What Is Life? The Physical Aspect of the Living Cell is a 1944 science book written for the lay reader by the physicist Erwin Schrödinger. The book was based on a course of public lectures delivered by Schrödinger in February1943, under the auspices of the Dublin Institute for Advanced Studies, where he was Director of Theoretical Physics, at Trinity College, Dublin. The lectures attracted an audience of about 400, who were warned "that the subject-matter was a difficult one and that the lectures could not be termed popular, even though the physicist’s most dreaded weapon, mathematical deduction, would hardly be utilized."Margulis, Lynn. & Sagan, Dorion. (1995). What Is Life? (pg. 1). Berkeley: University of California Press. Schrödinger's lecture focused on one important question: "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?"
In the book, Schrödinger introduced the idea of an "aperiodic solid" that contained genetic information in its configuration of covalent chemical bonds. In the 1940s, this idea stimulated enthusiasm for discovering the chemical basis of genetic inheritance. Although the existence of some form of hereditary information had been hypothesized since 1869, its role in reproduction and its helical shape were still unknown at the time of Schrödinger's lecture. In 1953, James D. Watson and Francis Crick jointly proposed the double helix structure of deoxyribonucleic acid (DNA) on the basis of, amongst other theoretical insights, X-ray diffraction experiments conducted by Rosalind Franklin. They both credited Schrödinger's book with presenting an early theoretical description of how the storage of genetic information would work, and each independently acknowledged the book as a source of inspiration for their initial researches.. Page 28 details how Watson came to appreciate the significance of the gene.
The "What is Life?" Lecture Series What is Life? is based on the lecture series of the same name delivered in February 1943 under the auspices of the Dublin Institute for Advanced Studies (DIAS) at Trinity College, Dublin. The lectures were attended by the Irish Taoiseach (Prime Minister) Éamon de Valera and his cabinet. At that time, DNA was known to be a constituent of cell nuclei but had not yet been identified with certainty as the molecular basis of inheritance; the concept of a "heredity molecule" was strictly theoretical, with various candidates. One of the most successful branches of physics at that time was statistical physics. Schrödinger himself is one of the founding fathers of quantum mechanics, a theory which posits a statistical focus for understanding the natural world at subatomic scale.
In 1940, Éamon de Valera had invited Schrödinger to become the first Director of the School of Theoretical Physics at DIAS. Schrödinger, an Austrian physicist who had shared the Nobel Prize in Physics in 1933 for his development of wave-mechanics, had left Nazi Germany and, after sojourns at Oxford, Graz and Ghent, settled in Dublin for what became a sixteen-year tenure, the longest of his life. One of DIAS's statuary requirements was to organise an annual lecture series for a general audience. Schrödinger devised the 1943 series himself and delivered it at Trinity College Dublin (TCD).
Under the collective title “What is Life?”, Schrödinger presented three public lectures on consecutive Fridays (the 5th, 12th, and 19th) in February 1943 at TCD. The lectures were held in the Physics Lecture Theatre in the Fitzgerald building. By Schrödinger's accounting in What is Life?, the audience numbered about 400 for each lecture. As the Physics Lecture Theatre held only about 150, each lecture is thought to have been repeated on each of the following Mondays. The lectures were widely publicised in The Irish Times, and even received coverage in Time magazine in the U.S. Indeed, on February 6, 1943, The Irish Times published a photograph of Schrödinger with Dr Ernest Alton, Provost of Trinity, and Éamon de Valera standing outside the Physics Building at Trinity before the lecture.
Schrödinger’s stated objective was to address the question: “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?” The subject matter was acknowledged in advance as difficult and not a popular science lecture in the usual sense. Schrödinger explored how physical concepts might underlie the stability of hereditary structures, mutation, and the organisation of living cells. He introduced the concept of the “aperiodic crystal” to describe the proposed hereditary material. He also argued that living systems are consistent with the laws of thermodynamics and compensate for the reduction in entropy associated with their growth by feeding on “negative entropy” from their environment. While the molecular mechanism of heredity in the form of DNA had not yet been discovered, in 1935 Nikolay Timofeev-Ressovsky, Karl Zimmer, and Max Delbrück had presented the first experimental evidence of a molecular basis of heredity at the Kaiser Wilhelm Institute for Chemistry in Berlin. Schrödinger knew Delbrück from his time in Berlin and was deeply intrigued by his experimental work; his speculations in Dublin brought attention to Delbrück's work and helped stimulate further inquiry.
What is Life? has been widely credited with influencing the young molecular biologists James D. Watson and Francis Crick, both of whom acknowledged his work as formative in their discovery of the DNA double helix in 1953. The Dublin Institute for Advanced Studies holds a 1953 letter from Crick to Schrödinger noting that he and Watson had been influenced by the book.
The lectures have been regarded as “the most important and exciting thing to happen in Trinity in all of the six dark years of wartime.” Time reported that “Only in the precarious peace of Eire could Europe today provide such a spectacle. At Dublin’s Trinity College last month crowds were turned away from a jampacked scientific lecture. Cabinet ministers, diplomats, scholars and socialites loudly applauded a slight, Vienna-born professor of physics. Erwin Schrödinger was speaking on the subject ‘What Is Life?’.”
Beginning in 1995 and continuing to this day, Trinity College has commemorated the original 1943 lecture series with an annual “What is Life?” lecture, usually delivered in the Physics Lecture Theatre. In 2018, Trinity also organised the 75th-anniversary conference “Schrödinger at 75: The Future of Biology”. The Trinity Library also produced an exhibition celebrating Schrödinger’s Dublin years and the impact of the lectures.
Background Max Delbrück's thinking about the physical basis of life was an important influence on Schrödinger. However, long before the publication of What is Life?, the American geneticist Hermann J. Muller, who would later win a Nobel Prize in 1946, had in his 1922 article "Variation due to Change in the Individual Gene" already laid out all the basic properties of the "heredity molecule" (not yet known to be DNA) which Schrödinger re-derived in 1944 "from first principles" in What is Life? (including the "aperiodicity" of the molecule), properties which Muller specified and refined additionally in his 1929 article "The Gene As The Basis of Life"Proceedings of the International Congress of Plant Sciences 1 (1929) and during the 1930s. Muller himself wrote in a 1960 letter to a journalist regarding What Is Life? that whatever the book got right about the "hereditary molecule" had already been published before 1944 and that Schrödinger's were only the wrong speculations; Muller also named two famous geneticists, including Delbrück, who knew every relevant pre-1944 publication and had been in contact with Schrödinger before 1944. DNA as the molecule of heredity became foremost only after Oswald Avery's bacterial-transformation experiments published in 1944; before those experiments, proteins were considered the most likely candidates. DNA was confirmed as the molecule in question by the Hershey–Chase experiment conducted in 1952.
Content of What is Life? book In Chapter I, Schrödinger explains that most physical laws on a large scale are due to chaos on a small scale. He calls this principle "order-from-disorder". As an example he mentions diffusion, which can be modeled as a highly ordered process, but which is nevertheless caused by random movement of atoms or molecules. As the number of atoms is reduced, the behaviour of a system becomes increasingly random. He states that life greatly depends on order and that a naïve physicist may assume that the master code of a living organism has to consist of a large number of atoms.
In Chapter II and III, he summarizes what was known at the time about the hereditary mechanism. Most importantly, he elaborates on the role mutations play in biological evolution. He concludes that the carrier of hereditary information has to be both small in size and permanent in time, contradicting the naïve physicist's expectation. This contradiction cannot be resolved by classical physics.
In Chapter IV, Schrödinger presents molecules, which are indeed stable even if they consist of only a few atoms, as the solution. Even though molecules had long been known to exist, their stability could not be explained by classical physics due to the discrete nature of quantum mechanics. Furthermore, mutations are directly linked to quantum leaps.
He continues to explain, in chapter V, that true solids, which are also permanent, are composed of highly ordered crystals. The stability of molecules and crystals is due to the same principles, and a molecule might be called "the germ of a solid". On the other hand, an amorphous solid, without crystalline structure, should be regarded as a liquid with a very high viscosity. Schrödinger writes that the heredity material is likely to be a molecule, which unlike a crystal does not repeat itself. He calls this an "aperiodic crystal". Its aperiodic nature allows it to encode an almost infinite number of possibilities with a small number of atoms. He finally compares this picture with the known facts and finds it in accordance with them.
In Chapter VI, Schrödinger states:
...living matter, while not eluding the "laws of physics" as established up to date, is likely to involve "other laws of physics" hitherto unknown, which however, once they have been revealed, will form just as integral a part of science as the former.
He anticipates that this statement will be open to misconception and tries to clarify it. The main principle involved with "order-from-disorder" is the second law of thermodynamics, according to which entropy only increases in a closed system (such as the universe). Schrödinger explains that living matter evades the decay to thermodynamical equilibrium by homeostatically maintaining negative entropy in an open system.
In Chapter VII, he maintains that "order-from-order" is not absolutely new to physics; in fact, it is even simpler and more plausible. But nature follows "order-from-disorder", with such exceptions as the predictable movement of the celestial bodies and the behaviour of mechanical devices such as clocks. Even those are influenced by thermal and frictional forces. The degree to which a system functions mechanically or randomly depends on the temperature. If sufficiently heated, a clock melts into a puddle of randomly moving molecules. Conversely, if the temperature approaches absolute zero, any system behaves more and more mechanically. Some systems, such as clocks, approach this mechanical behaviour even at room temperature.