  We are discussing Schwinger'idea that physical mechanism of sonoluminescence
is a physical vacuum excitation. This theory was based on the assumption that
the sudden change of the rate of bubble collapse leads to the jump of
dielectric constant of the gas trapped inside the bubble. We show that the
dependence of the dielectric constant on the gas density really leads to the
jump of the dielectric constant at shock-wave propagation in a collapsing gas
bubble.
