# Research

**Group leader Prof. Dr. Robin Santra**

Theory plays a critical role in modern science. With the help of theory, scientific tools such as free-electron lasers can be put to use more efficiently, i.e., searches for scientific opportunities can be greatly accelerated. Theory often predicts new, previously unobserved phenomena, which inspire the planning and execution of experimental investigations. Moreover, without support by theory the outcome of an experiment lacks a conceptual foundation. It is through theory that we understand nature. The general strategy in theory proceeds in two steps. In the first step, a mathematical model is developed to describe the physical situation under consideration. This mathematical model rests either on fundamental natural laws already established, or on approximations to these laws, or on entirely new theoretical concepts. In the second step, mathematical tools are identified that allow one to find a solution to the mathematical model. In rare cases, it is possible to find a so-called analytic solution, i.e., an explicit equation that represents the solution. In most practical cases, however, the mathematical model can only be solved using numerical techniques. For this reason, theorists make heavy use of computers.

In the CFEL-DESY Theory Division, we develop theoretical and computational tools to predict the behavior of matter exposed to intense electromagnetic radiation. Examples for intense electromagnetic radiation include the visible light emitted by modern table-top lasers and the X-rays produced by the latest generation of free-electron lasers. We employ quantum-mechanical and classical techniques to study ultrafast processes that take place on the characteristic time scales on which atoms, and the electrons within atoms, move.