What is Life?

What is Life? is a non-fiction book on science for the lay reader written by physicist Erwin Schrödinger (ISBN 0521427088). Francis Crick cited What is Life? as the best theoretical description, before the actual discovery of DNA, of how genetic storage would work. In the book, Schrödinger introduced the idea of an "aperiodic crystal" that contained genetic information in its configuration of covalent chemical bonds. This idea both stimulated enthusiasm for discovering the genetic molecule and could be seen (in retrospect) as having been a well-reasoned theoretical prediction of what biologists should have been looking for during their search for the genetic material.

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 Francis Crick and James D. Watson elucidated the structure of DNA in 1953.

One of the most successful branches of physics at this time was statistical physics, to which also Albert Einstein had contributed by explaining Brownian motion, 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[1].


 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 mentions diffusion, 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 he 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. Life is based on a different principle, "order-from-order."

In chapter VII Schrödinger 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 approaches absolute 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 view, common in Indian mysticism, that each individual's consciousness is only a manifestation of a unitary consciousness pervading the universe. In the final paragraph, though, he appears to contradict this opinion somewhat by emphasizing the uniqueness of each human being's store of memories, thoughts and perceptions.

 

Scientific Approaches To The Meaning of Life  Entropy and Life  What is Life?   Philosophical views on the meaning of life 

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