1. We prove a theorem that says, essentially, that it is impossible for an infinite slab to exist which absorbs, reflects, or transforms a fraction>0 of the energy in weak gravitational waves. But it is (essentially) possible to make a 100% efficient emitter of such waves.2. We prove a theorem that says, essentially, that General Relativity (GR) is soluble forward in time but not backward in time, in a manner like the 1D "heat equation" F_t = F_xx.3. We discuss other (known) time-direction asymmetries of GR. All of these ultimately arise, essentially, from the positivity of mass.Hence we argue that, contrary to common false claims, gravitational physics is not time-reversal invariant and defines a unique direction of time as an output not input. Making this argument involves re- solving several subtle apparent paradoxes. But non-gravitational physics (quantum field theories) obey charge-parity-time reversal invariance CPT. We conclude with the grand Manifesto that all the time-irreversibility phenomena observed in everyday life, including "quantum measurement," the 2nd law of thermodynamics, and outgoing-only radiative boundary conditions, ultimately arise from gravitational physics. We outline how that happens and discuss the implications about future quantum gravity theories.