SOME time ago, students at the University of Tennessee were handed an unusual assignment. Imagine yourself as a Lilliputian, they were told, as they stared at a miniature model of their communal lounge, complete with furniture and figurines. The students were asked to put themselves in the little people’s shoes, relaxing on the tiny chairs with minuscule cups of coffee. Then they had to say when they felt 30 minutes had passed.
For the notionally shrunken students, time flew. Their estimates fell well short of clock time. Even more curiously, the acceleration in their felt time was proportional to the scale of the model lounges in which they were immersed.
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This bizarre result, , is occasionally invoked by neuroscientists to suggest that space and time are folded together in the brain as they are in the universe. It is also one of many intriguing demonstrations of how malleable our perception of time is – and how mysterious.
Time’s passage is perhaps the most fundamental feature of our experience, and yet modern physics can’t decide if it is a fundamental property of the universe. So what is time, and why does it flow? How come it seems to slow and surge? And what, if anything, does the time we experience have to do with the time defined by the laws of nature?
The search for answers takes us into the strange borderlands between neuroscience and physics – a foggy, treacherous place that exposes the limits of our ability to see reality as it really is, forcing us to confront the idea that time is all in the mind.
It all made sense for Isaac Newton, whose classical laws of motion played out against the metronomic tick-tock of some “master clock” outside the universe. “All motions may be accelerated and retarded, but the flowing of absolute time is not liable to any change,” he declared. Time is the same, everywhere.
Alas, absolute time fell apart in the wake of Albert Einstein’s theories of relativity. They revealed that space and time are unified as four-dimensional space-time, a medium that is warped by both gravity and motion so that no two observers can ever agree on what happened when. Past, present and future are a matter of perspective, not something universal. Hence Einstein insisted that the flow of time is a “stubbornly persistent illusion”, and many physicists today maintain that there is no such thing as an objective “now”.
“Like everyone else, I feel this discrepancy between what is in the equations and my own experience,” says Carlo Rovelli, a theoretical physicist at Aix-Marseille University in France. Can the two ever be reconciled?
The obvious place to look is the lump of grey matter between our ears. But time isn’t like other things we sense. Uniquely, it is woven through everything we perceive. What’s more, there is no dedicated sensory organ or neural pathway for time, as there is for sight, smell, touch, hearing and taste. And there is no known clinical condition in which people lose all ability to perceive it, which makes time in the brain tricky to study.
We do know that there is a complex assortment of neural machinery tasked with imposing temporal order. “It is crystal clear that there is no one, single clock to keep time for a range of intervals, as your wristwatch does,” says , a neuroscientist at the University of California, Los Angeles. Indeed, the brain can seem like a Victorian clock-maker’s workshop, stuffed with all manner of contraptions, each designed to measure a different aspect of time.
There is the master circadian clock in the suprachiasmatic nucleus, which relies on pendulum-like oscillations inside proteins to keep us in sync with the rhythms of the sun. Then there are collections of brain cells that govern the sub-second timing for quick-fire physical reactions. Buonomano is among those to have demonstrated that pretty much any collection of neurons can track this sub-second time by firing in repeating patterns, .
Order in the brain
Yet neither of these can account for the familiar sense that time passes, or that we pass through events as if being dragged along by a current. “Our experience of the world depends on the ordering of what we interpret as events and causalities, and they take place over seconds and minutes and hours,” says Edvard Moser at the Kavli Institute for Systems Neuroscience in Norway. The brain marks out what happens when in real time to create episodic memories, without which there would be no past to recall and no sense of time’s passage. To understand how that works, says Moser, “you have to understand how the brain makes sequences”.
Moser and , also at the institute, won a share of the 2014 Nobel prize in medicine for their role in the discovery of “grid cells” that track rats’ position in space to create a navigational map in the brain. More recently, they have also identified what they think are equivalent cells for the arrangement of events in time.
Working with Albert Tsao, now at Stanford University in California, they recorded activity from collections of neurons as rats foraged for bits of chocolate in a maze. At first, there was no discernible pattern. But eventually the researchers were able to see that the cells’ activity was changing according to what each animal was experiencing. Only when they placed a rat in a figure-of-eight loop, forcing it to come back to the same place to get chocolate, did they see a .
The Mosers think these cells are marking the order of events. The idea fits nicely with the local neural landscape. Both the lateral entorhinal cortex, where they found these time cells, and the medial entorhinal cortex, where grid cells live, send signals directly into the neighbouring hippocampus. This is the centre of memory formation. So it is here, the Mosers suggest, that representations of space and time from specialist cells come together to create our unified experience of what, where and when. “It makes total sense because both space and time are essential components in episodic memory,” says Edvard Moser.
But it also begs the question: what sort of time do our brains mark out? At a recent meeting on the subject of space and time in the brain, Gyorgy Buzsaki, a neuroscientist at New York University, repeatedly pointed out that it is not clock time. Clocks are a cultural invention, after all, and clock time is presumably not something neurons have evolved to track. Intuitively, the time we perceive does seems to be Newtonian. It appears to proceed at a regular rate, providing the backdrop against which neurons track sequences of events. In evolutionary terms, it is difficult to see why it would be anything else, because it is hard to see how the ability to perceive that motion and gravity warp space and time brings a fitness advantage. We happen to live in a gravitational field of moderate strength and we go about our lives at roughly the same low speeds, so our senses haven’t been under pressure to develop the ability to perceive the odd things time does in Einstein’s theories of relativity.
Except there are reasons to suspect that we do perceive some Einsteinian aspects of time, primarily its intimate relationship with space. In that sense, time for us depends as much on motion as it does in Einstein’s relativity. There is evidence of this “spatialisation of time”, as Buonomano calls it, in how we talk about time (see “Does language shape the flow of time?”). Brain imaging studies have also indicated that and time are generated in the same regions as each other. And there are hints that some neurons are capable of encoding both: grid cells, for example, .
The warp factor
There is even some suggestion that perceptions of motion warp our perception of time. Many experiments have demonstrated the Kappa effect, for instance, when people overestimate the amount of time between two repeated visual stimuli as the physical space between them grows. Researchers have also shown that when people are themselves in motion, their perception of the duration of a given stimulus is . Then there is that strange Lilliputian experiment, which suggests that our perception of time is almost perfectly proportional to our perception of spatial scale. And yet Buonomano warns that we shouldn’t over-interpret such effects. “Time for us is certainly relative, because the brain interacts with everything, but not in the relativistic sense of Einstein,” he says.
Indeed, motion and spatial scale are far from the only things that warp time. We have all felt it: time flies when you are having fun and drags when you are bored. Most dramatically, time seems to go into super-slow motion when you are gripped by fear, as in a near-death experience or even a minor car accident.
At first approximation, focus appears to be the big factor. The less occupied we are, the more we pay attention to time’s passing – and thus it feels as if it is slipping by more slowly. But there is more to it than that. For Sylvie Droit-Volet, a psychologist at the University of Clermont Auvergne in France, emotions are the key. She has shown that people overestimate the duration of emotionally arousing clips lasting up to a few seconds. She has also demonstrated that horror films such as The Shining make .
“Explain time? Not without explaining existence. Explain existence? Not without explaining time”
John Wheeler, physicist
In one of the best-known time-perception experiments, David Eagleman, now at Stanford University National Institute on Drug Abuse in Maryland, had volunteers fall backwards off a tower into a safety net suspended 30 metres below. As they plummeted, they looked at a wrist-worn LED display showing a flickering number that alternated with its negative image 20 times a second. This is far too fast for humans to perceive unless time can somehow slow down in our brain at certain moments. Although the volunteers did report that the drop lasted longer than the actual time it took, none of them was able to discern the number. This led Eagleman and Pariyadath to suggest that the time expansion was an artefact of memory: intense situations cause our brains to soak up more sensory detail, laying down a richer memory, which explains why the .
But we also know that in certain situations, time dilation can occur in real time. Marc Wittmann at the Institute for Frontier Areas in Psychology and Mental 91ɫƬ in Freiburg, Germany, and his colleagues simulated the experience of a threatening scenario such as a car accident. Participants were shown circles on a screen, some of which grew larger to suggest they were approaching and some shrunk as if they were receding. Sure enough, when faced with the approaching circle, the amount of time that passed.
So, the warping of felt time, both in retrospect and in the moment, is as much about emotion as anything else. When you scratch the surface of emotions, however, you realise they are mental states induced by physiological responses to the world.
Several functional MRI studies have revealed a correlation between the and neural activity in the insular cortex, the brain region where bodily signals are processed. This suggests subjective time is shaped by physiological sensations, says Wittmann – and experiments seem to indicate as much. Wittmann has shown, for example, that people who are reproduce the duration of a sound they have just heard more accurately than others. He has also demonstrated that when people watch horror films and time slows, it feels as if it when the volunteers are encouraged to focus on their bodily sensations.
All of which suggests that heart rate, or more likely a sum of various bodily states, could serve as an intrinsic measure of duration. The number of accumulated pulses would represent the amount of time that has passed, and the brain would record them. In which case, we get the impression that time is moving more slowly than it is when bodily processes ramp up.
That makes more sense in the framework of Newtonian time than it does in the context of the relative space and time conceived by Einstein. Ultimately, however, our ability to sense and warp time doesn’t require either of these abstract concepts, says Wittmann. “You could say that our perception of time is calibrated to some average emotional state. When we suddenly find ourselves in a highly aroused emotional state, this can lead us to overestimate it relative to that average.”
That makes sense. Our experience of the world is by nature subjective, so why should we expect to see anything other than a blurry approximation of objective reality? In this picture, time flows not because our brains measure time according to some scientific concept or other, but because its cells have evolved to track sequences of events. We construct time, without having to impose clock time, to make sense of our experiences.
Similarly, time for us quickens and slows primarily in response to our emotions, themselves shaped by our physiological response to our environment. Our sense of time, then, is bound up more with our sense of self than anything else.
“Time by itself does not exist… It must not be claimed that anyone can sense time apart from the movement of things”
Lucretius, Roman poet
The extent to which this perception of time represents some real property of the universe is another question entirely. And here is where we enter the daunting terrain with neuroscience on one side and physics on the other. For his part, Buonomano finds it hard to believe our brains are deceiving us. “In the case of time perception, I think our senses are probably telling us something real about the universe,” he says. “I struggle to see why we would have evolved to see the present as special if it really wasn’t.”
Time gets real
But that isn’t to say our brains are telling us everything. Even the senses we understand don’t produce a perfectly accurate facsimile of the world: it’s why we are so susceptible to visual illusions.
Rovelli thinks something similar is going on with our perception of time. Physicists these days argue that the flow of time results from the second law of thermodynamics, which insists that in a closed system the overall disorder, known as entropy, always increases. But it is far from clear that the universe is a closed system. And even if it is, some people question the assumption that entropy was so low in the distant past. Rovelli has sought more satisfying answers in the uncertainties inherent in the quantum realm, the most fundamental level of nature we know of, from which the classical world we observe emerges.
As a result, he argues that our perception of time’s flow arises from our peculiar perspective on the universe, our inability to see it in all its details. “We sit on a spinning rock, but instead of perceiving the rotation, we perceive the stars moving around,” he says. That sounds a lot like an illusion. But Rovelli prefers to call it an approximation – “our most relevant approximation of physical reality”. The flow of time is a figment of consciousness that allows us to operate, without revealing the world as it really is.
Ultimately, to reconcile the physics and neuroscience of time will require a better understanding of both. To figure out how there can be anything but subjective reality, neuroscientists will have to solve the mystery of how the brain creates consciousness, and physicists will have to grapple with the fact that quantum systems don’t seem to become definite objects until they are observed. In any estimation, that will take time.
Does language shape the flow of time?
When we talk about time, we frame it in terms of space. English speakers look “forward” to good times ahead and leave the past “behind”. A day flies by just as a ball does, while a deadline approaches the same way a tiger might. But the spatial metaphors we use vary from language to language, and some people think those differences affect our perception of time.
The idea that language shapes thought is controversial. Critics argue that the words we use simply reflect thoughts that have emerged or been constructed through other means. Yet when we compare mental representations of time in speakers of different languages, there are hints that they influence the way people think about it.
In Mandarin, earlier events are said to be “up” and later events to be “down”. Lera Boroditsky, a cognitive scientist at the University of California, San Diego, has found that when people are asked to arrange photographs depicting a sequence in time, Mandarin speakers are more likely to line them up vertically than English speakers, who tend to place them from left to right. What’s more, people who speak both languages are more likely to arrange time vertically if the test is .
It is difficult to know for sure if language is constructing particular representations of time, rather than just reflecting them. But Boroditsky says the most compelling evidence comes from a 2017 study in which she taught English speakers novel metaphors for time, such as “Tuesday is above Wednesday”. When she tested them on their awareness of passing time, she found that their performance changed in a way that suggested they had also .
As ever, there is a lot more work needed. But these sorts of studies remind us that the ability to create mental representations of something as abstract as time, which we can’t see or touch, remains one of the great mysteries of the mind.
