The search for life beyond Earth is hampered by a fundamental conundrum: there is no agreed definition of what life actually is.
Living organisms are so extraordinary, so unlike other complex systems we know, that they seem to be made of some sort of magic matter. And for centuries it was widely supposed that organisms were indeed infused with a unique essence or life force to animate them.
A hundred years ago, however, these mystical musings gave way to a mechanical view of life, according to which organisms are just immensely complex machines obeying the same basic laws of physics as non-living systems.
Yet doubt has persisted. Seventy-five years ago, the theoretical physicist Erwin Schrödinger, one of the founders of quantum mechanics, delivered a series of lectures in Dublin entitled What is Life? He openly questioned whether life could in fact be explained by known physics, urging his colleagues to keep an open mind about the possibility of fundamentally new laws.
Schrödinger pointed to life’s ability to buck the trend of the second law of thermodynamics, the universal tendency for physical systems to become progressively more disordered over time. Life, by contrast, creates order from order. So maybe something was missing from the mechanistic model. But what?
The first hint of an answer lay in a letter written by the physicist James Clerk Maxwell to a friend in 1865. Maxwell imagined a tiny being, soon to be called a demon, that could observe individual molecules in a box of gas and sort them into fast and slow categories. By this means the demon could take a gas at a uniform temperature and manipulate the molecules to create a disequilibrium, with hotter gas at one end and cooler at the other.
A heat engine could then be run off the temperature gradient, thus extracting work from heat, in violation of the cherished second law of thermodynamics.
Maxwell’s demon lay like an inconvenient truth at the heart of physics and is still not completely understood. On careful analysis, the key to the demon’s prowess is found to be its ability to garner information about molecules and process that information in its diminutive brain, using the output to work the margins of thermodynamics and gain an advantage.
Recently it has become apparent that living organisms are replete with demonic nano-machines doing just that: manipulating information for thermodynamic advantage.
Strictly, there is no violation of the second law, so long as information is treated as a physical quantity in the laws of thermodynamics. Physicists have recently come to think of information as a type of fuel, and using nanotechnology they are busy designing “information engines” and even an information powered refrigerator!
For their part, biologists now regard living organisms as networks of information flow coupled to networks of chemical reactions. Just as evolution shapes the architecture of cells and bodies, so it sculpts the networks that support the swirling patterns of information that make organisms tick.
Regarding life as an intimate amalgam of chemistry and information is akin to the complementary roles of hardware and software in computers. Just as we need hardware engineers to design circuits and chips, so we need software engineers to design programs.
Any attempt to explain life’s origin has to account not merely for the complex molecules that serve as the substrate of life, but also for the informational modules that control it. Information isn’t restricted to the innards of cells, but is exchanged between cells and whole organisms, and in fact across entire ecosystems.
On Earth, organised biological information envelops the planet.
Astrobiologists have deliberated at length on the chemical signatures that life may imprint on, for example, the atmosphere of an extra-solar planet. But informational signatures of life might be more fundamental because they are likely to be universal, transcending the actual molecular hardware, and thus display common features across many alien life forms. NASA engineers have to confront this issue on a forthcoming mission to Enceladus, a moon of Saturn that is spewing organic material into space through fissures in its icy crust. A spacecraft is being designed to fly through the plume and sample the material.
What, exactly, should it look for? A simple inventory of molecules would not suffice as there are many abiotic ways to produce complex organics. What would stand out is if the molecules were not a random assortment, but formed a chemical reaction network with the type of organisation that, on Earth, life uses to process information – a striking example being the universal genetic code.
The challenge for astrobiologists is to work out the generic features of such networks in case extraterrestrial life uses a different molecular basis.
If Schrödinger were alive today, he might be surprised to learn that the answer to his question, What is Life? is not that living systems are made of the right stuff, but that they encode the right bits.