It might seem like a bit of an odd question, but what speed does time travel at?
The obvious answer is that it ticks by at exactly the rate of 60 seconds every minute. But new research into our perception of time shows that for us humans, time is a lot more complicated.
Our feelings about time are captured in common sayings - how time flies when you're having fun, or how it drags when you're bored.
Associations between this feeling of time passing and enjoyment run so deep that researchers in the United States were able to make people feel that they were having fun, just by simulating the experience of time flying.
By a surreptitious switching of stopwatches, the study at the University of St. Thomas in Minnesota was able to make those taking part in the experiment believe that a task had lasted 10 minutes, while in reality it lasted either five or 20.
"In controlled circumstances we could manipulate people's feelings of time quite easily," says Professor Aaron Sackett, who led the research.
HOW TO MAKE TIME FLY
Remove time cues - clocks and watches
Drink tea, coffee or other stimulants
Allow yourself to become absorbed in what you are doing
They found that people who were made to believe time had flown reported enjoying the task more, and those who believed time had dragged reported the opposite.
The study shows just how important the feeling of time passing at different speeds is in our evaluation of how much we enjoy life.
But while the feeling of time passing is essential to the experiment, Prof Sackett is "agnostic" about how we actually experience time.
He is perhaps wise to steer clear of the question.
While there are numerous anecdotal examples of dramatic distortions in our perception of time - from the intense slowing of time reported in car crashes, to the extreme speeding up of time reported by people under the influence of certain intoxicants - the actual brain processes involved are something of a mystery.
What then is time? If no one asks me, I know what it is. If I wish to explain it to him who asks, I do not know.
"There's no agreement on the biological basis for time perception. A lot of papers are published but they are all contradictory," explains Professor John Wearden, a time psychologist at Keele University.
The study of other areas of psychology are grounded in firm objective foundations, he says. With vision, for example, scientists already understand the wavelengths of colours and the biology of the retina, "so you know where to start".
But with the feeling of time passing, there is nothing to go on except what people say about time passing. These reports, says Prof Wearden, often seem to contradict themselves - citing the example of his mother, who often reports that days seem to last forever, but the months fly by.
The difficulty comes because what people say about time passing is based both on the experience of time, and their inferences about its passing.
The hospital waiting room is a hot spot for slow time perception
Because there is just one word to capture both "subjective" and "objective" time, people's feelings about how long something lasted become quickly entangled with the judgements they make from looking at their watch and calculating how long similar things usually last.
So Prof Wearden's mother feels like time is passing slowly in the moment, but when she reflects on the month that has just passed and the things she has done, it looks like it went quickly, hence the apparent contradiction.
Nevertheless, he says, it is possible to measure the passing of time, even if the results are "not like physics".
In countless experiments, psychologists have shown that people report that their perception of time is faster in exciting situations, such as watching action films, and slower in boring situations, like sitting in a waiting room.
While these experiences are only one part of people's judgements of how long has passed, he says, there is no doubt that they do take place.
Given Prof Wearden's warnings, it would perhaps be sensible to take new research from psychologist Dr Mark Wittmann, based at the Institute for Frontier Areas of Psychology and Mental Health in Freiburg, with a pinch of salt.
Space and time are not conditions in which we live; they are simply modes in which we think.
His research, however, does seem to give a persuasive account of what is going on in the brain when we experience time.
By using an fMRI machine to scan people's brains as they made timing estimates, he found a "significant neural activation signature" in one brain area, the insular cortex.
This region, he says, is the main brain area where our bodily sensations are processed, leading him to theorize that the feeling of time passing is based on the amount of stimulation our brain receives from our body.
When we are engaged in activities where we forget about our body, immersed in an exciting film or an absorbing task, time seems to fly, he says.
But when we are acutely aware of our own body, and this region of our brain becomes highly stimulated - as in threatening situations - our time perception slows down.
Emotional music could alter your perception of time
Previous research has demonstrated that bodily sensation is heightened in when we get emotional, which if Dr Wittmann is correct, is why time seems to fly in absorbing action films, but seems strangely slow in emotional dramas, especially those which touch on issues in your own life.
"If you lead an emotional life, if you are more aware of yourself and what is happening, you would feel that time passes slowly," he concludes.
Dr Wittmann's research is still in its infancy, and fits, he admits, into a research area where there are "five researchers and 10 theories".
So before you launch yourself into a psychological journey of discovery into the meaning of subjective time, or try to extend your life by being more emotionally attuned, it is perhaps best to finish with a word of warning from Professor Wearden.
"Remember, time doesn't really go fast or slowly, it can't do. We are stuck in time, it just goes at the speed it goes," he says.
"There's only one objective time, and that's measured by a clock."
Have you had experiences of extreme time dilation? Do you think our feelings of time passing are important? Let us know using the form below or join the debate on Twitter.
I think that time is not an absolute variable but a relative one. it is different for different people, and also perhaps different when you are travelling beyond the speed of light. Maryam Hassan, Lahore, Pakistan
My guess is that events are recorded (experienced) at different rates, but only played back (recalled) at a constant rate. Thus in body-threatening situations, many events are recorded, but in abstract situations fewer events are recorded. Play these back at the same rate, and the threatening situation appears in slow motion detail, whilst the abstract one is less detailed and played back in a short time. Richard Payne, Leicester, UK
I think to an extent time is subjective - i.e if your mind can process things quicker, wouldn't the same time period feel slower to you than someone else and vice versa? Simon, stafford, UK
When I was a student, I worked as a security guard one summer. Looking back at it, all those nights as a guard were pretty much identical in their rhythms and routines and thus equate one night only in my memory (even though time went slowly at the time). During my nights off, I went out partying a lot. Recollections from those nights are strong, hence each night fills up so much of my memory that I'm tempted to say each single night lasted for two (even though time flew while it was all going on). Jorn Bonesmo, London
The interview about this on today's programme felt like it went on for over an hour. Lou, London
As we now know, time is not a single entity. It is inextricably linked to space, hence 'space-time', and we also now know, due to Einstein's Theory of Special Relativity (not really a theory any more, as it has been proven experimentally - GPS!!!), that objects moving relative to each other experience their own measurements of distance and of time, ie They have their 'own time'. How this associates with people sensing time differently when enjoying themselves or when bored, is obviously not due to moving close to the speed of light (as they'd need to do in order to experience a noticeable time dilation), but it does show that time and space are not as simple as we first thought!! KG, Leicester
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