Why do they only consider S-matrix in QFT?

[ndung200790]

Please teach me this:
Why do they only consider S-matrix(or Green functions) in Quantum Field Theory?How about other physics observations as considering in normal Quantum Mechanics,e.g angular momentum...of particles?If we consider the other physical observations,how could we represent them in language of Quantum Field Theory?
Thank you very much in advance.

 

[Bill_K]

ndung, Scattering cross-sections are a main focus of calculations in quantum field theory because most experiments are scattering experiments. There are plenty of other calculations too which are not related to scattering: linewidths and lifetimes, anomalous magnetic moment, Lamb shift, etc