There were a number of different ether theories proposed during the nineteenth century, associated with names such as Thomas Young, Augustin Fresnel, and G. G. Stokes, but the concept of a stationary ether with the Earth moving through it was the easiest to reconcile with the phenomenon of aberration.

Some of the experiments that were performed are described in Whittaker (1958).

moving with speed v, a local time, t', had to be used, related to the general time by t' = t — vx¡c1. In 1899, Lorentz wrote down the transformations that were later (in 1904) given his name by Poincaré:36

x ' = y (x — vt), y' = y, z' = z, t' = y (t — vx ¡c1), Y = (1 — v1/c1)—1/1.

It follows that x = y (x1 + vt') and, hence, that if a body has length l = x1 — x1 in an inertial frame (at all times), then the length in the new frame (measured at time t') is x1 — x1 = y—1(x1 — xi). Now, y—1 = (1 — v1/c1)1/1 = 1 — v1 ¡1c1 to second order in v/c, which agrees with Eqn (11.3). Exactly the same set of equations, together with a derivation of the FitzGerald-Lorentz contraction,

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