  X-ray free electron lasers (FELs) have been proposed to be constructed both
at SLAC in the form of the so-called Linac Coherent Light Source as well as at
DESY, where the so-called XFEL laboratory is part of the design of the
electron-positron linear collider TESLA. In addition to the immediate
applications in condensed matter physics, chemistry, material science, and
structural biology, X-ray FELs may be employed also to study some physics
issues of fundamental nature. In this context, one may mention the boiling of
the vacuum (Schwinger pair creation in an external field), horizon physics
(Unruh effect), and axion production. We review these X-ray FEL opportunities
of fundamental physics and discuss the necessary technological improvements in
order to achieve these goals.
