The Law of Parity in Physics

The Law of Symmetry in Physics

 

by: lim ju boo (lin ru wu)  

Recently, the world lost one of the most important scientific personality - James Dewey Watson (April 6, 1928 – November 6, 2025) an American molecular biologist, geneticist, and zoologist who died at the age of 97. 

In 1962, Watson, Crick, and Maurice Wilkins were awarded the Nobel Prize in Physiology or Medicine for their discoveries concerning the molecular structure of nucleic acids -deoxyribonucleic (DNA) and its significance for information transfer in living material. 

I have written quite a lot of articles here on molecular biology, DNA and also its applications in medicine, forensic science, agriculture among others, and I shall not repeat them here.  

In this year (2025) alone, we also lost another Nobel Prize winners - Yang Chen-Ning who won the Nobel Prize in physics in 1957, 

I shall today write an article on physics instead concerning Yang Chen-Ning. 

I received this piece of news on the demise of China's first Nobel laureate, Yang Chen-Ning

https://www.chinadaily.com.cn/a/202510/18/WS68f3170ea310f735438b5bf2.html

I heard about this first Chinese scientist who won the Nobel Prize in Physics in 1957 from my classmate when I was only in Form 4 in school, and naturally, I could not understand  what this theory of parity was all about till I studied physics and mathematics in a university years  later.

Friends and former colleagues of mine who are medical doctors have not much clue in physics or chemistry are also now asking me what is this theory of parity all about?

The theory of parity non-conservation is the principle that some subatomic particle interactions, specifically the weak nuclear force, are not mirrored in a mirror image. "Parity" is the physics concept of mirror symmetry, and before the work of Yang and Lee, it was believed that all physical laws were symmetric, meaning a process and its mirror reflection should behave identically. Yang and Lee proved this wasn't true for the weak interaction, a discovery that revolutionized physics by showing that a fundamental symmetry of nature was not universal. 

Parity symmetry: Think of a mirror. In a mirror, left and right are swapped, but up and down are not. Parity conservation was the idea that the laws of physics should hold true even if you viewed them in a mirror. For example, if you dropped a ball, its mirror image would also drop. This was thought to be a fundamental law for all forces.

Weak interaction: This is one of the four fundamental forces of nature, responsible for things like radioactive decay.

Yang and Lee's discovery: They theorized and later proved that the weak interaction is "chiral," meaning it is not mirror symmetric. This is like a left-handed glove not fitting a right hand. The weak force can distinguish between "left-handed" and "right-handed" particles, breaking the principle of parity conservation.

Impact: This discovery was a major breakthrough, upending a long-held assumption in physics. It demonstrated that the laws of nature are not universally symmetrical and led to a deeper understanding of elementary particles and the fundamental forces. 

Let me now explain this in another simpler way:  Understanding Parity - Using A Simple Idea and Life Illustrations.

 

Let me now explain them under this title:

When the Mirror Lied: Yang Chen-Ning and the Hidden Asymmetry of Nature

A Tribute to a Giant of Physics

On 18 October 2025, the world lost one of its greatest scientific minds - Professor Yang Chen-Ning (Yang Zhenning) who passed away peacefully in Beijing at the age of 103.

Born in 1922 in Anhui Province, China, Yang grew up during a turbulent era of war and political change but rose through hardship to become one of the most brilliant theoretical physicists of the 20th century.

After graduating from the National Southwest Associated University during wartime China, Yang later pursued his Ph.D. at the University of Chicago under the legendary physicist Enrico Fermi. In 1957, at the age of just 35, he and his colleague Tsung-Dao Lee became the first Chinese-born scientists to receive the Nobel Prize in Physics for their groundbreaking theory that parity is not conserved in weak nuclear interactions. This discovery fundamentally changed modern physics and reshaped our understanding of the universe.

What exactly did Yang discover that was so profound? To understand this, we must take a journey, not through complex mathematics or quantum jargon, but through simple observation, biology, and ordinary life which my gentle readers here can understand

Parity is nothing more than mirror symmetry. It asks a simple question:

Would the laws of nature still work if everything were flipped in a mirror - left becomes right and right becomes left?

For example, if a person kicks a football with their right leg, in the mirror it looks like they are kicking with their left, but the action still makes sense. So we’d say: football respects parity.

For over a century, physicists believed that all natural laws behaved this way, that nature does not care about left and right.

Yang and Lee challenged this belief, and they were right.

Biology Already Told Us: Nature Is Not Perfectly Symmetric

Even before physics discovered this, biology already knew that nature breaks mirror symmetry.

1. Our Bodies Are Not Mirror - Symmetric

If you look inside your body, your heart is slightly to the left, your liver mostly on the right, and even the two halves of your brain have different functions. If humans were perfectly symmetric, flipping us into a mirror would still make perfect biological sense, but it doesn’t.

2. Life Prefers One Hand

Chemically speaking, molecules can be left-handed or right-handed (a property called chirality). But all amino acids in living organisms are left-handed. The right-handed versions simply do not function in the body.

3. DNA Twists Only One Way

The DNA double helix twists in a right-handed spiral, never the mirror version in normal biology. Life has chosen a direction.

So symmetry is not universal. Nature often chooses one side over the other.

Yang and Lee’s Revolutionary Question

In the 1950s, there was a puzzle in nuclear physics called the tau-theta problem. Two particles that looked identical behaved differently, something strange was going on. Yang and Lee proposed a bold idea:

What if the weak nuclear force does not obey mirror symmetry (parity)?

This was unthinkable. It challenged decades of unquestioned belief in physics. Many dismissed the idea, but Yang and Lee persisted. They worked with experimental physicist Chien-Shiung Wu, and in 1957 she performed a brilliant experiment using cobalt-60, a radioactive isotope.

The results shocked the world:

The electrons emitted during radioactive decay did not go out evenly in all directions.
They preferred a specific direction.
 In the mirror version, this behavior would be impossible.

For the first time in history, science had proof:

Nature breaks mirror symmetry. Parity is violated in the weak nuclear force.

Simple Everyday Life Analogies to Understand 

Everyday / Biological Analogy	Meaning
Right-handed scissors don’t work well in the left hand	Nature doesn’t always treat left and right the same
Screws only turn in one direction	Some processes prefer a direction
Only left-handed amino acids work in life	Nature has a built-in preference
DNA twists in one direction	Symmetry is not absolute

The weak nuclear force is like this, it has handedness. It only interacts with left-handed particles, ignoring right-handed ones completely.

Why This Was So Important

Yang and Lee didn’t just answer a question in nuclear physics, they opened a door into a deeper truth about the universe:

1. Nature is not perfectly symmetric

2. The universe has structure and preferences

3. There are hidden rules still waiting to be discovered

Their discovery reshaped physics and helped build the Standard Model, our modern theory of fundamental particles.

Summary:  The Legacy of Yang Chen-Ning

Yang Chen-Ning taught the world a profound lesson—not only in physics but in thinking itself:

Never assume. Question even the most “obvious” truths.

His life’s work reminds us that science advances not by accepting tradition, but by challenging it boldly yet intelligently. Today, as we remember this towering figure of human thought, we honor not just a Nobel laureate, but a mind of courage, clarity, and curiosity who dared to ask a question the world had ignored.

And with that single question, he changed science forever.

Summary: 

The law of parity in physics, or conservation of parity, states that a physical system and its mirror image should behave identically, meaning nature is indifferent to left and right.  This law was long thought to apply to all fundamental interactions, but experiments in 1956 revealed that parity is violated in weak interactions (such as beta decay), though it remains conserved in strong interactions, electromagnetism, and gravity.