X-rays have been invaluable in determining the atomic structure of matter. They allow determining the position of atoms and thereby the 3-dimensional steady-state atomic structure of the system under scrutiny by diffraction or microscopy. More recently, the technique has been extended to recording snaps of X-ray or electron diffraction images of dynamically-evolving systems by using ultrashort X-ray or electron pulses, respectively. From a series of such snapshots the temporal evolution of the structure of matter can be determined. This extends imaging from the three spatial dimensions to the fourth, temporal, dimension. The temporal resolution of four-dimensional imaging is dictated by the duration of the X-ray or electron pulses. The shortest X-ray and electron pulses currently have a duration of several hundred femtoseconds. This limits their applications to relatively slow motions in the microworld, such as molecular rotation and rearrangement of heavy constituent atoms. Structural dynamics of molecules made up of light constituents, such as biomolecules, as well as almost all kinds of electronic dynamics have remained out of reach of 4-dimensional diffraction imaging so far.

Attosecond X-ray and/or electron pulses, which are being pursued at LAP, will radically change this unsatisfactory state of affairs, see Fig. 1. Attosecond diffraction imaging will provide real-time access to all microscopic motions outside the atomic core and radically change our insight into the workings of the microcosm.