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What Would Happen If You Tried to Hit a Baseball Pitched at 90% the Speed of Light?

If the question "What would happen if you tried to hit a baseball pitched at 90% the speed of light?" has been keeping you up at night, you'd be pleasantly surprised that Randall Munroe of xkcd has devoted a lot of time researching the answer:

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 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.

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.

Read what happened when the ball reaches the batter a fraction of a second later, over at xkcd: Link - via metafilter

The pitcher and hitter are in the same reference plane (i.e. they are both traveling the same speed with the same acceleration), so their perception of the event would be the same.

There's also not a blast wave in the sense you're thinking about. A blast wave in the traditional sense is a wave of more compressed (higher pressure) air. In the scenario described here, the author is claiming the molecules in the air are destroyed rather than accelerated (at least for non-tangental impacts), so no increased energy air molecules exist from its passage that are heading towards the batter. The number of air molecules thus destroyed seems unlikely to generate enough high energy radiation to vaporize the batter due to the low number of molecules that would be struck by the ball. Possibly a dangerous dose of radiation though. I didn't get a chance to read the full article, so I don't know what the other suggested outcomes are.
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Relativistic effects? Isnt an observers perception of the speed of light a constant? The hitter would see the ball slow to an almost imperceptible speed while the pitcher would see the ball take a lifetime to reach the target?

Also wouldn't the hitter simply vapourise without moving perceptibly since the blast wave would have enough energy to rip apart his atoms at the moment of transfer of momentum?

Pure guesswork on my part
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Assuming the baseball didn't disintegrate under the G force of being accelerared to lightspeed in 3 feet, the expression on the pitchers face as Newtons second law kicks in and accelerates him backwards at several thousand G's would be a treat
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