  In special relativity, spacetime algebra (STA) provides a powerful and
insightful approach to an invariant formulation of physics. However, in this
geometric algebra of spacetime, relativistic physics is usually considered a
misnomer: STA provides us with an invariant (coordinate-free) formulation and
hence it seems that (passive) Lorentz transformations are misplaced. However,
in order to interpret experimental results, it is important to relate invariant
physical quantities to some reference frame. In this paper, passive Lorentz
transformations are derived using STA but without the second postulate on the
speed of light used by Einstein. This derivation sheds light on the physical
background of STA and, at the same time, clarifies Lorentz transformations in
the proper setting of (Minkowski) four-dimensional spacetime.
