  Historically, the observation of half-spin particles was one of the most
surprising features of quantum mechanics. They are often described as "objects
that do not come back to their initial state after one turn but do after two
turns". There are macroscopic implementations using constraints such as
clamping a belt or ribbon that purport to show similar behavior (the "Dirac
belt trick"). However, a demonstration of an unconstrained macroscopic object
with half-spin behavior remains elusive. In this article, we propose to fill
this gap and introduce the spinorial ball. It consists of a translucent plastic
ball with internal LED illumination that behaves as a freely movable
macroscopic half-spin object. It provides a new tool to introduce and visualize
half-integer spins as well as the covering group homomorphism from SU(2) to
SO(3), and offers in particular a clear visualization of the different homotopy
classes of SO(3). We discuss its development and function, and how one can
mimic quantum measurement and wave function collapse using this the spinorial
ball. The entire system is open source hardware, with build details, models, 3d
printing files, etc., provided under an open source license.
