Fearful Symmetry

Author: A. Zee
Publisher:  New Science Library Edition, Princeton University Press 2016

This book is probably the best popular physics book I have ever read.  However, the author has gone way overboard with an unnecessary assumption, it was painful to read.  I will give 5 stars to science content of the book.  But immediately take away two stars for making that unnecessary assumption.

Written by a theoretical physicist, this book reveals how a modern theoretical physicist approaches the subject.  Zee begins with a William Blake poem.

Tyger! Tyger! burning bright

In the forests of the night,

What immortal hand or eye 

Could frame thy fearful symmetry?

The author refers to symmetries in nature by “Tyger” throughout the book.  Modern physics is based on something called “the action principle”.  When physicists dream of writing down the entire theory of physical universe on a cocktail napkin, they mean to write down the action of the universe.  To say that physics possesses a certain symmetry is to say that the action is invariant under the transformation associated with that symmetry.  That is, to detect a symmetry of nature one has to check the invariance of the action.  

Amy Noether, a mathematician, made a giant leap in the 20th century by proving the following theorem known as the Noetherian theorem.

Noetherian theorem: For every continuous symmetry of action, there is a conserved quantity, and vice versa.

The experimental observation of a conserved quantity tells us that the nature has an associated continuous symmetry by the Noetherian theorem.  For example, the electric charge has been known to be conserved since the 18th century.  After Noether’s discovery, physicists were prompted to reexamine the theory of electromagnetism and to search for the symmetry responsible for charge conservation.  The symmetry was duly found and became known as the “gauge symmetry”. 

Zee takes you through the theoretical development of physics throughout the 20th century like no other.  He is thorough and uses historical developments to enhance the story.  The following is an example.

In 1930’s the Japanese physicist Hideki Yukawa wondered why the strong interaction was short ranged, in contrast to the two better known gravitational and electromagnetic interactions.  Physicists now understand force as being due to quantum exchange of a particle.  Two massive bodies exchange gravitons when they interact and two electrons exchange photons when they interact.  The long range of these interactions implies the rest mass of graviton (or a photon) is zero.  Since the strong interaction is short ranged, the exchange particle must have a non-zero mass.  Yukawa calculated the mass and other properties of this exchange particle.  It was known as the “Yukawa particle” until it was experimentally discovered and later renamed “pion”.  Yukawa won the Nobel price for this feat.

Zee tells us a story of a Chinese artist (totally unrelated to how I am going to use it.)  There was an artist in China greatly skilled in painting snakes.  His work was much admired, but nevertheless, he was not satisfied.  The snakes he painted did not look right to him.  Finally, picking up his brush, he painted feet on the snakes.  The Chinese expression “painting feet on snakes” is now used to describe the destruction of a design due to excessive embellishments.

It is disheartening to see a masterpiece getting tarnished due to an excessive use of an irrelevant assumption.   It is like adding feet to a very beautiful snake.