What Is Life? is a 1944 non-fiction science book written for the lay reader by physicist Erwin Schrödinger. The book was based on a course of public lectures delivered by Schrödinger in February 1943, under the auspices of the Dublin Institute for Advanced Studies 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."^[1] 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?"^[1]

In the book, Schrödinger introduced the idea of an "aperiodic crystal" that contained genetic information in its configuration of covalent chemical bonds. In the 1950s, this idea stimulated enthusiasm for discovering the genetic molecule. Although the existence of DNA had been known since 1869, its role in reproduction and its helical shape were still unknown at the time of Schrödinger's lecture. In retrospect, Schrödinger's aperiodic crystal can be viewed as a well-reasoned theoretical prediction of what biologists should have been looking for during their search for genetic material. Francis Crick, co-discoverer of the structure of DNA, credited Schrödinger's book with presenting an early theoretical description of how the storage of genetic information would work, and acknowledged the book as a source of inspiration for his initial research.^[2]

Background

The book is based on lectures delivered under the auspices of the Institute at Trinity College, Dublin, in February 1943 and published in 1944. At that time DNA was not yet accepted as the carrier of hereditary information, which only was the case after the Hershey-Chase experiment of 1952. One of the most successful branches of physics at this time was statistical physics, and quantum mechanics, a theory which is also very statistical in its nature. Schrödinger himself is one of the founding fathers of quantum mechanics.

Max Delbrück's thinking about the physical basis of life was an important influence on Schrödinger.^[3] Geneticist and 1946 Nobel-prize winner H.J. Muller had in his 1922 article "Variation due to Change in the Individual Gene"^[4] already laid out all the basic properties of the heredity molecule that Schrödinger derives from first principles in What is Life?, properties which Muller refined in his 1929 article "The Gene As The Basis of Life"^[5] and further clarified during the 1930s, long before the publication of What is Life?^{[6][verification needed]}

Content

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 mentionsdiffusion, which can be modeled as a highly ordered process, but which is caused by random movement of atoms or molecules. If the number of atoms is reduced, the behaviour of a system becomes more and more random. He states that life greatly depends on order and that a naive 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 this time about the hereditary mechanism. Most importantly, he elaborates the important role mutations play in evolution. He concludes that the carrier of hereditary information has to be both small in size and permanent in time, contradicting the naive 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 were known before, their stability could not be explained by classical physics, but is 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 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 believes the heredity material to be a molecule, which unlike a crystal does not repeat itself. He calls this an aperiodic crystal. The aperiodic nature allows 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 knows that this statement is 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. Schrödinger explains that living matter evades the decay to thermodynamical equilibrium by feeding on negative entropy.

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 some exceptions as the movement of the celestial bodies and the behaviour of mechanical devices such as clocks. But even those are influenced by thermal and frictional forces. The degree to which a system functions mechanically or statistically depends on the temperature. If heated, a clock ceases to function, because it melts. Conversely, if the temperature approachesabsolute zero, any system behaves more and more mechanically. Some systems approach this mechanical behaviour rather fast with room temperature already being practically equivalent to absolute zero.

Schrödinger concludes this chapter and the book with philosophical speculations on determinism, free will, and the mystery of human consciousness. He is sympathetic to the Hinduconcept of Brahman, by which each individual's consciousness is only a manifestation of a unitary consciousness pervading the universe. In the final paragraph, he points out that what is meant by "I" is not the collection of experienced events but "namely the canvas upon which they are collected." If a hypnotist succeeds in blotting out all earlier reminiscences, he writes, there would be no loss of personal existence - "Nor will there ever be."^[7]

Schrödinger's paradox

The concept of Schrödinger's paradox originates from the ideas presented in What is Life?^[8] A quick account of the Schrödinger paradox and its solution: In a world governed by thesecond law of thermodynamics, all closed systems are expected to approach a state of maximum disorder. In contrast, life approaches and maintains a highly ordered state - which seems to violate aforementioned second law. The solution to this paradox is that life is not a closed system. The increase of order inside an organism is more than paid for by an increase in disorder outside this organism. By this mechanism, the second law is obeyed, and life maintains a high order state, which it sustains by causing a net increase in disorder in the Universe.