3 ms·

What does this mean: "the photons must simultaneously excite several multipoles within the particle, which scatter the beam" ?

• jessriedel 16y ago
  When trying to find out how an object is going to interact with the electromagnetic field, it's generally useful to decompose the charge distributed on the object into "multipole modes". It turns out that any arbitrary distribution of charges can be expressed as a sum of such multipoles, and the interaction of each multipole with the E&M field can be solved even though the object itself may be too complicated. Usually, only the first few multipoles are important, which allows efficient computation of the objects behavior by summing up the contribution of the first few multipoles. The first multipole ("monopole") is just the net charge of the object. The second multipole ("dipole") is the degree to which the object has positive charge distributed toward one side and negative charge toward another. A single isolated point charge is a perfect monopole, with zero higher-order multipole components. An object composed of two opposite charges of equal magnitude, separated by a distance, is a perfect dipole, with zero monopole and higher-order multipole components. And so on. Apparently, the point is that objects which are electromagnetically simple (such as a single point charge) cannot be pulled by a tractor beam. Instead, the object must be complicated such that it has several large multipole components which interact in the right ways to produce a net pull.