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October 7, 2022

Evolving Time’s Arrow
Why do we perceive time marching in one direction? Combining physics, evolutionary biology and cognitive science could close the gap between the symmetrical notion of time in fundamental science and our everyday experience.
by Anil Ananthaswamy
FQXi Awardees: Craig Callender
May 9, 2011
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University of California, San Diego
"The distinction between past, present and future is only a stubbornly persistent illusion." It was none other than Einstein who uttered these words. He was speaking about how our perception of time differs from the fundamental nature of time in physics.

Take our perceptions first: We have a clear sense of the present moment, what came before, and what might come after. Unfortunately, physics treats time rather differently. Einstein’s theory of special relativity presents us with a four-dimensional spacetime, in which the past, present and future are already mapped out. There is no special "now," just as there’s no special "here." And just like spacetime does not have a fundamental direction—forcing us to move inexorably from east to west, say—time does not flow.

"You have this big gap between the time of fundamental science and the time we experience," says Craig Callender, a philosopher at the University of California, San Diego. It’s this gap that he has set out to narrow, using ideas from physics, evolutionary theory and cognitive science.

The question of why time marches in one direction is tough to answer, says Callender, not least because if you want to talk about an arrow of time, you have to be specific about exactly which arrow you mean. While Einstein’s naked spacetime—without any fields and particles—may not differentiate between the past and the future, Callender notes that physical processes have directionality. That brings us to the first arrow: thermodynamic systems become more disordered, moving towards greater and greater entropy. There is also a second arrow: the causal arrow of time, which we take for granted in everyday life. "Actions I do now can change where I’ll die, but nothing I can do will change where I was born," says Callender.

Slicing Up Spacetime

These arrows and our commonsense notions treat time as very different than space. Is there some connection between these different ways of treating time? To find out, Callender has been slicing up Einstein’s spacetime, as though it were a 4-dimensional loaf of bread. You can imagine taking a 3-dimensional slice of spacetime along the time axis and another 3-dimensional slice along, say, the east-west axis. The idea is to evolve each slice along its respective axis and get to the next slice. In the first case, you are varying time and, in the second, you are varying time and space.

Could the story of the cosmos have been told sideways rather than from
past to future?

The slicing allows Callender to investigate if can we tell the story of the universe sideways, from east to west, rather than from past to future. Given that Einstein’s picture puts time and space on an equal footing, with time having no special properties, we might expect the answer to be yes. "Well, the answer is no, surprisingly, for some specified class of equations," Callender says.

Intriguingly, these equations, which suggest that time and space are different on a fundamental level, tend to be the ones that physicists use most often to describe nature. Callender wants to see whether this holds generally across physics because it might provide a link between the thermodynamic and causal arrows of time.

If physics can explain how a causal arrow of time emerges, then biology will do the rest, says Callender. Evolution, he argues, selects for creatures that care more about the future than about the past. "Because of a causal arrow, a creature can’t do anything about the past to increase its fitness, but can towards the future," he explains.

If physics can explain how
a causal arrow of time emerges
then biology will do the rest.
- Craig Callender
We would have developed the perception that now is distinct from the past and future in order to communicate and survive in the world. Our brains gather information, via the eyes, ears and other senses, and integrate them to create an impression of an event that’s occurring now. It’s necessary to, say, figure out that a tiger is about to attack you now, and you have to run. Or if two people are communicating, there is an implicit understanding that what’s being said is being said now (contrast this with exchanging letters, which have to dated). The brain automatically timestamps anything that’s being said as being said now.

Research in cognitive science backs Callender’s claim that caring more about the future than the past is important. Eugene Caruso a behavioral scientist at the University of Chicago in Illinois, and colleagues asked 121 volunteers at Harvard University to imagine working for five hours entering data into a computer and to think of a fair payment for the job. Some volunteers were asked to imagine that they had already completed the work a month ago, while others were asked to think about it as work to be done in a month’s time. The result: the volunteers felt they deserved about 101 per cent more for future work than for past work.

Emotional Value

There’s another interesting twist to this notion of "value asymmetry": If you are asked to value some future action that you would undertake versus the value of the same action done by someone else, relative to the same action done in the past, you’ll tend to ask for more money for yourself, while estimating someone else’s past and future actions as having equal value.

Caruso and colleagues have shown that there are two main factors that contribute to this value asymmetry: the desire to reduce uncertainty and the ability to exert control over one’s life. Our emotions play a key role accounting for value asymmetry. "Emotions tend to be aroused most effectively by events that actions can meaningfully influence," says Caruso.

Callender will use part of his $102,263 FQXi grant to test whether this value asymmetry extends to kinship. He plans to set up a study in which people will be asked to compare the value of past versus future actions, not just for themselves and unrelated individuals, but also for people who are emotionally close to them, such as their spouses and kids. Such an experiment would specifically test whether evolution has selected for such emotions to increase our future fitness, says Callender, and Caruso agrees.

But don’t be mistaken into thinking that these ideas about an asymmetric overvaluation of the future are anything new. Humans have always been doing it, prompting Seneca the Younger, the Roman philosopher, to rather bluntly warn over 2000 years ago: "All the future is uncertain, and more certain to be worse than otherwise."

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