Laser fields and proxy fields
  The convention in Atomic, Molecular, and Optical (AMO) physics of employing the dipole approximation to describe laser-induced processes replaces four source-free Maxwell equations governing laser fields with a single Maxwell equation for a "proxy" field that requires a virtual source current for its existence. Laser fields are transverse, but proxy fields are longitudinal; there can be no gauge equivalence. The proxy field is sometimes serviceable, but its limitations are severe. One example is the "above-threshold ionization" (ATI) phenomenon; surprising by proxy-field understanding, but natural and predicted in advance of observation with a laser-field method. An often-overlooked limitation is that numerical solution of the time-dependent Schr\"odinger equation (TDSE) is exact for proxy fields, but not for laser fields. Acceptance of proxy-field concepts has been costly in terms of inefficiently deployed research resources. Calculations with a nearly-40-year old transverse-field method remain unmatched with proxy fields. The transverse-field method is applicable in the "tunneling" domain, the "multiphoton" domain, and, as shown here, in the low-frequency "magnetic" domain. Attempts to introduce low-frequency magnetic field corrections into TDSE cannot be expected to produce meaningful results. They would be based on inappropriate Maxwell equations, a non-existent virtual source, and would approach constant electric field properties as the field frequency declines. Laser fields propagate at the speed of light for all frequencies; they cannot approach a constant-field limit. Extremely strong laser fields are unambiguously relativistic; a nonrelativistic limit that connects continuously to the relativistic domain is simpler conceptually and mathematically than is a theory constructed with a proxy field that is certain to fail as intensities increase. 
H. R. Reiss
physics.atom-ph quant-ph