Wheeler–Feynman absorber theory

Lore & Background

Wheeler and Feynman began by observing that classical electromagnetic field theory was designed before the discovery of electrons, treating charge as a continuous substance. They reconsidered the fundamental problem of a collection of point charges, taking up a field-free action at a distance theory developed separately by Karl Schwarzschild, Hugo Tetrode, and Adriaan Fokker. Unlike instantaneous action at a distance theories of the early 1800s, these 'direct interaction' theories are based on interaction propagation at the speed of light. They differ from classical field theory in three ways: no independent field is postulated, point charges do not act upon themselves, and the equations are time symmetric. After that, Einstein told them of previous papers of Tetrode and Ritz. The Tetrode-Fokker work left unsolved two major problems: the conflict between equal action-reaction and causality, and the explanation of radiation reaction force without self-action. To overcome these issues, Wheeler and Feynman postulated the 'universe' of all other electrons as an absorber of radiation. Rather than considering an unphysical isolated point charge, they modeled all charges in the universe with a uniform absorber in a shell around a charge. As the charge moves relative to the absorber, it radiates into the absorber which 'pushes back', causing radiation resistance. They obtained their key result by considering all charged particles as emitters generating time-reversal symmetric waves.

Reader's Guide

The Wheeler–Feynman absorber theory is significant for its radical departure from classical field theory, proposing that electromagnetic interactions are direct, time-symmetric actions between particles without an independent field. This approach immediately removes the problem of electron self-energy giving an infinity in the energy of an electromagnetic field, as elementary particles are not self-interacting. The theory is invariant under time-reversal transformation, consistent with the lack of any physical basis for microscopic time-reversal symmetry breaking. Its legacy includes influencing later work on action-at-a-distance formulations and providing a conceptual framework for understanding radiation reaction through the absorber mechanism. The theory also highlights the tension between causality and time symmetry in electrodynamics, a problem that remains relevant in modern physics.

Did You Know?

• The theory postulates no independent electromagnetic field; it is encapsulated entirely by a Lorentz-invariant action of particle trajectories.
• The theory is invariant under time-reversal transformation, consistent with the lack of any physical basis for microscopic time-reversal symmetry breaking.
• Elementary particles are not self-interacting in this theory, which removes the problem of electron self-energy giving an infinity in the energy of an electromagnetic field.