What\u2019s his answer? Spoilers ahead …<\/p>\n
Most physicists would say that time runs in only one direction because of the Second Law of Thermodynamics<\/a>. That law states that the level of entropy, or disorder, must always increase over time in an isolated system. The inability to reverse time would be related to the inability to reverse the rise of entropy.<\/p>\n Some physicists have gone so far as to speculate that other universes might exist where the flow of time goes in the opposite direction<\/a>, from high entropy to low entropy.<\/p>\n Al-Khalili, who is a professor emeritus at the University of Surrey as well as a science presenter for the BBC, specializes in quantum physics. So, it shouldn\u2019t come as a surprise that his explanation for the directional arrow of time leans heavily on the weird workings of the quantum realm. Specifically, the phenomena of quantum entanglement and quantum decoherence.<\/p>\n \u201cI argue that the arrow of time is indeed real, and it\u2019s baked into the structure of the universe,\u201d he says. \u201cTime is real, and time does have a direction. But it\u2019s not the Second Law of Thermodynamics that is the most fundamental explanation for the direction of time, but rather this notion that emerges from the quantum realm.\u201d<\/p>\n In Al-Khalili\u2019s view, the most fundamental concept in quantum mechanics is entanglement \u2014 \u201cthe fact that a quantum entity like an electron or a photon doesn\u2019t exist in isolation.\u201d<\/p>\n When quantum entities interact, their properties can become linked to such an extent that they can exhibit what Albert Einstein called \u201cspooky action at a distance.\u201d<\/a> That\u2019s entanglement.<\/p>\n But when quantum entities are observed, their \u201cquantumness\u201d \u2014 for example, the alive-and-dead quality of Erwin Schr\u00f6dinger\u2019s famous cat<\/a> \u2014 leaks away. That\u2019s quantum decoherence. \u201cYou can\u2019t undo decoherence once you\u2019ve observed,\u201d Al-Khalili says. \u201cOnce you\u2019ve opened Schr\u00f6dinger\u2019s box to see if the cat is dead or alive, that\u2019s it. You fixed it. You can\u2019t close the box again and start again.\u201d<\/p>\n The way Al-Khalili sees it, every observation contributes to the entire universe\u2019s entanglement \u2014 and to quantum decoherence.<\/p>\n \u201cThink about what it means to make a quantum measurement; in order to observe or learn something about anything, we must partition that thing off from the rest of the universe,\u201d he writes in the book. \u201cThen the very act of measuring it involves becoming quantum entangled with it (we are its surrounding environment). In fact, since our detectors and our senses reside in the macroscopic, classical world, the act of measurement also causes irreversible decoherence. Observing anything is what gives time its direction.\u201d<\/p>\n If observation is the bowstring that sets the arrow of time in motion, could the arrow fly in a different direction if we change the way we observe? Do we each create our own reality?<\/p>\n That\u2019s an oft-used hook for science-fiction tales, and even for serious scientific speculation<\/a>. In an Apple TV series called \u201cDark Matter,\u201d which has just begun its second season<\/a>, a physicist builds a box that shields its occupants from quantum interaction with the outside world. By taking a consciousness-altering drug, the occupants can shift from one version of the universe to another that may be wildly different.<\/p>\n Al-Khalili says that version of the observer effect will always remain solidly in the realm of science fiction.<\/p>\n \u201cWe have to remember that observing a quantum system or a quantum state doesn\u2019t require a human. It doesn\u2019t require consciousness,\u201d he says. \u201cThe air molecules surrounding an electron can do the observing, can cause the decoherence. Essentially, its state is then recorded somewhere within those air molecules, even if no observer has examined the air molecules \u2014 let alone someone with a Ph.D. or wearing a white lab coat.\u201d<\/p>\n Nevertheless, \u201cDark Matter\u201d touches upon a theme in real-world quantum physics: the many-worlds interpretation<\/a>, which holds that all possible outcomes of a quantum event are physically realized in separate, branching realities. This hypothesis has spawned dozens of sci-fi sagas, ranging from \u201cBack to the Future\u201d<\/a> to \u201cDoctor Strange in the Multiverse of Madness.\u201d<\/a><\/p>\n Al-Khalili leaves room in his ruminations for the many-worlds view, but says that we\u2019re already too tangled up in our version of the universe to leap into a different quantum stream. \u201cWhile the universe as a whole can be regarded as timeless, this timelessness can only be perceived from outside of spacetime (the God\u2019s eye view). From our vantage point within the universe, entanglement is increasing,\u201d he writes.<\/p>\n In a bonus chapter at the end of \u201cOn Time,\u201d Al-Khalili lays out his perspective on time travel. His verdict? The current laws of physics provide a narrow opening for taking a shortcut through spacetime. Future physicists would have to unlock the mysteries of quantum gravity and negative energy to create a closed time loop.<\/p>\n In science fiction, that\u2019s done by whipping up a wormhole that connects two distant spots in spacetime. But in the real world, Al-Khalili suspects that there are yet-to-be-discovered features of spacetime that would rule out traveling back in time.<\/p>\n \u201cTimelike curves are allowed in general relativity, but of course, time travel to the past leads to a whole load of paradoxes that are really awkward to deal with,\u201d he says. \u201cPhilosophically and in terms of common sense, we would argue that time travel to the past surely must be impossible. But as yet, the door hasn\u2019t closed on it, according to our best theories of physics.\u201d<\/p>\n If Al-Khalili could fast-forward through time, what would he want to ask a future physicist who has learned everything there is to know about spacetime?<\/p>\n \u201cTop of my wish list would be for someone to find the true, correct, single interpretation of the meaning of quantum mechanics,\u201d he said.<\/p>\n Al-Khalili would also love to know whether his views about the quantum arrow of time are correct. \u201cI\u2019m pretty bullish and confident about the way I\u2019ve presented the work,\u201d he said. \u201cBut I like to think I\u2019m a good enough scientist to acknowledge that I may be way off beam when I may just turn out to be, you know, completely wrong. It\u2019d be nice to know that.\u201d<\/p>\n \u00a0<\/p>\n For more about Jim Al-Khalili and what he\u2019s up to, visit JimAl-Khalili.com<\/a>. And to take a deeper dive into the themes covered in \u201cOn Time: The Physics That Makes the Universe Tick,\u201d<\/a> check out physicist Sean Carroll\u2019s interview with Al-Khalili for the Mindscape podcast.<\/a><\/p>\n Check out the original version of this item on Cosmic Log<\/a> for time-travel movie recommendations and links to eight of my favorite time-travel stories from the past 23 years. My co-host for the Fiction Science podcast is Dominica Phetteplace, an award-winning writer<\/a> who is a graduate of the Clarion West Writers Workshop<\/a> and lives in San Francisco. To learn more about Phetteplace, visit her website, DominicaPhetteplace.com<\/a>.<\/p>\n Fiction Science is included in FeedSpot\u2019s 100 Best Sci-Fi Podcasts<\/a>. Feedspot is an RSS Reader<\/a> that lets you follow your favorite podcasts alongside blogs and news in one place. Stay tuned for future episodes of the Fiction Science podcast<\/a> via Apple<\/a>, Spotify<\/a>, Player.fm<\/a>, Pocket Casts<\/a> and Podchaser<\/a>. If you like Fiction Science, please rate the podcast and subscribe to get alerts for future episodes.<\/p>\n","protected":false},"excerpt":{"rendered":"Scientists and philosophers have argued over the nature of time since \u2026 well, since time immemorial. 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<\/a> Physicist Jim Al-Khalili is the author of “On Time: The Physics That Makes the Universe Tick.” (Credits: Princeton University Press \/ JimAl-Khalili.com)<\/p>\n