Einstein’s Postulates

Introduction to Special Relativity

In this course, Professor David Griffiths (Reed College) gives an introduction to Einstein’s Theory of Special Relativity. In the first mini-lecture, we discuss Einstein’s two postulates of Special Relativity — the principle of relativity and the universal speed of light — and compare Galileo’s velocity addition rule with Einstein’s velocity addition rule. In the second mini-lecture, we use a thought experiment to understand the relativity of simultaneity, where we see that observers in different reference frames do not agree on what is considered simultaneous. In the third and fourth mini-lectures, we use two more thought experiments to understand length contraction and time dilation, respectively. Finally, in the fifth mini-lecture, we introduce the Lorentz transformations, a set of equations that allows us to translate the coordinates of an event as assigned by an observer in one reference frame into the coordinates assigned to that same event by an observer in a different reference frame.

Einstein’s Postulates

In this mini-lecture, we introduce Einstein’s two postulates of Special Relativity: the principle of relativity and the universal speed of light. In particular, we consider: (i) a thought experiment for the principle of relativity to help us understand that it only applies to systems moving at constant velocity; (ii) the universal speed of light, which is 3×10⁸ m/s; (ii) Galileo’s velocity addition rule and how it contradicts Einstein’s postulate of the universality of the speed of light; (iii) Einstein’s velocity addition rule, which corrects that of Galileo; and (iv) the reasoning behind why Galileo’s velocity addition rule is a very good estimate for velocities at everyday speeds.