  The Square Kilometre Array (SKA) is a planned large radio interferometer
designed to operate over a wide range of frequencies, and with an order of
magnitude greater sensitivity and survey speed than any current radio
telescope. The SKA will address many important topics in astronomy, ranging
from planet formation to distant galaxies. However, in this work, we consider
the perspective of the SKA as a facility for studying physics. We review four
areas in which the SKA is expected to make major contributions to our
understanding of fundamental physics: cosmic dawn and reionisation; gravity and
gravitational radiation; cosmology and dark energy; and dark matter and
astroparticle physics. These discussions demonstrate that the SKA will be a
spectacular physics machine, which will provide many new breakthroughs and
novel insights on matter, energy and spacetime.
