The central problem of physics at the turn to the 20th century concerned the emission and absorption of radiation as interaction between matter and light, formulated as the problem of blackbody radiation.
This is analogous to the interaction between a loudspeaker as matter and sound waves in the surrounding air.
There are two approaches to this problem, depending on the nature of light as streams of light particles or as light waves, one trivial and one non-trivial. One may compare with sound as “streams of sound particles” or as sound waves.
In the trivial theory light is viewed as particles with a trivial interaction between different particles, between matter as material particles and light as immaterial particles named photons. The mathematical model is a trivial particle model with material particles absorbing and emitting photons.
In the non-trivial theory a distinction is kept between matter as a system of material resonators and light as electromagnetic waves. The mathematical model is non-trivial as a wave equation with small damping subject to forcing describing a non-trivial interaction between material resonators and radiation as a balance of forces.
An unfortunate result of the trivial model, which has become very popular because of its triviality, is that the term “radiation” is used with two different meanings, as
- electromagnetic waves
- transfer of heat energy between material bodies.
The confusion has led to the popular idea of two-way transfer of heat energy between two material bodies with transfer of heat energy both from warm to cold and from a cold to a warm as “backradiation”. This idea is a result of the confusion: Since electromagnetic waves can propagate spontaneously in all directions, also heat can spontaneously transfer in all directions, both from warm to cold and from cold to warm.
In particular, the trivial model has obscured the essential difference between spectrum and heat transfer.
Maintaining the confusion the debate on “backradiation” can continue for ever, while understanding the distinction between 1. and 2. will end the debate.