Wheeler-Feynman absorber theory[1] was originally conceived as a time-symmetric alternative to conventional electromagnetism which, unlike the latter, imposes no ad hoc time direction on electromagnetic processes. It is essentially a set of boundary-condition rules arising from the requirement of time-symmetry which were restated in AT1 as follows: (1) The process of emission produces an electromagnetic wave consisting of a half-amplitude retarded wave and a half amplitude advanced wave which lie along the same four-vector but with opposite time directions; (2) The process of absorption is identical to that of emission and occurs in such a way that the wave produced by the absorber is 180o out of phase with the wave incident on it from the emitter; and (3) An advanced wave may be reinterpreted by an observer as a retarded wave by reversing the signs of the energy and momentum (and therefore the time direction) of the wave, and likewise an observer may reinterpret a retarded wave as an advanced wave.
Fig. 1 illustrates this emitter-absorber protocol schematically using a Minkowski diagram. Here the relative phases of the waves are schematically represented as sinusoids inscribed on the lightlike world lines of the waves. The emitter-wave is shown as a solid line and the absorber-wave as a dashed line. In regions where they have opposite phases they will cancel and in regions where they have the same phase they will reinforce.
This combination of advanced and retarded waves specified by rule (1) describes both emission and absorption with the same time-symmetric combination of advanced and retarded radiation. In interacting with this time-symmetric field which it has produced, the emitter (or absorber) cannot change its energy or momentum, for such changes are intrinsically unsymmetric in time and therefore cannot result from interactions with a time-symmetric field. Thus, this simultaneous emission of a pair of waves, advanced and retarded, can produce no energy or momentum change in the emitter.
The emission of these time-symmetric electromagnetic waves therefore raises some immediate problems in its correspondence with observation, for the emitter experiences neither recoil (i.e., momentum transfer) nor energy loss in the act of emission. However, if absorption of the emitted retarded wave occurs sometime later, the correspondence with observation is restored. The observed recoils during emission and absorption occur because the respective electrons move in the electromagnetic fields of the waves, advanced and retarded, respectively, sent to them by the other electron, as demonstrated by Wheeler and Feynman[1].
As mentioned previously, the process described above can be thought of as the emitter sending out a probe wave in various allowed directions, seeking a transaction. An absorber, responding to one of these probe waves, sends a verifying wave back to the emitter verifying the transaction and arranging for the transfer of energy and momentum. This and the previous point are discussed more fully in AT1.
Of course, these transactions must be time symmetric and therefore need not take place in the ``emitter-absorber'' time sequence described above. If there were only time-symmetric constraints on the system, the absorption could have just as well have involved the advanced wave and have occurred before the emission, giving an ``absorber-emitter'' time sequence. It is the purpose of the present paper to explain why emitter-absorber events are observed in nature but not absorber-emitter events by explaining the origin of the time-asymmetric constraints on the system. We will return to this point later.