The idea of contraction was first suggested by a relatively unknown Irish physicist, George Francis FitzGerald. FitzGerald expressed his idea in a short communication to the American journal Science in 1891, ten years after Michelson’s first reported result, and he also suggested a reason.
I would suggest that almost the only hypothesis that can reconcile this [conflict] is that the length of material bodies changes, according as they are moving through the ether or across it, by an amount depending on the square of the ratio of their velocities to that of light. We know that electric forces are affected by the motion of the electrified bodies relative to the ether, and it seems a not improbable supposition that the molecular forces are affected by the motion, and that the size of a body alters consequently.
While FitzGerald referred to the ether, which was believed to be the carrier for light waves at the time, the reasoning holds with or without the ether. Somewhat later his rather timid suggestion that molecular forces are affected by motion was repeated and refined by the most famous physicist of the time.
While FitzGerald was little known outside Ireland, the Dutch scientist Hendrik Lorentz was recognized as the greatest physicist since Maxwell. In 1902 he and Pieter Zeeman received the second Nobel Prize ever awarded for discovering the “Zeeman effect” that led to the discovery of electron spin (Chapter 6). Einstein called Lorentz “the most well-rounded and harmonious personality he had met in his entire life” (P1982, p. 169). Upon Lorentz’s death, Europe’s greatest physicists attended his funeral and three minutes of silence were observed throughout Holland.
Lorentz had not seen FitzGerald’s paper, but he too realized that Michelson’s strange result would make sense if the apparatus contracted along the direction of motion. However he went further than FitzGerald; he did the calculation (not an easy one) using Maxwell’s equations. When he found that the theoretical contraction exactly compensated for the extra travel distance, this was surely one of the great “Eureka” moments in physics, comparable to those of Newton and Einstein.
When Lorentz learned of FitzGerald’s work, he wrote to him to be sure he was not usurping credit,… and thereafter Lorentz was careful to acknowledge FitzGerald’s priority. The contraction is sometimes called the FitzGerald contraction, some¬times the Lorentz contraction, and sometimes the FitzGerald-Lorentz (F-L) contraction. Fig. A-3 shows a modern version of Lorentz’s calcu¬lation done by John Bell with the aid of a computer.
Misconception #1. Some writers claim that the F-L contraction was an ad hoc explanation offered without any theoretical basis. In fact it was based on a deep understanding of how fields behave when in motion and how this behavior affects the molecular configurations.
Surprising as this hypothesis may appear at first sight, yet we shall have to admit that it is by no means far-fetched as soon as we assume that molecular forces are also transmitted through the ether, like the electric and magnetic forces of which we are able at the present time to make this assertion definitely. If they are so transmitted, the translation will very probably affect this action between two molecules or atoms in a manner resembling the attraction or repulsion between charged particles. Now, since the form and dimensions of a solid body are ultimately conditioned by the intensity of molecular actions, there cannot fail to be a change of dimensions as well. – H. A. Lorentz (E1923, p. 5-6)
Intuitive explanation. While I hope you can accept, as did FitzGerald and Lorentz, that length contraction happens because the field equations require it, it would be nice to have some intuitive insight into the phenomenon. We must recognize that even if the molecular configuration of an object appears to be static, the component fields are always interacting with each other. The EM field interacts with the matter fields and vice versa, the strong field interacts with the nucleon fields, etc. These interactions are what holds the object together. Now if the object is moving very fast, this communication among fields will become more difficult because the fields, on the average, will have to interact through greater distances. Thus the object in motion must somehow adjust itself so that the same interaction among fields can occur. How can it do this? The only way is by reducing the distance the component fields must travel. Since the spacing between atoms and molecules, and hence the dimensions of an object, are deter¬mined by the nature and configuration of the force fields that bind them together, the dimensions of an object must therefore be affected by motion.