Redshift, certainly, but there's also the issue of recoil. A photon goes out of the laser at one end and momentum in the laser system must be conserved. Althoug

Redshift, certainly, but there's also the issue of recoil. A photon goes out of the laser at one end and momentum in the laser system must be conserved. Although we're talking about impossible "perfect" devices, a laser must also necessarily create heat.