Einstein's principle of relativity


The main difference between Newtonian mechanics and the mechanics based on Einstein's principle of relativity is that in the latter the velocity of light, c, is the same in all inertial reference systems. Therefore, when comparing the velocity of an object measured in two reference systems 1 and 2 that are moving relative to each other, the Newtonian rule of addition of velocities, v__2_ = v__1_+v__R_, where v__R_ is the velocity of one system with respect to the other one, is not valid; if it were, the speed v__1_ and v__2_ of light in the systems 1 and 2 would not be the same. This introduces surprising conceptual consequences, and algebraic complications in the formulas relating the values of measurements, in the systems 1 and 2, of time, space and everything else that is related to that.


This post is thus about Einstein's principle of relativity and the consequences of the velocity of light being the same in all inertial reference systems. Although the topic is often considered advanced, the concepts, as shocking as they are, are easy to understand, and the algebra is still tractable in simple terms. The presentation, following Landau & Lifshitz [1], Chapter 1, is at a basic level, with no prerequisite expertise required, and illustrates well how to handle the basic algebraic aspects of special relativity using computer algebra.


Finally, it seems to me not useful to just present the algebra when the concepts behind Einstein's theory are straightforward and surprising. For that reason, the short sections 1 and 2 are all about these concepts, and the algebra only starts in section 3, with the Lorentz transformations (which was recently the topic of a Mapleprimes post at a more advanced level ). To reproduce the computations shown in this worksheet, please install the Maplesoft Physics Updates v.1314 or any subsequent version.


Download Einstens_principle_of_relativity.mw

Edgardo S. Cheb-Terrab
Physics, Differential Equations and Mathematical Functions, Maplesoft


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