Sterling service from Stirling engines

The hype about quantum computing continues to grow, but the laws of thermodynamics will continue to keep us grounded

I f there’s an underlying theme to this column, which may be doubted, then it’s the difference between the physical and the digital worlds. I can sum it up in an aphorism I’ve employed far too many times: “You can order a pizza online, but you can’t eat it online”. I’ve been living in this gap between worlds for 40 years now: my first toe in the digital water was via a Commodore PET in 1981 at the start of the personal computer revolution, though it wasn’t until the coming of WWW that we all got properly connected together.

Of course, I was born into the physical world, and inhabited it with increasing curiosity throughout a childhood filled with Meccano (I built the travelling gantry!) and model aeroplanes with glow-plug engines (I was in a control-line combat team). At school I immersed myself in science and my college lab days were pretty physical: hot, smelly and toxic in chemistry; warm, wet, salty and mildly radioactive in biochemistry. I dropped out and stumbled into the digital world when Dennis Publishing acquired Personal Computer World magazine in the late 1970s, merely because I was the most numerate person present, though my previous experience had been handing a printout to a man in a lab coat and getting the results back on Tuesday.

The physical world is filled with palpable, even edible, objects made of atoms and molecules that whizz round subject to rules we were taught in physics and chemistry, while the digital world consists of bits with which we construct representations of those physical objects to calculate and simulate their relationships. More precisely, bits live between the physical and yet another world, the quantum world, which follows different, weirder, rules that allow “entanglement” to eradicate distance and separation. Bits are electrical charges stored in silicon capacitors or similar, and at today’s tiny feature sizes they feel that weirdness. My scepticism toward the current hype over quantum computers grows from a suspicion that many quantum enthusiasts think “quantum” is going to free us from the confines of boring old physicality, which it isn’t.

The physical world is ruled by the laws of thermodynamics. The most readable account of those laws is Peter Atkins’ 1984 book

The Second Law, in which he explains that many things in the world are engines that take in energy, turn some of it into work, and expel the “waste” as heat. For example, we take in glucose and run around thinking, while chips steer electrons to perform Boolean operations.

I was introduced to Atkins’ book by my brother-in-law Pip Hills, who like me was fascinated by motors and machines – we once drove to Prague together in his 1937 Lagonda, fitted with a large Gardner diesel engine ( pcpro.link/326lagonda). Pip studied philosophy, but we share that faith in thermodynamics as a way to understand the limits of the physical world. A few years ago, he bought a rather fine old working model of an odd-looking engine in Portobello Road: we took it home, plonked it on a gas ring and it started running.

Once back home in Scotland, Pip began to study the history of this unusual heat engine, becoming so intrigued that he’s written a book about it called The Star Drive (Birlinn, 2021). He explains how the engine was invented by Scottish parson Robert Stirling in 1816, and it runs by expanding hot air rather than steam. It differs from internal combustion, petrol or diesel engines by using an external heat source.

Simpler and with fewer moving parts than steam engines, Stirling engines nevertheless lost out as the high temperatures they required broke available materials such as cast iron and leather, and also because their speed isn’t easy to control.

But, more recently, their ability to operate completely sealed from the outside world except for a heat source has opened up several interesting niche applications.

The Swedish navy operates small non-nuclear submarines powered by Stirling engines burning liquid oxygen and diesel fuel catalytically to charge batteries. One of these humiliated the USS Ronald Reagan during 2005 war games by penetrating its defences to “paintball” it. NASA uses closed-cycle Stirling engines heated by nuclear reactors to generate electricity in spacecraft travelling to the outer solar system, where they operate for years without sunlight, lubrication or maintenance. And miniature Stirling engines driven backward by external electromagnets make highly efficient heat pumps for cryogenic cooling – perhaps to keep some future quantum computer within its own chilly world.

Bits live between the physical and yet another world, the quantum world, which follows different, weirder, rules

Many quantum enthusiasts think “quantum” is going to free us from the confines of boring old physicality, which it isn’t