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# 1. Einstein’s Postulates

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#### About this Lecture

Lecture

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 3×10^{8} 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.

Course

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.

Lecturer

David Griffiths is an Emeritus Professor from Reed College in Portland, Oregon in the United States. He is known for his highly regarded undergraduate physics textbooks, including *Introduction to Elementary Particles* (published in 1987, second edition published 2008), *Introduction to Quantum Mechanics* (published in 1995, third edition published 2018), and *Introduction to Electrodynamics* (published in 1981, fourth edition published in 2012). He was also the 1997 recipient of the Robert. A Millikan award (renamed the Lillian McDermott Medal in 2021), an American Association of Physics Teachers (AAPT) award that recognises those who have made notable and intellectually creative contributions to the teaching of physics.

#### Cite this Lecture

**APA style**

Griffiths, D.
(2022, January 13).
*Introduction to Special Relativity - Einstein’s Postulates* [Video]. MASSOLIT. https://massolit.io/courses/introduction-to-special-relativity/einstein-s-postulates-e0caa429-e84d-40e7-a428-888323a733b6

**MLA style**

Griffiths, David.
"Introduction to Special Relativity – Einstein’s Postulates." *MASSOLIT*, uploaded by MASSOLIT,
13 Jan 2022,
https://massolit.io/courses/introduction-to-special-relativity/einstein-s-postulates-e0caa429-e84d-40e7-a428-888323a733b6