Universe Today
Scientists and philosophers have argued over the nature of time since … well, since time immemorial.
Does time flow unceasingly in one direction? Is our perception of time merely an illusion? Does each instant exist eternally, and are we just running through a sequence of instants like film in a movie projector? And if that’s the case, can we run the film backward? Could we even change what happens in the movie?
In a new book called “On Time” — and in the latest episode of the Fiction Science podcast — British physicist Jim Al-Khalili lays out what he thinks is the answer to such questions.
What’s his answer? Spoilers ahead ...
Fiction Science podcast: Physicist explains the mysteries of time
Most physicists would say that time runs in only one direction because of the Second Law of Thermodynamics. 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.
Some physicists have gone so far as to speculate that other universes might exist where the flow of time goes in the opposite direction, from high entropy to low entropy.
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’t 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.
“I argue that the arrow of time is indeed real, and it’s baked into the structure of the universe,” he says. “Time is real, and time does have a direction. But it’s 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.”
In Al-Khalili’s view, the most fundamental concept in quantum mechanics is entanglement — “the fact that a quantum entity like an electron or a photon doesn’t exist in isolation.”
When quantum entities interact, their properties can become linked to such an extent that they can exhibit what Albert Einstein called “spooky action at a distance.” That’s entanglement.
But when quantum entities are observed, their “quantumness” — for example, the alive-and-dead quality of Erwin Schrödinger’s famous cat — leaks away. That’s quantum decoherence. “You can’t undo decoherence once you’ve observed,” Al-Khalili says. “Once you’ve opened Schrödinger’s box to see if the cat is dead or alive, that’s it. You fixed it. You can’t close the box again and start again.”
The way Al-Khalili sees it, every observation contributes to the entire universe’s entanglement — and to quantum decoherence.
“Think 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,” he writes in the book. “Then 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.”
Physicist Jim Al-Khalili is the author of "On Time: The Physics That Makes the Universe Tick." (Credits: Princeton University Press / JimAl-Khalili.com)
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?
That’s an oft-used hook for science-fiction tales, and even for serious scientific speculation. In an Apple TV series called “Dark Matter,” which has just begun its second season, 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.
Al-Khalili says that version of the observer effect will always remain solidly in the realm of science fiction.
“We have to remember that observing a quantum system or a quantum state doesn’t require a human. It doesn’t require consciousness,” he says. “The 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 — let alone someone with a Ph.D. or wearing a white lab coat.”
Nevertheless, “Dark Matter” touches upon a theme in real-world quantum physics: the many-worlds interpretation, 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 “Back to the Future” to “Doctor Strange in the Multiverse of Madness.”
Al-Khalili leaves room in his ruminations for the many-worlds view, but says that we’re already too tangled up in our version of the universe to leap into a different quantum stream. “While the universe as a whole can be regarded as timeless, this timelessness can only be perceived from outside of spacetime (the God’s eye view). From our vantage point within the universe, entanglement is increasing,” he writes.
In a bonus chapter at the end of “On Time,” 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.
In science fiction, that’s 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.
“Timelike 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,” he says. “Philosophically 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’t closed on it, according to our best theories of physics.”
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?
“Top of my wish list would be for someone to find the true, correct, single interpretation of the meaning of quantum mechanics,” he said.
Al-Khalili would also love to know whether his views about the quantum arrow of time are correct. “I’m pretty bullish and confident about the way I’ve presented the work,” he said. “But I like to think I’m 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’d be nice to know that.”
For more about Jim Al-Khalili and what he’s up to, visit JimAl-Khalili.com. And to take a deeper dive into the themes covered in “On Time: The Physics That Makes the Universe Tick,” check out physicist Sean Carroll’s interview with Al-Khalili for the Mindscape podcast.
Check out the original version of this item on Cosmic Log 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 who is a graduate of the Clarion West Writers Workshop and lives in San Francisco. To learn more about Phetteplace, visit her website, DominicaPhetteplace.com.
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