Understanding the Physics of Living Matter

"What is life?" asked Erwin Schrödinger, the Nobel laureate who developed fundamental

results in quantum theory, in his public lectures in 1943 in Dublin. The question he

posed was profound: "How can the events in space and time which take place within the

spatial boundary of a living organism be accounted for by physics and chemistry?" After

all, the atoms and molecules of the organic and inorganic materials that make a living

organism are no different to those found in non-living matter. At what scale, then, does

life emerge?If not at the scale of a single atom, it must surely be an emergent

phenomenon effected by the collective interaction of many components.

It is winter and you happen to come across a murmuration of starlings at dusk. You are

captivated by what you see — the shape-shifting patterns that the birds make — and

you wonder if all this has been rehearsed before. You are reminded of similar patterns

of schools of sardines escaping sea lions that you have watched on TV. These living

creatures do not have the well-developed tools of language that we humans possess.

How then do they create such choreography? In fact, we now know that even simple

rules — a bird trying to orient itself with its nearest neighbours — can result in vivid

patterns as the number of members in the group increase. Such dynamical patterns

typical of living systems, therefore, emerge through collective interactions.

The tools to understand the collective behaviour of many particles form the bread and

butter of engineers and physicists. Physicists have developed the field of statistical

mechanics that explains, for example, how the many molecules of a gas exert pressure

on the holding container. You can thus measure the air pressure in your bicycle tyre

without caring about the trajectory of each individual gas molecule. Any good naval

engineer would know how the hull shape affects the wake pattern behind ships. Clearly, 

he does not and cannot account for the motion of every molecule of water in the 

ocean!

These tools, in simple words, involve the concept of averaging where a large number of

variables can be averaged down to a manageable number. This has already been

achieved for solids and fluids through the theories of elasticity and the Navier-Stokes

equations. We are now using these concepts from solids and fluids to model the

collective behaviour of living organisms. The key difference is that living systems are

out of thermodynamic equilibrium, for life continuously consumes and dissipates

energy. Water in a bottle equilibrates to the temperature of its surroundings, we as 
humans do not. Life is about being out-of-equilibrium. Equilibrium is tantamount to
death.

In a recent work, we considered how the out-of-equilibrium nature of even simple

systems can lead to rich dynamics. We were motivated by the observation that

biological cells use liquid-liquid phase separation to organise their interior. This phase

separation is like the separation of oil and vinegar in the vinaigrette dressing in your 

kitchen. Cells use phase separation to create droplets within, which can then serve

as chemical factories. The key difference with your vinaigrette, however, is that

phase-separation inside cells may occur in the presence of chemical reactions that

are constantly driven out of thermodynamic equilibrium.  In our work, we developed

a minimal model of the system — a two-component fluid mixture with the simplest

of chemical reactions — and found that the system can exhibit complex non-linear

dynamics reminiscent of the Rayleigh-Benard convection of a fluid between two

plates with the lower plate being heated. Establishing such relationships between

vastly disparate physical phenomena is what makes mathematical physics so

powerful. The goal is to use this faculty now in the realm of biology in order to

understand and appreciate life around us better.

Reference:

What is life?: With mind and matter and autobiographical sketches. Erwin Schrödinger, Cambridge 
University Press, 1992.

Fluid flow and spatiotemporal chaos in chemically active emulsions,  Charu Datt, Jonathan

Bauermann, Nazmi Burak Budanur, Frank Julicher, (to appear), Physical Review Letters

2026 (https://doi.org/10.1103/z1hs-y)