What is Life?
The simplest definition:- the idea that life is a 'self sustained chemical system capable of undergoing Darwinian evolution'. is perhaps one of the most accurate. Life needs the correct chemistry set; an energy source and a medium through which chemical processes of life can play themselves- on earth this medium is water.
Through avoiding rapid decay into the inert state of equilibrium that an organism appears so enigmatic, so that from earliest times of human thought some special non-physical or supernatural force was claimed to be operative in the organism and in some quarters is still claimed.
How does the living organism avoid decay. The obvious answer is by drinking, breathing and, in plants, assimilating, readily described as metabolism. Living things feed on energy. A living organism continually increases its entropy, produces positive entropy thereby approaching the dangerous state of maximum entropy which is death.
Schroedinger notes the second law of thermodynamics introduces a tricky physical quantity called entropy. How then does life assemble itself in seeming contradiction of the second law? This is known as Schroedinger's paradox.
In What is Life? Schroedinger in 1944, noted there are two questions arising from life's intriguing order. We saw how ordered structures emerge quite naturally, in accord with the laws of thermodynamics, in non-equilibrium conditions. A second question is equally pressing if we seek a a full explanation for the complexity of the living things we see today. How did the first, relatively simple biological molecules gradually become more complex? How did simple living things come to assemble that most spectacular monument to emergent complexity, our 4 billion year old ecosystem?
Order has clearly been built on order, year by year, millennium by millennium. But how? Schroedinger answered that there is memory in the system, once a complex process has emerged, not requiring re-invention. He did not know the what the memory mechanism was but he deduced a great deal of its properties. He called it an aperiodic crystal, a molecule that must be unusually stable and be able to pass information from one generation to the next. Inspired partly by Schroedinger's little book, Watson, Crick, Wilkins and Franklin discovered the precise structure of that aperiodic crystal a decade later in 1953 - the double helix of DNA, the precision with which it can encode precious order from generations past and pass it on faithfully to the next is fascinating.
Sources: What is Life? Erwin Schroedinger, Cambridge University Press, 1944; What is Life lecture, Erwin Schroedinger, Trinity College Dublin, 1943; Wonders of Life, Brian Cox and Andrew Cohen, Collins, 2013