  Collisions of atomic nuclei at relativistic velocities allow to recreate the
conditions encountered in neutron stars or in the early universe micro-seconds
after the Big Bang. These reactions are performed in today's largest
accelerator facilities, e.g. at CERN in Geneva, at the Relativistic Heavy Ion
Collider at Brookhaven, NY or in the planned FAIR facility in Darmstadt
Germany. During such a collision the matter is heated up to hundreds of MeV
(billions of degrees) and compressed to densities of $3-10$ times the density
inside ordinary atomic nuclei (i.e. $10^{17}-10^{18}$ kg/m$^3$). Usually these
collisions are studied via the measurement of a multitude of strongly
interacting particles, called hadrons, that are emitted at the end of the
collision. However, also some photons are created. These photons are of special
interest as they allow to look into the early stage of the collisions, because
they are only very weakly (namely only electro-magnetically) interacting with
the hadrons of the created fireball. This paper elucidates the physics of the
photons and what can be learned from them.
