Niels Bohr was one of the most influential physicists of the first half of the twentieth century. Like Alfred North Whitehead, he discussed the philosophical implications of the new physical theories, especially those of quantum mechanics. Although neither Bohr nor Whitehead made direct reference to each other in their publications, both thinkers seem to share several fundamental ideas concerning the description of the physical phenomena, as some authors have suggested recently. After presenting Bohr’s life and main hypotheses concerning the interpretation of quantum mechanics, I will analyze the alleged connections between the two thinkers. In particular, I will argue that their philosophical stances are in fact divergent: while Whitehead’s aim is to elaborate a new ontology, Bohr focuses exclusively on observable phenomena and avoids all metaphysical speculations.

### 1. Brief *Vita*

Bohr was born in Copenhagen on October 7, 1885, and grew up in a cultivated family. He entered Copenhagen University in 1903 where he received an education in physics. He prepared a thesis on the electron theory of metals, and took his Doctorate in 1911. The same year, he went to Cambridge University, working several months under the guidance of Joseph John Thomson, before moving to Manchester in the laboratory of Ernest Rutherford.

There, he developed, and proposed in 1913, his famous model of the structure of the hydrogen atom—for which he was awarded the Nobel Prize in 1922. This model disagreed with classical electrodynamics, but could explain the stability of the hydrogen atom. Incorporating Max Planck’s idea of the quantification of energy, Bohr suggested that the atom has only a discrete set of energy levels. According to his model, the atom emits radiation only when its state “jumps” to a lower energy level; when stable, it emits no radiation. During the following years, this “quantum theory” of 1913 has been corroborated by several experiments. Today it is referred to as the “old quantum theory.” Yet it was only this theory with which Whitehead was familiar (Folse 1981; Tanaka 2004).

In 1916, Bohr became Professor of Theoretical Physics at Copenhagen University. With his assistant Hendrik Kramers, he carried on his research on the radiations of atoms. According to his “correspondence principle,” the quantum theory had to be developed so as to rediscover the results of classical physics at the “asymptotic” limit (i.e. in the experimental domain of large quantum numbers).

In 1921, he founded the Institute of Theoretical Physics in Copenhagen where he invited many promising physicists (including Wolfgang Pauli, Werner Heisenberg and Paul Dirac), and supervised their research. In 1925, as the (old) quantum theory was facing insurmountable problems, Heisenberg, with the aid of Max Born and Pascual Jordan, formulated a new quantum theory in terms of matrixes. The following year, Erwin Schrödinger proposed another quantum theory in terms of waves, and established its mathematical equivalence with Heisenberg’s theory. “Quantum mechanics” in its current form was born.

Although well established at the formal level, this theory was very difficult to interpret, mainly due to its probabilistic nature. Although he did not himself publish a paper on the mathematical formulation of quantum mechanics, Bohr was at the center of discussions on the interpretation of the theory. In 1927, he gave a talk at Como where he proposed the idea of “complementarity.” According to this interpretation, quantum mechanics provides a complete description of microscopic phenomena, even though it gives only probabilistic predictions concerning the different experiments: in order to get a complete description of a microscopic system one has to combine the complementary pictures associated with the outcomes of the mutually exclusive measurements made on this system (see below § 2.2). The notion of “indeterminacy relations” (which implies that two conjugated variables, such as position and momentum, cannot be well-defined simultaneously for the same system), established the same year by Heisenberg, could support this idea. Most of the physicists who came to Copenhagen to work or talk with Bohr, came to endorse and promote his view, so that the so-called “Copenhagen interpretation” became dominant.

Nevertheless, some physicists were reluctant to accept it. Among them was Albert Einstein who, in collaboration with Boris Podolsky and Nathan Rosen, published a paper in 1935 that presented a thought experiment designed to show that quantum mechanics is “incomplete.” In a paper of the same year, Bohr responded to this so-called “*EPR* argument” by proposing a new hypothesis of “contextuality” (see below § 2.1). Later, this famous debate on the scope of quantum mechanics has been partially settled when in 1964 John Bell proved that any local theory (i.e. any theory compatible with special relativity) supposed to complete quantum mechanics with additional variables involves inequalities conflicting with this theory. In the 1980’s, several experiments (see for instance Aspect et al. 1981) “violated” these inequalities and corroborated quantum mechanics; thus, the *EPR* argument was experimentally refuted. This does not mean, however, that the argument is not logically consistent (concerning this point, see Hájek and Bub 1992), but that one (or more) hypothesis taken as a premise is wrong.

In 1943, when Nazi Germany occupied Denmark, Bohr fled to Sweden, then to England and the United States. In 1945, he returned to Copenhagen and pursued his research, particularly on superconductivity. He received the Atoms for Peace Award in 1957. He died in his native city on November 18, 1962.

### 2. Bohr’s Interpretation of Quantum Mechanics

Two ideas at the core of Bohr’s interpretation of quantum mechanics can be related to Whitehead’s philosophy—contextuality and complementarity. I will here attempt to present these ideas in more detail.

#### 2.1. Contextuality

According to Bohr, the formalism of quantum mechanics corresponds merely to a mathematical tool that enables one to make statistical predictions concerning the measurement outcomes in microphysics (Bohr 1958, 73; 1963, 60 and 92). In other words, it has no representative function. How then can we represent the microscopic systems under investigation?

Let us analyze their experimental manifestations. Surprisingly, the same system can appear either like a wave or a particle, depending on the experimental context. Yet, these concepts of wave and particle are incompatible: they cannot be applied simultaneously to the same system. In response to this fundamental dilemma, Schrödinger even suggested the need to forge new concepts (Letter to Bohr, May, 5, 1928).

In Bohr’s view, however, such a strategy is unhelpful: there cannot be an “intuitive representation” of microscopic systems. The reason is that, in a measurement outcome, it is impossible to identify what is due to the microscopic system and to the apparatus. The observed phenomena form an “individuality,” i.e. an indivisible unity consisting of the system, the apparatus and their interaction (Bohr 1934, 53; 1935, 697). This implies the “contextuality” of the microscopic phenomena: “the unambiguous account of proper quantum phenomena must, in principle, include a description of all relevant features of the experimental arrangement” (Bohr 1963, 4).

Bohr did not give a clear justification of individuality, or, equivalently, contextuality. In his first publication on this subject (*Atomic Theory and the Description of Nature*, 1934), he argues as follows: because in each interaction, there is always a minimal quantity of energy exchanged (a “quantum action”), any apparatus induces an uncontrollable perturbation of the state of the measured system (with which the apparatus interacts), so that it is impossible to make a sharp distinction between the apparatus and the system.

Nevertheless, the notion of “perturbation” appears to be unsatisfactory. For, if the apparatus perturbs the state of the system, this means that the system was in a well-defined state before its interaction with the apparatus. But this conflicts with Bohr’s idea of contextuality: the state of a system cannot be defined without referring to a given experimental arrangement.

In their 1935 paper (*cf*. §1), Einstein, Podolsky and Rosen had imagined an experiment intended precisely to neutralize Bohr’s justification in terms of “perturbation.” The idea is to determine the value of a physical quantity on a system indirectly by making a measurement on a distant system, so that the apparatus does not perturb the first system.

In his answer (1935), Bohr admitted that in such an experiment there is no “mechanical disturbance of the system under investigation.” Nonetheless, he argued that the remote measurement modifies the whole experimental context which is relevant for the definition of any physical quantity on the studied system. This has been analyzed as a change in Bohr’s conception of contextuality (Jammer 1974, 194-98; Faye 1991, 205; Bitbol 1996, 251-55). As Max Jammer has noticed, the notion of relation has become central: contextuality means now that “the description of the state of a system, rather than being restricted to the particle (or system of particles) under observation, expresses a *relation* between the particle and all the measuring devices involved” (1974, 197-98, my emphasis).

Although Bohr’s justification of contextuality remains obscure, his intuition was right. Indeed, in 1967, Simon Kochen and Ernst Specker eventually provided a mathematical proof of the contextual nature of quantum mechanics (*cf*. the simple proofs given by Mermin 1990 and Peres 1990).

#### 2.2. Complementarity

In Bohr’s view, the description of an experiment is “objective” if and only if it can be communicated “unambiguously” to the other physicists (Bohr 1958, 67; 1963, 3). As a necessary condition, the description has to make a clear distinction between the described system and the means of knowledge (e.g. the measurement apparatus) (Bohr 1958, 91). When we apply the concepts of classical physics (e.g. space, time, causality) to a system, we tacitly assume such a distinction (Bohr 1958, 69). This is why these concepts should be used as far as possible in the microscopic domain.

Let us consider a given measurement of a microscopic system. As Bohr has noticed (1963, 3 and 92), although the interaction between the system and the apparatus is “uncontrollable,” it provides an outcome which appears to us as a stable property of a macroscopic part of the apparatus (e.g. a pointer in a given position). Can the classical concepts be used to interpret such stable properties? Bohr’s answer is positive. However, contextuality restricts the validity of each of these classical concepts to a given set of experimental contexts. For instance, the space-time and causality descriptions cannot be applied simultaneously. The context in which space-time can be applied and the one in which causality can be applied correspond to “mutually exclusive experimental arrangements” (Bohr 1958, 41).

This reciprocal limitation of the applicability of the concepts eliminates the possibility of subsuming the different representations of the phenomena under one single picture, as in classical physics. It is necessary to take explicitly into account the context to which each of these representations is relative. For this reason, Bohr thinks that the conceptual frame of classical physics has to be generalized to the “complementarity” frame: “the impossibility of combining phenomena observed under different experimental arrangements into a single classical picture implies that such apparently contradictory phenomena must be regarded as *complementary* in the sense that, taken together, they exhaust all well-defined knowledge about the atomic objects” (Bohr 1963, 25, my emphasis). As he writes elsewhere, complementarity provides a “rational synthesis of the wealth of experience in [the atomic domain]” (Bohr 1958, 19).

### 3. Connections between Bohr’s and Whitehead’s Philosophies

Despite the fact that both Bohr and Whitehead worked extensively on the interpretation of the quantum theory, neither refers to the other in their publications.[1] Nevertheless, several links between the two thinkers have been put forward by various authors. The discussion of these alleged links will lead us to compare Bohr’s and Whitehead’s views concerning metaphysics.

#### 3.1 Contextuality and the Fallacy of Misplaced Concreteness

The contextual nature of the microscopic phenomena, emphasized by Bohr, has two interesting consequences with regard to Whitehead’s philosophy. First, let us recall that an “objective” description of a system, according to Bohr, presupposes the separation of this system from our means of knowledge (e.g. from the measurement apparatus). For this reason, when describing a system, one tends to disregard the systems with which it interacts. However, contextuality implies that two interacting microscopic systems cannot be considered as separate systems in two well-defined states. This is the reason for the following warning from Bohr:

it must be kept in mind that […] radiation in free space as well as isolated material particles are

abstractions, their properties on the quantum theory being definable and observable only through their interactions with other systems (Bohr 1934, 55-56, my emphasis).

This is similar to Whitehead’s criticism of the “Fallacy of Misplaced Concreteness” (*SMW* 51), the fallacy of taking abstractions as the concrete objects of immediate experience. Henry Folse makes this very comparison when discussing Bohr’s idea of contextuality: “Any attempt to represent the state of [two interacting systems] separately involves an ‘abstraction’ from the concrete physical situation which the quantum formalism is designed to represent. Such an attempt would be precisely what Whitehead called the fallacy of misplaced concreteness” (Folse 1981). At the same time, both Bohr and Whitehead consider the method of abstraction as unavoidable. In one passage, Whitehead writes: “you cannot think without abstractions” (*SMW* 59; *cf*. *MT* 77). Thus, neither forbids abstraction, only warns against forgetting that abstractions focus narrowly on a specific part of a whole situation.

To avoid the deficiencies of the abstractions of classical physics in the microscopic domain, the two thinkers propose different strategies. Whitehead would ultimately reconstruct the entire conceptual scheme of science, and replace classical mechanism with a theory of “organism,” most extensively in* Science and the Modern World*, *Process and Reality*, and *Modes of Thought*. Bohr, by contrast, thought it sufficient for the physicist’s purpose to “generalize” the conceptual scheme inherited from classical physics: classical concepts merely have to be adapted to the “complementarity” frame.

To take one example, both thinkers point out the limits of the concept of substance, so central to classical mechanics. This concept may be useful in describing experiences at the macroscopic level, but not at the microscopic level, where only quantum mechanics can provide appropriate predictions. Indeed, quantum mechanics conflicts with the substantialist picture of a particle having a well-defined trajectory in space and time. Bohr established this point by appealing to the indeterminacy relations deduced from quantum mechanics (i.e. the new quantum theory). Whitehead’s rejection of the concept of substance also drew upon his understanding of the (old) quantum theory:

one of the most hopeful lines of explanation is to assume that an electron does not continuously traverse its path in space [contrary to the substantialist picture]. The alternative notion as to its mode of existence is that it appears at a series of discrete positions in space which it occupies for successive durations of time (

SMW34).

Moving beyond mere criticism of an ontology of substance, Whitehead suggested replacing it with the “hypothesis that the ultimate elements of matter are in their essence vibratory” (*SMW* 36), i.e. they are waves of energy with given frequencies. This ontology is compatible with the new conceptual scheme of Whitehead’s theory of organism (see especially *SMW* Chapter 8; *PR* 36, 327). We cannot discuss this new ontology in detail, but here one can only point to “a sharp discrepancy” (as Shimony 1993, 298-300 describes it) between Whitehead’s ontology and the standard formulation of quantum mechanics, where the wave function may be defined on an infinite dimensional space, and hence, can no longer be assumed to represent a physical wave propagating in space and time.

For his part, Bohr refrains from developing an entirely new picture of nature. Even though the description of the phenomena in terms of particles moving in space and time is disconfirmed by the principles of quantum mechanics, one should not discard it altogether. Such a description still has a restricted validity: it can be applied in specific experimental contexts (which exclude the application of the complementary description in terms of causality). In other words, Bohr makes a pragmatic interpretation of the concepts used in science; he does not give them ontological scope.

#### 3.2. Contextuality and Relations

Let us turn now to the second consequence of contextuality which is relevant for Whitehead’s philosophy. As discussed above (§ 2.1), in Bohr’s view, contextuality means that the properties of a microscopic system are defined relationally: they depend on the relations of this system to all the other systems with which it has interacted, even if these systems are spatially separated.

This relational conception has been compared by Henry Stapp with Whitehead’s event theory of reality:

It is fundamental to Whitehead’s theory that the potentia of each event is conditioned by the entire preexisting world. This feature corresponds to the fact, often stressed by Bohr, that in describing quantum phenomena, the whole experimental arrangement must be taken into account. Indeed the basic conceptual problems of quantum theory disappear once it is admitted that the potentia for each event is conditioned by the entire preexisting world (Stapp 1977).

It is true, Bohr and Whitehead consider the notion of relation as fundamental in the description of reality. Nevertheless, they certainly would not have agreed on the definition of the term “reality.” As a realist, Whitehead refers to reality as it is in itself, independently of our means of knowledge (he does so in a “focus imaginarius” mode). In this respect, he assigns an ontological status to the relations between the described systems (*SMW* 142; *PR* 58-59, 66, 194-195, 286, 308-309; *MT* 91-92). He furthermore claims that “each eternal object has a ‘relational essence’” (*SMW* 160), an “eternal object” being an object that “transcend[s] particular concrete occasions of actual happenings” (*SMW* 159). For Bohr, on the other hand, any described reality is always what appears (or can appear) to us by means of our instruments.[2] This is so precisely because of contextuality:

The fact that in atomic physics […] objective description can be achieved only by including in the account of the phenomena explicit reference to the experimental conditions, emphasizes in a novel manner the inseparability of knowledge and our possibilities of inquiry (Bohr 1963, 12).

In fact, because of its ontological dimension, Whitehead’s relational picture of the world is more in the spirit of some recent interpretations of quantum mechanics, such as that of Simon Saunders with his idea of “relative facts” (1995; *cf*. Everett 1957), or that of David Mermin with his notion of “correlations without correlations” (1998).

#### 3.3. Complementarity and the Two Ways of Analyzing an Actual Entity

Yutaka Tanaka (2004) has recently compared the complementarity interpretation of Bohr with Whitehead’s description of an “actual entity,” which are defined as “the final real things of which the world is made up” (*PR* 18). For, Whitehead distinguishes two possible ways of analyzing such an actual entity. On the one hand, the “genetic” analysis describes it as a process, the process of “concrescence” (*PR* 283). Whitehead argues that this process “does not occur in the physical time,” because each of its phases involves the entire quantity of time of the process (*PR* 283). On the other hand, the “coordinate” analysis divides the quantity associated to the actual entity into successive temporal quantities (*PR* 283-84). The actual entity is here considered as a “concrete” entity.

Now, recall that Bohr considers the causal and space-time descriptions of the microscopic phenomena as two complementary descriptions. In Tanaka’s view, there is a correspondence (1) between the genetic analysis and the causal description, and (2) between the coordinate analysis and the space-time description.

Nevertheless, I suspect this correspondence to be apparent only. Indeed, the “causal description,” as Bohr understands it, refers to the conservation laws: the total energy and total momentum of two interacting systems are always conserved (*cf*. Chevalley 1991, 385). Yet, these conservation laws allow us to decompose the process into successive instants of time—contrary to Whitehead’s genetic analysis. What the causal description excludes is the possibility of describing, in the mean time, the studied system as a particle that occupies a well-defined position at each instant.

Furthermore, the genetic and coordinate analyzes are two distinct ways of considering the same object, namely an actual entity. The causal and space-time descriptions, by contrast, are relative to two mutually exclusive experimental contexts, and thus, depict two distinct objects, namely two observable phenomena (occurring in the two distinct experimental contexts). Once more, we see a major divergence in the two thinkers’ views: Whitehead speaks of reality as it is in itself, a reality that can be analyzed from different points of view, while Bohr refers always to a contextualized reality, one that cannot be dissociated from the instruments used to observe it.

### Notes

[1] These facts regarding the publications of Bohr and Whitehead have been confirmed for me by the Niels Bohr Archive in Copenhagen, and by the Center for Process Studies in Claremont, California.

[2] Not all the specialists of Bohr agree on this point. For another interpretation of Bohr’s definition of reality: see, for instance, Folse 1985, 246.

### Works Cited and Further Readings

#### Bohr’s main Publications on the Interpretation of Quantum Mechanics

Bohr, Niels. 1934. *Atomic theory and the description of nature* (Cambridge, Cambridge University Press).

Bohr, Niels. 1935. “Can Quantum-Mechanical Description of Physical Reality be Considered Complete?” *Physical Review*, 48, 696-702.

Bohr, Niels. 1958. *Atomic Physics and Human Knowledge* (New York, J. Wiley & Sons).

Bohr, Niels. 1963. *Essays 1958-1962 on Atomic Physics and Human Knowledge* (New York, Interscience).

#### Secondary Literature on Bohr’s Interpretation of Quantum Mechanics

Bitbol, Michel. 1996. *Mécanique Quantique. Une Introduction Philosophique* (Paris, Flammarion). See especially § 3.2.

Chevalley, Catherine. 1991. “Introduction” and “Glossaire,” in Niels Bohr, *Physique atomique et connaissance humaine*, French translation of Bohr 1958 (Paris, Gallimard), 17-140 and 345-567.

Faye, Jan. 1991. *Niels Bohr. His Heritage and Legacy* (Dordrecht, Kluwer).

Faye, Jan and Folse, Henry (eds.). 1994. *Niels Bohr and Contemporary Philosophy* (Dordrecht, Kluwer).

Folse, Henry. 1985. *The Philosophy of Niels Bohr: The Framework of Complementarity* (Amsterdam, North Holland).

Honner, John. 1987. *The Description of Nature: Niels Bohr and the Philosophy of Quantum Physics* (Oxford, Clarendon Press).

Jammer, Max. 1974. *The Philosophy of Quantum Mechanics* (New York, J. Wiley & Sons). See especially § 6.4 and 6.5.

Murdoch, Dugald. 1989. *Niels Bohr’s Philosophy of Physics* (Cambridge, Cambridge University Press).

Scheibe, Erhard. 1973. *The Logical Analysis of Quantum Mechanics* (Oxford, Pergamon). See especially Chapter I.

#### Papers Comparing Whitehead’s and Bohr’s Philosophies

Folse, Henry. 1981. “Complementarity, Bell’s Theorem, and the Framework of Process Metaphysics,” *Process Studies*, 11, 4, 259-73.

Stapp, Henry. 1977. “Quantum Mechanics, Local Causality, and Process Philosophy,” *Process Studies*, 7, 3, 173-182.

Tanaka, Yutaka. 2003. “The Individuality of Quantum Event: Whitehead’s Epochal Theory of Time and Bohr’s Framework of Complementarity,” in* Physics and Whitehead: Quantum, Process, and Experience*, edited by Timothy Eastman and Hank Keeton (Albany, SUNY Press), 164-79.

#### Papers providing a Proof of Contextuality in Quantum Mechanics

Kochen, Simon and Specker, Ernst. 1967. “The Problem of Hidden Variables in Quantum Mechanics,” *Journal of Mathematics and Mechanics*, 17, 1, 59-87.

Mermin, David. 1990. “Simple Unified Form for the Major No-Hidden Variables Theorems,” *Physical Review Letters*, 65, 27, 3373-76.

Peres, Asher. 1990. “Incompatible Results of Quantum Measurements,” *Physics Letters A*, 151, 3, 4, 107-108.

#### Other Cited Publications

Aspect, Alain, Grangier, Philippe and Roger, Gérard. 1981. “Experimental Tests of Realistic Local Theories via Bell’s Theorem,” *Physical Review Letters*, 47, 7, 460-67.

Bell, John. 1964. “On the Einstein-Podosky-Rosen Paradox,” *Physics*, 1, 195-200.

Einstein, Albert, Boris Podolsky, and Nathan Rosen. 1935. “Can Quantum-Mechanical Description of Physical Reality be Considered Complete?” *Physical Review*, 47, 777-80.

Everett, Hugh. 1957. “‘Relative State’ Formulation of Quantum Mechanics,” *Reviews of Modern Physics*, 29, 3, 454-62.

Hájek, Alan and Bub, Jeffrey. 1992. “E*PR*,” *Foundations of Physics*, 22, 3, 313-32.

Mermin, David. 1998. “What is Quantum Mechanics Trying to Tell Us?” *American Journal of Physics*, 66, 9, 753-67.

Saunders, Simon. 1995. “Time, Quantum Mechanics, and Decoherence,” *Synthese*, 102, 2, 235-66.

Shimony, Abner. 1993. *Search for a Naturalistic World View. Vol. II: Natural Science and Metaphysics* (New York, Cambridge University Press).

### Author Information

Manuel Bächtold

Universität Dortmund

baechtold@fb14.uni-dortmund.de

### How to Cite this Article

Bächtold, Manuel, “Niels Bohr (1885–1962)”, last modified 2008, *The Whitehead Encyclopedia*, Brian G. Henning and Joseph Petek (eds.), originally edited by Michel Weber and Will Desmond, URL = <http://encyclopedia.whiteheadresearch.org/entries/bios/contemporaries/niels-bohr/>.