Time is suspect!

Abstract ‘That it is possible to find the form of the Lorentz transformations without requiring the speed of light to be the same in every inertial system is rediscovered about every thirty years and, rightly, forgotten.’ This proposition alongside the doctoral dissertation of Hendrik Smid in 1986 provides food for thought: Is the light postulate of special relativity, which gives the speed of light c in vacuum its status of maximum and unchanging speed, indeed not necessary? What kind of physical world are you then left with? What could be the value of discussing this in class? In this paper we show how the Lorentz transformation, which describes the relationship between two coordinate systems moving relative to each other at a constant speed, can indeed be derived by replacing the light postulate by another assumption, namely the requirement of closure under composition of transformations that relate inertial frames of reference (i.e. two successive coordinate transformations of certain type can be written as one transformation of the same type). Although well known in specialised literature of physics research, the remarkable insight that c is somehow buried in nature is hardly mentioned in pre-university and bachelor’s physics courses and textbooks. Yet, the presented comprehensible approach may support the understanding and plausibility of the abstract subject of special relativity. Mathematics may go too far for an average pre-university student, but this subject would certainly not be out of place in an undergraduate physics course or a physics teacher course. Although many teachers do not know that the light postulate of special relativity can be replaced by another assumption or may not appreciate such replacement as part of their teaching of the topic, it offers in our opinion an opportunity to inform students that a critical look at postulates and an investigation of alternatives is an essential part of the nature of science.