IN “TIME’S Arrow”, a novel by Martin Amis, the protagonist experiences time backwards. Eating involves regurgitating food into his mouth, sculpting the mush with his tongue, packaging it up and selling it to a grocery store. The passage on defecation is best left undescribed. Such a comic device would once leave physicists cold. They used to think that time does not have a direction, at least at the subatomic level, though they now agree that it does. Ordinary people, of course, have always known this. Nearly all cultures have a version of the arrow of time, a process by which they move towards the future and away from the past. According to a paper to be published in Psychological Science this has an interesting psychological effect. A group of researchers, led by Eugene Caruso of the University of Chicago, found that people judge the distance of events differently, depending on whether they are in the past or future.

The paper calls this the “Temporal Doppler Effect”. In physics, the Doppler effect describes the way that waves change frequency depending on whether their source is travelling towards or away from you (think of the change in pitch of an ambulance siren as it passes). Mr Caruso argues that something similar happens with people’s perception of time. Because future events are associated with diminishing distance, while those in the past are thought of as receding, something happening in one month feels psychologically closer than something that happened a month ago.

This idea was tested in a series of experiments. In one, researchers asked 323 volunteers (recruited through Mechanical Turk, Amazon's micro-job portal) and divided them into two groups. A week before Valentine's day, members of the first were asked how they planned to celebrate it. A week after February 14th the second group reported how they had celebrated it. Both groups also had to describe how near the day felt on a scale of one to seven. Those describing forthcoming plans were more likely to report it as feeling "a short time from now", while those who had already experienced it tended to cluster at the "a long time from now" end of the scale. To account for the risk that recalling actual events requires different cognitive functions than imagining ones that have not yet happened, they also asked participants to rate the distance of hypothetical events a month in the past or future. The asymmetry remained.

Interestingly, the effect can be reversed by manipulating time’s arrow. In another experiment, participants were plugged into a virtual reality machine, with some moving forwards along a tree-lined street others backwards. Those who were moving backwards reported that past events began to feel closer.

Mr Caruso speculates that his research has implications for psychological well-being. He suspects that people who do not show this bias—those who feel the past as being closer—might be more subject to rumination or depression, because they are more likely to dwell on past events. He is also keen to test whether there are cultural differences in this perception (his past research has suggested that, for example, North Americans place more value on future events while East Asians place more value on history) and to study more closely those that do not follow the standard spatial metaphors of time, such as the Aymara, an Andean tribe, which talks of the past as being ahead of them.

There may also be lessons for politicians and business leaders. Talking of future plans may be more effective than boasting about past successes. “People want to know what are you going to do for me next, not what have you done for me lately,” suggests Mr Caruso.