Why, or perhaps how, is 100% of the mass in matter-antimatter annihilation turned into energy? I thought no reaction could be 100% efficient, i.e. 2nd law of thermodynamics? Isn't the Carnot heat engine or the conversion of matter into energy in the accretion disk of a black hole the most efficient systems observed? Entropy, as I understand it, is a time biased quantity, which can increase or stay the same in a system as time moves forward. Does this mean that matter-antimatter annihilation is a reversible reaction and that subatomic particles aren't biased by the direction of time(like macroscopic objects are)? Can these reactions reduce the entropy of a system? I believe that I may be missing out on a fundamental aspect of the physics underlying this reaction.