Time

• 1. The Fourth Dimension

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  Time is what a clock reads. Physicists settled on that blunt, almost evasive definition to escape a trap, because defining time any other way tends to fold back on itself in circles. Scholars across every field have tried and failed to pin down a meaning of time that works everywhere without becoming circular. And yet business, sport, science, and the performing arts all build measuring systems around it.

  Why do twelve and thirteen recur across so many cultures? Why did a French government try to remake the hour itself? How can two people moving past each other disagree on when an event happened, and both be right? The answers run from a Paleolithic moon-watcher to a clock built to keep time for ten thousand years. Each touches the same strange subject from a different side.

• 2. Carving Up The Day

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  Artifacts from the Paleolithic suggest the moon was used to reckon time at least 6,000 years ago. Lunar calendars came among the first, with years of either 12 or 13 lunar months, running to 354 or 384 days. Without intercalation, adding days or months to some years, the seasons drift quickly against a calendar built on twelve lunar months alone. A full year runs to about 365.24 days, so lunisolar calendars wedge in a thirteenth month to close the gap. The numbers twelve and thirteen took on prominence partly from this relationship of months to years.

  The Egyptians are credited with splitting the day into smaller parts, thanks to their sundials, which ran on a duodecimal system. The importance of twelve traces to the lunar cycles in a year and the number of stars used to count the passage of night. A sundial casts shadows from a gnomon onto markings calibrated to the local hour. Obelisks built as gnomons rose as early as around 3500 BC. An Egyptian device from around 1500 BC, shaped like a bent T-square, read the time from a shadow on a nonlinear rule. The T faced east in the morning, then at noon was turned around to cast its shadow toward evening.

  The Maya built their own systems in Mesoamerica, religiously and astronomically grounded. Their Haab' calendar held 18 months of 20 days, plus five epagomenal days at the end of the year. Alongside it ran the Tzolk'in, a 260-day sacred calendar. The reforms of Julius Caesar in 45 BC moved the Roman world onto a solar calendar, but the Julian calendar let the solstices and equinoxes drift by about 11 minutes per year. Pope Gregory XIII corrected this in 1582, and the Gregorian calendar spread nation by nation over centuries to become the most common calendar in the world.

• 3. Ten Hours In A Day

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  During the French Revolution, reformers built a new clock and calendar to replace the Gregorian one. The French Republican Calendar broke the day into ten hours, each of a hundred minutes, each minute of a hundred seconds. This was a deliberate break from the base-12 duodecimal system that so many cultures used in their devices. The system was abolished in 1806.

  In medieval philosophical writing, the atom was the name for the smallest possible division of time. Its earliest known appearance in English is in Byrhtferth's Enchiridion, a science text of 1010 to 1012, where it was defined as 1/564 of a momentum, making it equal to 15/94 of a second. It was used in the computus, the process of calculating the date of Easter. The word clock probably comes from the Middle Dutch klocke, which traces back through medieval Latin clocca to Celtic roots, all cognate with words meaning bell. Hours were marked by bells in abbeys and at sea alike.

• 4. Machines For Counting Hours

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  Horology is the study of devices built to measure time, driven by gravity, springs, or electrical power, and regulated by various means. The most precise timekeeper of the ancient world was the water clock, or clepsydra, one of which was found in the tomb of the Egyptian pharaoh Amenhotep I. These clocks could measure the hours even at night, but needed manual refilling. The ancient Greeks and the people of Chaldea, in southeastern Mesopotamia, kept timekeeping records as part of their astronomical work.

  Alarm clocks reportedly first appeared in ancient Greece around 250 BC, in a water clock made by Plato that set off a whistle. It worked by gradually filling a series of vessels until water emptied through a siphon. The inventor Ctesibius revised the design, using a float as the drive and a sundial to correct the flow rate. In the 11th century, Chinese inventors built the first mechanical clocks driven by an escapement.

  Richard of Wallingford, who lived from 1292 to 1336 and was abbot of St. Alban's abbey, built a mechanical clock as an astronomical orrery around 1330. The hourglass measured the flow of time with sand and served at sea, where Ferdinand Magellan carried 18 glasses on each ship for his circumnavigation of 1522. Galileo Galilei and especially Christiaan Huygens advanced accuracy with pendulum-driven clocks, while Jost Burgi invented the minute hand. Levi Hutchins built what has been credited as the first American alarm clock, though it could only ring at 4 a.m. Antoine Redier was credited with patenting the first adjustable mechanical alarm clock in 1847, and one modern project, the Clock of the Long Now, was designed to keep time for 10,000 years.

• 5. The Caesium Second

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  Since 1967, the second has been defined by the properties of caesium atoms. SI sets the second at 9,192,631,770 cycles of the radiation matching the transition between two electron spin energy levels of the ground state of the caesium-133 atom. Atomic clocks, accurate to seconds across many millions of years, probe caesium with microwaves to read the frequency of these electron vibrations, and they calibrate other clocks. The invention of the caesium atomic clock in 1955 led to the replacement of older astronomical standards such as sidereal time and ephemeris time for most practical purposes.

  Time is one of the seven fundamental physical quantities in both the International System of Units and the International System of Quantities. A minute is 60 seconds, an hour is 3600 seconds, and a day is usually 86,400 seconds, though its length can shift with daylight saving time and leap seconds. A portable timekeeper meeting precision standards is a chronometer, a term first applied to the marine chronometer used to find longitude by celestial navigation, a precision first reached by John Harrison.

  International Atomic Time, or TAI, is the primary international standard from which others are calculated. Universal Time, UT1, is mean solar time at 0 degrees longitude, drawn from astronomical observation, and it drifts from TAI because Earth's rotation is irregular. Coordinated Universal Time approximates Universal Time and is kept within 0.9 second of UT1 by inserting leap seconds. The Global Positioning System broadcasts a precise signal based on UTC, and the smallest measured time interval uncertainty stands on the order of 12 attoseconds, about 3.7 times 10 to the 26th Planck times.

• 6. Wheels And Straight Lines

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  In Hindu philosophy, time turns as a wheel called the Kalachakra, the Wheel of Time, carrying the universe through endless cycles of creation, preservation, and destruction. Many ancient cultures of the East held this cyclical view, seeing events recur in predictable patterns. The Mayans, Aztecs, and Chinese shared similar beliefs, tied to astronomical observation and calendars. The Vedas, the earliest texts of Indian philosophy, dating to the late 2nd millennium BC, describe cycles each lasting 4,320 million years.

  The Islamic and Judeo-Christian world-view, by contrast, treats time as linear and directional, beginning with God's act of creation. The traditional Christian view sees time ending teleologically, at the eschatological end of the present order. Augustine of Hippo and Aquinas held that God stands outside time, seeing all events at once, and that God created time. In Ecclesiastes, traditionally ascribed to Solomon, who lived from 970 to 928 BC, time is depicted as cyclical and beyond human control, with a season for every activity.

  The Greek language splits time into two principles. Chronos is numeric, chronological time. Kairos is the right or opportune moment, qualitative and tied to the divine. In Greek mythology Chronos personifies time, pictured as an old, wise man with a long gray beard, the figure of Father Time. His name seeds English words like chronology, chronometer, chronic, anachronism, synchronise, and chronicle. Kabbalists, meanwhile, called time a paradox and an illusion, and rabbis sometimes saw it as an accordion expanded and collapsed at will.

• 7. Container Or Idea

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  Isaac Newton believed time was part of the fundamental structure of the universe, a dimension independent of events, flowing the same everywhere. This realist position is sometimes called Newtonian time, and he held that humans can grasp only relative time while merely occupying the absolute. Gottfried Leibniz held the opposite, that time and space are relational, not containers but a framework within which we sequence and compare events. Their clash came to a head in the Leibniz-Clarke correspondence.

  Plato, in the Timaeus, identified time with the period of motion of the heavenly bodies, and held it was made by the Creator at the same instant as the heavens. Aristotle, in Book IV of his Physica, defined time as the number of movement in respect of the before and after, believing time did not exist on its own but was relative to the motion of objects. In Book 11 of his Confessions, Augustine of Hippo asked, "What then is time? If no one asks me, I know: if I wish to explain it to one that asketh, I know not." He ended by calling time a distention of the mind, by which we grasp the past in memory, the present by attention, and the future by expectation.

  Immanuel Kant, in the Critique of Pure Reason, described time as an a priori intuition that lets us comprehend sense experience, neither a substance nor an entity that flows. Henri Bergson called time a Duration, bound up with creativity and memory. Martin Heidegger went further, holding that we do not exist inside time, we are time, able to step out of sequential time in our thoughts through ecstasis. Against them all stood the doubters. In 5th-century BC Greece, Antiphon the Sophist wrote that time is not a reality but a concept or a measure. Parmenides called time, motion, and change illusions, feeding the paradoxes of his follower Zeno, and J. M. E. McTaggart argued in his 1908 article The Unreality of Time that the idea contradicts itself.

• 8. Warped By Speed And Mass

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  Until Einstein's reinterpretation in 1907, time was thought identical everywhere, with all observers measuring the same interval for any event. Non-relativistic classical mechanics rested on that Newtonian assumption. Then trouble arrived through electricity and magnetism. Maxwell's equations of the 1860s held that light always travels at a constant speed in a vacuum, but classical mechanics assumed motion measured against a fixed frame. The Michelson-Morley experiment contradicted that assumption, the same experiment that first publicly demonstrated that the time for light to cross a given distance is identical for all observers.

  Einstein, in special relativity, postulated the constant and finite speed of light for every observer. From this, distances appear compressed and time intervals lengthened for objects moving relative to an inertial observer. A spaceship flying near light speed feels nothing unusual aboard, since the clock, the crew's thoughts, and their bodies all slow at the same rate. To a stationary watcher, the ship looks flattened in its direction of travel and its clock crawls. What seems like seconds to the crew might be hundreds of years to the observer, yet causality holds. The past is the set of events that can send light signals to an entity, the future the set to which it can send them.

  General relativity adds that time runs slower in stronger gravitational fields, an effect called gravitational time dilation. A subject approaching the event horizon of a black hole experiences time normally, while an outside observer sees them slow, appear frozen, and fade to nothing as the returning light diminishes. Time has one direction, the arrow of time, with examples including the entropic arrow tied to the second law of thermodynamics. Brian Greene theorizes that entropy tends to increase in either direction, making our low-entropy universe a statistical aberration, like tossing heads ten times in a row, though local experiment does not support it.

  Time appears to live in the brain as a place. There is growing evidence that humans conceptualize time spatially, organizing it as a mental timeline, the MTL. Literacy shapes it. In Western cultures the line tends to run rightward, past on the left and future on the right, matching how people read and write. Speakers of Arabic, Farsi, Urdu, and Hebrew read right to left, and their timelines unfold the other way, past on the right.

  The Yupno people of Papua New Guinea showed something stranger in one study. Their timeline runs uphill, so they gestured downhill toward the past, where the valley river flows to the ocean, and uphill toward the future, toward its source, no matter which way they faced. A study of the Pormpuraawans, an aboriginal group in Australia, found they arranged photos of a man aging from east to west, again regardless of facing, while an American group ran the photos left to right. Both indigenous groups use an allocentric timeline, one tied to the world rather than the body.

  The brain's judgment of time is a distributed system, drawing on at least the cerebral cortex, cerebellum, and basal ganglia. The suprachiasmatic nuclei govern the circadian rhythm, while other cell clusters handle shorter ultradian timing. Stimulants lead humans and rats to overestimate intervals, while depressants do the reverse, an effect linked to dopamine and norepinephrine changing how fast neurons fire. The sense of time is impaired in Parkinson's disease and attention deficit disorder. Two- and three-year-olds know only now and not now, five- and six-year-olds grasp past, present, and future, and seven- to ten-year-olds can read clocks and calendars, each age reaching a little further along a line it cannot see.