molecular biology and physics

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Bringing physics to bear on the phenomenonof life: the divergent positions of Bohr,

Delbruck, and Schrodinger

Andrew T. DomondonCommittee on Conceptual and Historical Studies of Science, University of Chicago,

1126 East 59th St. Room 205, Chicago, IL 60637, USA

Received 2 April 2005; received in revised form 6 January 2006

Abstract

The received view on the contributions of the physics community to the birth of molecular biol-ogy tends to present the physics community as sharing a basic level consensus on how physics shouldbe brought to bear on biology. I argue, however, that a close examination of the views of three lead-ing physicists involved in the birth of molecular biology, Bohr, Delbruck, and Schrodinger, suggeststhat there existed fundamental disagreements on how physics should be employed to solve problemsin biology even within the physics community. In particular, I focus on how these three figures dif-fered sharply in their assessment of the relevance of complementarity, the potential of chemicalmethods, and the relative importance of classical physics. In addition, I assess and develop Roll-Hansens attempt to conceptualize this history in terms of models of scientific change advancedby Kuhn and Lakatos. Though neither model is fully successful in explaining the divergence of viewsamong these three physicists, I argue that the extent and quality of difference in their views help elu-

cidate and extend some themes that are left opaque in Kuhns model. 2006 Elsevier Ltd. All rights reserved.

Keywords: Niels Bohr; Max Delbruck; Erwin Schrodinger; Biology; Physics; Complementarity

1369-8486/$ - see front matter 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.shpsc.2006.06.014

E-mail address: [email protected]

Stud. Hist. Phil. Biol. & Biomed. Sci. 37 (2006) 433458

www.elsevier.com/locate/shpsc

Studies in Historyand Philosophy ofBiological andBiomedical Sciences
mailto:[email protected]:[email protected]


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1. Introduction

The role of physicists in the rise of molecular biology has been a favorite topic of manyhistorians of science. Evelyn Fox Keller captured this state of affairs nicely when she

remarked, The story of the role of physics in the development of molecular biology hassomething of an obsessive appeal to historians of science.1 Although this topichas indeedgenerated much literature, most of it tends to fall into one of three groups.2 One groupconsists of works that focus on a particular physicist and his or her contributions tomolecular biology. Some examples ofthis approach include Kays articles on Delbruck3

and Yoxens article on Schrodinger.4 A second group consists of works that focus onthe role of forces larger than a single individual (e.g. the Rockefeller Foundation) in shap-ing the molecular biology revolution. This group includes works such as Kays Molecularvision of the life5 and Kellers articlecited above. A third group consists of works suchasOlbys The path to the double helix6 and Moranges A history of molecular biology7 inwhich key experiments and discoveries in molecular biology form the central narrative.

Together these approaches have provided us with a rich picture of the emergence ofmolecular biology, but one important aspect that has been neglected in the received viewis the extent to which individual physicists brought differing philosophical and methodolog-ical commitments to their biological investigations. The relative inattention to this point isunfortunate because it conceals the variety of views that different physicists brought to theirbiological investigations. Moreover, the omission of this perspective perpetuates the falseperception that the early physicists working on biological problems shared a basic levelconsensus on how physics might contribute towards the advancement of biology.

Some works that are sensitive to this problem include those by Olby and Kay citedabove. Two more notable exceptions are Donald Flemings8 article on emigre physicistsand Niels Roll-Hansens9 article on the history of the complementarity from Bohr to Del-bruck. This paper follows the lead of Fleming and Roll-Hansen by undertaking a compar-ative examination of some of the methodological and philosophical assumptions of threeleading physicists involved in the rise of molecular biology: Niels Bohr, Max Delbruck,and Erwin Schrodinger.

What links these three physicists together is not simply their eminence as scientists buttheir considerable intellectual engagement with each other, especially on the topic of howphysics should be brought to bear on problems in biology. Bohr, of course, was instrumen-

tal in shifting Delbrucks research focus from physics to biology. Schrodingers What islife? was a major factor in popularizing Delbrucks ideas, especially his model of the

1 Keller (1999), p. 389.2 The references that I list under each group are not meant to be an exhaustive list of works that belong to a

given group. Rather, they are meant to serve as examples of works that exemplify the particular approach to thestudy of molecular of biology that characterizes each group. For an extensive list of references that deal with therole of physicists in the emergence of molecular biology see Keller (1999). Some other notable works includeHausmann (2003), Judson (1996), andSarkar (1996).3 Kay (1985a, 1985b).4 Yoxen (1979).5

Kay (1993).6 Olby (1974).7 Morange (1998).8 Fleming (1968).9 Roll-Hansen (2000).

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aperiodic crystal. Schrodinger also engaged seriously with Bohrs model of the atom andhis interpretation of quantum mechanics. For each of them life was a phenomenon thatseemed to defy the laws of physics, but they differed on what exactly they saw as paradox-ical or mysterious about the phenomenon of life and on what tools they thought should be

employed to understand that phenomenon. The tools they had at their disposal includedthe emerging discipline of quantum mechanics, increasingly powerful interventionist meth-ods based on chemistry, and the entirety of classical physics. Bohr, Delbru ck, and Schro-dinger, however, were hardly in agreement in their assessment of the relative importance ofthese tools. The aim of this paper is to understand the character of their disagreements byhighlighting their differing assessments on the value of complementarity, chemical meth-ods, and classical physics as means to unravel the mysterious phenomenon of life. In addi-tion, I will assess and develop Roll-Hansens attempt to understand the views of threephysicists in terms of models of scientific change advanced by Kuhn and Lakatos.

In Section 2, I will highlight and clarify some of Bohrs ideas on the relationshipbetween physics and biology. Section3will focus on Delbruck and how his views differedfrom many of the positions originally formulated by Bohr. Section 4will treat Schroding-ers attempt to develop a position on the relationship between physics and biology thatdraws on the work of Bohr and Delbruck but radically differs from both of them. Section5 will examine Roll-Hansens attempt to characterize these physicists engagement withbiology by employing ideas from Kuhn and Lakatos.

2. Bohrs purported influence on Delbruck

Although Bohrs name is most often mentioned in connection to the atomic model thatbears his name and his debates with Einstein over the status of quantum mechanics, healso played an important role in the birth of molecular biology. Unlike his contributionsto physics, his contributions to molecular biology are not linked to any specific theoreticalmodel. Rather, his main contribution was in stimulating physicists to look at the phenom-enon of life. One of the physicists upon whom he exerted a tremendous influence was MaxDelbruck. In particular, Bohr stimulated Delbrucks interest in biology through a lectureentitledLight and life, which he gave at the International Congress on Light Therapy in1932.10 Leon Rosenfeld noted Delbrucks reaction to this lecture as follows:

It would be a romantic exaggeration to say that we were fascinated by the lecture,

but it is a fact that when Delbru ck afterward read the text and pondered over it,

he was so enthusiastic about the prospects it opened up in the vast field of biology

that he there and then decided to take up the challenge.11

Rosenfelds observation was more than substantiated by Delbrucks subsequent move intobiology.

The deep connection between Delbruck and Bohr has been fairly well documented. Asnoted earlier, Kay and Roll-Hansen have written fine pieces on Delbruck. I am in agree-ment with many of the points made by these two scholars. Their focus, however, tendstowards the ways in which Delbruck was influenced by Bohr or followed his lead. This

10 Bohr (1933).11 Cited inOlby (1974), p. 231.

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perspective is valuable, but also tends to underplay the degree to which Delbruck departedfrom Bohrs original formulations. I will highlight these departures to show how theyattest to Delbrucks originality, and how they were important in defining his biologicalresearch. In short, I acknowledge that Bohr influenced Delbruck tremendously, but I stress

that he, like all good disciples, rethought and departed from many of the tenets held by hismentor. To make this case, I will focus on some key tenets that Bohr expressed in hisfamed Light and Life lecture. In particular, I highlight three tenets that are often citedin accounts that try to claim Delbruck as a follower of Bohr as opposed to a creative inter-preter of his ideas. These include: (1) the doctrine of complementarity, (2) the dismissal ofinterventionist methods, such as chemical methods, in probing the riddle of life, and (3) aconviction that new non-classical physical laws would arise from the study of life. Tounderstand how Delbruck parted with Bohr, this section will focus on clarifying Bohrsinitial formulation of these three themes.12 The next two sections will deal with how thepositions taken as Delbruck and Schrodinger differed from those taken by Bohr.

Of the three features noted above, the concept of complementarity has probablyreceived the most scholarly attention. This is not surprising because Bohrs ideas on thissubject are both profound and complex. To understand what he means by complementar-ity in the context of his Light and Life lecture, it is helpful to look first at his account ofcomplementarity as he originally formulated it. Bohr first articulated the idea of comple-mentarity in a paper he delivered at the Alessandro Volta commemoration conference in1927. This paper, often known as the Como paper, was later published with the title Thequantum postulate and the recent developments of atomic theory.13 Bohr scholars such asHenry Folse have also noted that Bohr himself considered this paper to be his more or less

fundamental statement.

14

Hence, it constitutes an ideal point from which to exploreBohrs understanding of complementarity.Folse have given a very detailed paragraph-by-paragraph exegesis of the Como paper. I

will not recapitulate that analysis here. Instead, I will focus on two particular aspects thatare especially important to understanding Bohrs usage of the term complementarity andhow Delbruck later draws upon that concept. The first concerns how he motivates theneed for the concept of complementarity. The second concerns his view of complementar-ity as a necessary perspective in understanding some features of nature. Regarding the lat-ter point, Bohr seems to have held that complementarity was only one of many possibleperspectives that an individual could bring to the study of nature. He thought, however,

that there are some phenomena (e.g. atomic phenomena, life) that are best understoodthrough the perspective of complementarity.15

12 I do not mean to suggest that Bohr or Delbruck remained set in their positions. As I will point out, it is truethat both of them changed their positions on the various issues discussed here. What I try to elucidate here is howDelbrucks views may be seen as re-interpretation of Bohrs early views on the relationship between physics andbiology, and how the views of these two figures differed from each other despite Bohrs great influence onDelbruck.13 Bohr (1934).14 Folse (1985).15

There is considerable debate on whether Bohrs concept of complementarity was an ontological claim or anepistemological one. That debate will not be resolved here, but in either case I believe it is clear that Bohr believedthat our ability to make measurements on the atomic level is hindered by limitations that had no counterpart inclassical physics. For an in-depth treatment of the philosophical status of complementarity in Bohrs thought, seeFolse (1985), Murdoch (1987), Faye (1991), Kanamori (1998), andBeller (1992).



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In the Como paper, Bohr motivates the idea of complementarity by noting the limita-tions of classical physics. In particular, he notes that classical physics is of limited use inunderstanding atomic phenomena. This is because classical physics assumes that the phe-nomenon and observer of the phenomenon are separable. Such an assumption, however, is

problematic at the atomic level. He says:

our usual description of physical phenomena is based on the idea that the phenom-

ena may be observed without disturbing them appreciably : : : Now the quantum

postulate implies that any observation of atomic phenomena will involve an interac-

tion with the agency of observation not to be neglected. Accordingly, an independent

reality in the ordinary physicalsense can neither be ascribed to the phenomena nor

to the agencies of observation.16

It is especially important to note that Bohr did not see quantum mechanics as simplya technical device that could be applied to circumvent the limitations of classical mechan-ics. Instead, he suggests that quantum mechanics must begin with an acknowledgementof the limitations of classical mechanics. He says, The quantum theory is characterizedby an acknowledgement of a fundamental limitation in the classical physical ideaswhen applied to atomic phenomena.17 Thus, for Bohr, the first step in accepting quantummechanics consists of embracing, not rejecting, the very real limitations of classicalmechanics.

According to Bohr, accepting the quantum postulate constituted an acceptance of aview of nature in which one cannot separate out the observer from the observed, at leastnot at the quantum level. More specifically, this constituted the framework for the doc-

trine of complementarity, which he first expressed in the following form:The very nature of quantum theory thus forces us to regard the space-time co-ordi-

nation and the claim of causality, the union which characterizes theclassical theories,

as complementarity but exclusive features of the description : : :18

In this passage Bohr is suggesting that position and momentum are complementary as-pects of description.19 At the atomic level these two descriptions are considered comple-mentary because one cannot simultaneously obtain absolute certainty in both values.This is the situation is expressed in the famous Heisenberg uncertainty relation, which im-plies that the complete certainty in one quantity comes at the expense of complete uncer-

tainty in the other. It should be noted, however, that Bohr also used the termcomplementarity in other contexts as well. For example, he used the term to describehow light could be understood as a particle as well as a wave.20

One further point that should be stressed concerning Bohrs understanding of comple-mentarity is that the limitations he speaks of are not limitations that can be overcome byemploying more ingenious measurement methods or better equipment. Bohr understoodthe limitations on our knowledge of the atomic world as arising from the fundamentalinseparability of the observer and observed object. It is also true that Bohr often discussed

16

Bohr (1934), p. 53.17 Ibid.18 Ibid., p. 54.19 Murdoch (1987).20 Bohr (1934), p. 56.



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how the process of observation itself disturbs an object. It is tempting to interpret thosediscussions as implying that a complementary perspective may be avoided altogether ifthe disturbances caused by those observations are minimized. A close reading of Bohrsessay, however, suggests otherwise. This point is brought out most clearly by the fact that

Bohr himself noted explicitly that at the quantum level the observer and the observed donot have independent realities. Folse has expressed this point nicely as follows:

Bohrs talk of a limitation on the classical descriptive concepts would appear to

refer to a limitation on knowledge of the classical mechanical states. That such a

reading of Bohrs words is not what he intended is clearly seen by the fact that it

is inconsistent with Bohrs primary conclusion: : : that we cannot describe an obser-

vation in a way which treats the observed object and the observing instrument as

having independent reality in the ordinary physical sense, i.e. in the sense of the

waves and particles of classical physics, because if the objects described by quan-

tum mechanics as such waves and particles did in fact independent reality in theordinary physical sense, it would be possible to define classical mechanical states

for them.21

For Bohr, accepting the principle of complementarity was a necessary precondition tograsping the character of nature.

How do the above aspects of Bohrs thought manifest themselves in his consideration ofthe phenomenon of life? He explores these issues in his Light and life lecture. One pointthat should be clarified immediately is that Bohrs attempt to import the concept of com-plementarity is not grounded in the fact that biological matter is also constituted from

atoms. In importing the concept of complementarity to the study of life, Bohr was suggest-ing that one cannot expect to understand the phenomenon of life solely in terms of itsparts. Instead, he suggests that life itself must be taken as a given:

the existence of life must be considered as an elementary fact that cannot be

explained, but must be taken as a starting point in biology, in a similar way as the

quantum of action, which appears as an irrational element from the point of view

of classical mechanical physics, taken together with the existence of the elementary

particles, forms the foundation of atomic physics. The asserted impossibility of a

physical or chemical explanation of the function peculiar to life would in this sense

be analogous to the insufficiency of the mechanical analysis for the understanding of

the stability of atoms.22

Bohrs importation of the concept of complementary into biology is not based on a reduc-tive argument linking living matter and atomic matter. Instead, he suggests that the studyof living phenomena and the study of atomic phenomena are similar in the sense that nei-ther of these phenomena can be understood by simply examining the parts that composethem. In both cases it is only with the acceptance or embracement of this limitation thatone can make further progress in understanding the phenomenon in question. In the sameway that the acceptance of the quantum postulate enables one to do quantum mechanics,Bohr argues that it is the acceptance of the phenomena of life as an elementary fact that

enables one to do biology.

21 Folse (1985), p. 111.22 Bohr (1933), p. 458.



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Although Bohr thought that a perspective incorporating complementarity was neces-sary to both the study of atomic phenomena and the study of living phenomena, he alsosaw a crucial difference between these two domains. In particular, Bohr thought that anyinvestigation of living phenomena was hampered by an additional degree of complexity.

He articulated this additional complexity as follows:

We must keep in mind, however, that conditions holding for biological and physical

researches are not directly comparable, since the necessity of keeping the object of

investigation alive imposes no counterpart in the latter. Thus, we should doubtless

kill an animal if we tried to carry the investigation of its or gans so far that we could

describe the role played by single atoms in vital functions.23

The preceding passage not only suggests that taking a reductionistic approach to the studyof life is problematic but also highlights the fact that this difficulty has no analogue inphysics. If a physicist ignores complementarity in studying atomic phenomena, then heor she may misunderstand the character of the phenomena in question. A crucial pointto note here is that even if the physicist makes such a mistake, the atomic phenomenathemselves are not destroyed. In contrast, Bohr stresses that a biologist who ignores com-plementarity in their dissections of living organisms risks destroying the very phenomenonthat he or she is trying to understand, namely the phenomenon of life.

In addition to the skepticism that Bohr voiced towards overly reductionistic approachestowards understanding the phenomenon of life, he was highly dismissive of the possibilitythat chemistry would provide any explanations that would advance or deepen our under-standing of life. More specifically, he says:

Analogies from chemical experience will not, of course, any more than the ancientcomparison of life with fire, give a better explanation of living organisms than will

the resemblance, often mentioned, between living organisms and such purely

mechanical contrivances as clockworks.24

This passage is noteworthy in that he suggests that chemistry provides no moreinsight than mechanical models in explaining the nature of life. Moreover, one should notethat Bohrs reason for rejecting chemistry as a means to explain life did not arise directlyfrom its early twentieth-century status as a science lacking in rigor when compared tophysics. Bohrs objection is a much more principled one. He contends that a chemical

explanation is insufficient because the chemical composition of a living organism is inconstant flux by virtue of its interaction with its environment. He expresses this point asfollows:

It is typical of biological researches, however, that the external conditions to which

any separate atom is subjected can never be controlled in the same manner as in the

fundamental experience of atomic physics. In fact, we cannot even tell which atoms

really belong to a living organism, since any vital function is accompanied by an

exchange of material, whereby atoms are constantlytaken up into and expelled from

the organization which constitutes the living being.25

23 Ibid.24 Ibid., p. 457.25 Ibid., p. 458.



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Hence, Bohrs position against chemistry did not stem from a simple prejudice of chemis-try as an inferior or less rigorous science but from deep theoretical considerations.

Furthermore, due to Bohrs close relationship with Delbruck, it is often assumed theBohr shared Delbrucks conviction that new non-classical physical laws would emerge

from the study of biology. As will be noted in the next section, it is true that Delbruck heldsuch a conviction, but Bohrs position is much more ambiguous. On the one hand, he cer-tainly seems to entertain the possibility that there exist some important aspects of the phe-nomenon of life that are yet undiscovered. He says, The question at issue, therefore iswhether some fundamental traits are still missing in the analysis of natural phenomena,before we can reach an understanding of life on the basis of physical experience. 26 Onthe other hand, the difficulty in understanding Bohrs position comes from understandingwhat Bohr meant by fundamental traits and in what sense they are before or prior tophysical experience. This is further complicated by the fact that Bohr rejects the possibilitythat an investigation into biological material will reveal a vitalistic force that is inexplica-ble in physical terms:

the wonderful features which are constantly revealed in physiological investigations

and differ so strikingly from what is known of inorganic matter, have led many biol-

ogists to doubt that a real understanding of the nature of life is possible on a purely

physical basis. On the other hand, this view, often known as vitalism, scarcely find its

proper expression in the old supposition that a peculiar vital force, quite unknown to

physics, governs all organic life. I think we all agree with Newton that the real basis

is the conviction that Nature under the same conditions will always exhibit the same

regularities.27

In rejecting vitalism Bohr seems to reassert that possibility that life can be explained inphysical terms. Moreover, he tries to clarify his original position by suggesting that the keyto understanding life resides in studying the organizationof living matter, not the materialcompositionof the living matter. In fact, Bohr states that he does not expect the investiga-tion of the composition of living matter to reveal any new laws: if we were able to push theanalysis of the mechanism of living organisms as far as that of atomic phenomena, weshould not expect to find any features differing from the inorganic properties of matter. 28

He did not see an inherent material difference between organic and inorganic matter. Inother words, he did not believe that investigating the material composition of life provides

any privileged access to the possibly missing fundamental traits of Nature. Instead, Bohrurged others to study how matter is organized in living organisms:

An understanding of the essential characteristics of living beings must be sought, no

doubt, in their peculiar organization, in which feature that may be interwoven with

typically atomistic traits in a manner having no counterpart in inorganic matter.29

For Bohr, if there is something novel to be discovered from the study of life, it resides inthe way that material components are organized.

26 Ibid., p. 457.27 Ibid.28 Ibid.29 Ibid.



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One difficulty that remains in Bohrs formulation is that even if it is the organization ofmatter that is important, it is still unclear how one should proceed towards uncovering thesupposedly missing fundamental traits without employing a reductionistic approach.Bohr might argue that one should take a complementary approach, not a reductionistic

approach, but it would appear that the essence of the complementary approach is anappreciation of a kind of holism. Yet holism, at least in Bohrs interpretation of it, seemsto deny thesoundness of any approach that tries to understand an organism by reducing itto its parts.30 Consequently, Bohr himself suggests that we should understand life as agiven, as an elementary fact.31 If Bohrs claims are understood in a concrete manner, thisleads to the question of how one can hold on to the holism of the complementarityperspective while searching for the missing supposedly undiscovered fundamentaltraits. This was the task that Max Delbruck, the central figure of the next section, madehis own.

3. Delbruck as an original interpreter of Bohr

There is little doubt that Bohr was a key influence in Delbrucks turn to physics. Onnumerous occasions Delbruck acknowledged the tremendous influence Bohr had onhim. Delbruck, in inviting Bohr to give the inaugural lecture for the dedication of the Insti-tute of Genetics at the University of Cologne, wrote: It was you who inspired me 30 yearsago to go into biologyand I believe I am the only one of your disciples who had made hisway in this direction.32 There is, however, an important irony to this invitation. In Mind

from matter, a book written by Delbruck towards end of his life, he notes that he had

hoped that Bohr would use the inaugural lecture to clarify and reassess the idea of com-plementarity that he had spoken of in his 1932 Light and life lecture, the lecture thattransformed Delbruck life.33 Delbruck undoubtedly felt Bohrs mark deeply upon his life,but it is uncertain whether Delbruck understood Bohr in the way that Bohr intended. Del-brucks final book leaves little indication that Delbrucks uncertainties were allayed withBohrs final lecture. In fact, I submit that such uncertainties forced Delbruck to interpretBohrs ideas from the 1932 lecture in his own way, and that these interpretative differencesmanifested themselves in Delbrucks biological studies. Though Delbruck is often por-trayed as a faithful disciple of Bohr, his own views differed from Bohr in important ways.He is better understood as an original and creative of interpreter of Bohrs ideas. To make

this case, I will focus on Delbrucks understanding of complementarity, his views on theproper means to probe the phenomenon of life, and his conviction that new laws wouldemerge from the study of life.

The principle of complementarity is perhaps the thickest line that is often thought tolink Delbruck and Bohr. Bohrs ideas on complementarity certainly had a great effecton Delbruck, but it is a mistake to understand Delbrucks subsequent research in biology

30 A commitment to holism by itself does not necessarily entail the rejection of studying a phenomenon in termsof its parts. Bohrs holism, however, seems to imply a commitment to seeing the phenomenon in question as anindivisible whole.31

Bohr (1933), p. 458.32 Cited inKay (1985a), p. 487.33 Delbruck (1986), pp. 236237. Bohrs lecture is found inBohr (1963). By this time Bohr had become more

appreciative of the potential of chemistry and other interventionist methods. Moreover, his discussion of theimportance of complementarity for biology took on a more historical character. See note 36 in this paper.



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as a straightforward application of Bohrs ideas. Given the abstract character of Bohrsformulation of complementarity, Delbruck re-interpreted Bohrs ideas in concrete exper-imental terms. Though inspired by Bohrs lecture, Delbruck, as observed by Fuerst,expressed considerable confusion in interpreting the meaning of complementarity.34 When

asked to reflect on Bohrs ideas on complementarity, Delbruck remarked that they con-sisted of

various degrees of vagueness, which I found both fascinating and very disturbing,

because it was always so vague : : : it was intriguing and annoying at the same time

: : : sufficiently intriguing for me, though, to decide to look more deeply specifically

into the relation of atomic physics and biologyand that means learn some

biology.35

Ultimately, Delbruck interpreted Bohrs ambiguous expressions of complementarity interms of a concrete project: the search for complementarity in livingorganisms. It is, how-ever, unlikely Bohr was arguing for such a concrete interpretation.36 For example, Mor-ange notes, Bohr was not a very clear speaker, and the lesson that Delbruck tookhome from the lecture was probably not what Bohr intended.37 Aage Bohr, Niels Bohrsson, echoed this view when he remarked to Delbruck that it is not my impression that he[Niels Bohr] took it [complementarity] quite as concretely as you did.38

Having given Bohrs ideas on complementarity a concrete interpretation, Delbrucksought to discover a paradox or, to borrow Kays term, an epistemological crisis in biol-ogy that resembled the waveparticle duality of light, a topic that had so deeply mystifiedthe physicists. The sense in which this search dominated Delbrucks research is nicely cap-

tured in the title of Gunther Stents introductory essay in Phage and the origins of molec-ular biology, a book celebrating Delbrucks sixtieth birthday and his career: Introduction:Waiting for a Paradox.39 In fact, speaking of what he saw as the main problem of biology,Delbruck said, In biology we are not yet at the point where we are presented with clearparadoxes and this will not happen until the analysis of the behavior of living cells hasbeen carried into far greater detail.40

In attempting to rectify this state of affairs, Delbrucks first step was to choose anorganism for his studies that had the greatest possibility of exposing or bringing the par-adox to the fore. His search for such an organism began with the Drosophila, but hemoved to viruses until settling on the phage. For Delbruck, the phage turned out to be

34 Fuerst (1982).35 Cited inibid., p. 263.36 Bohrs discussion of complementarity in relation to biology may also be interpreted as an attempt to make a

historical analogy with quantum mechanics. In particular, he may be interpreted as suggesting that thephenomenon of life seemingly defies or escapes description through known physical laws of his time in a waysimilar to how the phenomenon of complementarity and other quantum mechanical properties were inexplicablethrough classical physics alone. I am grateful to an anonymous referee for bringing this point to my attention.This interpretation of complementarity relation to biology appears more prominently in Bohrs later work,

especiallyBohr (1963).37 Morange (1998) p. 40.38 Cited inRoll-Hansen (2000), p. 439.39 Stent (1992).40 Delbruck (1992), p. 22.



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an ideal organism for study because it had a simple structure yet possessed rich replicativeproperties.41 Having determined the ideal object of study, a natural second step was tostudy it in a way that would allow for the paradox to reveal itself. To understand whatsuch an approach entailed for Delbruck, it is importantto give attention to the fact that he

often spoke of the phage as the atoms of biology.42 On first consideration, this moveappears to have reductionistic overtones, and to the extent that he chose to study simplemodels over more complex ones, one might call his approach reductionistic. It must bestressed, however, that Delbruck did not hold that complex phenomena were nothingmore than an aggregate of their parts. His aim was not to explain life in terms of its con-stituent parts. Instead, Delbrucks desire to work with simple systems was much moremotivated by the fact that they are more tractable and amenable to the kind of analysishe wished to undertake. He saw the phage as a kind of organic system upon whichone could impose various inputs and observe the consequent outputs to arrive at an under-standing of the inner workings of the system. As Kay put it:

That analogy of phage to atoms of biology corresponds closely to Delbru cks goal

of finding a simply and efficient model of replication requiring little physicochemical

intervention and amenable to numerical analysis. He conceptualized the riddle of life

(replicating phage) within the black box of the cell (host bacterium). The box did not

need to be pried open in order to study replication; biochemistry was irrelevant to his

epistemological program.43

Given Kays remarks above, it is tempting to see Delbruck as a faithful follower ofBohr. This view seemingly draws support from two characteristics of Delbrucks research:

his use of minimally invasive or non-interventionist methods to the study of life and hisdismissal of chemistry as a viable method for solving the riddle of life. Such a view is partlyjustified, but it also obscures some crucial differences between Bohr and Delbruck.Regarding Delbrucks use of non-interventionist methods, on first glance he does seemto respect Bohrs suggestion that an understanding of the essentialcharacteristics of livingbeings must be sought, no doubt, in their peculiar organization.44 Moreover, Delbrucksresearch appears motivated by a literal interpretation of Bohrs suggestion that there existsa complementarity relation between life and atomic physics analogous to thecomplemen-tarity encountered with the wave and particle aspects of atomic physics.45 In actuality,however, though Delbruck may have seen himself as carrying out Bohrs ideas by attempt-

ing to uncover such a relation through non-interventionist methods, it is a mistake tooverlook the fact that the two differed fundamentally in their understanding of how com-plementarity would manifest itself in living organisms. Bohr viewed complementarity as anartifact of the fundamental inseparability between the observer the object being observed.Delbrucks approach is in tension with Bohrs views because Delbruck believed thatnon-interventionist methods could be used to circumvent the problem of inseparabilityof the inseparability between the observer and object being observed. Bohr understood

41 Kay (1985b), p. 231. It may also be noted that Delbruck departs from Bohr in that he places emphasis on thereplicative properties of life. This is perhaps why the phage, with its rich replicative, was an especially attractive

object of study for Delbruck. This point is also explored in van Helvoort (1992).42 Ibid.43 Kay (1993), p. 135.44 Bohr (1933), p. 458.45 Delbruck (1976), p. 299.



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complementarity as a result of the fact that in any measurement process the observeralways interacts with the object in a way that depends on what feature is being measured.For example, light manifests itself as a wave when one measures it for properties of awave, but it manifests itself as a particle when it is measured for properties of a particle.

In contrast, Delbruck, understood complementarity as a property that exists independentof the observer. Consequently, it is not surprising that he thought that complementaritycould be exposed or surfaced through non-interventionist methods.

Delbruck is sometimes considered an opponent of chemistry because he often criticizedthe enterprise of biochemistry. For example, such an impression arises from his 1969 lec-ture A physicist turns to biology, where he claimed:

He [the physicist] may be told that the only real access of atomic physics to biology is

through biochemistry. Listening to the story of modern biochemistry he might

become persuaded that the cell is a sack of enzymes acting on substrates converting

them through various intermediate stages either into cell substance or into wasteproducts. They in turn must be brought into position by maneuvers which are yet

understood, but which at first sight do not necessarily differ in nature from the rest

of biochemistry : : : this program of explaining the simple through the complex

smacks suspiciously of the program of explaining atoms in terms of complex

mechanical models.46

These negative remarks, however, must be balanced against evidence that suggests Del-bruck viewed chemistry in much more positive terms. In fact, in a 1976 lecture given atCentennial of the Carlsberg Laboratory at Copenhagen entitled Light and life III, Del-

bruck gave a lecture that focused on the chemistry of photochemical reactions.

47

It was alecture filled with references to chlorophyll, phytochromes, and enzymes. The publishedversion of the lecture even included a reaction pathways diagram, a favorite schematic toolof biochemists. One might argue that it was only late in his career that he adopted a morefavorable attitude towards biochemistry. Even in his early works, however, he suggeststhat chemistry may be important in understanding the phenomenon of life. For instance,as early as 1937, he suggested that one should view replicationnot as complementarity toatomic physics but as a particular trick of organic chemistry.48

Given the seemingly two-sided character of Delbrucks stance towards chemistry, onemust ask what might account for it. In contrast to Bohrs very principled objection against

chemistry, Delbrucks objection was multifaceted but less firmly grounded in theoreticalconsiderations. Although Kay claimed earlier that biochemistry played little or no roleepistemological role in Delbrucks work, her own study on Delbruck suggests some rea-sons to doubt this conclusion, as well as insights for understanding Delbrucks complexstance toward chemistry. According to Kay, the more critical side of Delbruck likelystemmed from multiple factors, such as ignorance and old prejudices:

Delbru cks disdain for biochemistry was mostly based on ignorance. He objected on

several grounds: the social componentcontamination by the MD spirit; the philo-

sophical premisethe mechanistic approach to biology, and from an aesthetic point,

46 Delbruck (1992), p. 22.47 Delbruck (1976).48 Delbruck (1970), p. 1315. Also cited inFuerst (1982), p. 263.



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its cumbersome laboratory technique and its lack of theoretical elegance. Conjuring

up images of an alchemical laboratory, Delbru cks impressions were a throwback to

the beginning of the century, when primitive equipment andcrude procedures fre-

quently resulted in chemical artifacts and dubious theories.49

Kays remarks above describe Delbruck in 1947, the year in which he joined the faculty atthe California Institute of Technology. She also notes, however, that during his tenureDelbruck came to be impressed increasingly by the technological prowess (e.g. powerfulcentrifuges, electron microscopes, etc.) of the university and its interdisciplinary approachto scientific problems. Kay nicely captures how this change of environment likely affectedDelbruck as follows: With indirect mathematical analyses yielding only partial knowledgeof subcellular events, it was increasingly difficult to resist the temptation to tackle the rid-dle of life by penetrating the black box directly by physicochemical means.50 In an envi-ronment surrounded by new technology, Delbrucks ensuing work took on an increasingly

chemical character.Given that the traditional view of Delbruck portrays him as a physicist in biology, the

eventual openness he showed towards chemistry may come as a surprise.51 There are, how-ever, hints even in his early work that he thinks that physics will be challenged severely inaccounting for various biological phenomena. In contrast to Bohrs very ambiguous posi-tion towards the possibility of the emergence of new laws, Delbruck was deeply convincedthat new laws would indeed emerge for biological studies. In fact, he argued that oneshould not be afraid of such a development. In A physicist in biology, Delbruck arguedthat the failure of physics to account for living phenomena should be welcomed because itmay lead to the discovery of new laws. To illustrate his point, he notes that it would be

foolish to mourn the failure of classical mechanics because this failure opened the wayfor the development of quantum physics. He says, Therefore this renunciation [of classicalphysics] should not be considered as a loss but a liberation from unnecessary restrictionsand thus as the essential element of our advance which opened the widest possibilities forfuture developments.52 In a similar vein, he argues that physicists in biology should not beafraid of contradicting known physical laws: This analysis [of living cells] should be doneon the living cells own termsand the theories should be formulated without fear of con-tradicting molecular physics.53 The previous statement also reflects Delbrucks view thatbiology may emerge ultimately as an autonomous science on equal footing with physics.This sentiment is also echoed in the following passage in which he speaks of the possibilitythat the new laws that will emerge from the study of life will be related to the current lawsof physics in only a very indirect manner:

This idea [i.e. complementarity], which is due to Bohr, puts the relation between

physics and biology on a new footing. Instead of aiming from the molecular physics

end at the whole of the phenomena exhibited by the living cell, we now expect to find

natural limits to this approach, and thereby implicitly new virgin territories on which

49 Kay (1993), pp. 251252.50

Ibid.51 van Helvoort (1992) is one author who gives attention to how Delbrucks stance towards chemistry oftenshifted. See p. 572.52 Delbruck (1992), p. 20.53 Ibid., p. 22.



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laws may hold which involve new concepts and which are only loosely related to

those of physics : : :54

Despite the fact that Delbruck mentions Bohr here, it is unlikely that Bohr would have

held the same expectations as Delbruck regarding the emergence of new laws. In particu-lar, it is unlikely that Bohr would agree with Delbrucks suggestion that these new lawswill be only loosely related to those of physics. This again attests to the extent to whichDelbruck, though indebted to Bohrs insights regarding complementarity, had parted fromthe views of his mentor.

4. Rethinking Schrodingers contribution to biology

For many scientists, and perhaps even for those with some training in history of science,the first figure who comes to mind when asked to name a physicist with contributions to

twentieth-century biology is likely to be not Bohr or Delbruck, but Erwin Schrodinger.55

Such a perception is largely due to SchrodingersWhat is life?56, a book that presented themystery of life from the standpoint of a physicist. In one sense it is very tempting to seeSchrodinger as simply another recruit in the physicists attack on biological problems,an attack that was envisioned by Bohr and saw its first wave in Delbrucks work. I havesuggested, however, that viewing the physicists attack on biological problems as an attackunified in its philosophical and methodological orientation is problematic even in the caseof Bohr and Delbruck. This is no less the case for Schrodinger whose views diverged mark-edly from those of Bohr and Delbruck.

The first point of departure concerns Schrodingers understanding of the paradox of

life. For Delbruck, the paradox was yet to be discovered. This is most apparent in his fol-lowing statement from 1949 that biology is not yet at the point where we are presentedwith clear paradoxes and this will not happen until the analysis of the behavior of livingcells has been carried into far greater detail.57 It was the uncovering of a paradox thatdefined the course of Delbrucks research. In contrast, Schrodinger believed that healready had a sufficiently well posed paradox on his hands. For Schrodinger, the paradoxwas how individual traits could be passed down from one generation to another with suchhigh fidelity given the amount of thermodynamic noise at the molecular level. Heexpressed this paradox nicely in concrete terms by first asking readers to consider howa peculiar disfigurement of the lower lip in several members of the Hapsburg dynastycould be passed down for three centuries. He posed the paradox succinctly and preciselywhen said: The gene has been at a temperature of 98F during all that time. How are weto understand that it has remained unperturbed by the disordering tendency of the heat

54 Ibid., p. 21.55 There are number of works that deal with Schrodingers physics in relation to other disciplines.Keller (1995),

Symonds (1986), andDronamraju (1999)examine Schrodingers influence on biology. For some papers that dealwith Schrodingers views on biology and philosophy, see Gotschl (1992). Works in that volume that addressSchrodingers contributions to biology are Riedl (1992), Horz (1992), Leinfellner (1992), andWuketits (1992).

Kilmister (1987)also includes a nice collection of essays assessing the impact of Schrodingers physics upon otherareas. The essays byPauling (1987) and Perutz (1987)are especially relevant for the issues discussed treated in thispaper. I am grateful to an anonymous referee for bringing Kellers work to my attention.56 Schrodinger (1967).57 Delbruck (1992), p. 22.



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motion for centuries?58 This question encapsulates the central issue that Schrodingerattempted to address in his book.

Not surprisingly, Schrodinger differed from Delbruck and Bohr in his assessment ofwhat principles might be important in accounting for the phenomenon of life. In particu-

lar, he differed from them in his assessment of the importance of complementarity.Regardless of whether Delbruck had understood Bohrs original view of complementaritycorrectly, there is little doubt that Delbruck saw complementarity as a key concept inexplaining biological phenomena. In contrast, Schrodinger attached very little importanceto complementarity. Schrodinger held that quantum indeterminacy hardly plays any rolein elucidating the problem of life:

To the physicist I wish to emphasize that in my opinion, and contrary to the opinion

upheld in some quarters,quantum indeterminacyplays no biologically relevant role in

them [events in the body of the living beings], except perhaps by enhancing their

purely accidental character in such events as meiosis, natural and X-ray-inducedmutation and so onand this is in any case obvious and well recognized.59

In fact, Roll-Hansen notes that Schrodinger criticized Bohrs views on complementarity.60

Schrodinger, however, subscribed to an interpretation of the world that was no less mys-terious. In the last pages ofWhat is life?he suggests that Indian philosophy based on theVedanta may providea framework to understanding the seemingly contradictory findingsof quantum physics.61

Despite Schrodingers undeniable interest in Indian philosophy, it is a mistake to sup-pose that it played a role in Schrodingers worldview that is analogous to the role played

by complementarity in the worldviews of Bohr and Delbruck. He makes references toIndian philosophy only in the last few pages ofWhat is life?The main philosophical per-spective that seems to dominate a larger part of Schrodingers work is determinism, or atthe very least, a desire to save determinism. Schrodinger was an ally of Einstein in thefamous BohrEinstein debates over the status of determinism in quantum mechanics,but it is perhaps less known and underemphasized that his determinism also manifestsitself in his biological writings.62 A key passage in the book that reflects his commitmentto determinism is the following:

According to the evidence put forward in the preceding pages the space-time events

in the body of a living being which correspond to the activity of its mind, to its self-

conscious or any other actions, and any other actions, are (considering also their

complex structure and the accepted statistical explanation of physico-chemistry) if

not strictly deterministic at any rate statistico-deterministic.63

58 Schrodinger (1967), p. 47.59 Ibid., p. 86; original emphasis.60 Roll-Hansen (2000), p. 425.61 Schrodinger (1967), pp. 8790.62

Some authors who do pay attention to Schrodingers determinism are Olby (1974), Yoxen (1979), andFleming (1968). For an extended discussion of Schrodingers position in the BohrEinstein debates, see Ben-Menahem (1989). SeeBertotti (1985)for a discussion on how Schrodingers ideas on quantum physics comparedwith those of Einstein.63 Schrodinger (1967), p. 86.



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This passage is especially notable because Schrodinger speaks of the possibility of holdingon to a variety of determinism, either classical or statistico-determinism. In fact, many ofthe examples Schrodinger invokes in his book, such as the mechanical clock and the mo-tion of the earth around the sun, are suggestive of a classical deterministic picture of the

universe. Schrodinger uses these examples to make the point that even when thermody-namic forces are present their effects are not strong enough to prevent us from thinkingabout the objects of interest in classical terms.

Schrodingers commitment to determinism is also manifest in his prophetic suggestionthat there exists a miniature code or codescript in which the program of life is encoded.As he did with case of the clock and the earths rotation around the sun, Schrodinger con-siders the possibility of thermodynamic forces corrupting the codescript. What is, how-ever, most interesting is that way in which Schrodinger solves this problem: he suggests thecodescript is composed of the aperiodic crystals proposed by Delbruck.64 This is becausehe believed that HeitlerLondon forcesin solids are strong enough to withstand thermo-dynamic noise at the molecular level.65 For Schrodinger, the codescript is, as Keller put,safeguarded by quantum theory.66 In fact, he rules out all other explanations for the sta-bility of the codescript. He says, Consequently, we may safely assert that there is no alter-native to the molecular explanation of the hereditary substance. The physical aspect leavesno other possibility to account of its permanence. If the Delbruck picture should fail, wewould have to give up further attempts.67 What is notable about this suggestion is that itreflects Schrodingers attempt to accommodate the findings of quantum mechanics withhis picture of classical determinism. This contrasts greatly with Delbruck, who believedthat quantum mechanics forces us to alter radically our picture of the universe, especially

with respect to our understanding of living phenomena. Schrodinger, however, suggeststhat a quantum model underlies or underpins a world picture that is essentially determin-istic. For example, in speaking of the codescript itself, he says:

In calling the structure of the chromosome fibres a codescript we mean that the all-

penetrating mind, once conceived by Laplace, to which every causal connection lay

immediately open, could tell from their structure whether the egg would develop

under suitable conditions, into a black cock, or into a speckledhen, into a fly or a

maize plant, a rhododendron, a beetle, a mouse or a woman.68

For Schrodinger, the codescript may take the form of an aperiodic crystal, but what it rep-

resents, as alluded to in the reference to Laplace, is a return to a classical deterministicview of the world.

Another way in which Schrodinger differed from Bohr and Delbruck was in how hethought the riddle of life could be understood better. As noted earlier, Bohr made a prin-cipled objection against chemistry whereas Delbruck often wavered in his commitment tohis mentors view. Given the status of physics as the most prestigious science of the firsthalf of the twentieth century, one may be tempted to conclude that Schrodinger, one of

64 Fleming (1968)has rightly noted that one reason the differences between Delbru ck and Schrodinger are oftenmissed is because Schrodingers endorsement of Delbrucks aperiodic crystals is often misconstrued as indicating

agreement with Delbrucks worldview. See p. 186.65 SeePauling (1987)for why this argument is problematic from the standpoint of molecular physics.66 Keller (1995), p. 73.67 Schrodinger (1967), p. 57.68 Ibid., p. 21.



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the leading physicists of his time, must have also shared in the low estimate of chemistry.69

Even Roll-Hansen puts Schrodinger in this camp when he suggests that Schrodingeralsocommitted a misjudgment of the potential of chemistry to explain biological heredity.70 Aconsiderably different view, however, emerges from a close reading of Schrodingers book.

Early in the book he notes that organic chemists have come closer to an understanding oflife than the physicists:

In calling the periodic crystal one of the most complex objects of his research, I had

in mind the physicist proper. Organic chemistry, indeed, investigating more and

more complicated molecules, has come very much nearer to that aperiodic crystal

which, in my opinion, is the material carrier of life. And therefore it is small wonder

that the organic chemist has already made large and importantcontributions to the

problem of life, whereas the physicist has made next to none.71

Moreover, Schrodinger was not optimistic of the possibility that physicists would soonmake important contributions to the problem of life. This is perhaps most apparent inthe following quotation in which he suggests that future advances in the understandingof life are more likely to come from chemistry than physics:

Delbru cks molecular model, in its complete generality, seems to contain no hint as

to how the hereditary substance works. Indeed, I do not expect that any detailed

information on this question is likely to come from physics in the near future. The

advance is proceeding and will, I am sure, continue to do so, from biochemistry

under the guidance of physiology and genetics.72

As with the codescript, Schrodingers words would prove prophetic.Like Bohr and Delbruck, Schrodinger suggests that new laws are likely to emerge fromthe study of life, but he differs from them in his assessment of what kind laws these arelikely to be. On the one hand, he rejects the vitalistic perspective and endorses the viewthat the key to understanding living phenomena rests in, as Bohr and Delbruck had sug-gested, the organisms construction (i.e. its organization). His commitment to this view isexpressed as follows:

What I wish to make clear in this last chapter is, in short, that from all we have learnt

about the structure of living matter, we must be prepared to find it working in a man-

ner that cannot be reduced to the ordinary laws of physics. And that not on the

ground that there is any new force or what not, directing the behaviour of the single

atoms within a living organism, but because the construction is different from any-

thing we have yet tested in the physical laboratory.73

On the other hand, Schrodinger differs from both Bohr and Delbruck. Though Schrodingershares Delbrucks view that the study of living organisms has the potential to reveal newphysical principles, he did not see biological investigations as precipitating the kind of crisis

69 Perutz (1987) also shares the view that Schrodinger was dismissive of the importance of chemistry in

elucidating the mechanism of biological heredity; see p. 243.70 Roll-Hansen (2000), p. 426.71 Schrodinger (1967), p. 5.72 Ibid., p. 67.73 Ibid., p. 76.



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that Delbruck had envisioned. Rather, Schrodinger argued that any new principles thatmight be discovered would not be entirely alien or unaccountable by known physicallaws.74 In fact, in articulating the order-to-order principle, a new principle that he be-lieved was necessary to describe the orderliness of living things, Schrodinger stated that

it was nothing else than the principle of quantum theory over again.75Insofar as Schrodinger believed that the new principles that might be discovered

through the study of biology would be accountable in terms of known physical laws heseems to share Bohrs view that known laws for inorganic matter should be no less success-ful in accounting for organic matter. The essential difference between Schrodinger andBohr is in their understanding of how known physical laws, especially quantum mechan-ics, might be employed to account for the phenomenon of life. Bohr views life in quantummechanical terms. More specifically, he sees the phenomenon of life as a kind of macro-scopic quantum phenomenon that is best understood through the perspective of comple-mentarity. Schrodinger adopts a manifestly different approach. For him, the phenomenonof life can be understood in classical terms. This is not to say that Schrodinger denied thevalidity or importance of quantum mechanics. In fact, to explain why a clock may beunderstood in terms of classical dynamics despite the presence of thermodynamic noiseSchrodinger stresses the importance of the quantum mechanical character of the bondsin the solid materials used in the clock. He says, Clockworks are capable of functioningdynamically, because they are built of solids, which are kept in shape by LondonHeitlerforcesstrong enough to elude the disorderly tendency of heat motion at ordinary temper-ature.76 He analogizes this argument to the phenomenon of life as follows:

Now, I think, few words more are needed to disclose the point of resemblance

between a clockwork and an organism. It is simply and solely that the latter alsohinges upon a solidthe aperiodic crystal forming the hereditary substance, largely

withdrawn from the disorder of heat motion.77

The difference in perspective between Schrodinger and Bohr can be understood as result-ing from their differing assessments on the character and importance of quantum mechan-ics. Bohr and Delbruck gave great importance to the acausal and holistic character ofquantum mechanics. Their emphasis on those qualities of quantum mechanics led themto believe that quantum mechanics was also the ideal perspective with which to studythe seemingly non-classical character of life. In contrast, Schrodinger attached great

importance to the ability of quantum mechanics to give an account for the stability ofthe chemical bond. Not surprisingly, this aspect of quantum mechanics formed the center-piece of his account on why macroscopic phenomena may be treated in classical terms de-spite the fact that non-classical effects are present at the microscopic level.

5. Kuhnian and Lakatosian themes in these physicists engagement with life

The divergence in basic assumptions among Bohr, Delbruck, and Schrodinger raises thequestion of the extent to which their differences might be explained in terms of models of

74 Ibid., p. 81.75 Ibid.76 Ibid., p. 85.77 Ibid.



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scientific change. Tracing the development of the idea of complementary from Bohrthrough Schrodinger, Roll-Hansen has suggested that it is a history that exhibits someof the traits characteristics ofKuhns model of scientific change78 and Lakatoss modelof scientific research programs.79 Although I am sympathetic to Roll-Hansens suggestion

that both of these models may be useful in understanding a history of complementarity,especially if Delbruck is taken as a pivot of that narrative, the application of both of thesemodels must be qualified when they are used to understand the larger conceptual differ-ences in the views of Bohr, Delbruck, and Schrodinger.

Drawing upon Lakatoss model, Roll-Hansen suggests that the history of complemen-tarity from Bohr onward may be viewedas a research program with the idea of comple-mentarity at the core of the program.80 Although one could possibly make the case thatDelbrucks research could be described along those lines, it is much more questionablewhether a similar contention holds for Bohr and Schrodinger.81 Concerning Bohr, it is dif-ficult to characterize his view of complementarity as a core idea that he used to generateand solve research problems. In fact, as I have argued earlier, ascertaining the theoreticalstatus of complementarity in Bohrs thought is a highly complicated matter. At times,complementarity is best understood as a perspective or lens through which Bohr urgesus to see the world. At other times it seems to resemble the protective belt of Lakatossmodel because Bohr uses complementarity as a fallback position to deal with the bizarreimplications of quantum mechanics. In contrast, for Schrodinger it is clear that the idea ofcomplementarity had no place at all in his research program. The confusion in locating therole of complementarity in Bohrs research program and its absence in Schrodingers bothsuggest that neither shares the core features of Delbrucks program. Similar disagreements

also arise when attempting to articulate how each of these physicists would locate theirpositions on chemistry and classical physics within the theoretical structure of their respec-tive research programs. Hence, at least in the Lakatosian sense of sharing a common core,it is difficult to understand these three physicists as belonging to or constituting a singleunified research program.

Regarding Kuhns model, Roll-Hansen sees a link between Kuhns concept of anomaly,a discovery or finding that cannot be accounted for by the dominant paradigm of the time,and Delbrucks search for a paradox. It is tempting to characterize the history of thephysicists engagement with organic life using this concept because it resonates withhow many physicists spoke of organic life as a kind of mystery, or to borrow Delbrucks

term, a paradox. There are, however, at least ways two in which the physicists view oflife as a paradox does not completely fit Kuhns model. One way in which the character-ization noted above is problematic rests in the fact that the concept of anomaly as Kuhnformulated it is often understood as possessing an element of surprise or unexpectedness.It is true that Bohr, Delbruck, and Schrodinger all saw life as a mysterious phenomenon,

78 Kuhn (1996).79 Lakatos (1978).80

One point concerning Lakatoss concept of scientific research programs that is problematic is whether the corecan even be articulated as clearly and simply as Lakatos suggests. For example, it is questionable whether the coreof Newtonian mechanics is sufficiently captured by Newtons three laws.81 Fischer & Lipson (1988)have rightly noted that Delbruck was one of the few physicists who went beyond

theoretical speculations by carrying out biological experiments; see p. 164.



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but it is not clear that they came to regard life in that way because of any one specificanomalous discovery. One could also make the case that explaining the nature of livingthings was not even considered within the domain of phenomena that the physics of theirtime recognized as within its explanatory purview. In other words, the attempts to under-

stand life by the three physicists treated here were not prompted by a specific discovery orfinding that they had not known about earlier. If any discovery was unexpected, it was thebizarre character of quantum mechanics. In fact, for Bohr it was the bizarre aspects ofquantum mechanics that seemed to lend themselves towards a natural explanation of life.In contrast, for Schrodinger the phenomenon of life itself seemed to provide the idealsgrounds to refute, or at least question, the relative importance of quantum mechanics atthe macroscopic level.

The case of Delbruck is somewhat more complicated, but it also leads nicely into thesecond difficulty that emerges in applying Kuhns concept of anomaly to this case. It istrue that Delbruck undertook a deliberate search for a paradox that would bring intoquestion the adequacy and completeness of the physics of his day, but it is debatablewhether such a search should be understood as part of the Kuhnian model of science.More specifically, whether Delbrucks search can be understood as such depends onhow one understands what Kuhn calls normal science. On the one hand, the kind ofsearch that characterized Delbrucks scientific research appears to be in conflict withwhat Kuhn describes as normal science in his Structure of scientific revolutions (hence-forth I shall refer to this work simply as Structure). According to Kuhn, [Normal sci-ence is] the devoted attempt to force nature into the preformed and relativelyinflexible box that the paradigmsupplies. No part of the aim of normal science is to call

forth new sorts of phenomena.

82

Moreover, in Structure Kuhn appears to suggest thatanomalies are not discoveries that can be anticipated in advance, much less searched forin a deliberate manner.83 Consequently, one might argue that insofar as Delbruck sawhis own research as an aggressive search for an anomaly, his activity appears moreambitious and explorative than Kuhns picture of normal science as presented inStructure.

On the other hand, it is true that in some works other than StructureKuhn presents apicture of normal science in which Delbrucks search can be considered as an example ofnormal science. In those works Kuhn presents normal science as possessing an innovativeand exploratory side that complements the more conservative character of normal science

outlined inStructure. Morespecifically, in The function of dogma in scientific research84

and The essential tension85 Kuhn characterizes scientific activity as a tension betweentwo opposing forces. One of these forces is the scientific communitys expectation thatthe scientist will use the paradigms that he or she learned from his or her community toexplain all future phenomena, and that he or she will remain committed to those para-digms. Kuhn, however, notes that scientific research requires a second force that counter-balances those expectations. This second force is a drive or willingness to explore and test

82 Kuhn (1996), p. 24.83

Of course, this is not to say that a scientist in Kuhns model believes that his or her paradigm will able toaccount for all yet undiscovered phenomena. It is only to say that on this particular reading normal scientificactivity does not deliberately aim to discover such phenomena.84 Kuhn (1963).85 Kuhn (1977a).



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the limits of received paradigms. Kuhn has articulated the resulting tension between thesetwo forces as follows:

Though successful research demands a deep commitment to the status quo, innova-

tion remains at the heart of the enterprise. Scientists are trained to operate as puzzle-solvers from established rules, but they are also taught to regard themselves as

explorers and inventors who know no rules except those dictated by nature itself.

The result is an acquired tension, partly within the individual and partly within

the community, between professional skills on the one hand and professional ideol-

ogy on the other.86

If Kuhns concept of normal science is understood as possessing this kind of tension, thenactivities that test or challenge the received paradigms must also be considered as part ofnormal science. This suggests that a scientist often possesses some sense of the limits of agiven paradigm, andit is likely that those limits will become the target of systematic test-ing of the paradigm.87 In other words, to the extent that normal science includes suchactivities it may be argued that normal science does indeed aim at exposing or producinganomalies. Hence, on this reading of Kuhns concept of normal science Delbrucksaggressive search for a paradox can be considered as exemplifying normal scientificactivity.88

The fact that two deeply contrasting interpretations of normal science emerge fromKuhns own writings suggests that Delbrucks search cannot be categorized as normal sci-ence without giving considerable care and attention to the range of activities that Kuhnassociated with normal science. I will not pursue here the question of which of the two

interpretations of Kuhns concept of normal science outlined above is a more accuratereading of Kuhn what intended.89 What I do wish to note, however, is that the case of Del-bruck is not only important in that it showcases conflicting aspects of Kuhns accounts ofnormal science but in that it suggest problems in both of the readings of Kuhns accountsof normal science. In particular, the conservative reading of normal science seems toassume problematically that a search for anomaly, the feature that characterized Del-brucks research, is a feature that is rarely part of actual scientific practice. Such an inter-pretation seems to underplay the extent to which a great deal of actual scientific practice isconducted with a hope, or even with the goal, that some novel phenomenon, a phenom-enon that cannot be explained by the dominant paradigm, will be discovered. It is true that

Kuhns less conservative account of normal science, the account that claims innovativeand exploratory activities as part of normal scientific activity, does address this problemto some extent, but it raises the question of whether such activity is not more succinctly

86 Kuhn (1963), pp. 368369.87 This point is in tension with Kuhns suggestion in Structure that anomalies are essentially unexpected

discoveries. In particular, if one possesses a sense of the limits of a paradigm, this suggests that one would knowwhere and how to search for anomalies. Hence, they are no longer unexpected discoveries.88 I am indebted to an anonymous referee for suggesting that Delbrucks search may be claimed as normal

science under this interpretation.Hoyningen-Huene (1993)has also suggested that Delbrucks scientific activitiesmay be understood as exemplifying an aspect of normal science; see p. 188.89 This issue will be examined in a later paper. In particular, I will address the question of whether Kuhns

account of science should be understood as a descriptive or prescriptive account of science, and how his positioncompares with that of the logical positivists.



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expressed through Poppers idea of falsification.90 Popper has suggested that one of thedefining traits of scientific activity is the process of consciously attempting to falsify prop-ositions through experimental tests. To the extent that Delbruck was searching for a par-adox that would falsify, or at least question, the adequacy of quantum mechanics, his

scientific activity is captured nicely by Poppers account.91In comparing Kuhns model to Lakatoss model, Roll-Hansen says, Imre Lakatos

evolutionary epistemological analogy appears better suited than Kuhns to catch the rightbalance between independence and linkage of philosophical ideas and experimental sci-ence.92 Although I have argued that there are serious problems to applying either modelto the history of interest here, there are at least two ways in which Kuhns model, asopposed to Lakatoss model, may be more useful in understanding the diversity of viewsthat one sees among these three physicists. One of these ways relates to understanding whyBohr, Delbruck, and Schrodinger chose different paradigms to understand the phenome-non of life. Kuhn suggests that paradigm choice cannot be reduced to any strict algorith-mic procedure because the choice of paradigm is underdetermined by experimentalevidence. He noted that various non-transcendental factors suchas accuracy, consistency,scope, simplicity, and fruitfulness enter into paradigm choice.93 As Kuhn himself hasnoted, some read him to be suggesting that paradigm choice was determined by social,political, and psychological factors.94 Although I do not wish to assess which set of criteriaare more on the mark, I think it is worth noting that Bohr, Delbruck, and Schrodingershared many similar experimental and theoretical presuppositions but arrived at radicallydifferent views regarding the relationship of physics to biology. For each of them the pro-cess of importing physics to the study of life entailed a choice of what experimental facts

and what theories of physics were relevant or important in developing a theory capable ofaccounting for biological phenomena. Moreover, there was no strict algorithmic proce-dure by which to determine what was relevant or important for such a theory. Not surpris-ingly, the differing views that arose among these three physicists reflect the theoretical andexperimental aspects of physics they deemed as important for biological investigations.For example, on the one hand, Bohr and Delbruck viewed the experimental and theoret-ical findings of quantum mechanics that stressed its acausal and non-deterministic charac-ter as the most important features in bringing physics to bear on biology. On the otherhand, Schrodinger stressed how quantum mechanics can support a classical deterministicpicture of the universe as exemplified by how aperiodic crystals are extremely stable struc-

tures despite their quantum mechanical character.

90 The fact that this interpretation of Kuhns view of normal science partially resembles Poppers idea offalsification may not be problematic in itself, but it does evoke an image of science as a cumulative and systematicdiscipline insofar as it suggests that science can discover the limits of its own paradigms by systematically testingpropositions and using those results to generate new propositions. It should be noted, however, that thiscumulative and systematic picture of science is what Kuhn supposedly sought to overthrow in Structure. In fact,giving an overly Popperian reading ofStructurebetrays one of Kuhns expressed hopes for the work, namely toproduce a decisive transformation in the image of science by which we are now possessed. SeeKuhn (1996), p. 1.

For a stimulating discussion of Kuhns views in relation to those of Popper, see Fuller (2005).91 Popper (1959).92 Roll-Hansen (2000), p. 442.93 Kuhn (1977b).94 Kuhn (1996), pp. 198210.



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A second sense in which Kuhns model is useful in discussing the case of the physicistshere relates to what he terms the pre-paradigmatic stage, a stage during which multipleparadigms are proposed to solve an anomaly.95 To the extent that the three physicists trea-ted here held sharply conflicting views over how physics could elucidate the phenomenon

of life their situation seems to resemble what Kuhn called the pre-paradigmatic stage.This particular case also furthers our understanding of the pre-paradigmatic stage in thatit suggests one way in which the various paradigms that appear in the pre-paradigmaticstage arise. This is a point on which Kuhn is relatively silent. He seems to assume thatthe newly proposed paradigms are often modified versions of the once dominant paradigmin the domain of interest. In short, new paradigms that are proposed are often intradisci-

plinaryvariants, variants of paradigms that were once accepted or dominant in the samediscipline or domain of interest. For example, in the struggle over the Ptolemaic model, theCopernican model represented a modified version of the Ptolemaic model. Though themodification was a major one, it is important to note that the Copernican model was stillessentially derived from one that had already experienced some degree of success inexplaining planetary motion, the same phenomenon that the Copernican model soughtto explain.96 In contrast, the attempts of the three physicists treated here to propose par-adigms to explain the phenomenon of life constituted attempts at interdisciplinary para-digm import, the usage of paradigms of one discipline or domain to explain phenomenain another discipline or domain. This is because they each developed their views by draw-ing upon a well accepted paradigm (i.e. classical and/or quantum physics) of one domain,the physical world (i.e. the non-biological phenomena), to construct a theory aboutanother domain, the biological world. Consequently, the differences in the views of these

three physicists can be understood as following from differing positions regarding whatparts of physics each of them thought were important for biology.An additional point that attests to the pre-paradigmatic character of the conflicting

views among these three physicists is the fact that their views showed disagreement on avery basic level. The physicists lacked a basic level consensus regarding what kind of ideaswould be promising in attempting to explain the phenomenon of life. In fact, among thethree physicists treated here, it was often the case that an idea believed to be promisingby one of them was quickly dismissed by one or both of the other two physicists. For exam-ple, Delbruck considered the doctrine of complementarity as the driving force of hisresearch, but few others, even those in his research group, shared such a conviction. As

noted earlier, Schrodinger dismissed it completely and Aage Bohr suggested that Delbruckhad taken his fathers ideas too literally. Another case concerns one of Schro dingers keyideas, the idea of the codescript. According to Roll-Hansen, Delbruck found Schrodingersidea ridiculously simplistic when he first read in Schrodingers book.97 Olby, however,notes that in contrast to Delbrucks idea of complementarity, Schrodingers idea of thecodescript was quite inspirational to those such as Luria, Crick, Watson, and Wilkins.98

95 AsGiere (1999)has noted, the term pre-paradigmatic stage is a misnomer because it is not a stage in whichthere are no paradigms, but one in which multiple paradigms are competing for dominance; see p. 35.96 I am aware that there is some discussion on whether Ptolemy and Copernicus both believed that the heavenly

bodies were perfect bodies in Aristotles sense of the word. This, however, remains peripheral to my point thatKuhn tends to think of paradigm change in an intradisciplinary sense as opposed to the interdisciplinary sensediscussed here.97 Roll-Hansen (2000), p. 425.98 Olby (1974).



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Despite the differing fates of their models, the inability of Schrodinger and Delbruck to givemore principled criticisms for dismissing the others idea suggests a sense in which theabsence of a dominant paradigm of life made it difficult to mount such principled criticisms.In a sense, this situation seems to resemble what Feyerabend calls anarchy.99 In fact, Tsou

has recently argued that this type of situation, a situation where a dominant paradigm isabsent, may be one in which Feyerabends anything goes stance is defensible because itcan helpgenerate theories that are potentially useful in accounting for the phenomena inquestion.100

6. Conclusion

This paper has attempted to demonstrate that Bohr, Delbruck, and Schrodinger, threephysicists often associated with the migration of physicists into biology, held fundamen-tally different views regarding how complementarity, chemical or interventionist methods,and classical physics might contribute towards a deeper understanding of the phenomenonof life. In bringing these disagreements to the fore, I tried have to redress the relative lackof attention to the philosophical and methodological disagreements that has characterizedearlier accounts of these physicists engagement with biology. Moreover, I attempted tocorrect the misperception that these physicists shared a common core of assumptions thatguided their biological research by arguing they shared neither what Lakatos calls aresearch program nor what Kuhn calls a paradigm. It was also noted that some Kuhnianideas that are fruitful in helping to understand this history include his discussion of thenon-transcendental character of paradigm choice and his concept of the pre-paradigmatic

stage. I do not claim to have exhausted the Lakatosian or Kuhnian themes present in thishistorical episode or to have characterized the full history of the interaction between phys-ics and biology in the emergence of molecular biology. It is hoped, however, that thisexamination has conveyed some sense of the rich and complex character of the disagree-ments among Bohr, Delbruck and Schrodinger regarding how physics should be broughtto bear on the phenomenon of life.

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