| Each week Quanta Magazine explains one of the most important ideas driving modern research. This week, physics staff writer Charlie Wood describes some of the oddities and mysteries that swirl around the quantity that orders the universe and our lives — time. | | How Physicists Are Exploring and Rethinking Time By CHARLIE WOOD | | Time is inextricably woven into what might be the most fundamental goal of physics — prediction. Whether they're studying cannonballs, electrons or the entire universe, physicists aim to gather information about the past or present and project it forward to catch glimpses of the future. Time is, as Nobel Prize winner Frank Wilczek put it in a recent episode of Quanta's The Joy of Why podcast, "the master variable under which the world unfolds."
In addition to prediction, physicists face the challenge of understanding time as a physical phenomenon in its own right. They develop ever-sharper explanations of the most obvious feature of time in our daily lives: that it flows inexorably forward. And recent experiments showcase more exotic ways in which time can behave under the established laws of quantum mechanics and general relativity. As researchers deepen their understanding of time in these two cherished theories, they encounter puzzles that seem to bubble up from murkier, more fundamental levels of reality.
Einstein famously quipped that time is what clocks measure. It's a snappy answer. But as physicists fiddle with ever more sophisticated clocks, they're frequently reminded that measuring something is very different from understanding it.
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A major achievement has been understanding why time flows only forward, when most simple physical events can be done and undone with equal ease. The broad answer seems to stem from the statistics of complicated systems, and the tendency of those systems to move from rare, ordered configurations to more common, disordered configurations, which have higher entropy. Physicists defined a classical "arrow of time" in this way in the 1800s, and in modern times physicists have been recasting this probabilistic arrow in terms of growing quantum entanglement. In 2021, my colleague Natalie Wolchover reported on a new description of clocks as engines that require disorder to run smoothly, tightening the connection between time and entropy.
Meanwhile, experimentalists have delighted in exposing the bizarre bends and crackles in time that we don't experience but that are allowed by the counterintuitive laws of general relativity and quantum mechanics. On the relativistic side, in 2021 Katie McCormick described an experiment measuring how Earth's gravitational field slows the ticking of time over distances as short as a single millimeter. And on the quantum front, last year I reported how physicists got particles of light to experience time flowing forward and backward simultaneously.
It's when physicists face the formidable task of melding quantum theory with general relativity that delight turns to confusion; each theory has its own concept of time, but the two notions have almost nothing in common.
In quantum mechanics, time works more or less as you might expect: You start with an initial state and use an equation to step it rigidly ahead to a later state. Quantum shenanigans may occur due to the peculiar ways that quantum states can combine, but the familiar concept of change occurring with the ticking of a master clock remains intact.
In general relativity, however, there is no such master clock. Einstein stitched time into a space-time fabric that bends and ripples, slowing down some clocks and speeding others. In this geometric picture, time becomes a dimension on par with the three dimensions of space, albeit a funky one that admits travel only in one direction.
And in this context, physicist frequently strip time of even its hallmark one-way nature. Many of Hawking's foundational discoveries about black holes — scars in the space-time fabric created by the violent collapse of giant stars — sprang from the metering of time with a clock that ticked in imaginary numbers, a mathematical treatment that simplifies certain gravitational equations and makes time indistinguishable from space. His conclusions are now viewed as inescapable, despite the unphysical nature of the mathematical trick he used to reach them.
More recently, physicists used this same imaginary time trick to argue that our universe is the most typical universe, as I reported in 2022. They still ponder why the trick seems to work and what its usefulness means. "There may be something profound here that we have not quite understood," the renowned physicist Anthony Zee wrote of the imaginary time gambit in his textbook of quantum field theory.
But what of the real, one-way time in our universe? How can physicists reconcile the two pictures of time as they tiptoe toward a theory of quantum gravity that unites quantum theory with general relativity? This is one of the hardest problems in modern physics. While no one knows the answer, intriguing proposals abound.
One suggestion, as I reported in 2022, is to loosen the restrictive way that time works in quantum mechanics by allowing the universe to seemingly generate a variety of futures as it grows — an unpalatable solution to many physicists. Natalie has written about the growing suspicion that the passage of time emerges from the entanglement of quantum particles, much as temperature emerges from the jostling of molecules. In 2020, she also covered an even more out-of-the-box idea: that physics be reformulated in terms of imprecise numbers and relinquish its ambitions of making perfect forecasts of the future.
Whatever it is that clocks are measuring continues to prove elusive and mysterious. | | | PBS Space Time has a detailed explainer video in which Matt O'Dowd describes how disorder aims the arrow of time from the past toward the future. | | | New Scientist hosted a 2020 lecture about the nature of time, available on YouTube, by Carlo Rovelli, a quantum gravity researcher who wrote a book on the subject. | | | Wired published a playful and wide-ranging personal essay by K.C. Cole that explored the many manifestations of time in physics and philosophy. | | | | | | |
