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Traveling back in time could probably be ruled impossible because you are not only traveling back in time, you are traveling back in space. If you are in cleveland right now, and want to visit cleveland 100 years ago today, you have to take the following differences in position into account:

Rotation of the earth.
Earth revolving around the sun.
Our solar system moving around in the milky way galaxy.
The milky way galaxy continually moving away from the center of the universe.

So basically, if you are in cleveland right now and zap yourself back 100 years, you'll have to wait 100 years for the earth to catch up with you, because it's a loooooooooooooooong way away.

Ad to this conservation of matter and conservation of energy, and you cannot travel in time without breaking those two laws. Basically, if I do manage to drop back into the same spot cleveland was in 100 years ago, I have instantly added a whole bunch of matter to the universe as it existed then. And taken a whole lot of matter out of our present day universe. Now, the present day might be easy by converting myself to energy, but finding a way to convert energy out of thin air into matter 100 years ago is going to be a doozy of a problem.