Some Subtle Concepts in Fundamental Physics

In this paper, we discuss some subtle concepts, such as coordinate, measurement, simultaneity, Lorentz-FitzGerald contraction, singularity in fundamental physics. The explanations of these concepts in textbooks are usually incomplete and lead to puzzles. Some long-standing paradoxes such as the Ehrenfest one are caused by misinterpretation of these concepts. The analysis shows these concepts all have simple and naive meanings, and can be well understood using suitable logical procedure. The discussion may shed light on some famous paradoxes, and provide some new insights into the structure and features of a promising unified field theory.