Impossibility of photon absorption by a free electron. by superimpos- ing plots of the dispersion relations for photons and electrons, show that energy and momentum cannot both be conserved during the absorption or emission of a photon by a free electron, regardless of the initial velocity of the electron. 

pretty sure it's false

it's when something is going so fast it makes a loud noise, breaking the sound barrier, for example: a jet makes a loud noise, not only cause of the jet but because they reached a point where they went so fast that you can year the noise from various parts, kinda like a police car, lets say the car was going   so fast that you hear the siren of the cop car right next to you, but the car is probably a mile a way, it's cos the siren was next to you a few seconds if you understand me