Length Contraction
Length contraction is the prediction that an object in motion is measured to be shorter along its direction of travel than the same object at rest, by the factor √(1 − v²/c²). It is the spatial counterpart of time dilation and follows directly from the Lorentz transformation.
The standard account
The idea began as a rescue. To reconcile the null result of the 1887 Michelson-Morley experiment with motion through a stationary aether, George FitzGerald (1889) and independently Lorentz (1892) proposed that a body moving through the aether is physically compressed along its line of motion by exactly the amount needed to cancel the expected fringe shift. In Lorentz's electron theory the contraction was a real, dynamical deformation caused by the alteration of intermolecular forces by motion.
Einstein's 1905 treatment kept the formula and changed its meaning. Contraction became a kinematic consequence of how simultaneity is defined: measuring the length of a moving rod requires marking both ends at the same time, and observers in relative motion disagree about what "the same time" means. On this reading nothing happens to the rod. The effect is reciprocal — each observer measures the other's rod as shortened — and no observer travelling with the rod finds anything amiss. At everyday speeds it is negligible; at 0.87c the factor is about one half.
The important and often-glossed fact is that length contraction has never been measured directly. Time dilation has abundant direct confirmation (muon lifetimes, the Hafele–Keating and later clock experiments, GPS corrections). Contraction is supported indirectly: through the consistency of the Lorentz-invariant framework, through Michelson-Morley-type null results, and through the transverse electromagnetic field of a relativistic charged bunch in accelerators. No experiment has laid a ruler alongside a fast rod. This is acknowledged in the mainstream literature, not only by critics.
On this wiki
That evidential gap is the focus of most of the work catalogued here.
Jack Redver Harris's The State of Experimental Evidence for Length Contraction (2002) states the position plainly: both the classical Lorentz–Larmor–FitzGerald–Poincaré theories and Einstein's special theory build a physical contraction into their worldview, yet no direct measurement has ever been made. Harris analyses the one experiment that tried — Sherwin's — and argues that its underlying assumptions leave it equivocal rather than confirmatory. Hector A Munera reaches a blunter verdict for the founding period in The Evidence for Length Contraction at the Turn of the 20th Century: Non-existent (2006).
Curtis E Renshaw proposed an actual test. The Space Interferometry Mission as a Direct Test of Relativistic Length Contraction (2000) points out that the Earth's velocity against a field of background stars changes by 30 km/s every three months, which on special relativity implies a change in the apparent angular spacing between stars of up to 18 microarcseconds per degree of separation — well above the 1 microarcsecond per degree resolution NASA's Space Interferometry Mission was designed to reach. SIM was cancelled before launch, so the test remains undone.
Several researchers accept the formula but reject the physical reading. Boon Leong Lan's A Moving Rod Does Not Shrink gives a short argument that the orthodox interpretation of the contraction equation does not follow from it. Tom Van Flandern takes the same line in Lorentz Contraction (Apeiron, 2003): contraction "is not a change in the physical length of rods or meter sticks" but an illusion introduced by the lack of remote simultaneity — and in his own Lorentzian relativity, with the light-carrying medium entrained, the need for it disappears altogether. Paul Marmet's A Realistic Interpretation of Length Contraction (1996) argues a similar realist reinterpretation.
Others treat contraction as a symptom of a mis-set problem. Steven Bryant (The Twin Paradox: Why it is Required by Relativity) holds that time dilation, length contraction and the Twin Paradox all issue from one error: applying length-based equations to wavelength-based observations. Curtis E Renshaw's Galilean reconstruction (The Restoration of Space and Time from a Galilean Approach to Relativity's Second Postulate) eliminates contraction and dilation together while, he argues, reproducing every experimental result. Cameron Y Rebigsol documents claimed internal inconsistencies in Relativity's Length Measurement Inconsistency (Proceedings of the NPA, 2009).
See Category:Relativity for the wider body of work.