  We propose dynamical collapse models in which the stochastic collapse terms
affect only photons and/or gravitons. In principle, isolated systems comprising
only massive particles could evolve unitarily indefinitely in such models. In
practice, since photons and gravitons are ubiquitous and scatter from massive
particles, dynamical collapses of the former will effectively induce collapses
of the latter. In non-relativistic models in which particle number is conserved
and interactions are modelled by classical potentials, massive systems can be
modelled as collections of elementary massive particles bound by potentials,
interacting with an environment of photons and gravitons. In this picture,
although the photon and/or graviton collapse dynamics effectively localize
massive systems, these collapses take the effective form of approximate
measurements on the environment whose effect on the massive systems is
indirect. We argue that these environmental collapse models, like standard
mass-dependent spontaneous localisation models, may be consistent with quantum
experiments on microscopic systems while predicting very rapid effective
collapse of macroscopic massive systems, and hence a potential solution to the
quantum measurement problem. However, the models considered here have different
experimental signatures from standard mass-dependent spontaneous localisation
models. For example, they predict no deviations from standard quantum
interferometry for mesoscopic systems of massive particles isolated from a
decohering environment, nor do they predict anomalous spontaneous emission of
radiation from isolated matter of the type prediction by standard
mass-dependent spontaneous localization models. New experiments and analyses
are required to obtain empirical bounds on the decoherence rate in our models.
