Running a brain-twisting thought experiment for real shows that information is a physical thing – so can we now harness the most elusive entity in the cosmos?

The unseen agent 'No Shadow' by Makoto Tojiki

WE LIVE in the age of information. We are surrounded by it, and more of it year by year. It is the currency of human understanding, our indispensable guide to navigating a complex world. But what, actually, is information?

As we have wrestled with the question over the years, we have slowly begun to realise it is more than an abstraction, the intangible concept embodying anything that can be expressed in strings of 1s and 0s. Information is a real, physical thing that seems to play a part in everything from how machines work to how living creatures function.

Recently came the most startling demonstration yet: a tiny machine powered purely by information, which chilled metal through the power of its knowledge. This seemingly magical device could put us on the road to new, more efficient nanoscale machines, a better understanding of the workings of life, and a more complete picture of perhaps our most fundamental theory of the physical world.

For at its heart, information is a mystery bound up with thermodynamics. This set of iron rules explains how heat is converted to and from other forms of energy, and governs a huge variety of processes. Thermodynamics makes a vital distinction between heat – a melee of random motions of atoms and molecules – and work, energy directed towards a purpose, such as the action of an engine pushing a car along.

Perhaps the most cast-iron of the thermodynamic rules is the second law, which says that heat will not flow from a cool object to a warmer one unless you put in …