I have found a book from my father epoch of undergraduate student in Physics. It was apperently Physics 101, and was written with a typewriter and the formulae with pen. I have gone straight to Special Relativity section and, to my dismay, and even if all formulae(adsbygoogle = window.adsbygoogle || []).push({});

were indeed correct, the interpretation was not. It said that the new approach by Einstein, based on measurements, relegated the really physical nature of lenght contraction by Fitzgerald and Lorentz to the oblivion, and thus the effect could not and never could be measured.

We are talking of 1950s as late. Since many years ago, I have seen similar confusions when explaining or trying to understand special relativity. It seems that the principle of constancy of the speed of light, which gives foot to the length contraction and time dilation in a very direct way also confuses some neophites when approaching for the first time the subject. Here I think there is a confussion among the coordinates of distant events as measured by different observers, and the time, finite which takes a signal to communicate these events to the observer placed in the origin of spatial coordinates coordinates. Things get event worst and feed even more this misunderstanding when leading with the electromagnetic Doppler effect. As it is composed of 2 main parts, namely one emminently classical and the other purely relativistic (transversal

Doppler effect being the maximum expression of the latter), the student can interpret that the effects of time dilation and length contraction are as physical (or rather unphysical) as the Doppler effect, and that all is a pure problem with the measurements in two relatively moving systems. Things even get worst, if possible, when pure relativists insist on interpreting spacetime intervals and thus not giving any credence to a time measured by a clock.... or someetimes giving too much credence and putting proper time in a priviledged status above coordinated times. What do you think of all of this mess? What is the best approach to teach relativity?

Personally I would insist on time dilation and lenght,contraction coming from the postulates and being completely real, physical. Later I would explain simoultaneity and derive Lorentz transf., and from there the relativity of these effects and why one observer is convincing of her measurements and why she believes the measurements of her boyfriend are those which are wrong. Later would introduce Doppler and finally examples of calculating time intervals of light emitted and received, so it can be seen tht the time used forr a ray to go from point A to point B is outside the different rates of clocks measuring these events.

man

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# On the usual confusion among what it is and what is measure

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