WHAT IS LIFE? numerous sections were originally intended to be ERWIN SCHRODINGER marginal summaries, and the text of every First published 1944 chapter should be read in continuo. E.S. Dublin September 1944 What is life? The Physical Aspect of the Living Cell. Homo liber nulla de re minus quam de morte Based on lectures delivered under the auspices of cogitat; et ejus sapientia non mortis sed vitae the Dublin Institute for Advanced Studies at meditatio est. SPINOZA'S Ethics, Pt IV, Prop. Trinity College, Dublin, in February 1943. 67 To the memory of My Parents (There is nothing over which a free man ponders Preface less than death; his wisdom is, to meditate not on A scientist is supposed to have a complete and death but on life.) thorough I of knowledge, at first hand, of some subjects and, therefore, is usually expected not to CHAPTER 1 write on any topic of which he is not a life, The Classical Physicist's Approach to the Subject master. This is regarded as a matter of noblesse oblige. For the present purpose I beg to renounce This little book arose from a course of public the noblesse, if any, and to be the freed of the lectures, delivered by a theoretical physicist to an ensuing obligation. My excuse is as follows: We audience of about four hundred which did not have inherited from our forefathers the keen substantially dwindle, though warned at the longing for unified, all-embracing knowledge. outset that the subject-matter was a difficult one The very name given to the highest institutions and that the lectures could not be termed popular, of learning reminds us, that from antiquity to and even though the physicist’s most dreaded throughout many centuries the universal aspect weapon, mathematical deduction, would hardly has been the only one to be given full credit. But be utilized. The reason for this was not that the the spread, both in and width and depth, of the subject was simple enough to be explained multifarious branches of knowledge by during without mathematics, but rather that it was much the last hundred odd years has confronted us too involved to be fully accessible to with a queer dilemma. We feel clearly that we mathematics. Another feature which at least are only now beginning to acquire reliable induced a semblance of popularity was the material for welding together the sum total of all lecturer's intention to make clear the fundamental that is known into a whole; but, on the other idea, which hovers between biology and physics, hand, it has become next to impossible for a to both the physicist and the biologist. For single mind fully to command more than a small actually, in spite of the variety of topics specialized portion of it. I can see no other involved, the whole enterprise is intended to escape from this dilemma (lest our true who aim convey one idea only -one small comment on a be lost for ever) than that some of us should large and important question. In order not to lose venture to embark on a synthesis of facts and our way, it may be useful to outline the plan very theories, albeit with second-hand and incomplete briefly in advance. The large and important and knowledge of some of them -and at the risk of very much discussed question is: How can the making fools of ourselves. So much for my events in space and time which take place apology. The difficulties of language are not within the spatial boundary of a living organism negligible. One's native speech is a closely fitting be accounted for by physics and chemistry? The garment, and one never feels quite at ease when preliminary answer which this little book will it is not immediately available and has to be endeavor to expound and establish can be replaced by another. My thanks are due to Dr summarized as follows: The obvious inability of Inkster (Trinity College, Dublin), to Dr Padraig present-day physics and chemistry to account for Browne (St Patrick's College, Maynooth) and, such events is no reason at all for doubting that last but not least, to Mr S. C. Roberts. They were they can be accounted for by those sciences. put to great trouble to fit the new garment on me and to even greater trouble by my occasional STATISTICAL PHYSICS. THE reluctance to give up some 'original' fashion of FUNDAMENTAL W DIFFERENCE IN my own. Should some of it have survived the STRUCTURE mitigating tendency of my friends, it is to be put That would be a very trivial remark if it were at my door, not at theirs. The head-lines of the meant only to stimulate the hope of achieving in the future what has not been achieved in the past. calling the periodic crystal one of the most But the meaning is very much more positive, viz. complex objects of his research, I had in mind that the inability, up to the present moment, is the physicist proper. Organic chemistry, indeed, amply accounted for. Today, thanks to the in investigating more and more complicated ingenious work of biologists, mainly of molecules, has come very much nearer to that geneticists, during the last thirty or forty years, 'aperiodic crystal' which, in my opinion, is the enough is known about the actual material material carrier of life. And therefore it is small structure of organisms and about their wonder that the organic chemist has already functioning to state that, and to tell precisely made large and important contributions to the why present-day physics and chemistry could not problem of life, whereas the physicist has made possibly account for what happens in space and next to none. time within a living organism. The arrangements of the atoms in the most vital parts of an THE NAIVE PHYSICIST'S APPROACH TO organism and the interplay of these arrangements THE SUBJECT differ in a fundamental way from all those After having thus indicated very briefly the arrangements of atoms which physicists and general idea -or rather the ultimate scope -of our chemists have hitherto made the object of their investigation, let me describe the line of attack. I experimental and theoretical research. Yet the propose to develop first what you might call 'a difference which I have just termed fundamental naive physicist's ideas about organisms', that is, is of such a kind that it might easily appear slight the ideas which might arise in the mind of a to anyone except a physicist who is thoroughly physicist who, after having learnt his physics imbued with the knowledge that the laws of and, more especially, the statistical foundation of physics and chemistry are statistical throughout. his science, begins to think about organisms and For it is in relation to the statistical point of view about the way they behave and function and who that the structure of the vital parts of living comes to ask himself conscientiously whether organisms differs so entirely from that of any he, from what he has learnt, from the point of piece of matter that we physicists and chemists view of his comparatively simple and clear and have ever handled physically in our laboratories humble science, can make any relevant or mentally at our writing desks. It is well-nigh contributions to the question. It will turn out that unthinkable that the laws and regularities thus he can. The next step must be to f compare his discovered should happen to apply immediately theoretical anticipations with the biological facts. to the behaviour of systems which do not exhibit It will then turn out that -though on the whole his the structure on which those laws and regularities ideas seem quite sensible -they need to be are based. The non-physicist cannot be expected appreciably amended. In this way we shall even to grasp let alone to appreciate the gradually approach the correct view -or, to put it relevance of the difference in ‘statistical more modestly, the one that I propose as the structure’ stated in terms so abstract as I have correct one. Even if I should be right in this, I do just used. To give the statement life and colour, not know whether my way of approach is really let me anticipate what will be explained in much the best and simplest. But, in short, it was mine. more detail later, namely, that the most essential The 'naive physicist' was myself. And I could not part of a living cell-the chromosome fibre may find any better or clearer way towards the goal suitably be called an aperiodic crystal. In physics than my own crooked one. we have dealt hitherto only with periodic crystals. To a humble physicist's mind, these are WHY ARE THE ATOMS SO SMALL? very interesting and complicated objects; they A good method of developing 'the naive constitute one of the most fascinating physicist's ideas' is to start from the odd, almost and complex material structures by which ludicrous, question: Why are atoms so small? To inanimate nature puzzles his wits. Yet, compared begin with, they are very small indeed. Every with the aperiodic crystal, they are rather plain little piece of matter handled in everyday life and dull. The difference in structure is of the contains an enormous number of them. Many same kind as that between an ordinary wallpaper examples have been devised to bring this fact in which the same pattern is repeated again and home to an audience, none of them more again in regular periodicity and a masterpiece of impressive than the one used by Lord Kelvin: embroidery, say a Raphael tapestry, which shows Suppose that you could mark the molecules in a no dull repetition, but an elaborate, coherent, glass of water; then pour the contents of the glass meaningful design traced by the great master.