(also called "emitter theory") was a competing theory for the special theory of relativity
, explaining the results of the Michelson-Morley experiment
. Emission theories obey the principle of relativity
by having no preferred frame for light
transmission, but say that light is emitted at speed "c"
relative to its source instead of applying the invariance postulate. Thus, emitter theory combines electrodynamics
with a simple Newtonian theory.
The name most often associated with emission theory is Isaac Newton. In his Corpuscular theory Newton visualised light "corpuscles" being thrown off from hot bodies at a nominal speed of c with respect to the emitting object, and obeying the usual laws of Newtonian mechanics (although he did also assign wave properties to light).
Albert Einstein is supposed to have worked on his own emission theory before abandoning it in favour of his special theory of relativity.
Special relativity's geometrical simplicity was persuasive, but a convincing general disproof of emission theory proposed was still difficult to find, and some considered the main competitor to Einstein's special theory to be the emission theory proposed by Walter Ritz.
Many years later R.S. Shankland reports Einstein as saying that Ritz' theory had been "very bad" in places and that he himself had eventually discarded emission theory because he could think of no form of differential equations that described it, since it leads to the waves of light becoming "all mixed up".
In 1913 Willem de Sitter wrote that the expected consequences of emission theory on the appearance of double stars, an extreme scrambling of their lightsignals, did not happen. This was widely accepted as definitive proof that emission theory was not viable.
Problems with emission theory
The simplest form of emission theory says that radiating objects throw off light with a speed of "c" relative to their own state of motion, and (unless we have reason to believe that the light changes speed in flight), we then expect light to be moving towards us with a speed that is offset by the speed of the distant emitter (c ± v) ).
This description generates three "odd" results:
- If a radiant star moves across our field of vision, light given off by differently-moving atoms in its atmosphere should take different amounts of time to reach us. Since the retreating atoms would have a "red" Doppler shift, and the approaching ones a "blue" Doppler shift, the passing star might be expected to appear as a "rainbow streak".
- Similarly, if a radiant star is eclipsed, one might expect the eclipsing shadow to appear to intercept different colours of Doppler-shifted light in sequence - the eclipse might appear to have coloured fringes.
- For the case of a double-star system seen edge-on, light from the approaching star might be expected to travel faster than light from its receding companion, and overtake it. If the distance was great enough for an approaching star's "fast" signal to catch up with and overtake the "slow" light that it had emitted earlier when it was receding, then the image of the star system should appear completely scrambled.
De Sitter argued that none of the star systems he had studied showed the extreme optical effect behaviour in , and this was considered the death knell for Ritzian theory and emission theory in general.
Newton appears to have enquired whether or not moons of Jupiter showed coloured fringes at eclipse, suggesting that he may have already been aware of these arguments and problems.
Variations on a theme
- The idea that perhaps the speed of light only has an effective value of cEMITTER while it is local to the emitter, as a "light-dragging" or "proximity" effect has been considered in detail. This can be expressed in terms of the "extinction effect", and it arguably undermines the cogency of deSitter type evidence based on optical stars. However, similar observations have been made more recently in the x-ray spectrum, which have a long enough extinction distance that it should not affect the results. The observations confirm that the speed of light is independent of the speed of the source. In addition, terrestrial experiments have been performed, over very short distances, where no "light dragging" or extinction effects could come into play, and again the results confirm that light speed is independent of the speed of the source, conclusively ruling out emission theories.
Furthermore, quantum electrodynamics places the propagation of light in an entirely different, but still relativistic, context, which is completely incompatible with any theory that postulates a speed of light that is affected by the speed of the source.
see also: de Sitter double star experiment
- J. G. Fox, "Evidence Against Emission Theories," Am. J. Phys. 33 1, (1965).