In baseball, a fielder's ability to follow a fly ball is a marvel of perception that has defied explanation. Now physicist Peter J. Brancazio suggests the fielder ''may actually be judging fly balls by ear.''
As the head swivels and tilts to keep the eyes on the ball, sensors in the inner ear pick up the complex motion. It's a little like a radar set that senses the direction and elevation of a target, and the rate of change of these angles, by the way the antenna swivels and tilts in its gimbals.
''Thus,'' Professor Brancazio says, ''it's possible that the sudden and rapid motion of the fielder's head as he looks upward to follow the flight of the ball off the bat may provide the sensory information that directs the player's body toward the eventual landing point.''
Brancazio, who teaches physics at Brooklyn College, doesn't claim to have solved the fielding mystery completely. But if his suggestion proves correct, he will have put research on the right track, having abandoned what he says are misleading theories proposed in the past.
He has laid out his research in a presentation prepared for the annual meeting of the American Physical Society and the American Association of Physics Teachers in New York.
Brancazio notes there appears to be no way to describe fielding skill adequately in words so that it can be taught. Instead, he says, it ''must be completely self-taught on a nonverbal level.'' You just have to work at it until you get a ''feel'' for it.
Yet there is nothing uncertain about the skill once learned. Good fielders seem to have an uncanny sense of where the ball is going to come down.
Some earlier investigators suggested that fielders were doing unconscious geometry or trigonometry. Brancazio notes, in particular, that Cornell University physicist Seville Chapman proposed 15 years ago that the angle between the line of sight to the ball and the ground (the angle of elevation) may be critical.
Professor Chapman suggested that a fielder works with the tangent of this angle. (The tangent is equal to the height of the ball above ground divided by the horizontal distance from the fielder to the point on the ground directly under the ball.) In Chapman's theory, a fielder would see that tangent increasing at a constant rate if he were standing correctly at the landing point.
cl11 Brancazio says he had doubts about this elegant theory because it neglects air resistance. He computed the effects of such resistance and found that, under game conditions, a batted ball travels only about 60 percent as far as it would in vacuum. Also, air drag distorts the ball's trajectory.
Brancazio also noted that one-eyed fielders can learn to follow the ball as well as fielders with normal vision. So binocular depth-perception doesn't appear to be critical either. Such considerations led him to suspect the inner ear.
He explains: ''Whenever an individual follows a moving object with his eyes, he ordinarily moves his head as well - and the motion of head and eyes must be finely coordinated. . . . It turns out that these compensatory motions of the eyes are primarily guided not by visual feedback, but rather by signals triggered by the motion of the head from sensors in the inner ear. . . .''
Brancazio warns that the mystery of fielding is part of the larger mystery of how a human determines the spatial location of rapidly moving objects and coordinates this information with his own body movement. Scientists have only begun to understand this. But it may be that the same inner-ear sensors that help people tell up from down and to keep their balance, also keep a fielder on the ball.