The artistically inclined folks over at XKCD (authors of one of my favorite web comics ever) devoted one of their What If? segments today to the logically insane yet weirdly interesting scenario of what would happen if someone pitched a baseball at 90 percent the speed of light. Using actual science and helpful illustrations, the short answer is that if this experiment was a) feasible and b) conducted within the confines of a populated baseball stadium, many thousands of fans would die before they knew what actually happened. Yay, science!
The constant fusion at the front of the ball pushes back on it, slowing it down, as if the ball were a rocket flying tail-first while firing its engines. Unfortunately, the ball is going so fast that even the tremendous force from this ongoing thermonuclear explosion barely slows it down at all. It does, however, start to eat away at the surface, blasting tiny particulate fragments of the ball in all directions. These fragments are going so fast that when they hit air molecules, they trigger two or three more rounds of fusion.
After about 70 nanoseconds the ball arrives at home plate. The batter hasn't even seen the pitcher let go of the ball, since the light carrying that information arrives at about the same time the ball does. Collisions with the air have eaten the ball away almost completely, and it is now a bullet-shaped cloud of expanding plasma (mainly carbon, oxygen, hydrogen, and nitrogen) ramming into the air and triggering more fusion as it goes. The shell of x-rays hits the batter first, and a handful of nanoseconds later the debris cloud hits
And that's when things get really ugly, like something out of The Day After. But outside of Aroldis Chapman, we've probably got little to worry about here.