Einstein's view of phenomenological and dynamical treatment

Special relativity has always had more than one interpretation. FitzGerald's original 1889 paper first hypothesized the length contraction as a logical consequence of the Michelson-Morley experiment. That is how textbooks often introduce it today. But he also suggested an explanation in terms of motion affecting the electromagnetic forces holding the apparatus molecules together. This "dynamical" explanation is commonly badmouthed on metaphysical grounds, even tho it is consistent with all known mathematical and physical evidence. Today it is preferred by philosopher Harvey R. Brown and no one else.

Lorentz's 1892 and 1895 reasoning was very similar to FitzGerald's, but independent as FitzGerald's letter was published in AAAS Science, which was then an obscure journal to Europeans.

Einstein has said that his famous 1905 paper presented the constructive theory only because he could not get the dynamical theory to work. He defines distances in terms of rigid measuring rods (meter sticks), ignoring the fact that the rods are really just a bunch of molecules in an electromagnetic equilibrium that is upset by motion.

Marco Giovanelli addresses these issues in a new paper:

"But One Must not Legalize the Mentioned Sin". Phenomenological vs. Dynamical Treatment of Rods and Clocks in Einstein's Thought

The paper offers a historical overview of Einstein's oscillating attitude towards a "phenomenological" and "dynamical" treatment of rods and clocks in relativity theory. ...

Einstein’s waving attitudes towards the role, indispensable or provisional, of rods and clocks in both of his theories, has been usually cast in the well-known opposition between ‘constructive’ and ‘principle’ theories, which Einstein had explicitly introduced in 1919 (Einstein, 1919b), but addressed in several occasions starting at least from 1907 (see Howard, 2005, for more details). In Einstein’s original stance toward special relativity as a ‘principle theory’, the geometry of space-time appeared to be ‘defined’ through the behavior of ‘rods’ and ‘clocks’, whose contractions and dilations were postulated, without introducing a realistic microscopic model of their material constitution. However, when a suitable ‘constructive theory’ of matter would eventually be at hand, rods and clocks will be thought as rather complicated physical systems obeying fundamental dynamical laws. Symmetry properties of space-time will turn to be nothing but a codification of the symmetries of the laws governing matter. In particular ‘length contraction’ and ‘time dilation’ in special relativity should be described in the final analysis as a consequence of the dynamical laws that govern rods and clocks. ...

Einstein’s unease towards the ‘sin’ of treating rods and clocks as simple, unstructured entities rather than as complicated dynamical systems did not intend to address the question around which the contemporary debate is centered: to establish which is the ‘cart’ and which are the ‘horses’ between abstract geometrical structure space-time and the physical laws governing material spacetime devices. It was rather an attempt to give a balanced answer to a more general philosophical question, which he did not hesitate jokingly to compare Pilate’s question: ‘what is the truth?’. More humbly, it was the question whether a theory should describe its own means of verification or whether this description should lie outside its domain.

The preferred view today is that special relativity is a non-Euclidean geometry of spacetime, as discovered by Poincare and Minkowski, so this stuff is mainly of historical interest.