24 Jun Curveball

It’s an old debate: Does the curveball really bend, or is it just an illusion, like the river that runs uphill at Marshy Hope? Both says Arthur Shapiro, Associate Professor of Psychology at American University in Washington DC. Shapiro’s demonstration of the illusory component won the Neural Correlate Society‘s Best Illusion of the Year Contest.

In the game of baseball, a pitcher stands on a mound and throws a 2.9-inch diameter ball in the direction of home plate. The pitcher creates different types of pitches by releasing the ball at different velocities and with different spins. A typical major league “curveball” travels at about 75 mph, and spins at an oblique angle at about 1500 rpm; this means that the travel time from the pitcher’s hand to home plate is about 0.6 sec, during which time the ball undergoes about 13 rotations.

The spinning of the curveball creates both a physical effect (“the curve”) and a perceptual puzzle. The curve arises because the ball’s rotation creates an imbalance of forces on different sides of the ball, which leads to a substantial deflection in the path of the ball. The perceptual puzzle arises because the deflection of the ball should appear gradual, but from the point of view of the batter standing near home plate, the flight of the ball often appears to undergo a dramatic and nearly discontinuous shift in position (this sudden shift is referred to as the curveball’s “break”).

Try it for yourself here.