The Jamaican sprinter Asafa Powell set a new world record for the 100-meter dash on June 14, with a time of 9.77 seconds. The previous record, set by American Tim Montgomery, was 9.78 seconds—just one-hundredth of a second slower. How accurate are the devices used to time these events?
They can record to the ten-thousandth of a second, although that level of precision is not commonly used. That's because every track (and every lane) differs in length by a minuscule amount, so two runners who race at exactly the same speed might cross the finish line with times that differ by a tiny fraction of a second. In the 1972 Olympics, for example, two swimmers finished a 400-meter race within two-thousandths of a second—or a few millimeters—of each other. As it turned out, each lane of the swimming pool had been constructed to a precision of just 10 millimeters, so there was no way to determine the winner.
At short events (including the 100-meter, 200-meter, and 400-meter races), times are measured to the thousandth of a second and reported to the hundredth. (Exact times are always rounded up to the next highest hundredth of a second.) In longer races, official times are often less precise: Marathon times are given to the whole second. And any race that's 800 meters or longer can be hand-timed with a stopwatch, according to international rules.
To set a world record in the 100-meter and 200-meter races, a runner's performance must be recorded with a photofinish system. The best systems use a digital camera positioned on the stripe that marks the finish line. This camera has a very narrow field of view and records only the thin slice of space directly above the line. As runners cross the line and into its field of view, the camera takes a rapid series of time-stamped pictures. These create a composite image (like this one) that shows when each athlete passed through the slice and over the line.
In track events, the race is over as soon as the runner's "body"—meaning his or her torso, not including arms, legs, or head—crosses the finish line. (Other sports use more specific markers: In speed-skating, it's the front of a skate.) Before an official time can be posted, timing personnel must review the photofinish to determine where each "body" begins.
Officials can get a quicker and less accurate reading by placing light beams across the finish line. Times are recorded as soon as an athlete passes through the beams, although there's no way to tell whether it was a hand, a head, or an abdomen that broke the plane of the finish line. As a result, these times don't always match up with those gleaned from the photofinishes. When Powell first completed his record-breaking race, his time showed up as 9.78 seconds—which would only have tied the previous record. But after a review of the digital images, his time was corrected to 9.77. (Initial readings are often padded with some extra time, so officials never have to adjust up—and take away a world record.)
How does the clock get started? International rules hold that it must begin to tick within .001 seconds of the official gun. The "gun" can be either a real gun (at least .32 caliber for outdoor events and .22 for indoor) or a "silent gun," without real bullets, that triggers the emission of a clicking sound from speakers positioned just behind each runner. The flash of the gun—which is fired near the finish line—provides an instantaneous signal to the photofinish camera, which then starts its clock.
Explainer thanks Giles Norton of Lynx System Developers, Inc., and Imre Matrahazi and István Gyulai of the International Association of Athletics Federations.