Does Time Work Differently in the Quantum Realm?
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February 26, 20255 min readIn the Quantum Realm, Times Arrow Might Fly in Two DirectionsScientists studying a centuries-old mystery of physics suggest two arrows of time control the evolution of quantum systemsBy Gayoung Lee edited by Lee Billings MirageC/Getty ImagesIf a cup of water spills on the floor, the water cant unspillthat is, its inconceivable that each water molecule would exactly reverse its course to slip back into the cup. To do so would be to turn back timesomething that, as far as we know, cant be done. The water either spills or it doesnt, but if it does, itll stay that way.In that way, time as we experience it is asymmetric. We have memories of the past rather than the future, and spilled water doesnt flow back to its cup, just as an arrow that has been let fly doesnt return to its bow. In our everyday lives, the arrow of time goes only in one direction: forward.We know [this] is something thats part of our common experience, says Andrea Rocco, a theoretical physicist at the University of Surrey in England. But how exactly times arrow arises is less clear to physicists, in part because the math they use to describe most of the world makes no distinction between time that moves forward and time that moves backward; either direction is perfectly viable, as far as their equations are concerned.On supporting science journalismIf you're enjoying this article, consider supporting our award-winning journalism by subscribing. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.Relatedly, the concept of time becomes somewhat illusory in the absence of change. For instance, if our cup of water was held in stasis in a magic, perfectly insulated and physically impervious box floating in deep space, this system would seem the same whether it was examined five years ago or five millennia from now. So which way is times arrow flying inside the magic stasis box? For such isolated systems, time is thus considered to be symmetric; only when it is open to influence from the external environment is this symmetry broken, whether via water evaporating or the cup tipping over to spill its contents.And yet open and isolated systems are inherently linked. Even if our cup of water was isolated from the external world, its molecules would still be randomly jostled by microscopic effectschanges that potentially break times symmetry, like the ticking of the cosmic clock. So why does this discrepancy exist, and what does it imply about the validity of the models physicists use to study the role of time in the reality we experience?Different researchers have tackled these questions in different ways, but Rocco and his colleagues revisited some of the math behind the inconsistency to see if an alternative approach could resolve the apparent asymmetry. Their conclusions, recently published in Scientific Reports, detail the existence of not one but two opposing arrows of time within open quantum systems.This is a bit like if our water cup were precariously balanced on a knife-edge: it could topple to spill down either side, each side being an opposite-facing arrow of time. But the cup falling one way versus the other doesnt make the water spill differently. Either way, mathematically speaking, the end result is exactly the sameit preserves the symmetry for both possible instances. In a sense, we are stuck in this universe in which time actually goes in one direction, Rocco explains. But the equations of motion that we are considering would have allowed the universe to go in the other direction.This way, as the conventional understanding goes, both instances display time symmetry without requiring the assumption that some external influence from the environment is imposing or defining an arrow of time, Rocco says. In other words, the researchers work suggests that times arrow is a spontaneous feature of an open quantum system rather than one that is set by its history.The new work adds to some interesting questions about what physicists deem relevant in their studies of time. Michele Campisi, a theoretical physicist at the Italian National Research Councils Nanoscience Institute, who was not involved in the study, commends the paper for its bold take but notes that it also implies a strange, subjective malleability to the origins of times arrow. The hows and whys of the way time flies are a reflection of an approximation, he says, dependent on a physicists preferred interpretation of quantum mechanics itselfof which there are several. Ones view of a quantum system is set to some degree by ones expectations, he says. For instance, a more global view of events could also see a purported open system as just another part of a much larger, isolated one in which complications with time asymmetry wouldnt occur at all.Nevertheless, this paper demonstrates how our quest to understand the quantum realm is still far from complete, says James Cresser, a now retired professor of physics. This [helps] shake up a commonly held idea that certain equations describing dissipative behavior are not time-reversal invariant and are therefore a theoretical indicator of a particular physical state of affairs, Cresser says. (Cresser was thanked for illuminating discussions in the study and had taught its lead author Thomas Guff as an undergraduate, but he did not directly contribute to the work.)For instance, the way water acts when it spills is akin to a type of dissipative behavior in quantum mechanics and thermodynamics called decoherence, in which a system progressively spreads out, or decoheres, and loses information. In this example, the information is the particular arrangement of water molecules previously inside the cup. Dissipative processes are considered asymmetric with regard to time because the initial configuration ends up irrelevant to the final state of affairs, explains Nicole Yunger Halpern, a physicist at the National Institute of Standards and Technology, who was not a part of the new study.But even this kind of interpretation necessarily depends on our expectations of how a system must evolve as energy flows through it. For example, if we were to see two movies, one in which the water spills and another in which the water preternaturally returns to the cup, wed be inclined to say that the second movie is illusory and impossible and that it is merely the first played backward. Thus, an asymmetric arrow of time is, in some ways, an emergent phenomenon, Yunger Halpern says. Sometimes people call [this] a psychological arrow of time.All that said, perhaps the concept of timearrow or nois nothing but scaffolding for the human brain to grasp at as we try to label all the events that take place in the world around us, Cresser says. But the events themselves dont rely on the scaffolding. And what were doing with these equations is applying one kind of temporal scaffolding as compared to another possible temporal scaffolding.In that case, our questions about time might never have answersmaking them, well, timeless. We are unavoidably buried in timethat is a profound part of our existence that we can never, ever escape, Cresser says.
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