Motion and observation in a single-particle universe

We outline an argument that a single-particle universe can be described, which is observationally indistinguishable from the world we see around us despite the absence of any meaningful frame of reference relative to which motion can be defined. Our argument uses a formal model of spacetime that can be considered either relational or substantivalist depending on ones prefered level of abstraction, suggesting that this long-held distinction is to some extent illusory.