What is Living Matter?

Perhaps the most fundamental hallmark of living matter is multiscale organization, dense with interconnected cycles and loops. In particular life can be presented, at least approximately, as a hierarchical cascade of machines making machines. This cascade stretches between "almost nothing" (electrons, protons and photons) to "almost everything" (the whole biosphere):

Space-time scaling of the double cascade of machines, with its critical point (red) at the scale of the microbial cell, the smallest self-reproducing machine (1 micron, 1000 sec). The micro-cascade describes how the cell is made from smaller machines, down to the atomic scale (green), and the macro-cascade depicts the construction of larger machines from cells, up to the whole biosphere (1 year is ~3·107seconds). The cascade of machines has two branches: The micro-cascade, proceeding from the bacterium down to its tiniest parts: the membrane that envelopes the cell, the molecular machines inside the membrane, ribosomes, and cytoskeleton, the submachines that make the machines, proteins, RNA, and DNA, the parts that make the submachines, amino acids, nucleotides, and lipids, and down to atoms, electrons, protons, and photons. The macro-cascade, which assembles bacteria into larger, more intricate organizational machines: communities of bacteria, eukaryotic cells formed by the symbiosis of archaea and bacteria, tissues made of eukaryotic cells, organs constructed from tissues, organisms composed of organs, populations of organisms, ecosystems made of populations, including human society, up to the whole biosphere (and perhaps other planets).

This leads us to the following provisional definition of life:

Life is a cascade of machine-making machines that solves the problem of survival by realizing in the world of water an autotrophic self-reproducing machine. The main features of the cascade are as follows:

1. Scaling down from the cell to atomic-size machines that interface with salty water.
2. Scaling up from the cell, filling all physically accessible scales, up to the whole biosphere.
3. Entanglement of matter and information: All machines have coded images.
4. "Self" defined by singularities in space, time, energy, and information:
   • a semipermeable 2D boundary separates the self-reproducing machine from the water world.
   • to self-reproduce: (i) high fluxes of matter and information are pumped through the boundary. (ii) the fluxes are used to double the machines and their images. (iii) the doubled matter and information are then cut into two new machines.
   • a semipermeable 2D boundary separates the self-reproducing machine from the water world.
5. Open-ended evolution: Exponentially doubling self-reproducing machines compete over finite resources, leading to the appearance of new machines and new ways to organize them in a cascade.