Does a physics lecture from 1943 still have anything to teach a biologist, a coder, or a curious reader in 2026? I didn’t expect Erwin Schrödinger’s What Is Life? The Physical Aspect of the Living Cell to answer that so directly, but eighty-two years after Cambridge University Press first printed it, this short book still explains why a living cell doesn’t fall apart the way everything else in the universe eventually does.
I read it expecting a historical curiosity. I finished it convinced it is one of the few science books written before 1950 that a person can pick up today and still use.
At a glance
- What it is: A Nobel Prize-winning physicist’s attempt to explain heredity, mutation, and the stability of life using thermodynamics and the early language of quantum mechanics.
- Where it came from: A series of public lectures Erwin Schrödinger gave at Trinity College Dublin in February 1943, published as a short book by Cambridge University Press in 1944.
- Why it matters: Nearly ten years before James Watson and Francis Crick worked out the double helix in 1953, Schrödinger predicted that the hereditary material had to be an “aperiodic crystal,” a molecule irregular enough to carry a code. Watson later said the book pointed him toward finding the secret of the gene, and Crick, whose own training was in physics, cited it too.
- The famous idea: Living things stay ordered by feeding on “negative entropy,” pulling order out of their surroundings faster than the Second Law of Thermodynamics can pull them apart.
- The twist most reviews skip: Schrödinger’s original Dublin publisher, Cahill & Co, refused to print the book once they saw his closing epilogue on free will, because it clashed with Catholic teaching. Cambridge University Press picked it up instead.
- The catch: Physicist colleagues, and later Nobel laureates Linus Pauling and Max Perutz, argued that “negative entropy” was a shaky metaphor and that Schrödinger missed work already being done on genes and enzymes. Schrödinger admitted as much himself.
- My verdict: Still worth reading in full, still occasionally wrong, and still more honest about its own limits than most popular science published this year.
What Is Life? is Best for:
- Readers who want to understand how a physicist’s outsider question changed the direction of biology
- Anyone curious about the actual scientific reasoning behind DNA before DNA was found
- Students of the history of science who want a primary source instead of a summary of one
- People who enjoy short, dense, opinionated nonfiction they can finish in an afternoon
Not for:
- Readers wanting current, textbook-accurate genetics; parts of What Is Life? are almost eighty years out of date
- Anyone who wants a strictly secular argument; the final third moves into Hindu philosophy and religious language
- Readers who dislike a little math; there is no calculus, but there are ratios, exponents, and a few equations
Table of Contents
Introduction
What Is Life? The Physical Aspect of the Living Cell is a 1944 nonfiction book by the Austrian physicist Erwin Schrödinger, first published by Cambridge University Press and based on a course of public lectures he delivered under the auspices of the Dublin Institute for Advanced Studies at Trinity College, Dublin, in February 1943.
The edition I read pairs the original text with two later pieces, Mind and Matter (first published in 1958, drawn from his 1956 Tarner Lectures at Cambridge) and his Autobiographical Sketches, and opens with a foreword written by physicist Roger Penrose in August 1991, nearly three decades before Penrose himself won a share of the 2020 Nobel Prize in Physics for his own work on black holes.
That combined Canto Classics edition first appeared in 1992 and, according to Cambridge University Press’s own printing history, had reached its twenty-third printing by 2018, which is a remarkable run for a book built around thermodynamics and chromosome biology.
Schrödinger was not a biologist. He was a theoretical physicist who had already won the 1933 Nobel Prize in Physics, shared with Paul Dirac, for the wave equation that carries his name and that still sits at the center of every undergraduate quantum mechanics course.
By the time he stood in front of an audience of roughly four hundred people in Dublin, he had left Nazi Germany in 1933 over his opposition to the persecution of Jewish colleagues, bounced through Oxford, Graz, and Rome, and finally settled at the newly founded Dublin Institute for Advanced Studies, brought there at the personal invitation of Irish Taoiseach Éamon de Valera.
He later called his seventeen years in Dublin the happiest of his life.
So What Is Life?‘s purpose is stated almost apologetically in its own preface: Schrödinger admits he is not a master of the subject and is, in his own words, prepared to risk “making fools of ourselves” in order to attempt a synthesis that no single specialist could manage alone.
His central question, laid out in the opening chapter, is direct: how can physics and chemistry account for the events that happen inside a living organism? His preliminary answer is that they can, once we understand that a gene is not built like ordinary matter.
Background
To appreciate why a physicist felt entitled to lecture biologists, it helps to know the moment this book was written in. It was February 1943, in the middle of the Second World War, and genetics as a field had spent thirty years accumulating breeding data (chromosome maps, dominant and recessive traits, X-ray-induced mutation rates) without a clear physical explanation for how a structure that small could stay stable for generations.
Meanwhile, quantum mechanics, the field Schrödinger himself had helped build in the 1920s, had already solved a related mystery in chemistry: why molecules hold together at all. Schrödinger’s contribution was to notice that nobody had connected these two bodies of knowledge.
Even amid wartime paper shortages and a Dublin lecture hall that, according to Time magazine’s coverage from April 1943, still drew a full house despite an explicit warning that the material would not be simplified, Schrödinger pressed ahead.
He was, by his own account in the Autobiographical Sketches included in this edition, “an ardent follower of Darwinism” since childhood, after devouring On the Origin of Species in conversations with his botanist father, so the subject was not a passing interest. It had been building for decades.
What Is Life? Summary
I am going to walk through this chapter by chapter, because the book’s real reward is watching Schrödinger’s argument accumulate. You should not need to reread the original after this section.
Chapter 1: The General Character and Purpose of the Investigation. Schrödinger opens with an oddly simple question: why are atoms so small, relative to us? His answer reframes the question entirely. The real puzzle is why our bodies are so large relative to atoms. He explains that physical laws are statistical.
A single atom behaves unpredictably, but a large enough group of atoms obeys precise laws, with the accuracy of those laws improving in proportion to the square root of the number of particles involved, what he calls the “√n rule.”
An organism built from too few atoms would be at the mercy of random heat motion and could never sustain reliable behavior, let alone thought.
This sets up his most famous piece of terminology: a gene, he argues, must be an “aperiodic crystal,” a molecule that behaves like a stable solid but, unlike an ordinary crystal’s repeating pattern, is irregular enough at the molecular level to store a code.
Chapter 2: The Hereditary Mechanism. Here Schrödinger walks through what genetics already knew by 1943: chromosomes carry a “code-script” for the organism’s full development, cells duplicate this code faithfully through mitosis, and the reductive division called meiosis halves the chromosome set so that each parent contributes exactly one copy to their offspring.
He calculates, using the accepted figures of his day, that a gene occupies a space of roughly 300 angstroms, small enough to contain “certainly not more than about a million or a few million atoms.” He also states, following the standard genetics of 1944, that the human cell contains 48 chromosomes.
Chapter 3: Mutations. Schrödinger explains Hugo de Vries’s 1901 discovery that heredity changes in sudden, discrete jumps rather than Darwin’s gradual, continuous variation, and calls this, only half figuratively, “the quantum theory of biology.”
Drawing on the 1935 paper by Max Delbrück, Nikolai Timoféëff-Ressovsky, and Karl Zimmer, he shows that X-ray-induced mutation rates rise in exact proportion to radiation dosage and are independent of wavelength, which lets him calculate that a single mutation is triggered by an ionization event inside a target volume of only about ten atomic distances across, containing roughly a thousand atoms.
Chapter 4: The Quantum-Mechanical Evidence. This is the most technical chapter, and also the one where the payoff arrives. Schrödinger uses the 1927 Heitler-London theory of the chemical bond to explain why a molecule holds a stable shape: switching from one configuration to another requires crossing an energy threshold, and the higher that threshold is relative to average heat energy (a ratio he calls W:kT), the longer the molecule stays put.
He offers a striking table of figures: a threshold thirty times the average heat energy gives a molecule a life expectancy around a tenth of a second, but raise that ratio to fifty and the expected lifetime jumps to sixteen months, and at sixty it reaches thirty thousand years.
A gene, he argues, only needs a modest jump in chemical stability to last for centuries, which finally explains a fact that had been sitting unexplained in front of geneticists all along: why a trait like the “Habsburg lip,” documented in family portraits across four centuries, could be passed down essentially unchanged.
Chapter 5: Delbrück’s Model Discussed and Tested. Schrödinger formalizes the picture: a gene is a large, irregular molecule capable of flipping into a different stable arrangement (an isomer), and that flip is what a mutation actually is at the atomic level.
To make the storage capacity plausible, he reaches for the Morse code as an analogy, noting that a code using just two symbols in groups of ten already allows tens of thousands of combinations, and one using five symbols in groups of twenty-five allows for more than sixty trillion. His point is that a molecule does not need to be enormous to hold an almost unlimited amount of hereditary information.
Chapter 6: Order, Disorder and Entropy. This is the chapter most people quote and the one Schrödinger himself later had to defend.
He argues that a living organism avoids decaying into the “dangerous state of maximum entropy,” which is death, by continuously drawing “negative entropy” from its environment, essentially eating order in the form of complex organic molecules and exporting disorder as heat and waste.
He grounds this in Boltzmann’s statistical definition of entropy as the logarithm of molecular disorder, and reframes negative entropy as simply order measured with a negative sign.
Chapter 7: Is Life Based on the Laws of Physics? Schrödinger closes the scientific portion of What Is Life? by distinguishing two kinds of order-producing processes: “order from disorder,” the statistical kind that governs ordinary physics and chemistry, and “order from order,” the kind that governs how a single, stable gene can dictate the reliable, repeated unfolding of an entire organism.
He compares this to a clock, arguing that a clock only behaves predictably because it is built from a solid, and a solid is held together by the same quantum-mechanical bonding that keeps a gene stable even at room temperature.
He credits Walther Nernst’s Third Law of Thermodynamics for showing why some systems behave almost mechanically even without being anywhere near absolute zero.
Epilogue: On Determinism and Free Will. Schrödinger closes with what he calls, honestly, his own subjective opinion, separate from the science before it. He argues that if the body operates as a physical mechanism, and a person nonetheless experiences directing that mechanism, then the only logically consistent conclusion is that consciousness is singular rather than plural, an idea he traces to the Vedanta concept of Atman equaling Brahman, the self equaling the universal self.
He writes plainly that this leads, in Christian terms, to a statement that “sounds both blasphemous and lunatic,” namely, “Hence I am God Almighty,” and asks the reader to set that reaction aside long enough to consider the argument.
This was the section that, according to a 2024 retrospective published in the journal Structural Chemistry, caused Schrödinger’s intended Dublin publisher, Cahill & Co, to withdraw from the project entirely, since the claim ran against Catholic Church teaching. Cambridge University Press took the book on instead, and it has never been out of print since.
What Is Life? Analysis
Reading this as a document of intellectual history rather than a modern biology textbook, I think What Is Life? succeeds at exactly what it set out to do, and Schrödinger says as much in his own preface: convey one idea, not a survey.
That idea, that hereditary stability requires a physical explanation and that explanation has to come from quantum-level chemical bonding rather than classical statistics, holds up.
The evidence Schrödinger marshals for it, the X-ray mutation data from Timoféëff-Ressovsky, Delbrück, and Zimmer, the Heitler-London bonding theory, and the observed permanence of traits like the Habsburg lip, is used carefully and consistently, and he is scrupulous about flagging where he is speculating versus where the physics is settled.
What genuinely surprised me is how often What Is Life? reads like it is thinking out loud, correcting itself in real time (he adds two “amendments” to his own molecular model mid-chapter, which is not something popular science books do anymore).
Where What Is Life? strains is in the second half, once the argument leaves quantum chemistry and enters metabolism and consciousness. The negative entropy argument is elegant as a hook but loose as physics, a point Schrödinger later conceded in his own note appended to Chapter 6. And the epilogue is not really an extension of the scientific argument at all; it is a separate essay wearing the same book jacket.
I don’t think that sinks the book. I think it is honest about what it is, which is rarer than it should be in science writing that tries to say something about meaning.
Strengths and Weaknesses
What I found compelling:
The aperiodic crystal prediction. This is the reason the book is still assigned in physics and biology courses. Schrödinger had no access to X-ray crystallography of DNA, no knowledge of its double helix, and yet he correctly reasoned that the hereditary molecule had to be irregular rather than repetitive to carry information.
Historian Horace Freeland Judson, in his history of molecular biology The Eighth Day of Creation, credited Schrödinger with the earliest mention of what became known as the coding problem, years before anyone knew what that code was made of.
The writing itself. There is no jargon dumped on the reader without explanation, and Schrödinger repeatedly stops to check that a non-specialist can follow him, including a genuinely charming digression comparing the Ångström unit to a tailor measuring a suit.
Intellectual honesty. Few authors append a note to their own book admitting that named colleagues found a central metaphor unconvincing. Schrödinger does exactly that in the note following Chapter 6.
What did not hold up, and why it matters:
The chromosome count is wrong, through no real fault of the author. Schrödinger states that human cells carry 48 chromosomes, which was the accepted figure in genetics from 1912 until 1956, when Joe Hin Tjio and Albert Levan, working in Lund, Sweden, proved the true diploid number is 46.
This was a field-wide error that predates Schrödinger and outlived his book by twelve years, but it is a useful reminder that even a physicist working from the best available biology can inherit a mistaken premise.
He appears not to have engaged with the “one gene, one enzyme” hypothesis. George Beadle and Edward Tatum had already published this idea in 1941, two years before Schrödinger’s lectures, and Nobel laureate Max Perutz later criticized the book, in a 1987 essay in Nature, for not seeming to have heard of it.
Negative entropy drew real pushback. Linus Pauling argued Schrödinger’s treatment of biological specificity at the molecular level was too vague, and Schrödinger himself admitted in a note he added to the book that, writing only for physicist colleagues, he “should have let the discussion turn on free energy instead.”
The epilogue will not land for every reader. Whether you find the free will argument bold or overreaching will depend heavily on your own prior views, and Schrödinger gives you little room to disagree quietly.
He tells you directly that the conclusion is his personal opinion, which I respect, but it is still a large philosophical claim resting on a fairly short chain of reasoning.
Comparison With Similar Books
The most natural comparison is not a contemporary of Schrödinger’s but his own intellectual descendant: Paul Nurse’s 2020 book What Is Life? Five Great Ideas in Biology.
Nurse, who won the 2001 Nobel Prize in Physiology or Medicine and now directs the Francis Crick Institute in London (named for one of the scientists Schrödinger’s original book helped inspire), reuses the same title on purpose, and where Schrödinger reasons from first principles toward the gene, Nurse writes from eighty additional years of confirmed biology back toward the same question of what separates living matter from everything else.
Reading them together feels like watching a question get asked twice, once before the answer existed and once long after.
For readers who want the free will argument from the epilogue in a different key, I would point to Mark Twain’s philosophical dialogue What Is Man?, which I reviewed here on Probinism, and which reaches a similarly deterministic conclusion about human agency from a completely different, entirely secular starting point.
It is worth reading the two side by side.
Readers who enjoyed how Schrödinger extends a single scientific idea into a case about meaning and human nature may also want to look at Daniel Dennett’s Darwin’s Dangerous Idea, which I have also reviewed on Probinism, and which asks a structurally similar question about evolution instead of thermodynamics.
And for the other half of this story, the actual 1953 discovery that confirmed Schrödinger’s prediction, James Watson’s memoir The Double Helix is the natural next read; Watson names What Is Life? directly as the book that pointed him toward genetics in the first place.
Conclusion
I recommend What Is Life? to anyone who wants to see a scientist think in public, mistakes and all, rather than read a tidy retrospective account of how DNA was found.
It rewards a reader with some patience for a stray equation and works equally well for a general audience curious about the history of science and for students who want the primary document instead of a paraphrase of it. It is not a genetics textbook, and treating it as one will only highlight where it has aged.
Read as what it actually is, a physicist’s honest, occasionally wrong, and remarkably forward-looking attempt to answer one hard question in under two hundred pages, it remains, eighty-two years on, worth the afternoon it takes to finish.
Quick Answers (FAQ)
What is Schrödinger’s “aperiodic crystal”?
It is Schrödinger’s term for the molecule that carries heredity, a structure stable like a crystal but irregular enough, unlike a repeating crystal lattice, to encode information. It anticipated the actual structure of DNA by roughly a decade.
What does “negative entropy” mean in What Is Life?
It is Schrödinger’s description of how living things stay ordered: by drawing order out of their environment (through food, in most animals) faster than the Second Law of Thermodynamics can break that order down.
Did Schrödinger actually predict DNA?
Not the double helix itself, but he correctly predicted that heredity had to be stored in a stable, irregular molecule, which is exactly what Watson and Crick found in 1953. Both scientists later credited this book as an influence.
Is What Is Life? hard to read for a non-scientist?
It requires some patience with ratios and a handful of equations, but Schrödinger avoids calculus and explains every technical term as he introduces it, so a motivated general reader can follow it without a science background.
Why did Schrödinger’s original publisher refuse What Is Life?
His first choice of publisher, Cahill & Co in Dublin, withdrew once they read his epilogue on free will, which conflicted with Catholic teaching. Cambridge University Press published it instead in 1944.
