  We investigate the vacuum properties of a massless scalar field theory in
constrained spatial geometry, namely, the instantaneous appearance of a thick
Dirichlet boundary inside a one-dimensional (1D) Dirichlet cavity and divides
it into two parts. Our work presents the calculations for the energy density
and particle number density created. The expression of the energy density is
found to be dependent on the size nature of the appearing wall, more precisely,
it depends on whether the length of the wall is rational or irrational.
