rpenner. If a neutrino and photon are emitted from same position a million miles away from supermassive black hole event horizon, photon will maintain light speed and increase frequency as it closes on event horizon due to gravitational potential increasing energy of photon which cannot "fall" any faster than lights peed it had from emission. But the neutrino has non-zero rest mass (whatever it is in fact even if not measured). Therefor the neutrino is affected by gravity as all particles with rest mass are, they "fall" faster and faster the longer the gravitational acceleration acts while it closes on event horizon. You say that neutrino, which can accelerate by gravity from its initial emission speed, cannot reach light speed. I agree. I agree also that invariant mass (whatever its rest mass is) is invariant mass. But can you tell me what prevents neutrino being accelerated to higher than emission speed (or even to light speed), since gravitational acceleration is "always on" all the way from a million miles away and until it "falls" down to the event horizon the rest or invariant mass is responding to gravitational gradient potentials and associated accelerations?