Emission theory (also called emitter theory or ballistic theory of light) 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 and mechanics 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).
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.
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:
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.
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