What is Thermodynamic ‘Negative Entropy’?

Fine tax visible from the microscope

In the Western philosophical and scientific tradition, theorists have endlessly tried to determine what separates life from non-life. Does life contain some special or enigmatic vital principle that other inanimate natural and artificial objects do not? Is it the fact that life can reproduce itself, evolve and set its own goals that distinguish it from, say, rocks, chairs and sowing machines?

One of the most important responses to this question can be found in Schrödinger’s 1944 book What is Life. In this book, which is mostly remembered for initiating the idea that life stores information about itself in helix shaped ‘aperiodic crystals’, Deoxyribonucleic acid (DNA) will later be discovered to be just as Schrödinger suggested, he also famously claimed that one of the most fundamental characteristics of living organisms is that they delay the onset of ‘rapid decay’ and death that comes as the result of an increase in ‘positive entropy’ by ‘by continually drawing from its environment negative entropy,’ in the shape of food and nutrients.

After Schrödinger’s book, many figures (as wide as the psychologist Bowlby and the physicist Davies) have taken this to mean that life somehow bucks the trend when it comes to the universal increase in entropy, that life’s special vitalist feature is that it is somehow negentropic and not entropic. But is this really what Schrödinger meant by saying that one of the features of life is that it draws ‘negative entropy’ from its environment? By aligning negative entropy with the delay of rapid decay through the consumption of food, did he really mean that life was somehow separate from every other system which is necessarily entropic in nature?

A large portion of the misunderstanding surrounding ‘negative entropy’ arises from the two related but distinct definitions of positive entropy. The first stems from thermodynamics and physics, how Schrödinger uses it above, and the second stems from individuals like physicist Brillouin and cyberneticist Weiner who used the term in a different but related information theoretical context. This blog post shall concentrate on the first thermodynamic notion since this is the notion of ‘positive entropy’ that Schrödinger first referred to when coining entropy’s so-called opposite.

As one might be able to infer from the use of ‘positive entropy’ by Schrödinger, the notion of entropy in thermodynamics and physics is related to an energetic system, whether that be living or non-living, that can ‘decay.’ As Schrödinger writes in this section of the book, decay is understood as ‘the whole system’ fading away into ‘a dead, inert lump of matter,’ something that physicists call the ‘state of thermodynamical equilibrium,’ or of ‘maximum entropy.’

The term ‘entropy’ (Entropie) was itself first coined by physicist Clausius in 1865 and given the mathematical symbol S most likely to honour entropy’s initial thermodynamic theorization by the young military engineer Carnot. Carnot, in 1824, asserted that the dissipation of heat (as a form of energy) always moves spontaneously in one direction from hot to cold and that this movement was the cause for motive power (think of a piston being moved by the increase in water pressure achieved by heating water to steam). In other words, if one did want to heat something up then that heat would have to be supplied from outside of the system itself, through the transformation of one type of energy (stored potential energy) into heat. The example given in the last blog of boiling water is a great example, since the water does not boil of its own accord but requires the burning of a fuel that converts chemical energy into actual heat.

What is important, what is lawlike in this principle, is that you cannot get that chemical energy back into its initial state after it has been used and transformed into heat, at least not without producing more heat. If no more chemical energy was supplied to the gas burner, for example, the gas burner would gradually stop burning and the temperatures between the stove and the water would equalize. In short, thermodynamic ‘positive entropy’ as theorized by Carnot and Clausius quantifies the quality of the energy available to a system, whether there is any available energy left for the system to remain out of ‘thermal equilibrium.’

The example of a hot cup of tea is useful for intuitively grasping what is at stake in positive entropy. If left and not reheated the temperature of the tea and the room in which it is placed will gradually equalize, the only way to keep this difference of temperature in place is by heating the tea again, but as we have just discovered that is only possible if there remains an amount of available chemical energy to burn that will maintain the heat of the stove and reheat the tea!

This cycle, or downward spiral of events, where entropy, the amount of energy already converted to heat (already-dissipated-energy), always increases came to be known by the ‘second law of thermodynamics’ and still remains one of the most watertight laws of the universe: ‘The entropy of the world strives towards a maximum’ as Clausius put it in 1965.

From this definition of thermodynamic entropy, it becomes easier to understand what Schrödinger meant by ‘negative entropy’ in relation to life since like the reheated cup of tea, life can delay its fall to maximum entropy by consuming not-yet-dissipated energy in the form of food and nutrients; energy that organisms end up dissipating through all of the energetic activities involved in their metabolisms (pumping hearts, breaking down complex carbohydrates, thinking, walking and so on).

In the notes for Chapter 6 written after the initial publication, the chapter in which Schrödinger first coined negative entropy, he recounts how after publication he had received doubts and opposition to this term from physicists given that the tendency for entropy to spontaneously decrease is unobserved. He then states that if he had been ‘catering’ to these physicists he would have used the term ‘free energy,’ which is a fair more precise term for what he was trying to describe, but that the audience would likely have been confused if he used the adjective ‘free.’ Free energy, sometimes called ‘available energy’ as used by physicist Gibbs simply means that amount of energy not yet dissipated, what is left in the store house of the energetic system under observation. Free energy is how much gas is left in the cylinder, how much energy has yet to become entropically dissipated. Instead of being the negation of entropy, one should define it as not-yet-dissipated energy, or not-yet-entropy.

Indeed, this is why Boltzmann argued, following on from Darwin’s insistence that the evolution of life is related to the ‘struggle for existence,’ argued that what life really struggle for, which is to say is in competition for is ‘entropy.’ What Boltzmann meant by claiming that the evolution of life is related to the ‘struggle for entropy’ was precisely a struggle for the store of not-yet-dissipated energy present in nutrition.

Taken from this angle, we cannot say that life itself is ‘negentropic,’ as some suggest, indeed this was not at all the argument made by Schrödinger either. All that Schrödinger was trying to argue, in agreement with Boltzmann, was that the mechanism by which the relatively stable organization of a living organism maintains this relative stability (sometimes called homeostasis or metastability) is by ‘continually sucking orderliness from its environment.’

But is this mechanism not the same as any other energetic system that requires an input of energy to maintain its ‘orderliness,’ with orderliness defines as a complex thermodynamic system that is not yet at maximum entropy? Can we really say that life’s capacity to maintain its orderliness is singular to life? Surely any engine that requires fuel is likewise maintaining the relative stability of its ‘orderliness’ by sucking and consuming not-yet-dissipated energy from its environment? Indeed, in many machines, the demand and consumption of negative entropy in the form of fuel is regulated by the machine itself. What would the difference be between a plant that spreads its leaves to catch the sun’s rays, sucking in the negative entropy supplied by the sun, and a solar powered vacuum cleaner taking itself outside to bath in the sun’s light when its battery light comes on? From a thermodynamic perspective, perhaps very little, if nothing at all.

Joel White