# Physics Colloquium

Date: Thursday, March 5, 2020

Speaker: **Markus ****Luty**, UC Davis

Title: **Classical Radiation Reaction**

Abstract: Every physics undergraduate knows that when electrically charged particles accelerate, they radiate away energy and momentum. This means that they must experience a “radiation reaction” force in addition to the Lorentz force. Over 100 years ago, Lorentz and Abraham showed that this force is proportional to the derivative of the acceleration. However, this makes a number of paradoxical predictions, such as runaway solutions and the absence of radiation from charges undergoing constant acceleration. This has led to over a century of conflicting claims in the literature and in textbooks. This talk will give an account of radiation reaction at the advanced undergraduate level that aims to be both understandable and correct. I will give an elementary derivation of the Abraham-Lorentz force using energy and momentum conservation. I then show that in a theory of classical electrons with radius smaller than the classical electron radius, the total energy is unbounded from below, and the theory is catastrophically unstable. If instead the classical approximation breaks down at a radius larger than the classical radius, the structure of the electron cuts off the instability. Although the electron is an elementary particle with no structure, quantum effects cause the classical theory to break down, saving us from this instability. I will close with some comments on radiation reaction for constant acceleration

Host: Spencer Chang