  It is well known that a constant energy density of the vacuum produces a
negative pressure. We show that, in the presence of special boundaries for the
photon field, the virtual photons in the quantum vacuum may induce a negative
pressure of this type. In this context, we find in particular that an anomalous
quantum vacuum radiation is responsible for the appearance of this negative
pressure in a system, which generates the tangential Casimir force [Z. Zhang,
New J. Phys. 24 (2022) 113036] experienced by parallel conducting plates.
