Evolution of the Approch to Length Contraction

Fitzgerald (1889) — Ad hoc hypothesis

Context: Michelson-Morley experiment (1887) failed to detect the motion of the Earth relative to the ether.
Idea: To explain the negative result, George Fitzgerald proposed that bodies contract in the direction of motion through the ether.
Nature: Ad hoc hypothesis, without a clear mechanism, introduced solely to save the ether.

Lorentz (1892–1904) — Electron theory

Context: Hendrik Lorentz developed an electrodynamics of the ether.
Idea: A body moving relative to the ether undergoes a real contraction of its dimensions in the direction of motion, due to the electromagnetic forces binding matter.
Formula: 

Einstein (1905) — Special relativity

Context: Einstein eliminated the ether and introduced two postulates: invariance of the speed of light and relativity of physical laws.
Idea: Length contraction is no longer a physical effect on bodies, but a geometric consequence of coordinate transformations between inertial frames.
Nature: Apparent contraction, relative to the observer.
Key: It does not exist in itself: a body is never contracted in its own frame of reference.

Minkowski (1908) — Geometrization

Context: Hermann Minkowski formalized special relativity in terms of four-dimensional spacetime.
Idea: Length contraction becomes a geometric projection of a worldline onto a spacetime hyperplane.
Nature: Purely geometric, integrated into the structure of spacetime.

General relativity (1915 and after)

Context: Einstein generalized relativity to gravitation.
Idea: Contraction of distances depends on the gravitational field and the local metric.
Example: Time dilation and radial contraction near a massive body.
Nature: Dependent on the curved geometry of spacetime, measured by the metric.

Modern experimental approaches

- Optical and interferometric tests (improved Michelson-Morley, Kennedy-Thorndike experiments, cryogenic resonators): confirm isotropy of the speed of light at extremely high precision.
- Shapiro effect (1964): delay of light in a gravitational field, interpreted as an “effective contraction” of the light path.
- GPS and applied physics: relativistic corrections (time and distances) are indispensable.

Alternative interpretations

- Revived ether theories: some models continue to interpret contraction as real.
- Relational approaches (e.g., Mach, Rovelli): emphasize the relative character of spatiotemporal quantities.
- Metrological interpretations (Lachièze-Rey, Eddington): contraction is seen as a consequence of measuring standards (clocks, rods) being affected at the same time as the observed phenomena.

Relational approach to space and motion — working hypothesis

In this framework, length contraction is neither purely apparent (Einstein) nor purely real through the ether (Lorentz). It results from a relational mechanism:

- Directional contraction acts on the proper mass of the body.
- This modification of proper mass then induces an omnidirectional contraction that tightens the structure.
- Contraction thus becomes a mediation between the particular (the body) and the general (space).

This relational approach provides a causal mechanism that previous interpretations lacked, and opens the way to a broader vision of physics, potentially leading us to the idea of a universal driving principle. After all, Étienne Klein himself questions the existence of a driving principle of time: here, the proposal is to extend this question to the understanding of space and motion.

Experimental importance and distinction from Lorentz and Einstein

The relational approach presented in point 8 is distinct from both Einstein’s and Lorentz’s.

- For Einstein, contraction is a coordinate effect: purely apparent and relative to the observer. An interferometer onboard a shuttle cannot then detect any difference, because spacetime geometry exactly compensates all possible effects.

- For Lorentz, contraction was an ad hoc hypothesis introduced to account for the negative result of the Michelson-Morley experiment (1887). It was conceived as a real contraction of matter moving within a privileged reference frame (the ether), but it was not based on the idea that the speed of light might depend on spatial configuration. In this sense, it radically differs from a relational approach.

- In the relational approach, contraction is neither a simple geometric illusion nor a mechanical rearrangement in the ether. It is the result of a causal process: directional contraction acts on the proper mass of the body, which then induces an omnidirectional contraction. Proper mass here plays a mediating role between the body and the surrounding spatial configuration.

Consequence: An interferometer onboard a spacecraft could reveal an experimental residue, since the compensation is not perfect. Point 8 thus becomes the key to an experimental test: if the speed of light truly depends on spatial configuration, such a device would make it possible to detect its effects.

The text and equation on this page were written with the assistance of ChatGPT, based on my analysis.

Sincerely,
Philippe de Bellescize