gather ye rosebuds while ye may/Old Time is still a-flying." So wrote 17th-century English poet Robert Herrick, capturing the old adage that time flies when you're having fun.
And while our experience of time has intrigued scientists, philosophers and artists for, well, a really long time, vast blanks remain in our understanding of time perception. After all, we can't see, hear or touch time. And time doesn't have a taste or smell.
As physicist Paul Davies of Arizona State University in Tempe points out, we don't really observe the passage of time; what we observe is that later states of the world differ from earlier states that we remember.
Some scientists think our perception of time is a by-product of memory and our senses. Others are searching for specialised timekeeping devices in the brain. One thing is clear: the human mind is prone to time distortions.
After all, time waits for no man, but occasionally it stands still; time heals all wounds but is also the great destroyer; we don't know where the time goes, but there's all the time in the world; time crawls, marches and flies; we make time, save time and kill it.
The quest to uncover our inner chronometer has driven scientists to the extremes of perception: to leaping off towers and living in underground caves. And by focussing on our personal time warps – the ordinary and extraordinary – scientists are unravelling the mystery of what make us tick.
IN JULY 1962, FRENCH GEOLOGIST Michel Siffre lost his grip on time. Deliberately. He descended 130 m underground into a glaciated cave in the southern Alps where he stayed for 60 days. The 23-year-old wanted to know what would happen to his sense of time when cut off from external cues such as clocks, caffeine and cycles of light and dark.
Siffre used a battery-powered lamp to take notes, but the cost of lighting meant he spent most of his time in the dark. His only contact with the outside world was via a telephone he used to let his team know when he lay down to sleep, when he woke, and how much time he thought had passed. Gradually, Siffre lost touch with time.
"When, for instance, I telephone the surface and indicate what time I think it is, thinking that only an hour has elapsed between my waking up and eating breakfast, it may well be that four or five hours have elapsed," he wrote in his diary. At one point, Siffre woke from what he thought was a post-lunch nap. In reality he'd slept for more than eight hours.
On September 14, a rope ladder was lowered into the cave and a confused Siffre was hauled to the surface. His diary entry from hours earlier was dated August 20 – somehow he'd lost 25 days. But while his consciousness experienced a time warp, Siffre's body had been running like clockwork: each day in his cave lasted 24.5 hours, of which he spent 16 hours awake.
Since then, numerous other experiments have confirmed that the human body is governed by a reliable internal clock. From the 1960s, scientists at the Max Planck Institute for Behavioural Physiology in Seewiesen, Germany, studied students isolated in subterranean apartments near Munich.
In 1972, Siffre was observed by NASA scientists during 205 days spent underground in Texas. More recently, scientists have studied volunteers living above ground in 'time-free environments', that is, in apartments without clocks or natural light.
In all of these experiments, after a period of adaptation, the body falls into a precise rhythm of days lasting 24.5 to 26 hours, depending on the individual. This rhythm is set by a biological timepiece scientists call the circadian clock – from the Latin circa ('about') and diem ('a day').
"It's incredibly precise, within minutes per day," says Jay Dunlap, a chronobiologist at Dartmouth Medical School in New Hampshire, USA.
The circadian clock controls when we feel tired and when we feel alert, but its influence extends much further. It governs daily cycles of body temperature, blood pressure, hormone secretion, metabolism and gene activity, with cycles expressed in almost every cell in the body.
Decades of research have given chronobiologists a fairly good understanding of the cogs and gears that drive the circadian clock. Two clusters of 10,000 nerve cells, one in each hemisphere of the brain, form what Dunlap refers to as the clock's "central pacemaking tissue". These suprachiasmatic nuclei (SCN) sit a couple of centimetres behind the bridge of the nose, in a brain region called the hypothalamus.
Light isn't needed for the SCN to establish a cycle, as the experiments of Siffre and others have shown. But without daylight, our internal timepiece lags behind the 24-hour succession of day and night. Normally, to calibrate the clock to the environment, dedicated cells in the retina of the eye transmit information about light levels to the SCN.
But as Dunlap explains, our circadian cycles are deeply hardwired, so trying to adjust the time by more than a few minutes a day – in a flight from Sydney to San Francisco, for example – can cause debilitating jetlag. "The body has an enormous amount of temporal inertia," he says. "It's like trying to turn around an aircraft carrier."
So if the body has such a powerful and pervasive internal clock, why did Siffre lose his grip on time? "I feel motionless, but at the same time I feel as though I am being pulled along by the uninterrupted flow of time," he wrote. "I try to grab hold of it but...realise I have failed."
According to researchers, while Siffre's body clock was ticking steadily along, his conscious perception of time was being governed by a much more malleable timepiece.
BIOLOGISTS TRADITIONALLY DIVIDE human timekeeping abilities into three domains. At one end of the scale is the circadian clock that set up Siffre's predictable rhythm.
At the other end is the millisecond timing involved in fine motor tasks and speech. Both of these systems are fairly well understood and fairly inflexible. Not so the third domain – the minutes-to-seconds range known as interval timing – which is responsible for our conscious perception of time.
"The mind is a time machine," says Catalin Buhusi of the Medical University of South Carolina in Charleston, USA. He studies the interval timing system – the system that warps time to make it fly when we're having fun and crawl along when we're bored. But while the feelings associated with this flexible system might be familiar, its biological basis remains something of a mystery.
According to Buhusi, the conventional understanding of interval timing is based on a 'pacemaker-accumulator' model. This proposes that the brain has an internal pacemaker that emits regular pulses, a bit like a ticking clock. These pulses are temporarily stored in an accumulator, so when we need an estimate of how much time has passed – how long you've been reading this article, for example – we access the contents of the accumulator and count up the ticks.
The pacemaker-accumulator model is a "powerful theoretical tool," which is good at explaining how people perform when asked to judge the length of a short interval – the duration of a tone or flashing light, for example. But it runs into serious problems at the physiological level. The idea of a neural 'accumulator' that counts ticks indefinitely is especially problematic. As Buhusi explains, "There's no way for something like this to be coded in the brain."
In recent years, however, neuroscientists have explored the brain's timing mechanisms using measurements of electrical activity and imaging techniques such as positron emission tomography (PET) and magnetic resonance imaging (MRI). They have also studied people whose perception of time has been distorted by brain damage or disease. The result is a more complex model of interval timing known as 'coincidence-detection'.
"This model is much more physiologically realistic," says Warren Meck of Duke University in North Carolina, USA. In late 2005, Meck and Buhusi brought together the theoretical and physiological work in a paper published in Nature Reviews Neuroscience. They proposed a model in which a brain region called the striatum – part of the basal ganglia, deep inside the brain – acts as the control centre for interval timing.
According to their model, the striatum monitors activity in other areas of the brain, including the frontal cortex. As neurons in these regions perform their usual functions – coordinating memory and movement, for example – they emit repeating pulses of electrical activity. As Meck explains, "All of these neurons are oscillating on their own schedules; none of them are synchronised."
But when a stimulus grabs our attention – when we place a pot on the stove, for instance – it prompts all the neurons in the cortex to fire simultaneously, causing a spike in electrical output. Afterwards the neurons resume their disorderly pulsing, but because of their synchronised start, they now create a distinct, reproducible pattern of oscillation.
"This pattern is like a symphony played by an orchestra," says Buhusi. "And each structure in the brain is a different instrument." Then, at the instant the stimulus ends – when the pot on the stove starts to boil – the striatum records the pattern of cortical oscillations. As Buhusi explains, "It's like noting where the orchestra is up to in the melody."
That point in the melody then acts as a reference for other encounters with the same stimulus. So when the striatum 'detects' the refrain after which a future pot should have boiled, it sends an electrical pulse to another brain region called the thalamus. The thalamus then communicates with the cortex so that higher cognitive functions such as memory and decision-making kick in, and the pot gets taken off the stove.
So while the circadian timepiece is like a clock that works without external cues, the interval-timing system functions more like a stopwatch. In other words, our conscious perception of time relies on constantly checking the duration of familiar external events. This explains Siffre's feeling of disorientation: in the darkness and isolation of his cave, his interval timing system lost its usual markers.
According to Meck, the neurotransmitter dopamine (which is central to our reward system) is also key to our interval-timing system. In his model, a burst of dopamine at the onset of a stimulus starts our internal stopwatch, and a second burst stops it at the end. He thinks dopamine also affects the frequency of neural oscillations (the orchestra's tempo), effectively speeding up or slowing down our inner clock.
This theory is backed by observations that diseases and drugs that affect dopamine levels distort our perception of time. Patients with untreated Parkinson's disease, for example, have low dopamine levels, and their slow internal clock makes them underestimate durations of time. Patients with schizophrenia, by contrast, have heightened dopamine activity, "and their clocks run so fast it feels like the whole world is crazy," says Meck.
Drugs that regulate dopamine activity can bring the internal clocks of Parkinson's and schizophrenic patients back to relatively normal speeds. On the other hand, recreational drugs that affect dopamine levels can wreak havoc with our perception of time.
Stimulants such as nicotine, caffeine and cocaine speed up the 'ticks' of the internal clock, making users feel as though time is passing more quickly. This leads them to overestimate how much time has passed, so five minutes might feel like fifteen. On the other hand, sedatives like marijuana and Valium slow down the internal 'ticking', leading to the opposite effect.
Extremes of temperature can also warp our experience of time. In the 1930s, American physiologist Hudson Hoagland noticed that when his wife was sick she seemed to overestimate periods of time. So, being a consummate professional, he conducted an experiment: throughout her illness Hoagland asked his wife to count to 60 while he took her temperature; the hotter she was the faster she would count.
Hoagland suspected the heat made his wife's internal clock speed up, so she felt as if more time had passed than actually had. Over the next few decades his observations inspired a series of bizarre experiments in which subjects wore specially-designed heating helmets or sat in sweat rooms kept at 63 °C, until some were on the verge of collapse.
"You'd never be allowed to do these experiments today," says John Wearden, professor of psychology at Keele University in Staffordshire, England. But you can draw on the results, and a few years ago Wearden analysed the data. His conclusion: while the mechanism remains unknown, raising the brain's temperature can alter a person's sense of time by more than 20 per cent.
BUT WE ALL KNOW there are safer and more natural ways to experience a time warp. Which brings us to why time flies when we're having fun. It turns out our perception of time also depends on how much attention we pay to our internal clock. "When we're having fun our attention is diverted elsewhere," explains Meck. "It's as if we're missing ticks, so time seems to fly." On the other hand, when we pay close attention, we count every monotonous tick. Hence a watched pot never seems to boil.
This makes intuitive sense, but how can we account for the paradoxical complaint of some elderly and unemployed people – that days seems to drag on forever while the years flash by? Wearden thinks the answer might be that our perception of time depends on whether we're thinking about time 'in the moment' or after an event.
Wearden tested this idea in his well-known Armageddon experiment. In the study, one group of volunteers watched nine minutes of the movie Armageddon – an action-packed science fiction thriller involving a menacing asteroid on a collision course with Earth. Another group sat for the same length of time in a waiting room.
As expected, the Armageddon group reported that "in the moment", time seemed to pass more quickly than usual, while the group from the waiting room felt as if time dragged. But when both groups made a retrospective judgement of how much time had passed, the results were surprising: despite feeling that time had flown, the movie-watchers judged the period as about 10 per cent longer than the group in the waiting room.
The explanation, says Wearden, is that "in the moment" judgements depend on attention to the clock, while retrospective judgements depend on how much information we process in a period. So while the waiting room group paid attention to the clock and felt as if time dragged, not much happened, so in retrospect the period seemed short. By contrast, the movie-watchers were distracted from the clock "in the moment", but processed lots of information, so the period seemed long.
Since we learn and encode less information as we age, the years seem to pass quickly, says Wearden. But if we're bored we watch the clock, so days drag. David Eagleman of the Baylor College of Medicine in Houston, Texas, thinks novel events may also play a part.
"When you're a child, you lay down rich memories for all your experiences; when you're older, you've seen it all before and lay down fewer memories," he says. "Therefore, when a child looks back at the end of a summer, it seems to have lasted forever; adults think it zoomed by."
EAGLEMAN HAS LONG BEEN fascinated by another type of time warp – the feeling that time slows down during a traumatic event. As a child he fell off a roof in what felt like a slow-motion sequence; as a researcher he collected similar anecdotes from car crash and other trauma victims. And over the years he began to wonder whether people really experience the world in slow motion, or whether it only seems to have happened that way in retrospect.
To answer the question, Eagleman designed an unconventional experiment. One by one, he had 20 subjects jump backwards off a 50-metre tower, so they experienced free fall before landing in a net below. Strapped to each person's wrist was an LED screen alternately flashing a number and its negative image, at a rate slightly faster than would normally allow them to distinguish the number from a uniformly coloured screen. Eagleman wanted to know whether his subjects' ability to see the number improved during the fall – evidence of a slow-mo experience.
For those with fantasies of one day strolling round bullets like Neo in The Matrix, the results were disappointing: no one could read the number flashing on the screen. They did, however, experience a retrospective time distortion: after taking the plunge, on average they estimated that the fall took 36 per cent longer than estimates they made based on watching other people fall.
According to Eagleman, this time distortion after the fact is a result of a brain region called the amygdala kicking into gear to lay down extra memories. "Frightening events are associated with richer and denser memories.
And the more memory you have of an event, the longer you believe it took." This makes sense from an evolutionary perspective, since we use memory for forward planning. "Dangerous events are the ones you really need to remember – and avoid," he says.
So how do we account for elite athletes who report that the ball or an opponent seems to move in slow motion when they get into "the zone"? The amygdala lays down richer memories during any emotional event, so it could be partly responsible, says Eagleman. So might a surge of adrenalin that modulates dopamine levels and speeds up the inner clock.
But Wearden offers an alternative explanation: "With training, many of their reactions become automatic, so they have more time to think about other things."
While most time perception studies focus on how well we judge durations, there is a second closely related issue to consider. 'Relative timing' refers to our ability to judge whether one event happened before, after, or at the same time as another event. And according to Derek Arnold, a lecturer in psychology at the University of Queensland in Brisbane, who studies relative timing, "our ability to distinguish the order of events is quite poor".
In a recent study in the journal Neuron, Eagleman demonstrated how easy our relative timing skills are to manipulate. He asked subjects to press a button that triggered a flash of light, with the flash delayed by about 100 milliseconds. After several presses, the delay was removed – suddenly the subjects felt as though they could see the flash before the button press. "It's an illusory reversal of action and effect," says Eagleman. "Your brain recalibrates its expectations in time."
In fact, Eagleman thinks schizophrenia might be a relative timing disorder. A common symptom of the illness is 'credit misattribution', in which a patient will perform an action and then deny it was their own. "That's exactly what you'd expect if you had an illusory reversal of action and effect, if your timing was a little bit wrong," says Eagleman. Other symptoms might also be explained: "We all talk to ourselves, but if you get the timing wrong you attribute the voice to someone else – that's an auditory hallucination."
When it functions normally, however, the plasticity of our relative timing system – the brain's ability to recalibrate – serves an important evolutionary purpose. When someone speaks to you from a few metres away, since light travels faster than sound, the audio should register a few milliseconds after the sight of their lips moving. But as Arnold explains, our brains synchronise these sensory inputs so our social lives don't play out like a spaghetti western.
Recalibration also helps us learn about cause and effect. "We quite often need to determine the causal relationships between events and doing so can be important for our survival," says Arnold. Think 'snap of twig' coinciding with 'arrival of a hungry lion', or 'request for raise' coinciding with 'look of horror on a boss' face'. "Plasticity in relative timing might have evolved to link pieces of information together, rather than to separate them," he says.
OUR PERCEPTION OF TIME is far from objective, which makes it all the more remarkable that it sometimes appears to us as such. But even deceptions can be illuminating if they give us insight into the machinations of the mind. It's to this end that scientists such as Buhusi, Meck, Eagleman and many others hope to explore the strange world of time distortions.
The rest of us need to satisfy ourselves that time will fly when we want it to crawl, drag when we want it to race, and at the end of the day, we must make the most of it, since there's never quite enough to go round.
Erica Harrison is a writer, photographer and the Features Editor of Cosmos.
