Physicists Step Behind the Plate

Steeeee-rike! Umpires are up in arms about a computerized pitch tracker.

Amid a looming players' strike and allegations of widespread drug abuse, Major League Baseball now has another headache: physicists. Angry umpires have asked a panel of scientists to assess a controversial computer system that calls balls and strikes. The panel cites compelling reasons to question the system, but the league refuses to reveal how it works.

Major League Baseball introduced the Umpire Information System in 10 ball parks this year, ostensibly to help umpires improve their calls. It uses two cameras, suspended over the 1st and 3rd base lines, to follow the 60-foot, 6-inch (roughly 18-meter) flight of a pitched baseball. It then reconstructs the ball's path to see if it passed through the strike zone, an area about the size of an oven door.

The question is accuracy. System designers claim it can measure the position of the ball, which crosses the plate anywhere from 100 to 160 kilometers per hour, to within about a centimeter. But umps say the system's calls are shaky. One possible reason is that the system probably cannot measure the spin of the ball. That means it can't calculate the Magnus force--a differential pressure created by spin that accounts for curves, sinkers, sliders, and slurves. And it would have to be precisely aligned and remain stable for all nine innings in order to correctly position the strike zone.

To force Major League Baseball to address these difficulties, the union filed a lawsuit last month and impaneled seven physicists and engineers to vet the system. But the league has provided no information about the system in response to the suit and declined ScienceNOW's request for comment.

Scientists say the suit has merit. Panel member Robert Adair, a retired physicist from Yale University and author of the book The Physics of Baseball, notes that the view of one camera is blocked by the batter's body for the last 2 meters of the ball's trajectory. And the system could easily be thrown out of whack, adds LeRoy Alaways, an engineer at Exponent, a technical consulting company in Menlo Park, California, who is not on the panel. Alaways worked on a similar system during the 1996 Olympic Games, which was plagued by an unexpected alignment problem: "When the crowd did the wave, all bets were off," he says.

Alaways's site
A technical look at pitching
World Umpires Association
Major League Baseball

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