It’s a “debate” that just won’t die. Scientist after scientist has come along purporting to “prove” that a curveball is nothing more than an optical illusion. This time it’s Arthur Shapiro and Zhong-Lin Lu of USC, as chronicled by an overly credulous article over at Wired and accompanying video on Youtube.

But the fact remains that curveballs do curve. As much as 16-18 inches. As Dizzy Dean famously said, “Stand behind a tree 60 feet away, and I’ll whomp you with an optical illusion!” Because with a well-placed curveball, you actually can hit a man standing directly behind a tree 60 feet away, a man whom you cannot even see.

The curveball-as-optical illusion argument always boils down to the same line of reasoning. The laws of physics are immutable, scientists argue, so it is impossible for a ball to actually curve. Every curveball ever thrown follows a perfectly parabolic path.

Which is all perfectly true. But it is completely missing the point of what a ballplayer means when he says that a curveball “curves.” The ballplayer is not suggesting that the ball suddenly randomly, and unpredictably veers onto a physically impossible, non-parabolic path, but simply that its trajectory alters in mid-flight in a way that we would not usually expect under “normal” conditions.

Most people do not normally expect that you can throw a fairly heavy, round object and hit a target for which there is no unobstructed, straight-line path, even though the laws of physics are perfectly happy to allow this.

In the case of the curveball, the mathematical function that it charts on its way to the plate is always going to be a perfectly parabolic function, but just not the same function we would expect a smooth ball of the same size and weight to take. This is because a baseball has seams.

Due to the Magnus effect, as the ball slightly loses velocity due to air resistance, the air acts on the seams to reduce the speed even faster and impart a slightly different trajectory on the ball than we would expect from the original motion of the pitchers arm. The ball actually veers horizontally, which is not something we normally expect from something the size and weight of a baseball.

In other words, physics is not defied in any way. But importantly, our expectations are. A curveball is not an optical illusion because its motion has nothing (or at least very little) to do with the eyes. It’s all about expectations inside the brain.

Basically, these scientists who have never actually tried to catch or hit a curveball are reading way too much into the way non-scientists use the word “curve.”

Think about it this way. If I made a fancy paper airplane with all sorts of wings and flaps, and I threw it and it traced an arcing path, and then I said “Wow, that airplane curved!”, no scientist would come alone and try to convince me that the curving path of the paper airplane was just an “optical illusion.” It would be ridiculous!

And yet the way curveball “curves” is for all intents and purposes exactly the same as the way a paper airplane curves: air resistance is acting to alter the trajectory of the object in mid-flight. The path of both is perfectly parabolic, but that is entirely beside the point. The point is that the object moved in a way we did not expect based on “common sense.”

The endless parade of scientists who keep claiming that the curveball is nothing but an optical illusion should try to get a better handle on what laymen mean by “curve” (hint: it does NOT mean “non-parabolic, physics-defying motion”).

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