely defined by the oscillation frequency of cesium atoms.
Main Content
Time in Physics
Classical Mechanics and Absolute Time
In Newtonian mechanics, time t is an absolute background that flows identically everywhere. Along with Galileo's principle of relativity, classical physics treated time as a coordinate independent of space, and this view is still used as a valid approximation at everyday scales.
Theory of Relativity
Special relativity introduced the relativity of simultaneity and time dilation. Time flows more slowly for a rapidly moving observer, as explained by the extended lifetime of muons or the twin paradox. General relativity predicts that time flows more slowly as gravity becomes stronger, and GPS satellites apply a correction of about 38 microseconds per day to prevent positioning errors.
Thermodynamics and the Arrow of Time
The second law of thermodynamics states that the entropy of an isolated system increases, creating an 'arrow of time' that distinguishes past from future. Named by Eddington, this concept starts from the low-entropy state of the early universe and is a central topic in explaining why macroscopic irreversibility appears even though microscopic physical laws possess time symmetry.
Quantum Mechanics and Time
The Schrödinger equation treats time as an external parameter. In the Wheeler-DeWitt equation, which attempts to quantize spacetime, a 'problem of time' arises in which the time variable disappears, and attempts continue to reinterpret time as a relative relationship within the system, such as the Page-Wootters clock model.
Measurement of Time
History of Calendars and Clocks
Time measurement, which began with sundials and water clocks, became more precise through Huygens' pendulum clock in 1656 and Harrison's marine chronometer in the 18th century. Standard time and time zone systems were organized with the development of railways and telegraphs in the 19th century.
Atomic Clocks and the SI Second
In 1967, the International System of Units defined the second as 9,192,631,770 cycles of the transition frequency between the two hyperfine levels of the ground state of the cesium-133 atom. Since then, optical lattice clocks and ion clocks have achieved precision at the 10^-18 level, and the definition of the second was maintained unchanged in the 2019 SI redefinition.
Coordinated Universal Time and Leap Seconds
Coordinated Universal Time (UTC), introduced in 1972, has inserted leap seconds to reconcile atomic time with Earth's rotation. However, due to fluctuations in Earth's rotation and the burden on computing systems, the General Conference on Weights and Measures agreed in 2022 to abolish leap seconds by 2035.
Time in Philosophy and Psychology
Augustine said, "What then is time? If no one asks me, I know," interpreting time as a distension of the soul. McTaggart distinguished the A-series, which arranges events as past, present, and future, from the B-series, which arranges them by earlier-later relations, and this led to the debate between presentism and eternalism (the block universe). In psychology, research has accumulated showing that time perception is distorted by attention, arousal, and emotion, and that dopamine and the cerebellum are involved in internal time estimation.
Biological Time
Biological clocks coordinate circadian rhythms with a period of about 24 hours, centered on the suprachiasmatic nucleus (SCN). The 2017 Nobel Prize in Physiology or Medicine was awarded to Hall, Rosbash, and Young for revealing these molecular mechanisms, and their connections to sleep, metabolism, and immune function continue to be elucidated.
Latest Trends
In 2024, a 'nuclear clock' using the thorium-229 nuclear isomer transition operated for the first time, suggesting the possibility of stability surpassing atomic clocks. Optical lattice clocks have improved in precision to the 10^-19 level, and applications for measuring gravitational time dilation at the millimeter scale are being discussed. In the same year, an experiment on the 'negative time' of photons reported by a University of Toronto research team sparked public debate over the concept of time, but the prevailing rebuttal is that it was actually a result interpreting the phase response of an atomic medium. Meanwhile, with the abolition of leap seconds in 2035 approaching, standards for time synchronization in satellite navigation, financial transactions, and distributed systems are being reorganized, and research on quantum network-based time transfer and time crystals is also active.
Related Topics
- [[Theory of Relativity]]
- [[Entropy]]
- [[Atomic Clock]]
- [[Coordinated Universal Time]]
- [[Circadian Rhythm]]
- [[Time Travel]]