Emission theory (relativity)
Emission theory, also called emitter theory or ballistic theory of light, was a competing idea to Albert Einstein’s special relativity. It said that light is emitted with speed c relative to its source, not with the same speed in every frame. In this view, there is no preferred frame for light, but the speed of light can depend on how fast the source is moving.
Historically, Newton’s corpuscular theory imagined light as tiny particles shot from hot bodies at speed c relative to the emitter. If the source is moving, the light would arrive with a speed like c ± v. This is a simple Newtonian picture of how light might work.
Special relativity, introduced by Einstein, says the speed of light is the same in all inertial frames. This simple idea solved conflicts with electrodynamics and led to the familiar Lorentz transformations. By the early 1910s, relativity had become the mainstream view, and most scientists accepted it.
Emission theories tried to keep some version of relativity by allowing light’s speed to depend on the source, using a parameter that could range from 0 to 1. If the parameter is 0, you get the relativity result; if it’s greater than 0, light from moving sources would behave differently. Many experiments tested this idea.
A huge number of precise experiments—ranging from Earthbound interferometer tests to observations of stars, moving mirrors, and particles—found no evidence that light’s speed depends on the source’s motion. The results consistently show light travels at speed c regardless of how the source moves, setting very tight limits on any possible dependence.
Because of these results, emission theory is now considered incompatible with how light actually behaves. The evidence supports the view that the speed of light is independent of the source’s motion, as described by special relativity. Modern physics, including quantum electrodynamics, fits naturally with that view.
This page was last edited on 3 February 2026, at 13:27 (CET).