Time, as the late physicist John Archibald Wheeler liked to say, is nature’s way of keeping everything from happening all at once.
But time also has many other jobs. It keeps eggs from unscrambling, glass from unbreaking, and somehow accommodates the expansion of the universe. Time helps humans and other organisms function on a recurring daily schedule that alternates light with darkness. Time in the brain underlies synchronization of sights and sounds needed to make sense of the external world.
In its relationship to these tasks, time poses tough questions. Nobody really knows why time marches always forward, as the universe grows bigger and irreversible processes within it generate increasing disorder. The second law of thermodynamics is somehow involved, most physicists believe. But they can’t agree on how time’s direction was determined at the origin of cosmic history.
Biologists ponder time from the perspective of evolutionary history, wondering why and how life-forms acquired the internal clocks that guide daily life. Circadian rhythms associated with sleeping, waking, eating and the ebb and flow of bodily chemicals reflect a fundamental role for time in the way that living things relate to their environments. Researchers are pursuing multiple investigations into the origins of these biological clocks and how they influence life today.
Biological clockwork is especially important in the brain, where keeping track of time is essential for survival. The human brain possesses a posse of precision timekeepers tasked with keeping track of time on different scales and for different sensory purposes. Understanding the interplay of these neural timepieces is at the frontier of scientists’ efforts to explain time’s role in the life of the mind.
In the stories that follow are accounts of the latest scientific investigations into time’s place in the physical, biological and mental worlds — timely science, aspiring to solve eternal mysteries.
— Tom Siegfried
By Andrew Grant
Gravity may explain how time always runs forward, even though the laws of physics should permit it to run backward.
By Teresa Shipley Feldhausen
Humans have created timepieces over the millennia to organize daily life, guide ship navigation and now keep the world’s 7 billion people in sync.
By Laura Sanders
To perceive time, the brain relies on a diverse collection of internal clocks that precisely orchestrate movement, sensing, memories and learning.
By Tina Hesman Saey
Creatures large and very small keep time with the planet’s day/night cycle. Scientists are still debating how and why the circadian clocks that govern biological timekeeping evolved.