What can be measured asymptotically?

Understand

We consider asymptotic observables in quantum field theories in which the S-matrix makes sense.

• We argue that in addition to scattering amplitudes, a whole compendium of inclusive observables exists where the time-ordering is relaxed.
• These include expectation values of electromagnetic or gravitational radiation fields as well as out-of-time-order amplitudes.
• We explain how to calculate them in two ways: by relating them to amplitudes and products of amplitudes, and by using a generalization of the LSZ reduction formula.