Exact Results in Quantum Theory

In the literature, various methods are used to compute decay rates of excited states in QED. In the simpliest cases, lower-rank calculations motivate the belief that they lead to the same results. However, there is no such evidency in general and it is not even clear if the most common method is adequate from a conceptual point of view. We will discuss the problem on the example of two-photon decay processes in the excited hydrogen atom. In particular. we will make use of the Gell-Mann, Low and Sucher formula and compare it with alternative approaches.