Although special relativity and quantum mechanics revolutionised physics in the early 20th century,
the consequences of combining these two theories are still being explored a hundred years later,
usually using the formidable theoretical machinery of quantum field theory. However, a formalism
accessible to undergraduates has been recently developed which shows how the centre of mass and
internal dynamics of classical and quantum systems is relativistically coupled with interesting
consequences. Here we explore some of the implications of this coupling, first classically, where we
find that the dynamics of the system is time dilated when moving relative to another inertial frame.
We then apply the dynamics to a quantum 2-level atom bound in a one-dimensional infinite potential
well, and show that the coupling produces collapses and revivals in quantum interference. This
example provides an illustration of how the combination of special relativity and quantum mechanics
can be stud…