What distinguishes a living system from a non-living one, in physical terms?
We can recognise a living cell: it is active, maintains its membrane and electrochemical gradients, and exchanges matter and energy with its surroundings. We can also recognise when it is no longer capable of performing these functions. But can we identify a physical signature that accompanies the transition from one state to the other?
Thermodynamics offers a starting point. An organism is not an isolated system: it maintains its organisation through a continuous exchange of matter and energy.
Schrödinger described this condition in What Is Life? Later, Ilya Prigogine’s work on non-equilibrium thermodynamics helped explain how systems traversed by flows can sustain organised structures far from equilibrium.
I believe that life is, precisely, a negentropic phenomenon: a dynamic organisation of matter that remains far from equilibrium and resists the entropic tendency through its own autopoietic activity. In my view, this is what distinguishes life fundamentally from the other dissipative structures described by Prigogine.
This organisation is sustained by continuous flows of energy and matter and, in the process, produces and exports entropy to its surroundings. The negentropy of life does not contradict the overall entropy of the system. In my hypothesis, it describes the form of energy and organisation that enables a living system locally to resist disintegration. It is in this context that Schrödinger’s famous expression negative entropy acquires its significance. Léon Brillouin later shortened it to negentropy; Schrödinger made the expression famous by describing what organisms feed on in order to maintain their order.
The experimental question, then, might be this: what happens to the individual terms in a cell’s energy balance as it transitions from life to death?
In an earlier, modest contribution, 21 Grams, I suggested examining the energy balance before and after this transition. I expect the overall balance to close, though it would be truly astonishing to find otherwise.
Taking that idea further and adapting it to the present context, the experimental interest would lie not only in calculating the balance “before” and “after”, but also in tracking what happens to its individual terms. Even when the total balances, the distribution and pattern of the flows may change, leaving a kind of thermodynamic signature of the transition.
One could follow a cell—or a sufficiently homogeneous population of cells—as it moves from a clearly viable state to one in which it can no longer sustain its vital functions. Heat production, oxygen consumption, carbon dioxide production, exchanges of matter, chemical transformations and changes in electrochemical gradients could all be measured over time. The aim would be to determine whether any of these flows change in a recognisable way, and whether those changes are associated with the loss of the organisation that sustains life.
Nair-Collins interprets death as the irreversible cessation of an organism’s capacity to maintain homeostasis and resist entropy. His account provides a conceptual framework for examining this transition. I propose adding an experimental question: can we track the terms in the energy balance over time and determine whether changes in them are associated with the loss of vital organisation—and whether, together, they form a recognisable thermodynamic signature?
The question also applies in the opposite direction: when does a new life begin? Imagine following two gametes, each with its own flows and energy balance. After fusion, the new cell initiates a coordinated dynamic, involving exchanges and transformations that no longer belong to the gametes separately. At what point does this new organisation emerge? And might its emergence be accompanied by a recognisable thermodynamic signature?
If such a signature could be identified and reproduced, it might help us describe both the loss and the emergence of vital organisation. In the longer term, it could contribute to the detection of life processes on distant planets, regardless of their chemical basis.
References
E. Schrödinger, What Is Life? The Physical Aspect of the Living Cell, Cambridge University Press, 1944.
I. Prigogine and I. Stengers, La nouvelle alliance. Métamorphose de la science, Gallimard, 1979.
M. Nair-Collins, “We Die When Entropy Overwhelms Homeostasis”, in M. Cholbi and T. Timmerman (eds.), Exploring the Philosophy of Death and Dying: Classical and Contemporary Perspectives, Routledge, 2021.