Physical Quantities in Different Reference System According to Relativity, A. Gamba, Am. J. Phys., 35(2), February 1967
The relativistic transformation of measurements from one observer S to another observer S' is valid, of course, only when both observers are measuring the same physical quantity. What is the same quantity is, however, not a trivial problem in relativity. As a matter of fact, two different definitions have have already been used in the literature, without realizing that they were not equivalent. Some of the pitfalls of such a confusion are discussed.