Disputes over line calls used to be one of the main joys of tennis—this, after all, is John McEnroe's game. But fans rarely see players explode in rage anymore. In high-profile matches (i.e., those broadcast on TV), human umpires have largely been replaced by a machine called Hawk-Eye. The system is a kind of computerized ump that stitches together video footage from several high-speed cameras to produce a 3-D simulation of the ball as it approaches and bounces off the ground. Hawk-Eye's decisions are final: When a player challenges an umpire's call, the system displays its view of what just happened, then displays a judgment on the screen— in or out—that the human umpires are compelled to accept.
Hawk-Eye represents one extreme in the growing adoption of technology to solve disputes in sports. On the other side, you've got Major League Baseball, which has long resisted any kind of instant-replay system. Last week, pro baseball played its first games under a new rule that lets umpires review video in the limited scenario of "boundary calls"—essentially determining whether a home run was really a home run. (The replay system is so limited that in the week it's been active, it hasn't been deployed once.) Unlike tennis, which has made Hawk-Eye the ultimate authority, MLB, the NFL, the NBA, and the NHL all give human officials the final say in interpreting instant-replay footage.
Even so, instant replay alters how fans and players approach the game. Baseball officials have vowed not to expand the current replay system, but that will be a difficult promise to keep. If video can help an ump determine whether a ball went over the fence, why can't it help with every other call a baseball umpire has to make? That's the lure of video—it promises a measure of certainty in an otherwise uncertain endeavor. Place enough high-speed, high-resolution cameras at enough points around the field of play and you'll eventually get at the absolute truth of any play, the thinking goes. The trouble is, technology can introduce as much uncertainty as it solves.
For one thing, photography doesn't give clear-cut answers. From one angle, a ball may look in, while from another it looks out. Sure, umpires can decide which replay is most reflective of what actually happened. But umpires are biased—studies show that officials tend to favor certain players and teams based on race, jersey colors, and the size of the home crowd. Two umpires can look at the same instant replay and see different things. The whole point of going to replay was to move away from umps' screw-ups. Computers, on the other hand, are free of hate and idiosyncrasies. So why don't we move to the tennis model, letting a computer be the ultimate decision-maker?
Sure, I'm making a slippery-slope argument, and it may seem far-fetched to think that baseball or any other team sport will let machines analyze, rather than just record, what happens on the field. But you can't dismiss the slippery slope when some sports have already slid down: Tennis adopted Hawk-Eye after several high-profile matches were marred by bad calls. Hawk-Eye is installed at the U.S. Open's two stadium courts, as well as at Wimbledon, the Australian Open, and other major tournaments. (It's not necessary at the French Open because the ball leaves a visible mark on the red clay.) Since 2006, there have been more than 550 Hawk-Eye challenges at the U.S. Open, and 30 percent resulted in reversed calls. Players have occasionally questioned its decisions, but lately many have been agitating for Hawk-Eye to be used more widely. Roger Federer said recently that his biggest complaint about Hawk-Eye is that it isn't installed everywhere. Fans, too, seem to enjoy the challenge system—the crowds at the U.S. Open watch the replays intently and regularly cheer the results.
The most fascinating thing about Hawk-Eye is that it's perceived as such a success despite being demonstrably fallible. According to a fascinating paper by Harry Collins and Robert Evans of Cardiff University, the system's manufacturer reports its average error as 3.6 millimeters. The International Tennis Federation, which tests the line-calling equipment, allows for Hawk-Eye to be off by as much as 10 millimeters in some situations. This means that if a ball lands nine millimeters out, Hawk-Eye might call it in by one millimeter.