Celebrating the second law of thermodynamics

Sadi Carnot was a child of the Industrial Revolution. Steam power was widely used in manufacturing and mining by his birth in 1796, and the first steamboat and first steam railroad locomotive were invented while he was still a child. Steam technology was transforming society, and Europe was at the center of the action — a Silicon Valley for steam.

Carnot’s father was a prominent French engineer, so it’s not surprising that Carnot became an engineer himself and took on one of the biggest practical challenges of his time: how to make steam engines more efficient. Carnot’s great insight was that heat produced motion for doing work by dropping from a high temperature to a lower temperature. There would always be some energy loss, no matter how well designed. That led him to more theoretical questions about matter and energy. And in June 1824, Carnot became the first to articulate what we now call the second law of thermodynamics.

Simply stated, the Second Law observes that heat always flows from hot to cold. Or phrased another way: Not all heat can be converted into work, resulting in heat waste. We now describe that loss as entropy.

Two centuries later, big questions about the Second Law remain to be answered. To explore Carnot’s legacy and how physicists have fleshed out this fundamental law over the last 200 years (and continue to probe it today), we called on contributing correspondent Tom Siegfried, a former editor in chief of Science News and an ace at explaining the complexities of physics.

I wasn’t familiar with Carnot, and I loved learning about his life at a time of great scientific and political ferment (Carnot’s father had served Napoleon Bonaparte and was forced into exile for a time). There’s a lot we don’t know about Carnot’s thinking. As Siegfried explains, that may be because the engineer published his work in a book that wasn’t widely read. And he died tragically young, before the age of 40, so he never had the opportunity to gain greater recognition for his pioneering work.

I like to think of a law as something settled, but science is always advancing. There’s always more to investigate. Who could have imagined in Carnot’s day that studies of the Second Law might 200 years later become so intertwined with ideas about information and the principles of computation? “Computing is a whole different way to looking at it compared to steam engines,” Siegfried notes. “It’s approaching the thing from a totally different perspective.”

There’s plenty more physics to ponder elsewhere in this issue, particularly in the realm of the quantum world. Physics reporter and senior writer Emily Conover reports on new efforts to nail down certain quantum properties of the neutrino and describes two real-world experiments that demonstrate the potential of a technology that might support a quantum internet.

Just as Carnot was living through a golden age of steam power, we may be on the cusp of a golden era of technology powered by quantum physics, which could bring more efficient and secure communication and computing. It’s an exciting time, one that surely would astonish and delight Carnot.

Nancy Shute is editor in chief of Science News Media Group. Previously, she was an editor at NPR and US News & World Report, and a contributor to National Geographic and Scientific American. She is a past president of the National Association of Science Writers.