Cylinder with gas, diathermal walls, closed by two pistons, R and L, inside a plane-parallel capacitor with charge surface density . Laser (l), beam splitter (s), pistons of blocked mechanism (b) are used to assure that forces are applied simultaneously [62]. The gas in cylinder is compressed by forces acting on pistons. Force acts on piston L during time interval and displacement . Force acts on piston R during time interval and displacement . During the compression process, photons are emitted by cylinder walls with frequency and zero linear momentum in frame .