“What Is Life?” is a short but enormously influential book by the physicist Erwin Schrödinger, published in 1944. In it, Schrödinger asked a deceptively simple question: how can physics and chemistry explain the things that happen inside a living cell? His answers, drawn from thermodynamics and quantum mechanics, anticipated the discovery of DNA’s structure by nearly a decade and helped launch the field of molecular biology.
Where the Book Came From
The book grew out of a series of public lectures Schrödinger delivered at the Dublin Institute for Advanced Studies during World War II. Already a Nobel Prize winner for his work in quantum mechanics, Schrödinger turned his attention to biology. He wasn’t running experiments on cells or genes. Instead, he was asking what the known laws of physics demanded of living matter, and where those laws seemed to fall short.
Life as Organized Resistance to Decay
Schrödinger’s central insight was thermodynamic. Everything in the physical world tends toward disorder. A hot cup of coffee cools, a stone wall crumbles, and a dead organism decays. This drift toward randomness is what physicists call entropy. Living organisms, Schrödinger argued, are remarkable because they temporarily resist this process. They hold themselves in a state of high internal order even as the universe around them slides toward chaos.
How do they pull this off? Through metabolism. By eating, breathing, and excreting waste, an organism continuously imports order from its environment and exports disorder back out. Schrödinger described this as “feeding on negative entropy,” a phrase later shortened to “negentropy.” He put it vividly: a living thing “really consists in continually sucking orderliness from its environment.” When that process stops, the organism reaches thermodynamic equilibrium, which is another way of saying it dies.
This framing was powerful because it gave biologists a way to think about life in the language of physics. It also raised a more precise question: if life depends on maintaining order, what physical structure inside the cell is responsible for storing and transmitting that order from one generation to the next?
The Aperiodic Crystal
This was Schrödinger’s most famous prediction. He reasoned that genetic material had to store an enormous amount of information in a very small space. A regular crystal, like quartz, wouldn’t work. In a periodic crystal, the atoms repeat in the same pattern over and over. If you know the position of one unit, you can predict the position of every other unit. That kind of structure can’t carry much information, for the same reason that a wallpaper pattern, no matter how pretty, can’t encode a novel.
What was needed, Schrödinger proposed, was an “aperiodic crystal”: a structure with the stability and regularity of a crystal, but with a non-repeating arrangement of its components. The variation in that arrangement is what would carry biological information. He imagined the atoms of this structure rearranged in countless different configurations, each responsible for different instructions governing how an organism develops.
Nine years later, James Watson and Francis Crick revealed the double-helix structure of DNA, which is exactly that: a stable, crystalline molecule whose four chemical bases are arranged in a non-repeating sequence that encodes genetic instructions. DNA is neither periodic like quartz, nor random like glass. It is, as Schrödinger predicted, an aperiodic crystal.
The Code-Script
Schrödinger also introduced the idea of a “code-script” contained within chromosomes. This is sometimes credited as the first published suggestion that heredity works through a kind of coded message. He compared the idea to Morse code, pointing out how a small number of symbols, arranged in different sequences, could specify a vast number of distinct meanings.
The analogy wasn’t perfect. Schrödinger envisioned something closer to a set of operating rules (like a legal code or a highway code) than a cipher that gets translated symbol by symbol. He didn’t know about DNA or messenger RNA, and he initially assumed the genetic material was made of protein. Still, the core insight held up: biological information is stored as a molecular sequence, and the order of that sequence matters. As the science historian Horace Freeland Judson later wrote, “The earliest mention of coding that counts was Erwin Schrödinger’s, in 1944.”
Why Genes Don’t Fall Apart
One of the subtler arguments in the book drew on quantum mechanics. Schrödinger was puzzled by the stability of genes. Cells copy their genetic material billions of times across generations, yet the instructions remain remarkably faithful. Classical physics couldn’t easily explain this. At the molecular scale, thermal energy should constantly jostle atoms out of position, introducing errors.
Schrödinger argued that quantum mechanics provided the answer. Chemical bonds within molecules exist in discrete energy states, meaning it takes a specific minimum amount of energy to break or rearrange them. Small thermal fluctuations aren’t enough. This is why genetic material can persist through countless cell divisions without degrading into noise. He approached the gene “not as an algebraic unit but as a physical substance that had to be almost perfectly stable and yet express immense variety.” That combination of stability and variety is what makes heredity possible.
The Book’s Impact on DNA’s Discoverers
Few science books have had such a direct line to a major discovery. James Watson, who co-discovered DNA’s structure, described picking up the book as a 17-year-old college student: “In that little gem, Schrödinger said the essence of life was the gene. Up until then, I was interested in birds. But then I thought, well, if the gene is the essence of life, I want to know more about it. And that was fateful because, otherwise, I would have spent my life studying birds and no one would have heard of me.”
Francis Crick, Watson’s collaborator, called the book “peculiarly influential” and said it “attracted people who might otherwise not have entered biology at all.” After he and Watson published their DNA structure in 1953, Crick wrote directly to Schrödinger: “Watson and I were once discussing how we came to enter the field of molecular biology, and we discovered that we had both been influenced by your little book. We thought you might be interested in the enclosed reprints. You will see that it looks as though your term ‘aperiodic crystal’ is going to be a very apt one.” Maurice Wilkins, whose X-ray crystallography work was essential to the discovery, likewise credited the book with getting him interested in biological problems for the first time.
Where Schrödinger Got It Wrong
Not everything in the book held up. Schrödinger initially assumed that genes were made of protein, which was the prevailing guess at the time. DNA was considered too simple a molecule to carry hereditary information. He also treated his hypothetical genetic molecule as a solid whose atoms could be physically rearranged into different shapes, rather than a polymer whose information is encoded in the linear sequence of its subunits. The mechanism was wrong, but the functional requirement he identified was correct.
His thermodynamic framing has also been refined. The phrase “feeding on negative entropy” caused confusion even among physicists when the book was first published. In strict thermodynamic terms, organisms don’t consume negative entropy directly. They take in low-entropy energy sources (like food or sunlight) and release high-entropy waste (like heat and carbon dioxide). The net effect is what Schrödinger described: the organism maintains its internal order by increasing the disorder of its surroundings. Modern biologists have also found that the picture is more nuanced at the microbial level, where some organisms generate enough heat during growth that the entropy accounting works differently than Schrödinger assumed.
Why the Book Still Matters
At just over 90 pages, “What Is Life?” remains one of the rare works that genuinely bridged physics and biology. Schrödinger didn’t discover DNA, crack the genetic code, or run a single biological experiment. What he did was frame the right questions in a way that made physicists take biology seriously and made biologists think about information and thermodynamics. The book demonstrated that a living cell isn’t just chemistry. It is a system that stores, copies, and acts on encoded information while maintaining itself far from equilibrium. That framework still underpins how scientists think about life today.