  The special theory of relativity teaches us that, although distinct inertial
frames perceive the same dynamical laws, space and time intervals differ in
value. We revisit the problem of time contraction using the paradigmatic model
of a fast-moving laboratory within which a photon is emitted and posteriorly
absorbed. In our model, however, the laboratory is composed of two independent
parallel plates, each of which allowed to be sufficiently light so as to get
kickbacks upon emission and absorption of light. We show that the lightness of
the laboratory accentuates the time contraction. We also discuss how the photon
frequency shifts upon reflection in a light moving mirror. Although often
imperceptible, these effects will inevitably exist whenever realistic
finite-mass bodies are involved. More fundamentally, they should necessarily
permeate any eventual approach to the problem of relativistic quantum frames of
reference.
