Can I travel back in time and invest in cattle futures?
No, but I plan on going back and buying stock of an upstart company called Wal-Mart.
Yes, but when you return, everyone will be a vegetarian.
Only if you are a female democrat member of the Arkansas Mafia.
You could, but your wealth today has already been set. You would not be richer than before you went back to the past in order to alter the present (which then was in the future).
But, you could travel to the *future* and buy a copy of the Wall Street Journal, travel back to the present and invest wisely. Since you travelled to the future and now know with certainty what will happen, quantum mechanics says that the future, now, cannot be altered. Just make sure you do not meet yourself, else your future will also be already determined.
Now where's that Tylenol.