Almost half-a-century ago, the late great J. D. Eshelby advanced the notion of a
force on a defect or a singularity in a stressed solid. This notion is radically
different fi'om the usual Galilean or Newtonian concept of a force, which engineers
of various disciplines encounter daily in their work, concerned perhaps with the
determination of flight trajectories of launched satellites, or with stress, deformation
and structural stability analysis of high-rise buildings. The Eshelby-type force is
always to be understood as a relative change of the total energy of a given system
with respect to some quantity which alters the configuration of that system. The
latter quantity might be the displacement of a foreign or missing atom in a lattice,
the change in location of a dislocation, the change in size or shape of a crack,
cavity or inclusion or the change in location of a phase boundary in a material. All
such changes of configuration of certain objects occur within the material in which
they find themselves, by contrast to changes in the configuation of a bridge under
some Newtonian loadings, which occur in what might be called the physical space
of our surroundings, in which the bridge finds itself and in which Newton's laws are
valid.