User:FirstSamson/Sandbox

Description
Time dilation and length contraction are two well-known relativistic effects. Though they are apparently well understood, there is still some confusion swirling around them.

1. Length contraction (Lc) is an effect caused by observing a length L at rest with respect to an inertial reference frame (IRF) k from another inertial reference frame K in motion with respect to frame k.

The effect is "apparent" (not directly observable and measurable) since it requires two different IRF's for manifestation. Attempts to measure the effect directly from within the reference frame k have produced a null result. See the Trouton-Rankine experiment for the detailed description.

2. Time dilation (Td) is an effect caused by observing a time interval delta(t) measured on a clock (or time measuring device) at rest with respect to an IRF k from another IRF K in motion with respect to frame k.

In stark contrast with Lc, Td is a directly measurable effect: at Einstein's suggestion Ives and Stilwell (see Transverse Doppler effect ) staged an experiment meant to measure time dilation via an effect predicted by Einstein in his 1905 seminal paper. The effect in cause is the Transverse Doppler effect (TDE) and it is an effect charcteristic to special relativity only. Ives and Stilwell could observe directly the TDE by observing the displacement of the emission spectrum coming from a high speed fascicle of hydrogen ions.

Time dilation and length contraction occur together because they are caused by the pairing of IRF's k and K, as in the Kennedy-Thorndike experiment but this is not a necessary rule for all cases. For example, the Michelson-Morley experiment is explained by the apparent length contraction only. (the time dilation, though present is not relevant in explaining the experiment).

Another excellent reference is the page on Lorentz ether theory (LET) where a clear distinction is made between LET and special relativity (SR).

We can add to the arguments listed on the (LET) page that SR regards Lc as "apparent" and Td as measurable while LET regarded (at least until 1908, the date of the Trouton-Rankine experiment) Lc as measurable. The Trouton-Rankine experiment proved Lc to be "apparent" only.

Until 1915, LET regarded Td is "apparent", a view that he later corrected (see below). Had the Ives-Stilwell experiment been executed earlier, it is possible that Lorentz would have changed his mind earlier. The Ives-Stilwell experiment proved Td to be measurable.

"If I had to write the last chapter now, I should certainly have given a more prominent place to Einstein's theory of relativity (p 189) by which the theory of electromagnetic phenomena in moving systems gains a simplicity that I had not been able to attain. The chief cause of my failure was my clinging to the idea that the variable t only had to be considered as the true time and that my local t' must be regarded as no more than an auxiliary mathematical quantity." in "The Theory of Electrons," by H. A. Lorentz second edition (1915). Page 197 in the original 1915 edition, page 321 in Dover Publications 1952 reprint.