Summary: What Is Life?

The GIST

What Is Life? by Erwin Schrödinger is a very short book with a very long shadow. In February 1943, with Europe at war, the Austrian physicist who had shared the 1933 Nobel Prize for his wave mechanics stood up in a lecture theatre at Trinity College, Dublin, and spent three consecutive Fridays on a question that was not supposed to be his business. He put it this way: "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?" About four hundred people came to each lecture, among them the Irish prime minister Éamon de Valera and his cabinet, and crowds had to be turned away. The lectures were published the following year under the full title What Is Life? The Physical Aspect of the Living Cell.

At the time nobody knew what a gene was made of. Most scientists guessed protein, and DNA was thought too dull a molecule to matter. Reasoning from physics alone, Schrödinger argued that the hereditary substance had to be a particular kind of molecule, one that stores a code in its structure. A decade later James Watson and Francis Crick proposed the double helix, and both said this book had helped set them on the path. It is written for the general reader, with almost no mathematics, though it asks for concentration. It remains one of the best examples of a great mind thinking out loud about something outside its own field.

The Big Ideas

Order from disorder, and why size matters

Schrödinger begins as what he calls a naive physicist would. The reliable laws of physics we observe at large scale, he explains, are statistical. Individual atoms jitter about at random. It is only because there are unimaginably many of them that their average behaviour is steady and predictable. He calls this principle order-from-disorder. Diffusion is his example: it can be described as a smooth, lawful process, yet its cause is nothing but the random motion of atoms and molecules. One reviewer was struck by his related remark about Brownian motion, that the effect only appears when many particles are jostling one another.

The catch is that the law gets shakier as the numbers fall. With fewer atoms, chance takes over. Since life depends on exquisite order, the naive physicist would predict that whatever directs an organism must be built from an enormous number of atoms, so that the averages hold. That prediction sets up the rest of the book, because it turns out to be wrong.

The gene is too small and too stable

In the next two chapters Schrödinger reviews what biologists of his day knew about heredity. The carriers of inheritance sit in the chromosomes, and each gene, by the estimates then available, is tiny, a cluster of atoms far too few for statistical law to guarantee anything. Yet genes are astonishingly permanent. They are copied faithfully and handed down from generation to generation.

They are also not perfectly permanent, and that matters just as much. Now and then a gene changes abruptly. These mutations are rare, they come as jumps with nothing in between, and once made they are inherited. Schrödinger stresses that such discontinuous changes are the raw material on which natural selection works. So the physicist faces a real contradiction: the thing that governs life is small where it should be large, and durable where it should be fragile. Classical physics, he concludes, cannot explain it.

The quantum answer

The way out, in chapter four, comes from Schrödinger's own science. Quantum mechanics says that a small system cannot hold just any amount of energy. It can exist only in certain distinct states, with nothing permitted in between. That is why molecules are stable even when they contain only a handful of atoms. To rearrange a molecule into a different configuration, its atoms must be lifted over an energy threshold in a single quantum jump, and at ordinary temperatures such a jump may not happen for a very long time.

This gives him both properties at once. A gene, if it is a molecule, will stay the same for ages because nothing short of a jump can alter it. And when the rare jump does occur, the molecule lands in a new stable arrangement, which is exactly what a mutation looks like. Schrödinger was drawing here on the experimental work of Max Delbrück and his colleagues in Berlin, who had presented early evidence that heredity has a molecular basis, and his lectures brought that work to far wider notice.

The aperiodic crystal and its code-script

Chapter five holds the book's most famous idea. Schrödinger points out that true solids are crystals, in which atoms are locked in an orderly pattern by the same forces that hold a molecule together. A molecule, he says, might be called "the germ of a solid". But an ordinary crystal repeats the same small unit endlessly, and a repeating pattern can say almost nothing.

The hereditary material, he reasons, must be a solid of a different sort: an aperiodic crystal, a large molecule in which the arrangement does not repeat. Because each position can differ, a modest number of atoms can spell out an almost limitless variety of messages. He goes further and gives the idea a name that would echo through biology. The chromosomes, he writes, contain a code-script that determines "the entire pattern of the individual's future development and of its functioning in the mature state". The gene is at once the plan and the means of carrying it out.

This was, as the biologist Matthew Cobb has written, the first clear suggestion that genes carry a code. Schrödinger did not mean it quite as we do. He did not propose that parts of the script correspond to particular chemical reactions, and the notion of genetic "information" entered biology later, through the work of others. But when the structure of DNA was worked out, a long molecule whose sequence of bases does not repeat, his phrase proved to fit.

Life feeds on negative entropy

Chapter six turns from heredity to a second puzzle. The second law of thermodynamics says that disorder, or entropy, always increases, and that every system left alone runs down toward a dead equilibrium. Living things seem to do the opposite. They build and maintain intricate order for decades.

Schrödinger's answer is that an organism is not a closed system. It stays alive by continually drawing order from its surroundings, by feeding, as he puts it, on negative entropy. What we take from food is not merely matter or energy but orderliness, which the body uses to offset the disorder it is always producing. The second law is not broken. The order inside the organism is more than paid for by the disorder it exports, as heat, to the world outside.

In the same chapter he makes his boldest claim: that living matter, without escaping the known laws of physics, is likely to involve other laws not yet discovered, which will one day be as much a part of science as the old ones. In the final chapter he explains what he has in mind. Besides order-from-disorder, physics also knows order-from-order, the direct, mechanical lawfulness of a clock or of the planets. Any system behaves more like clockwork the colder it gets, and heat a clock enough and it melts into a puddle of randomly moving molecules. Yet some things, clocks among them, behave mechanically even at room temperature, and Schrödinger's suggestion is that the machinery of life belongs with them.

The epilogue: one mind

Schrödinger ends with philosophical speculation on determinism, free will and consciousness. If the body is a mechanism that obeys the laws of nature, and yet each of us knows from direct experience that we direct our own actions and are responsible for them, how can both be true? His answer is that the "I" which says so must be the very thing that governs the atoms. He connects this to the teaching of the Upanishads that the individual self and the universal self are one. Minds only seem to be many, he argues. In his words, "consciousness is a singular of which the plural is unknown". He admits that this conclusion is subjective, and readers are free to part company with him here.

Putting It to Work

• Read it as a way of thinking, not a textbook. Later critics pointed out that its biology is dated and that the talk of new laws and negative entropy was speculative. Its lasting value, historians argue, is as inspiration. It shows how to ask a precise question about a vague subject.
• Carry a question across a border. Schrödinger's outsider view let him see heredity as a problem of physical stability. The book drew a generation of physicists into biology, including Watson, Crick and Maurice Wilkins.
• Keep the credit straight. The geneticist Hermann Muller had described the key properties of a hereditary molecule in the 1920s, and he later complained that what the book got right was already in print. Schrödinger's contribution was to derive those properties from first principles and to make them vivid.
• Remember the two big pictures. Life is information stored in a stable, non-repeating molecule, and life is a process that keeps its order by exporting disorder. Both remain central to how biology is taught.
• Notice the power of a phrase. Aperiodic crystal and code-script gave researchers something to look for before anyone knew what it was.

Who Should Read It

This is for curious readers who enjoy watching a first-rate scientist reason from the ground up, and for anyone interested in how physics, chemistry and biology fit together. Students of genetics will find the origin of ideas they take for granted, and those drawn to questions of free will and consciousness will find the closing pages provocative. It is brief and free of equations, but dense. Expect to read slowly, and to disagree with parts of it.