  Electrostriction, the deformation of dielectric materials under the influence
of an electric field, is of continuous interest in optics. The classic
experiment by Hakim and Higham [Proc. Phys. Soc. 80, 190 (1962)] for a
stationary field supports a different formula of the electrostrictive force
density than the recent experiment by Astrath et al. [Light Sci. Appl. 11, 103
(2022)] for an optical field. In this work, we study the origin of this
difference by developing a time-dependent covariant theory of optical force
densities in photonic materials. When a light pulse propagates in a bulk
dielectric, the field-induced force density consists of two parts: (i) The
optical wave momentum force density carries the wave momentum of light and
drives forward a mass density wave of the covariant coupled field-material
state of light. (ii) The optostrictive force density arises from the atomic
density dependence of the electric and magnetic field energy densities. It
represents an optical Lorentz-force-law-based generalization of the electro-
and magnetostrictive force densities well known for static electromagnetic
fields and derived from the principle of virtual work. Since the work done by
the optostrictive force density is not equal to the change of the field energy
density during the contraction of the material, we have to describe this
difference by optostriction-related dissipation terms to fulfill the energy
conservation. The detailed physical model of the dissipation is left for
further works. The optostrictive force density can be understood in terms of
field-induced pair interactions inside the material. Because of the related
action and reaction effects, this force density cannot contribute to the net
momentum transfer of the optical field. We also use the theory to simulate the
propagation of a Gaussian light pulse through a dielectric material.
