Sort:  

A mass large enough to be a craft traveling at near light speed momentum stopping in the kind of timeframe expected would probably produce an itty-bitty baby neutron star and an instantaneous catastrophic dispersion of energy and potentially material, aka: nova
:]

The brilliant xkcd has written a hilarious hypothetical scenario in which we are guided through in detail exactly what would happen if you tried to hit a baseball pitched at 90% the speed of light - must read.

Yeah by serious G forces I wasn't thinking near light speed, that's for sure, to do that we'd need structural integrity fields, massive forcefields to divert space dust and micrometeorites out of the way, internal damping fields to maintain integrity of the individuals inside and stop them from pasting themselves on the nearest bulkhead, or some kind of space warping business. Unfortunately all a long way off unless you count an electromagnetically generated plasma sheath as a forcefield. :D

I'm glad I stumbled onto this post, been really enjoying this.

Going to read it now. This should be amazing.

Everything within roughly a mile of the park is leveled, and a firestorm engulfs the surrounding city. The baseball diamond is now a sizable crater, centered a few hundred feet behind the former location of the backstop.

Bahahaha, yes, this is something I can get behind.

I think my favourite part is:

"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)."

Followed by:

"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..."

Railgun x 1000. hahahahahaha.