I'm doing some research into time travel for a presentation I have to give in a month or so, and I'm currently looking at the compatibility of Time Travel and the Laws of Conservation. Sending an object back in time would increase the mass - and hence the energy - in the Universe at this time, and would therefore appear to violate the conservation of energy. However, I found http://www.weburbia.com/physics/time_travel.html" that says that in fact the conservation laws may not violated because conservation laws are local, whereas there may not be a global conservation law. I've tried to find more information on this, but couldn't nothing obvious came up, so I was wondering what people here thought of the idea. Assuming that there are no other objections to time travel, do conservation laws necessarily rule out time travel? Or can the two be compatible (even if it requires something strange to be happening)?