This question comes from Ellen, who asks: What would happen if you tried to hit a baseball pitched at 90% the speed of light?

Let's set aside the question of how we got the baseball moving that fast.

We'll suppose it's a normal pitch, except in the instant the pitcher releases the ball, it magically accelerates to 0.9c.

From that point onward, everything proceeds according to normal physics.

And "what proceeds" 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).

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 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 so hard that the atomic nuclei in the air molecules actually fuse with the atomic nuclei in the ball.