Absorber Theory of Radiation

[lilphil1989]

When space contracts, time will not go backwards. This would violate the second law of thermodynamics. The universe increases in entropy.

Time would start to slow down if the universe contracts, correct?

I believe that is correct.

What make you guys believe that?

This is a theory of radiation put forward by Wheeler and Feynman in 1945 which says that radiation can also flow backward into space in such a manner as to arrive back into the source (or the emitter) if the expansion of the universe begins to reverse.

Not quite, actually absorber theory says nothing about the expansion of the universe. Advanced and retarded potentials are general objects in physics, not specifically related to absorber theory.

Absorber theory is a way to try to re-cast radiation theory in a way that's invariant under time-reversal. Usually when you solve the Maxwell equations for some accelerating charge, you get two solutions, one which is a function of (x - ct) and one of (x + ct). Usually, the latter is disregarded as it is considered non-causal. Doing that, however, seems to breaks the time-reversal symmetry, so Feynman and Wheeler were looking for a way to build a theory in which you don't have to arbitrarily discard a solution.

It works, and you get sensible solutions, if you impose the condition that the retarded and advanced fields are equal at every spacetime point. It isn't necessary though, from the viewpoint of calculating things you get the same answers just using retarded fields. It does make the theory look nicer because you don't have to give privilege a priori to the retarded fields.


The real influence of this idea is that it was working on this that prompted Feynman to come up with the formulation of quantum mechanics using path integrals and starting from a Lagrangian (kinetic - potential energy) instead of a Hamiltonian (kinetic + potential energy), which is the way that almost all work in quantum field theory for particle physics is done these days.