Does the Law of Conservation of Matter/Energy Disallow Time Travel?

[Brian37]

In what context because time travel already exists. When you observe the sun, you are seeing it as it existed 8 minutes in the past. Those in orbit on the space station are subject to their own personal time.

I don't see right now how the si fi stuff of say going back and killing Hitler happening though. You are still talking about the problem of complexity at a quantum level and energy input vs output problems.