SAN DIEGO -- If you're light, it's fairly easy to travel at your own speed -- that is to say 186,282 miles per second or 299,800 kilometers per second. But if you are matter, then it's another matter altogether.Nothing we know of zips along more quickly than light. Einstein, nearly 100 years ago, said it's not possible. For us, the speed limit makes strange sense: Go faster than light, and you could return before you've left, become your own grandpa, or perform other leaps of cosmic logic.Fast forward a century. Astronomers are now measuring stuff -- material, matter, things --...