Current metrics-based approaches to visualise unfamiliar software systems face two key limitations: (1) They are limited in terms of the number of dimensions that can be projected, and (2) they use fixed layout algorithms where the resulting positions of entities can be vulnerable to misinterpretation. In this paper we show how computer games technology can be used to address these problems. We present the PhysVis software exploration system, where software metrics can be variably mapped to parameters of a physical model and displayed via a particle system. Entities can be imbued with attributes such as mass, gravity, and (for relationships) strength or springiness, alongside traditional attributes such as position, colour and size. The resulting visualisation is a dynamic scene; the relative positions of entities are not determined by a fixed layout algorithm, but by intuitive physical notions such as gravity, mass, and drag. The implementation is openly available, and we evaluate it on a selection of visualisation tasks for two openly-available software systems.