Physics & Math / Math

Time is fundamental to so many things that we do, people take it for granted,” says physicist Ronald Beard of the U.S. Naval Research Laboratory in Washington, D.C. “But it’s a generated thing, not a naturally occurring phenomenon we’re just monitoring. We actually make time.”

Nobody knows that better than Beard, who is trying to corral a rancorous constituency of experts into reaching an agreement on what time it is. The experts are arguing about the future of the leap second, an extra second added to the world’s clocks every year or so to keep the artificial constructs of hours and minutes in lockstep with the actual length of the day.

Leap seconds are to a clock what leap days are to the calendar. An extra day is added every fourth February because the planet takes 365 and a quarter days to circle the sun. So an ordinary calendar year is a few hours shy of the actual length of a year, and leap days keep the calendar in sync with Earth’s orbit. Similarly, the standard 24-hour day is slightly shorter than day length as determined by the rate at which Earth turns on its axis. That’s because the moon’s gravitational pull is constantly slowing Earth’s rotation.  It takes a network of ground-based radio telescopes, triangulating from the most distant objects in the universe, to determine Earth’s rotation day by day, hour by hour, minute by minute. Leap seconds “slow” clocks to match these calculations.

Leap seconds are also like leap days, says Judah Levine, a physicist in charge of time services at the National Institute of Standards and Technology in Boulder, Colorado, in that “once they’re inserted, they’re forgotten.”

But computer software designers haven’t adapted very well to the occasional added second, so experts in air traffic control, satellite communications, and electronic fund transfers have been lobbying to abolish the tinkering. A leap second may have caused the Russian satellite navigation system to crash for hours, and critics claim the added instants could cause commercial airliners to crash as well. “A one-second jump can cause significant problems for systems that require continuous, uninterrupted time,” Beard says.

If it seems strange to contemplate changing time to suit a computer, that’s because most people don’t realize how time is made. At the standards institute, Levine presides over more than a dozen atomic clocks—the most accurate, uniform timekeeping devices ever invented. Yet the clocks rarely agree. So Levine calculates an average time using a computer program that accounts for each clock’s offsets. Then he transmits the average in a series of digital ticks. People running communications networks, satellite navigation systems, and multimillion-dollar telescopes take note.

Levine and his counterparts at 50 timing labs in other countries also send their signals to the International Bureau of Weights and Measures in Paris, the agency that determines the world’s official atomic time. The bureau takes the weighted average of about 200 clocks and publishes it in Circular T, a list of official times for every fifth day of the preceding month. Clocks in timing labs across the globe can then be synchronized, albeit in retrospect, to a billionth of a second.

“It’s like, ‘Remember last Thursday? Well, last Thursday you were five nanoseconds off,’ ” says Levine.

And every so often, another international agency tells Levine and his peers to add an entire second to the time given by their atomic clocks. When the International Earth Rotation Service calls for it, timing labs in all time zones add a leap second just before midnight in Greenwich, England. Leap seconds create the civil time used by TV stations, hospitals, factories, schools, airports, and wristwatches. All told, 22 seconds have been added since the practice was formally adopted more than 30 years ago. “In 1972 the leap second was considered a step forward,” says astronomer Dennis McCarthy, who runs the Washington-based time directorate for the International Earth Rotation Service.

But today many computer-based systems, including Global Positioning System satellites, use uncorrected atomic time. Foes of the leap second fear that growing discrepancies between atomic time and civil time could sabotage military operations, disrupt financial markets, disable cellular phones, and even cause midair collisions. Commercial jets, for example, use atomic time–based GPS satellites to set their flight paths, but air traffic controllers typically use civil time. “You have to be very careful that everybody understands that the timing by which we navigate and the timing by which we live are very different,” adds astronomer William Klepczynski, a senior analyst in the State Department’s Office of Space and Advanced Technology. “It makes for an operational headache.”