Can physics explain biology? Shutterstock / Billion Photos
The following is an extract from our Lost in Space-Time newsletter. Each month, we hand over the keyboard to a physicist or two to tell you about fascinating ideas from their corner of the universe. You can sign up for the Lost in Space-Time here.
At the dawn of time, the universe exploded into existence with the big bang, kick-starting a chain of events that led to subatomic particles clumping together into atoms, molecules and, eventually, the planets, stars and galaxies we see today. This chain of events also led to us, although we often see life and the formation of the universe as separate, or “non-overlapping magisteria” to borrow biologist Stephen Jay Gould’s phrase.
To cosmologists, complex systems like life seem of little consequence to the problems they are trying to solve, such as those relating to the big bang or the standard model of particle physics. Similarly, to biologists, life is housed in a biosphere that is decoupled from the happenings of the grandiose universe. But is that right?
Notable scientists, including John von Neumann, Erwin Schrödinger, Claude Shannon and Roger Penrose, have entertained the idea that there could be insights to gather from looking at life and the universe in tandem.
Physicist Erwin Schrödinger’s views were particularly interesting, as his audacious speculations and predictions in biology have been hugely influential. In 1943, he gave a series of lectures at Trinity College Dublin that would eventually be published in a tiny, but mighty, book called What Is Life? In it, he speculated on how physics could team up with biology and chemistry to explain how life emerges from inanimate matter.
Schrödinger believed that the same laws of physics that describe a star must account for the intricate processes of metabolism within a living cell. He knew that the physics of his time was insufficient to explain some of the ingenious experimental findings that had already been made about living cells, but he ploughed on regardless, attempting to use the physics he knew to explain biology.
He said that quantum mechanics must play a key role in life, as it is necessary for making atoms stable and enabling them to bond in the molecules found in matter, both living and not. For non-living matter, such as in metal, quantum mechanics allows molecules to organise in interesting ways, such as periodic crystals – lattices of molecules with high-degrees of symmetry. But he believed that periodicity was too simple for life; instead he speculated that living matter is governed by aperiodic crystals. He proposed that this type of non-repetitive molecular structure should house a “code-script” that would give rise to “the entire pattern of the individual’s future development and of its functioning in the mature state”. In other words, he was stumbling across an early description of DNA.
An outsider’s approach
Before Schrödinger’s time, biologists had hit upon the idea of the gene, but it was just an undefined unit of inheritance. Today, the idea that genes are governed by a code that programs the structures and mechanisms of cells and determines the fate of living organisms seems so familiar, that it feels like common sense. Yet, exactly how this is accomplished at a molecular level is still a being teased out by biologists.
What is particularly remarkable is that Schrödinger used reasoning stemming from quantum mechanics to formulate his hypothesis. He was an outsider to biology, and this naturally made him bring a different approach.
Physics and biology have moved on a lot since Schrödinger’s day. What if we were to follow the same process and ask what is life today?
Over the years we, the authors of this newsletter, have developed a pattern. We meet up, sometimes over a drink, to exchange ideas and share our latest musings in cosmology or molecular biology. We have often stayed up late talking while listening to our favourite jazz or flamenco musicians. In part, our conversations are an exercise in deliberately generating an outsider perspective, like Schrödinger did, hopefully to benefit each other’s research. But it is also just a lot of fun.
Specifically, since 2014 we have developed a common intuition that there is a hidden interdependence between living systems and cosmology, as demonstrated in some of our publications. To understand this, we need to talk about entropy, a measure of disorder, and how it flows in the universe, both at biological and cosmological scales.
In the early universe, before there were stars and planets, space was mostly filled with an equal amount of radiation and matter. As this mixture warmed and moved about more, it became less ordered and its entropy increased. But as the universe expanded, it distributed radiation and matter in a homogenous, ordered fashion, lowering the entropy of the universe.
As the universe further expanded and cooled, complex structures such as stars, galaxies and life formed. The second law of thermodynamics says that entropy always increases, but these structures had more order (and therefore less entropy) than the rest of the cosmos. The universe can get away with this because the regions of lower entropy are concentrated within cosmic structures, while entropy in the universe as a whole still increases.
We believe this entropy-lowering network of structures is the main currency for the biosphere and life on planets. As the father of thermodynamics, Ludwig Boltzmann, said: “The general struggle for existence of animate beings is therefore not a struggle for raw materials… nor for energy which exists in plenty in any body in the form of heat, but a struggle for entropy, which becomes available through the transition of energy from the hot sun to the cold earth.”
Emergent phenomena
As the universe deviates from homogeneity, by seeding and forming lower entropy structures, entropy elsewhere in the universe continues to grow. And entropy also tends to grow within those structures. This makes entropy, or its absence, a key player in sustaining cosmic structures, such as stars and life; therefore, an early lifeless universe with low entropy is necessary for life here on Earth. For example, our sun radiates energy that is absorbed by electrons in plants on Earth and used in the functions they need to live. Plants release this energy in the form of heat, giving back to the universe more entropy than was taken in.
Unfortunately, it is difficult to explain with our current understanding of physics why the entropy was so low in the early universe. In fact, this problem of the low entropy we demand of the big bang is one of the major problems with this theory.
The biology side of the story stems from Salvador’s research into the genetic and ecological drivers that lead populations of harmless bacteria to evolve and emerge as pathogens. Crucial to the story is that it isn’t just a question of the genetic code of the bacteria. One of Salvador’s mantras is that life is an adaptive phenomenon responding to constant and unexpected changes in pressures from the environment.
This makes an organism an emergent phenomenon, where the final shape of it isn’t contained in the individual pieces that make it up, but can be influenced by a series of larger systems to which it belongs. Living things comprise a network of interactions mediated through the environment. A living system is able to regulate billions of cells to maintain its overall functioning. Beyond that, collections of organisms belong to a network called an ecosystem, which also maintains a dynamical equilibrium.
This extends all the way to networks at life’s largest scales. The idea of Earth being a self-regulating ecosystem was co-discovered by James Lovelock and Lynn Margulis in the 1970s, and it became known as the Gaia hypothesis. The takeaway for us is that the flow of negative entropy exists not only for individual living things, but for the entire Earth.
The sun sends free energy to Earth, and through a chain of complex interactions, the energy gets distributed through a network of interactions to living things, each relying on it to maintain its complexity in the face of increasing disorder. To contextualise the role of life within the framework of thermodynamics, we define these order-generating structures (such as a cell) as Units Of Negentropy, or UONs. But there’s no such thing as a free lunch. When UONs release this energy back into the environment, they mostly do so in a form that has higher entropy than was received.
This uncanny parallel between living systems, UONs and the evolution of the universe may seem like a coincidence, but we choose not to think of it this way. Instead, we propose that it is a central organising principle of the evolution of the cosmos and the existence of life. Salvador elected to call this the entropocentric principle, a wink at the anthropic principle, which, in its strong form, states that the universe is fine-tuned for life. This arises because the laws of nature seem to be just right for life. For example, if the strength of the nuclear force that binds the hearts of atoms differed by a few per cent, stars wouldn’t be able to produce carbon and there would be no carbon-based life.
The fine-tuning problem may not be as severe as it seems, though. In research Stephon conducted with colleagues, he showed that the universe can be fit for life even when we let the constants of nature like gravity and electromagnetism vary, so long as they vary simultaneously. Maybe we don’t need the anthropic principle after all. The entropocentric principle, on the other hand, is harder to shake. If the universe was unable to provide pathways that enabled it to create regions of lower entropy, then life as we know it wouldn’t exist. This leaves us wondering: do we live in a cosmic biosphere or is the universe a cosmic cell?
Stephon Alexander is a theoretical physicist at Brown University in Rhode Island who spends his time thinking about cosmology, string theory and jazz, and wondering if the universe is a self-learning AI. He is author of the book Fear of a Black Universe. Salvador Almagro-Moreno is a molecular biologist at the University of Central Florida who investigates emergent properties in complex biological systems, from protein evolution to pandemic dynamics.
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