The common theoretical model used to describe the sintering process is the two particle model. This model describes comprehensively the growth of inter-particle contacts driven by the Laplace pressure. In three dimensions this model fails as cooperative material transport (i.e., movements of powder particles) occurs. The fundamental understanding of these processes is still rather sketchy and is derived from the observation of 1D or 2D samples. To observe and quantify cooperative material transport new experimental methods are needed, for example high resolution computer tomography in conjunction with photogrammetric image analysis. Single-crystal copper spheres were marked by drilling holes into the surface of each particle and two samples were prepared using the marked powder. One sample was heated continuously to 1,050 A degrees C in a silica capillary and was analyzed by in situ synchrotron computer tomography (SCT) at the European synchrotron radiation facility in Grenoble. Free sintering was investigated by ex-situ SCT at Bessy II in Berlin. The novel technique of drilling holes into the sphere surfaces allows for the quantification of both intrinsic rotations and particle rolling by photogrammetry. The observed intrinsic rotations exceed the rolling by far and we conclude that the anisotropy of grain boundary energies results in grain boundary sliding. Furthermore, we present first computer simulations of sintering processes based on initial particle positions measured by SCT.