What Is Life?

Summary

What Is Life? The Physical Aspect of the Living Cell is a short book based on public lectures that Erwin Schrödinger gave at Trinity College Dublin in February 1943, under the auspices of the Dublin Institute for Advanced Studies. Writing as a physicist who admits he is not a biologist, Schrödinger asks how the events inside a living organism can be accounted for by physics and chemistry. In the first chapter he explains why ordinary physical laws are statistical: they hold reliably only because they describe enormous numbers of atoms, and their precision improves roughly with the square root of the number involved. That is why organisms must be large compared with atoms. Yet heredity seems to defy this rule. In the second and third chapters he reviews the hereditary mechanism, chromosomes, meiosis and Mendelian inheritance, and the mutations described by Hugo de Vries, and uses estimates from X-ray mutation experiments to argue that a gene may contain only about a million atoms or fewer, far too few for statistical reliability. How, then, do hereditary traits remain unchanged over many generations? His answer, in the fourth and fifth chapters, comes from quantum mechanics. Drawing on the 1935 work of N. W. Timoféeff-Ressovsky, K. G. Zimmer and Max Delbrück, he describes the gene as a large molecule whose configurations are separated by energy thresholds, so that it stays stable at body temperature and changes only by rare quantum jumps, which are the mutations. He calls the chromosome an aperiodic crystal: unlike a periodic crystal that merely repeats a pattern, it is an irregular structure that can carry a code-script specifying the whole development of the organism, as a Morse-like code can carry endless meanings with a few signs. The sixth chapter turns to thermodynamics. A living thing avoids decay to equilibrium by drawing order from its surroundings, feeding, in his phrase, on negative entropy, which plants obtain from sunlight. In the seventh chapter he contrasts two ways of producing order: order from disorder, the statistical regularity of physics, and order from order, the new principle by which the organism's small, stable structures govern its life, like the clockwork that runs by quasi-static mechanism. He concludes that living matter, while obeying the laws of physics so far established, probably involves other laws of physics hitherto unknown. A short Epilogue, On Determinism and Free Will, offers his personal view, drawn from the Upanishads, that the conscious self which feels itself directing the body's atoms is one with the universal mind. Cambridge University Press published the book in 1944, and later editions add a note to Chapter 6 on free energy.

Plot or Argument

• 1. The classical physicist's approach

  Physical laws are statistical and precise only for vast numbers of atoms, so organisms must be large; the puzzle is how heredity can depend on very small structures.

• 2-3. The hereditary mechanism and mutations

  Chromosomes, cell division, Mendelian crossing-over and estimates of gene size; de Vries's mutations as discontinuous jumps, and X-ray induced mutation rates as evidence of the gene's small target size.

• 4-5. Quantum-mechanical evidence and Delbrück's model

  Quantum theory explains why molecules are stable: changes require energy thresholds, so a gene can persist for centuries and mutate by rare quantum jumps. The gene is pictured as an aperiodic crystal carrying a code-script.

• 6. Order, disorder and entropy

  Life escapes decay to equilibrium by drawing order from its environment, a process Schrödinger describes as feeding on negative entropy, ultimately supplied by sunlight.

• 7. Is life based on the laws of physics?

  The organism exhibits order from order rather than the order from disorder of statistical physics, comparable to a clockwork; new physical laws may be found in living matter.

• Epilogue: On determinism and free will

  Schrödinger argues that the body runs by natural law while the self directs it, and concludes, in the language of the Upanishads, that the self is the universal consciousness; he presents this as his personal view.

Characters

• Gregor Mendel, founder of genetics

  His laws of inheritance are the starting point of Schrödinger's account of heredity.

• Hugo de Vries, Dutch botanist

  His observations of sudden, heritable changes give Schrödinger the concept of mutation as a jump.

• Max Delbrück, physicist turned biologist

  Co-author of the 1935 model of the gene as a stable molecule, which Schrödinger expounds.

• N. W. Timoféeff-Ressovsky and K. G. Zimmer, geneticist and radiation physicist

  Delbrück's collaborators, whose X-ray experiments allowed the estimate of gene size.

• Max Planck, founder of quantum theory

  Cited in the discussion of dynamical and statistical types of law.

Lessons

• Asking a large question well

  Schrödinger turns the vague question what is life into precise ones: why atoms are small, how a tiny gene can be stable, and how an organism avoids decay. His book shows how a question from one field can be sharpened by the tools of another.

  • Why does Schrödinger begin by asking why atoms are so small?
  • What makes the stability of heredity a puzzle for statistical physics?
  • Can you name a question in one subject that another subject might help answer?

• Information in matter

  The idea of an aperiodic crystal carrying a code-script anticipated the discovery that DNA stores hereditary information in a sequence of chemical units. Schrödinger saw that the important feature was not substance but arrangement.

  • How does an aperiodic crystal differ from an ordinary crystal?
  • Why does Schrödinger compare the code-script to the Morse code?

• Order sustained against decay

  A living thing stays alive only by taking order from its surroundings and releasing disorder. The picture of the organism as constantly sustained from outside gives a physical meaning to the old observation that life must be fed.

  • What happens to a system left to itself, according to the second law of thermodynamics?
  • Why did Schrödinger later add a note about free energy to Chapter 6?
  • What does it mean to say life is order from order?

• Science and the self

  In the Epilogue Schrödinger moves beyond physics to the question of who directs the motion of the atoms, offering a personal answer from Hindu philosophy. Readers can see a great scientist marking where his science ends and his metaphysics begins.

  • Why does Schrödinger separate the Epilogue from the scientific chapters?
  • Does physics alone settle whether we are free?

Context

In 1943 the chemical nature of the gene was unknown; many biologists thought hereditary material was probably protein, and the experiment of Avery, MacLeod and McCarty pointing to DNA appeared only in 1944. Physicists, meanwhile, were turning to biology, and the 1935 paper of Timoféeff-Ressovsky, Zimmer and Delbrück on mutation and gene structure gave Schrödinger his central model. He wrote in neutral Ireland during the Second World War, where Éamon de Valera had founded the Dublin Institute for Advanced Studies partly to give him a post.

Composition

Schrödinger delivered three public lectures at Trinity College Dublin in February 1943 and revised them into a book of seven chapters with an epilogue, published by Cambridge University Press in 1944. Later printings added notes, including one to Chapter 6 explaining that physicists would speak of free energy where he had written negative entropy. The book is still in copyright and is described here in summary.

Reception

Francis Crick, James Watson and Maurice Wilkins all later said the book had drawn them or encouraged them toward the problem of the gene, and it is widely credited with helping bring physicists into molecular biology after the war. Some biologists and historians, notably Max Perutz, argued that much of its biology was derivative or mistaken, while others praise its framing of heredity as information and of life as an open thermodynamic system. Its title question has been taken up by many later scientists.

Comparisons

• mendel-experiments-in-plant-hybridisation

  Mendel showed that heredity comes in discrete, stable units; Schrödinger asks what physical structure could make those units so stable and answers with the aperiodic crystal.

• on-the-origin-of-species

  Darwin needed heritable variation without knowing its source; Schrödinger identifies mutations as quantum jumps in the gene molecule that supply natural selection with material.

• heisenberg-physics-and-philosophy

  A fellow founder of quantum mechanics also writes for general readers about what the theory means beyond physics, though Heisenberg reflects on Greek philosophy where Schrödinger turns to biology and Vedanta.

• eddington-the-nature-of-the-physical-world

  Eddington's popular lectures likewise explain entropy and the arrow of time and close with reflections on mind and free will.

• the-bhagavad-gita

  The Hindu tradition of the self's identity with the universal spirit, which Schrödinger cites from the Upanishads in his Epilogue, is expressed in the Gita as well.

Quotes

For Students

A reading companion for classical Christian high school students. See the student guide.

Reading Guide

Watch for

• The square-root rule in Chapter 1 and why it makes organisms large compared with atoms.
• How X-ray experiments let Schrödinger estimate the size of a gene in Chapter 3.
• The comparison of the chromosome to an aperiodic crystal and to the Morse code in the fourth and fifth chapters.
• The two principles of order from disorder and order from order in Chapter 7.
• The shift in tone in the Epilogue, where Schrödinger speaks of his own convictions.

Key terms and names

Aperiodic crystal

Schrödinger's term for a solid that, unlike an ordinary repeating crystal, has an irregular structure able to carry hereditary instructions.

Code-script

The pattern in the chromosome that, on his view, contains the full plan of an organism's development.

Mutation

A sudden, heritable change, which Schrödinger explains as a quantum jump of the gene molecule to a new stable configuration.

Entropy

A physical measure of disorder that tends to increase in an isolated system; living things keep their own entropy low.

Negative entropy

Schrödinger's phrase for the order an organism takes in from its environment; a later note says free energy is the more exact term.

Order from order

His name for the way an organism's few, stable molecular structures govern orderly events, unlike statistical order arising from many atoms.

Seminar Questions

1. Comprehension Why, according to Chapter 1, must an organism be made of very many atoms?
2. Comprehension What does Schrödinger mean when he calls the chromosome an aperiodic crystal?
3. Comprehension How does an organism avoid decaying into equilibrium, according to Chapter 6?
4. Interpretation Why does Schrödinger think quantum mechanics, rather than classical physics, is needed to explain the stability of genes?
5. Interpretation What does the contrast between order from disorder and order from order reveal about how living things differ from other matter?
6. Interpretation Why might a physicist's outside view have helped biologists see heredity as a problem of information?
7. Big Question Can physics and chemistry fully answer the question what is life, or does the question reach beyond them?

Faith Connections

• Psalm 139:13-16 (Scripture)

  The psalmist praises God for forming him in the womb and writing all his members in a book before they existed. Schrödinger's code-script gives a striking natural image for that ordered plan of development.

• Genesis 2:7 (Scripture)

  God forms man of the dust and breathes into him the breath of life. Schrödinger's question is how dust, ordinary atoms, can be so organised as to live, and Genesis frames life as a gift rather than a product of matter alone.

• Augustine, De Genesi ad litteram (The Literal Meaning of Genesis) V-VI (Great Conversation)

  Augustine taught that God created seminal reasons, causal principles that unfold in time; readers have compared the idea to a hidden plan carried in living things, though Augustine's concept is theological rather than physical.

Logic and Rhetoric

Central argument: Physical laws are statistical and hold only for vast numbers of atoms. Hereditary traits are stable over centuries yet depend on genes of relatively few atoms. Quantum mechanics shows that a large molecule can be stable through energy thresholds. Therefore the gene is best understood as an aperiodic crystal carrying a code-script, and life works by order from order, possibly involving physical laws not yet known.

Rhetorical device: analogy. In explaining the code-script, Schrödinger compares the variety a few signs can carry to the Morse code, where dots and dashes in different orders spell any message.

Debate: Resolved: that life can be fully explained by the laws of physics and chemistry.

Writing Prompts

Essay: Explain Schrödinger's reasoning from the size of atoms to the aperiodic crystal, and evaluate which steps later discoveries confirmed.

Comparison with Experiments in Plant Hybridisation (in Bateson's Mendel's Principles of Heredity): Mendel inferred stable hereditary factors from pea ratios; Schrödinger asked what physical structure they could be. Compare how each reasons from evidence to an unseen cause.

Medical Lens

For the history of medicine the book marks the point at which heredity began to be conceived as stored molecular information, the conceptual root of molecular genetics and of later genetic medicine. Schrödinger discusses mutation as a rare physical event in a single molecule, its induction by X-rays according to the dose, and the fact that most mutations are disadvantageous and recessive, so that they can be carried hidden in a population; these arguments drew on the radiation genetics of the 1920s and 1930s. His picture of the organism maintaining itself by taking in order from food and sunlight became a common way of describing metabolism. The Epilogue's argument about the self that directs bodily processes speaks to the mind-body problem, but as philosophy rather than medicine.

• Chapters 2-3

  Mendelian inheritance, recessive mutations carried without effect in heterozygotes, and X-ray induced mutation rates used to estimate gene size.

  Heredity Degeneration
• Chapter 4

  The permanence of hereditary traits across many generations explained by the quantum stability of the gene molecule.

  Heredity Degeneration
• Epilogue

  The body as a mechanism obeying natural law, set against the direct experience of directing its motions, resolved in the identity of the self with universal consciousness.

  Mind Body

Creative Prompts

• Classroom

  Write the same message using a periodic pattern (one repeated symbol) and an aperiodic one (a few symbols in varied order), and discuss why only the second can carry a code-script.

• Essay

  Schrödinger expected new laws of physics in living matter. Using what was learned about DNA after 1953, argue whether his expectation was fulfilled, refuted, or reframed.

• Dialogue

  Imagine a conversation in 1953 between Schrödinger and Mendel about what the double helix revealed about their work.

Further Reading

• The Double Helix, James D. Watson

• The Eighth Day of Creation, Horace Freeland Judson

• Schrödinger: Life and Thought, Walter Moore

• Mind and Matter, Erwin Schrödinger

  Later lectures continuing the Epilogue's themes; often printed with What Is Life?.