  A relativistic transient absorption theory is derived, implemented and
validated within the dipole approximation based on the time-dependent Dirac
equation. Time-dependent simulations have been performed using the Dirac
equation and the Schr\"odinger equation for the Hydrogen atom in two different
attosecond transient absorption scenarios. These simulations validate the
present relativistic theory. The presented work can be seen as a first step in
the development of a more general relativistic attosecond transient absorption
spectroscopy method for studying heavy atoms, but it also suggests the
possibility of studying relativistic effects, such as Zitterbewegung, in the
time domain.
