Hitting the Books: How Planck’s ‘chain of tiny beads’ helps explain why lightbulbs work

by Joseph K. Clark

The laws of thermodynamics help govern virtually every aspect of the known universe — from the biological functions of single cells to the formation of black holes at our galactic core. Modern conveniences like refrigerators, light bulbs, central air, and jet engines have only come about because of our relatively new understanding of these fundamental forces of physics. And without the Herculean efforts of scientists, theorists, engineers, and tinkerers over nearly two centuries, humanity would not be enjoying even almost the level of technological advancement we do today. In his new book, Einstein’s Fridge, author, documentary filmmaker, and science communicator Paul Sen explores the works and quirks of these pioneering researchers — from Lord Kelvin and James Joule to Emmy Noether, Alan Turing, and Stephen Hawking — as they sought to understand the thermal underpinnings of the universe better.

“Excerpted from Einstein’s Fridge: How the Difference Between Hot and Cold Explains the Universe by Paul Sen. Copyright © 2021 by Furnace Limited with permission by Scribner, a division of Simon & Schuster, Inc. In 1900, Max Planck, a critic of Boltzmann’s science for nearly two decades, published papers that hinted at a change of heart. Even more unexpectedly, he seemed to say that Boltzmann’s statistical methods might have relevance far beyond thermodynamics. This reluctant conversion was forced upon Planck by the advent of new technology—the electric light bulb. In these, electric current flows through a filament, warming it and making it glow. This focused scientific minds on investigating the precise relationship between heat and light.


Heat can flow out of an object in conduction, convection, and radiation. All can be observed in most kitchens. Conduction is how electric hot plates transfer heat. The whole heated surface of the container is in contact with the underside of a pan, and the heat flows from one to the other. Kinetic theory explains this as follows: As the hot plate’s temperature rises, its constituent molecules vibrate faster and faster. Because they’re touching the molecules of the saucepan, they shake them. Soon all the saucepan molecules vibrate more vigorously than before, which manifests as the saucepan’s temperature rises.

Heat flow through convection occurs in ovens. The heating elements within the oven’s wall cause the air molecules nearby to zip more quickly. These then collide with moleculesmore profound molecules in the stove, increasing their speed, and the oven’s temperature rises. The third kind of heat transfer, radiation, is linked to light. Turn on a grill, and it glows red as the element’s temperature rises. In addition to the red light, it also gives off infrared light, which feels hot. When this strikes an object, say the sausages in the grill pan, it causes their constituent molecules to vibrate, raising their temperature.

Scientists’ understanding of radiating heat had improved in the 1860s thanks to James Clerk Maxwell, who published a set of mathematical equations describing “electromagnetism.” Imagine holding one end of a very long rope for a sense of Maxwell’s reasoning. It’s stretched relatively tight, and the other end is a mile away. Jerk the future; you’re holding up and down. You see kink travel away from you down the rope. Now move the end of the rope up and down continuously. A continuous undulating wave travels down the cord.

To see why imagine the rope as a chain of tiny beads. Each is connected to the next by a short stretch of elastic. When you move the first bead in the chain, it pulls the one adjacent to it. That then pulls the one beyond it, and so on. The up-and-down movement of the first bead is thus passed sequentially down all the dots, which looks like a wave moving down the rope.

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