It has been generally believed that hard core interaction is irrelevant to absorbing-type critical phenomena because the particle density is so low near an absorbing phase transition. We study the effect of hard core interaction on the
N-species branching annihilating random walks with two offspring and report that hard core interaction drastically changes the absorbing-type critical phenomena in a nontrivial way. Through a Langevin equation-type approach, we predict analytically the values of the scaling exponents, ${\ensuremath{\nu}}_{\ensuremath{\perp}}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}2$,
z=2, $\ensuremath{\alpha}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1/2$, and $\ensuremath{\beta}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}2$ in one dimension for all
N>1. Direct numerical simulations confirm our prediction. When the diffusion coefficients for different species are not identical, ${\ensuremath{\nu}}_{\ensuremath{\perp}}$ and $\ensuremath{\beta}$ vary continuously with the ratios between the coefficients.
The absorber theory of radiation is discussed with particular reference to its application to quantum optical phenomena. The essential feature of the theory is that the absorber is the source of the radiative reaction force. This is an advanced action of the absorber on the emitting atom and, in the quantum version of the theory, is highly directional and polarized. Direct action absorber theory, without fields, can reproduce effects such as spontaneous emission, the Lamb shift, the dynamic Stark effect and photoelectron antibunching, which are often described as properties of the quantized field. Its most interesting application, however, is in calculating the detailed dynamics involved in the Kocher-Commins experiment. It is this experiment which most clearly displays the anti-causal nature of the radiative reaction in absorber theory and shows the advantage of absorber theory over the conventional quantum electrodynamic model in explaining the effect.