How pitchers ‘doctor’ a baseball, explained

More than a day after Framber Valdez bested the Phillies in Game 2 of the World Series, baseball fans continued to pore over clips of the Astros pitcher’s hand movements, seeking to support — or refute — claims that he illegally “doctored” the baseball with a sticky substance.

Umpires found no such evidence upon checking the pitcher’s hands, and the Phillies said they had no concerns.

But throughout the sport’s history, plenty of other pitchers have used foreign substances to alter the flight of the ball. The practice became so widespread in recent years that Major League Baseball cracked down during the 2021 season, directing umpires to inspect players during games.

More below on the physics of how a sticky substance can alter the ball’s trajectory — it has to do with how fast the ball spins as it leaves the pitcher’s fingers — but there is no dispute that the crackdown had an impact.

In the weeks following the increased enforcement, some pitchers’ spin rates slowed by 200 to 300 revolutions per minute, according to a New York Times analysis.

That decline is enough to divert the path of a typical fastball by 1 inch, said Patrick Dufour, an associate professor of physics at the University of Montreal.

Athletes in many sports seek to control the flight of the ball by causing it to spin in midair, and it’s perfectly legal.

In tennis, for example, players “brush” upward with their rackets when hitting the ball to create topspin — spinning the ball away from them so that it dives down into the opponent’s half of the court.

Baseball pitchers do the same thing when throwing a curveball, causing the ball to drop faster than it normally would, said Alan Nathan, a professor emeritus of physics at the University of Illinois at Urbana-Champaign.

Or, when throwing a fastball, they can do the reverse — snapping their fingers so the ball spins backward as it speeds toward home plate. That causes the ball to drop less than it normally would — the opposite effect of a curveball.

What’s illegal is when players use a sticky substance to improve their grip. That allows them to spin the ball even faster, Dufour said.

“As the ball leaves your fingers, you can do this little flip, and you can put more spin on it,” he said. “You can have a fraction of a second more of pushing and rotating the ball.”

Before the league crackdown, one popular choice among rule-breaking pitchers was a product called Spider Tack, which was designed to give people a better grip when lifting large boulders.

Whether it is enhanced with foreign substances or not, this spin-induced phenomenon is called the Magnus effect.

It happens because of a “boundary layer” of air that clings to the surface of a moving ball, leaving a turbulent wake behind it — almost like that of a speeding boat, Nathan said.

Depending on which way the ball spins, the boundary layer peels off in such a way that it nudges the ball’s flight path in one direction or the other, like the rudder on a boat.

Altering the ball’s path of a pitch by 1 inch may not sound like much, but it can make a difference, given that a typical bat measures less than 3 inches in diameter, Nathan said.

“Let’s say it’s a high fastball, and it’s an inch higher than you expected,” he said. “You could end up swinging beneath it, and it could end up being a pop-up instead of a home run.”

Nathan said he had not analyzed the balls thrown by Valdez or other pitchers during the World Series. But he noted that if a player wanted to break the rules, the postseason would be a risky time to do it.

“It would be a little surprising to me if he really was doing something illegal on such a big stage,” Nathan said.