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When a batter goes up to hit in baseball, they have to be ready for anything the pitcher might throw at them.   It might be a fastball right across the plate, for instance, or it could be a curveball. Literally.   Curveballs, along with pitches like screwballs and sliders, are what’s known as a breaking pitch: a pitch that curves   significantly down or to the side.

And these breaking pitches can be hard to hit,   so much so that it’s become a figure of speech to   describe unexpected problems. Like, “That last question was a real curveball”.   Part of what makes these pitches hard to  hit, according to batters, is that they   seem to suddenly move in erratic ways,  right before they connect with the bat.   But... why? It turns out the answer might have more to do with the batter’s eyes than the pitcher’s arm.   Now, to be clear, curveballs do curve.

The secret is in how they spin and something   called the Magnus effect. In brief, the more spin   the pitcher can put on the ball, the more they can make its path curve.   In fact, in 2021, Major League  Baseball cracked down on pitchers   putting sticky substances  like pine tar on baseballs.   The extra grip lets the pitcher  put more spin on the ball,   which can lead to unusually large breaks. That curving movement, combined with the   pitcher doing their best to make it look like they’re actually about   to throw something else, can trick the batter into   swinging at the wrong place or time.

But even so, the ball is still   flying in a continuous arc. They don’t make sudden jumps.   To explain why they seem to do that,  we need to look at the batter’s eyes.   For instance, in a 2010  paper, scientists looked at   how different parts of our eye  process images differently.   They set up an experiment where they  asked people to watch moving images   with different parts of their vision: either  directly, or through peripheral vision.   Objects we look at directly  are captured by the fovea,   which is an especially  accurate area of the retina.   The scientists found that if you were  looking at something with your fovea   and then switched to peripheral vision,  a moving object could seem to jump.   This might happen in baseball  if the batter loses focus.   Gotta keep your eye on the ball, as they say. But it might happen naturally,   as the ball comes closer and takes up more of your   field of view than the fovea can capture.

Or because an optimal strategy for batters   is to track the ball until the very last second,   then shift focus away from the ball to where the bat and ball will meet instead.   That split second change of focus might result in your eyes seeing that jumping illusion.   This effect might be compounded by your  brain trying to compensate for the   sudden loss of visual precision by predicting where the ball is based on past information.   But those predictions might  not always be accurate.   The researchers in that 2010 paper say  this could potentially explain why   things in motion may seem to slow  down, shift position, or curve.   Not just baseballs, but other things we  have to perceive in a short amount of time,   like aircraft landing strips. Maybe there’s a bigger lesson here.   Like, the next time life seems  to throws you a curveball,   it might be worth pondering if  things really did get screwy,   or whether it was all an illusion. Or maybe the lesson is just that playing   professional baseball is harder than it seems.

Thanks for watching this episode of SciShow,   and thanks to our patrons for  stepping up to the plate.   You guys knock it out of the park every time. If you’d like to join our team, you can sign up at patreon.com/scishow. [♪ OUTRO].