One of the most efficient ways for a physicist to collect information about the outside world is through scattering processes: We aim a suitable beam of particles at the target or object to be investigated and observe the recoil particles or, more generally, the end products of the process as in figure 1. Classically, we use the results of this observation to derive, by theory and computation, some properties of the target that are assumed to be more fundamental than the mere scattering data. Alternatively we could take a very cautious attitude and assume that the scattering matrix, without further elaboration, fully describes our target, so far as that kind of primary particle is concerned. How do we get the most useful scattering matrix? And why, for so many years, did the only beam of “particles” exploited to any extent remain electromagnetic radiation within a certain limited frequency band—why did optics, the physics of visible light, develop first?

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