  Many evidences appear in the past decades and show that the negativity of
Casimir energy is responsible for exotic mechanical and gravitational effects.
We study in this work the Lorentz boost of a Casimir cavity, on which little
attention is paid to its momentum in historical works. We find that the vacuum
energy and momentum carried by the cavity transform differently from those of
point particles due to the cavity's extension feature. However, the mass-shell
condition of the two are identical as long as the cavity is finite along the
moving direction only.
