Putting aside the possibility of actually pitching a baseball that fast, this fascinating read on xkcd's what if section explains in laymen terms what would happen if the ball travelled at nearly the speed of light.
The answer turns out to be "a lot of things," and they all happen very quickly, and it doesn't end well for the batter (or the pitcher). I sat down with some physics books, a Nolan Ryan action figure, and a bunch of videotapes of nuclear tests and tried to sort it all out. What follows is my best guess at a nanosecond-by-nanosecond portrait:
The ball is going so fast that everything else is practically stationary. Even the molecules in the air are stationary. Air molecules vibrate back and forth at a few hundred miles per hour, but the ball is moving through them at 600 million miles per hour. This means that as far as the ball is concerned, they're just hanging there, frozen.
The what if section is updated every Tuesday to answer your hypothetical questions with physics, so be sure to check often.
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