If you assume that the law of conservation of energy is accurate the energy can not be created or destroyed, therefore the total amount of energy within the whole universe is constant. From E=MC^2 we know that mass has energy, so if a person travels backwards in time then energy is transfered from one point in time to another, altering the total energy in the universe, meaning the total amount of energy in the universe is not constant, the resulting contradiction means that either the law of conservation of energy is inaccurate or reverse time travel is impossible.