QED Prerequisites Principle of Least Action: Introduction

In this lesson we put the Principle of Least Action at the center of the study of mechanics. This will set the stage for our eventual flow into field theory. In the next lesson we will study the most essential Lagrangians for field theory: the elementary relativistic free particle, and the charged particle in an electromagnetic field.

The software I usually use to produce the lectures is:

[steveboigon]

I love when you review concepts like this. The reason is simple...you explain it in a manner that is far superior to professors I have had! Great lecture!!

[billbulgari]

Hello. Very nice lecture. I really like the rigor in your videos. Could you add this video to your QED Prerequisites playlist please? Also where is your "QED Prerequisites The 4-Potential and Field Tensor via Landau and Lifshitz" video?

[giampierochiavarotti]

Please could you add subtitles? For me it's very hard to folloow just audio. Thanks a lot for your exceptional lessons

[inGuy]

The principle of least action is how I live my life. Now where is my remote?

[JuliusBrainz]

I believe, at some point in history, the principle of least action has been weakened to the principle of stationary action, at least for two reasons. Firstly, sometimes zig-zag trajectories can minimize the action better than any smooth curve. Secondly, at some point, one can encounter a conjugate point past which the same curve does not minimize the action anymore. Already for the harmonic oscillator after t = pi/omega the solution to the obtained differential equation does not necessarily minimize the functional, and with infinite degrees of freedom, the actual minimization may actually never occur.

[jagdeep]

why actual path in configuration space has extremum action??