Lorentz ether theory

In physics, Lorentz ether theory (or "LET") proposed that light was transmitted through a light medium in which the motion of objects did not cause dragging effects, but did cause objects to contract in their direction of motion (Fitzgerald-Lorentz contraction). This was used as an explanation of why the Michelson-Morley experiment had failed to detect an aether drag effect.

Lorentz' exposition of what became known as Lorentzian electrodynamics (trans: "Electromagnetic Phenomena in a System Moving with any Velocity Smaller than that of Light") was published in 1904.

LET and special relativity

The following year, 1905, Einstein published his paper on what we now call special relativity (tr. "On the Electrodynamics of Moving Bodies" [1]. Special relativity adopted the same basic relationships as LET, but Einstein derived them without making reference to a physical light medium, using the more abstract (and seemingly incompatible) postulates:

1. that light could be said to propagate at an absolute speed of c in the observer's frame ("principle of the constancy of light"), and
2. that the principle of relativity could be applied to the behaviour of light.

Taken together, these two postulates seemed to offer only one mathematical solution: the mathematics of special relativity.

Since special relativity derives the "useful" parts of LET without postulating the presence of a real physical aether with an absolute state of motion, the special theory is usually conseidered to have superceded LET, and made Lorentz' concept of the aether redundant.

Explanations of special relativity do still often adopt the language of Lorentz, explaining that "moving" objects should contract in length and age more slowly according to the theory ("length contraction", "time dilation"). However, some people consider such explanations to be archaic and overly interpretative, and prefer to distance special relativity from LET by describing Einstein's special theory in terms of the relationships of the Minkwski metric.

Further reading:

various, The principle of relativity (Dover, NY, 1952) [2] - reprints a collection of historically-interesting papers on relaitvity theory, including both Lorentz' 1904 paper and Einstein's 1905 "electrodynamics" paper