📅 2020-Sep-13 ⬩ ✍️ Ashwin Nanjappa ⬩ 📚 Archive

In Ted Chiang's sci-fi short **Story of your life** he teases this phenomenon: *when light travels from air to water, how does it know to take the path shortest in time?* This led our bookclub to pick up **QED: The Strange Theory of Light and Matter**. This book published in 1985 is a compilation of 4 lectures delivered by famed physicist **Richard Feynman** about the theory of **quantum electrodynamics**. The lectures were designed to be accessible by a layman and that is the reason I was brave enough to attempt this.

The **opening lecture** is an introduction where Feynman says that QED was a theory developed to unify all the existing phenomenon in physics: mechanics, heat, light, electricity, magnetism and chemistry. Everything that is, except gravity and radioactivity, which are not unified to this day. At its core, QED focuses on the interaction of light with matter. QED is not a clear and exacting theory (like the classical ones), but it delivered probabilities, which matched experiments.

Feynman uses the phenomenon of light partially reflecting through glass as the primary example for the rest of the book. Light is composed of particles called **photons** and they can be individually detected by a device called a **photomultiplier**. Now, during partial reflection how does a photon *decide* whether to go through glass or reflect off it?

Using a method of drawing arrows and lining them up on each other, Feynman describes how we can deduce the probability of light reflecting off glass. The arrow direction is given by a stopwatch spinning at the frequency of the light and the arrow length is inversely proportional to the distance the light travels. The arrows are actually complex numbers (which I wish he mentioned in the book) and by lining them up we are performing addition of complex numbers. The values that we compute using this system are called **probability amplitudes**.

The second lecture (**Photons: Particles of Light**) applies the probability amplitude method on several experiments in light and matter to show why it seems like *light always takes the path of least time*. Some of the interesting phenomenon are mirror reflection, refraction of light through water, diffraction grating and focusing lens. When light has multiple interactions on its path, the complex numbers are multiplied by scaling the arrow and rotating to get the resulting probability amplitude.

**Electrons and their interactions** are the focus of the third lecture, where we get to Feynman's actual research (as I learnt later online). Electrons can emit or absorb photons. To illustrate the interactions of electrons and photons in space-time, a diagram is introduced which has space on X axis and time on Y axis. Motion of electrons is depicted as straight lines and that of photons as wavy lines. (This is called the **Feynman diagram** as I discovered online.)

The final lecture titled **Loose Ends** is a grab bag summary of all the new particles discovered by bombaring particles with each other. It provides a handy table to categorize and theorize about them.

This was my first real interaction with physics after almost a quarter century (high school). I already knew Feynman is a fantastic speaker and writer since I loved reading his **Surely You're Joking Mr. Feynman!**. But, QED was all-new territory for me and he certainly made it easy to understand the experiments and the theory. His technique of using the simplest diagrams, visualizations and explanations is particularly noteworthy. It is also refreshing to see self-deprecation and that of physics and research. I loved the first two lectures, got a bit lost in the third lecture and was fully lost in the fourth one (I need to get to that again sometime). I wish he mentioned real terminology when he introduced his methods and also some basic math or formulas would have been nice. This is an excellent introduction to know the theory used today for light and matter. Despite Feynman's assurances, I do not think this book is digestible by the lay person, but someone from a science background would thoroughly enjoy it.

**Rating:** 4/4