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Arrows of Time

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We care about your data, and we'd like to use cookies to give you a smooth browsing experience. Please agree and read more about our .AgreeAustralia’s first inhabitants, the ancestors of today’s aboriginal peoples, are believed to have embraced a timeless view of nature, in which the present and past are intimately connected. The spirits of long-dead ancestors, for example, were believed to inhabit the living. These spirits reflected a long-ago golden age sometimes known as the Dreamtime.A number of civilizations, including those in ancient Iran as well as Greece and Rome, embodied time in a deity, often called Chronos. Chronos was frequently depicted as a winged serpent, although in Rome he took on a more human form. The philosopher Pythagoras, in the 6th century BCE, described the god Chronos as the “soul” of the universe.Source art: Chronos and His Child by Giovanni Francesco RomanelliWhile Western culture has come to embrace a linear conception of time, many other cultures have focused on time’s recurring cycles. Hinduism and Buddhism, for example, adopted a cyclic view of time that suggested the eventual return of the world to its former state; nothing is permanent, and even death is merely a passage to rebirth and renewal.Source photo: Ipek Morel DiplikayaIn Physics, the Greek thinker Aristotle spelled out a fairly modern-sounding definition of time as “the calculable measure of motion with respect to before and afterness.” This idea of time as a fixed sequence of events would survive with only minor modifications until the work of Einstein in the early 20th century. Did time have a beginning? Aristotle balked at the notion, instead viewing time as unbounded.Source photo: Delphotostock“Absolute, true, and mathematical time, in and of itself and of its own nature, flows uniformly,” wrote Isaac Newton in his masterwork, the . Newton was arguing that, even if our clocks might be imperfect, real time flows at a steady rate, serving as a kind of master clock for the universe. Newton’s definition meshes with our commonsense impression of time — that it passes at the same rate for everyone.The daily cycles of plants, as they open and close their petals and flowers, were known to the ancient Greeks, but it was an 18th-century astronomer, Jean-Jacques d’Ortous de Mairan, who showed that plants are doing more than simply responding to sunlight. Mairan placed mimosa plants in a darkened room for several days and noticed that their cyclic behavior persisted “as if the plants could feel the sun.”Video: Celia Gorman for Quanta Magazine; source: Zygista/ShutterstockThe French scientist Sadi Carnot proved there’s an upper limit to the efficiency of any engine. Carnot’s rule, as we now call it, was the first statement of what came to be known as the second law of thermodynamics. The law states, roughly, that in a closed system, entropy (the amount of disorder) must always increase. The fact that things get more disordered — but never the other way around — appears to lend a directionality to time.Source art: Brockhaus’ Conversations-LexikonThe British writer H.G. Wells published The Time Machine, often described as the first modern time travel story. Though it was published a full decade before Einstein’s theory of relativity, the book treats time as a fourth dimension, independent of the three dimensions of space, . The protagonist travels thousands and even millions of years into the future, only traveling back in time to briefly return to his point of departure in Victorian England.In his , Albert Einstein showed that there can be no “master clock” for the universe, as Newton had imagined. Instead, measurements of time depend on the motion of each observer. An exception is the speed of light, which is seen to have the same value by everyone (about 300,000 km per second). The theory also implies that the speed of light is the ultimate speed limit for the universe.In the wake of Einstein’s special relativity, it became clear that space and time were intimately connected. The German mathematician Hermann Minkowski — one of Einstein’s former teachers — described the resulting four-dimensional entity: While space and time are relative, intervals in space-time are absolute. At a lecture in 1908, he proclaimed: “Henceforth space by itself, and time by itself, are doomed to fade away into mere shadows, and only a kind of union of the two will preserve an independent reality.”In his , Einstein showed that space and time can be warped by gravity (in fact, the theory describes gravity as a warping of space-time). While special relativity applies only to objects moving at constant speeds, general relativity also covers accelerating objects. The theory predicted the existence of and — both of which have now been confirmed by observation.A few months after Einstein worked out the equations for general relativity, the German physicist Karl Schwarzschild solved the equations for a specific case: a nonrotating spherical mass. But Schwarzschild’s solution had a surprising implication: If all of an object’s mass could be compressed to within a critical radius (called the ), it would undergo a catastrophic, unstoppable collapse, creating an object whose gravitational field was so intense that nothing — not even light — could escape. We now call such objects .Why does time appear to flow in just one direction? In The Nature of the Physical World, the British physicist Arthur Eddington called this “.” He believed that it was somehow connected to entropy. Eddington wrote: “If as we follow the arrow we find more and more of the random element in the state of the world, then the arrow is pointing towards the future; if the random element decreases the arrow points towards the past. That is the only distinction known to physics.”Using observations from the 100-inch telescope outside Los Angeles, the U.S. astronomers Edwin Hubble and Milton Humason showed that the farther a galaxy is from Earth, the faster it is speeding away from us. The discovery paved the way for what we now call of cosmology — the idea that all of the matter and energy we see around us was once concentrated in a much smaller space. The implication is that the universe — and possibly time itself — had a beginning.The German zoologist Gustav Kramer hypothesized that many animals and plants have a kind of internal clock governing their biological rhythms. He argued that this was related to the “sun compass” that birds and bees appeared to use for navigation. Further experiments by his compatriot Klaus Hoffmann showed that these “circadian rhythms,” with periods of approximately 24 hours, do in fact assist birds in navigation.Two U.S. scientists — the physicist Arno Penzias and the radio astronomer Robert Wilson — using a radio antenna at Bell Labs in Holmdel, New Jersey. This radiation was soon recognized as the “,” sometimes described as the echo of the Big Bang. Whether the Big Bang should be considered the beginning of time continues to be debated. Some cosmologists suggest that the universe runs through cycles of .Relativity was put to the test when commercial airliners carrying identical atomic clocks were flown around the world twice — first eastward and then westward — and the results compared to identical clocks that remained on the ground. The resulting discrepancy, known as time dilation, was very small (less than one-millionth of a second) but readily measurable with the clocks, and the results were in agreement with Einstein’s special and general theories of relativity.Two groups of scientists, led by Robert Moore and Victor Eichler in Chicago and by Friedrich Stephan and Irving Zucker in Berkeley, discovered the brain region that governs circadian rhythms. The key structure is the suprachiasmatic nucleus, or SCN, which processes information from the retinas about light and darkness. Later studies in mice found that damage to the SCN destroys the animal’s rhythms — and that an SCN transplant can restore them.Source art: Sobotta’s Human AnatomyThe British physicist Stephen Hawking published , a popular book on space, time and the universe that became a surprise bestseller. Hawking identified three distinct “arrows of time”: a psychological arrow (underpinning our memories of the past and how we imagine the future), a thermodynamic arrow (the direction in which entropy increases), and a cosmological arrow (the direction in which the size of the universe increases).Book cover: Bantam PressAn Italian woman named Stefania Follini emerged from a cave in New Mexico where she had spent 130 days without any contact with the outside world — and without clocks. Soon after the experiment began, her sleep-wake cycle shifted from 24 to 25 hours; by the end it was up to 36 hours. After the 130 days, she estimated that about 60 days had passed. Much remains to be learned about how our bodies track time, but such experiments show that our internal clocks are not simply biological equivalents of mechanical clocks and watches.In an influential , Noah Linden, Sandu Popescu, Tony Short and Andreas Winter argued that the arrow of time can be explained . As a physical system becomes entangled with its surroundings, it moves closer to equilibrium — and this one-way evolution determines time’s arrow. The team’s work built on ideas that Seth Lloyd had put forward in his doctoral thesis in 1988.Researchers investigating possible have begun to suspect that neither space nor time is a fundamental property of the universe. A key idea, developed by the physicists Juan Maldacena and Leonard Susskind in 2013, posits an between quantum entanglement and wormholes. The idea is sometimes called , after two papers published by Einstein and co-authors in 1935. If true, it would imply that quantum interactions .A team of Norwegian scientists, led by Albert Tsao, found a network of cells within the brain that appears to play a crucial role in connecting an . The , conducted on rats, suggests that certain groups of cells within a brain region known as the lateral entorhinal cortex, which is adjacent to the hippocampus, encode episodic memory by “allowing the hippocampus to store a unified representation of what, where and when.”Quanta Magazine moderates comments to&nbsp;facilitate an informed, substantive, civil conversation. Abusive, profane, self-promotional, misleading, incoherent or off-topic comments will be rejected. Moderators are staffed during regular business hours (New York time) and can only accept comments written in English.&nbsp;Show comments

We care about your data, and we'd like to use cookies to give you a smooth browsing experience. Please agree and read more about our .AgreeAustralia’s first inhabitants, the ancestors of today’s aboriginal peoples, are believed to have embraced a timeless view of nature, in which the present and past are intimately connected. The spirits of long-dead ancestors, for example, were believed to inhabit the living. These spirits reflected a long-ago golden age sometimes known as the Dreamtime.A number of civilizations, including those in ancient Iran as well as Greece and Rome, embodied time in...

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