Randall "XKCD" Munroe's new "What If?" feature answers one wild hypothetical per week. The first two are corkers: Relativistic Baseball baseball asks what would happen if a baseball pitcher could throw a ball at 0.9C; the second, SAT Guessing, looks at the (very long) odds against getting a perfect SAT by bubbling in random guesses. Here's a taste of Relativistic Baseball:
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 ideas of aerodynamics don't apply here. Normally, air would flow around anything moving through it. But the air molecules in front of this ball don't have time to be jostled out of the way. The ball smacks into them hard that the atoms in the air molecules actually fuse with the atoms in the ball's surface. Each collision releases a burst of gamma rays and scattered particles.
fusion illustration fusion zone of baseball
These gamma rays and debris expand outward in a bubble centered on the pitcher's mound. They start to tear apart the molecules in the air, ripping the electrons from the nuclei and turning the air in the stadium into an expanding bubble of incandescent plasma. The wall of this bubble approaches the batter at about the speed of light—only slightly ahead of the ball itself.