Do we need Feynman diagrams for higher order perturbation theory?

On Perturbation Theory, Dyson Series, and Feynman Diagrams

To study the transitions (scattering, making new particles, decays, etc.) caused by V̂ we want to use a fixed basis of Ĥ0 eigenstates, but we want to keep the transitions separate from wave-packet spreading and other effects due to Schrödinger phases e−iEt of the Ĥ0 itself. The picture of QM which separates these effects is the interaction picture. In the Schrödinger picture, the operators are t...

On Perturbation Theory, Dyson Series, and Feynman Diagrams

The calculation of Feynman diagrams in the superstring perturbation theory

The method of the perturbative calculation of the multi-loop amplitudes in the superstring theories is proposed. In this method the multi-loop superstring amplitudes are calculated from the equations that are none other than Ward identities. The above equations are derived from the requirement that the discussed amplitudes are independent from a choice of gauge of both the vierbein and the grav...

Perturbation theory via Feynman diagrams in classical mechanics

In this paper we show how Feynman diagrams, which are used as a tool to implement perturbation theory in quantum field theory, can be very useful also in classical mechanics, provided we introduce also at the classical level concepts like path integrals and generating functionals.

Ramadan fasting: Do we need more evidence?

Over a billion of Muslims fast worldwide during Ramadan each year. Through this religious custom, fasting contributes to their health as well as their spiritual growth. However, available evidence regarding the health-benefits of Ramadan fasting is scarce and highly contentious. Although Islam exempts patients from fasting, many fast conceivably, and their clinical condition is prone to deterio...