Emission theory (relativity)

Emission theory, also known as ballistic theory, was a historical alternative theory to special relativity that attempted to explain the observed speed of light's independence from the velocity of its source. It posited that light is emitted from a source with a velocity equal to c relative to the source's velocity, similar to how projectiles are emitted from a gun.

Several versions of emission theory were proposed, primarily in the late 19th and early 20th centuries, as attempts to reconcile Newtonian physics with the burgeoning evidence contradicting its predictions regarding electromagnetism and the speed of light. A key motivation was the desire to preserve the idea of an absolute frame of reference, which was undermined by the null result of the Michelson-Morley experiment.

One of the most prominent proponents of emission theory was Walther Ritz. Ritz’s emission theory predicted that the speed of light would be c + v when emitted from a moving source with velocity v.

However, emission theories were ultimately rejected for several reasons:

• Observational Contradictions: Several experiments provided strong evidence against emission theories. Observations of binary star systems, for instance, showed that if light's speed depended on the velocity of the star emitting it, we would observe distortions in their orbits and even multiple images, which are not seen. Other experiments involving the behavior of high-energy particles also contradicted the predictions of emission theories.
• Internal Inconsistency: Some emission theories encountered internal logical inconsistencies, particularly when combined with other well-established physical laws.
• Compatibility with Maxwell's Equations: Emission theories struggled to reconcile their postulates with Maxwell's equations of electromagnetism, which form the foundation of classical electromagnetism and predicted a constant speed of light independent of the source's motion.

The experimental successes of special relativity, including its accurate predictions for time dilation and length contraction, along with the absence of any empirical support for emission theories, led to the widespread acceptance of special relativity as the correct description of spacetime and electromagnetism. While emission theories played a role in exploring alternative explanations for observed phenomena in the early development of relativity, they are now considered obsolete and have been superseded by the more comprehensive and experimentally validated theory of special relativity.