This essay will consist of two parts each focusing respectively on the initial and conclusive sections of Bohr’s statement (below). The arguments emerging from the following commentary to Bohr’s view of physics aim to emphasize the importance of the adaptation of a balance between the realist and agnostic attitudes in scientific inquiry, as well as point out the conceptual crisis caused by the breadth of human experience which results in the impossibility of clear and comprehensible appreciation of that experience.
“Physics is to be regarded not so much as the study of something a priori given, but rather as the development of methods for ordering and surveying human experience.” - Niels Bohr
Before any polemic or commentary to the above statement is made, at least a superficial analysis (due to the limited length of this paper) of its implications needs to be carried out in order to provide a platform of unambiguous reference.
One can safely assume that here Bohr sees physics to as the most fundamental of the natural sciences, and therefore potentially unlimited in its scope (i.e. encompassing all other sciences) since he uses its role as an exemplification of the role of scientific inquiry in general. This can be inferred from the empirical declaration emphasizing the primary role of experience viz. a posteriori knowledge.
A more careful survey of the above will lead one to realize that the something that Bohr tentatively denies as “a priori given” object of inquiry is reality itself. In his view physics should limit itself to statements concerning our knowledge of nature and refrain from metaphysical speculation. This attitude is clearly echoed in the words of another “Copenhagist”, Werner Heisenberg who writes in his Physics and Philosophy: “What we learn about is not nature itself, but nature exposed to our methods of questioning”. Such a view renders physics metaphysically mute.
This limitation imposed on physics, legitimizing only statements about phenomena does remind one of Berkeley’s subjective idealism, where talk of a reality beyond perceptions is unjustified since it is only their content that one has access to. In fact that very content alone qualifies to have any ontological status at all. Direct parallels could be drawn between the role of perception in Berkeley’s system and measurement in Bohr’s physics and their exclusive status as the basis of our knowledge. There is a fundamental difference however. Our view of reality as “looked” upon through the eye of physics by Bohr’s definition is indeed necessarily tainted by our experience, but this is not to say that there is nothing causing the particular result of measurement. In other words the Danish natural philosopher indeed is not a dogmatic realist, but such a position does not imply anti realism of the Berkleyan type. He remains an agnostic.
Having briefly analyzed the Copenhagen position with respect to what should not be seen as the role of physics, the positive aspect of its activity as posited by Bohr will now be discussed. This will be done with the aid of a close analogical relationship mentioned earlier, between human experience and science (physics in particular). The latter can be seen as an extension of human senses which allowes an incessant broadening of human experience of the world in general. This increase in scope necessarily forces the constant development of language in order to accommodate this new experience.
New experiences not only broaden our knowledge base but also serve as platforms for further discoveries. This process has probably evolved hand in hand with the development of language in the human species. The capacity and ability of the human mind has been constantly revised in a positive feedback process between nature and the intelligent observer, magnified more so with the ability to communicate and record the acquired observations. That process has been subsequently formalized by the likes of Aristotle, Galileo and Francis Bacon just to name a few key figures in the human development of thought and refinement of methods of inquiry. This development consequently broadens the set of concepts which can be seen as the carriers of meaning in this dynamic interplay.
Subsequent millennia and finally centuries of scientific endeavor have exposed the limitations of our innate senses and hence immediate perception of the world. As good examples may serve the shape of our planet (which is locally flat), the mechanics of the heliocentric system or the breadth of the electromagnetic spectrum to which we only have a significantly limited access. However surprising this class of discoveries has been, the fundament had already been laid down by philosophical activity making these counter intuitive discoveries that much easier to absorb by the collective.
With the advent of early 20th century paradigm shifting theory of special relativity our cherished and fundamental intuitions harbored by the concepts of absolute space and time were seriously revised. Modern physics had ventured completely out of reach of the everyday tangible experience (i.e. we do not in any noticeable way experience the relativistic effects of time dilation and length contraction). In this new framework of broader experience the concept of simultaneity of space-like separated events turns out to be a meaningless conceptual fossil which should be abandoned. This means that our common sense notions about the world and causation, and even Newton’s laws do not hold true in general but turn out to be merely a limiting case of how the world is. Still prior to the quantum revolution, the need for a refinement of our language in describing our experience required the adaptation of a model containing the concept of space-time curvature in order to accommodate the verified predictions of general relativity. This new framework had boldly abandoned our intuitions.
Enough historical evidence shows that Bohr’s idea of the active role of physics has in fact been taking place all along. Seeing in retrospective how science indeed consisted of developing increasingly more sophisticated methods allowing a systematic and rigorous ordering of human inquiry (experience). However before moving on to the second part of Bohr’s statement, the importance of motive behind scientific inquiry in general should be emphasized.
Wouldn’t curiosity, arguably unique to the human species, serve as a candidate motive? Has it been the desire to categorize phenomena into a systematic and consistent system in order to predict still more phenomena and patterns emerging from them? Or maybe the motive is deeper than that? A closer analysis will in fact reveal that it has been the desire to explain and discover the causes of the appearances, emerging from the belief that there is some underlying and absolute reality behind the veil of phenomena. Einstein himself expresses the above as follows:”The belief in an external world independent of the perceiving subject is the basis of all natural science”. It has been the belief of high priests, philosophers and scientists that there exists some absolute, Arché, ideal forms, primary qualities, noumena from which all appearances emerge.
In fact it is necessary to adopt the metaphysical realist attitude at least to some degree in order to do any science at all. Only after this has been agreed upon, further philosophical discussion may commence concerning to what degree this attitude should be implemented before it becomes a hindrance. Even Bohr with his view of the role of physics is not entirely immune to the necessity of the presence of realism in science. One needs to make some fundamental assumptions about reality in order to make any sense of the results of experiments. For example, when recording the number of clicks of the Geiger counter caused by the current induced by electrons ejected from ionized gas molecules as a result of bombardment by alpha particles emitted by a radioactive isotope, the experimenter not only makes a series of assumptions about causality, but also the consistent behavior of phenomena previously observed. The assumption of causality is obviously a realist approach and the expectation of consistent results also assumes a permanent status quo feebly supported by induction.
As a fitting example of the importance of realist approach in discovery one needs to look no further than the EPR argument posed by Einstein and his colleagues, and the surprising result of Bell’s proof. It was the deeply cherished belief in an ultimately objective reality that drove the father of special relativity to devise a thought experiment aimed to expose the noumena behind the enormously successful quantum theory.
The core of the argument was the belief of objectively existing “hidden variables” intrinsic to quantum entities prior to observation. According to Einstein and contrary to Bohr these hidden variables were merely waiting to be revealed by measurement. In particular, the correlated pair of photon polarization attributes (spin) existed in a mixed rather than pure state. Since the two measurements (events) of these photons were to be space-like separated one could assume that the first measurement could not influence the second one unless it traveled faster than the speed of light. This fundamental assumption, so very realistic in character was to lead to one of most profound consequences of quantum theory.
It took almost thirty years before John S. Bell, fascinated by the EPR argument, approached it with the hope of proving Einstein’s intuitions with mathematical rigor. The proof consisted of formalizing the requirements of locality, setting out the expectations of two models (the Einstein mixed state hidden variable view, and the Copenhagen pure, entangled state) and finally calculating their consecutive predictions concerning the probabilities of correlated outcomes of the space-like separated measurements of the spin attribute. The conclusion of the proof turned out to be, contrary to Bell’s expectations, in favor of the quantum theory prediction. Furthermore, this purely mathematical result has been confirmed experimentally numerous times since Bell published his paper. What this amounts to is a rather paradigm shattering consequence, that no local model of reality can explain the result of the EPR experiment.
The above discovery serves as an ironic example how the realist approach has served as an arguably necessary means of revealing the contrary nature of its own set of principal assumptions and thus vindicating Bohr’s epistemic caution not to take anything as “a priori given”. One can safely conclude that a balanced interplay and implementation of both worldviews is necessary to fruitful inquiry.
“In this respect our task must be to account for such experience in a manner independent of individual subjective judgment and therefore objective in the sense that it can be unambiguously communicated in common human language.”
- Niels Bohr
Let’s venture a little further into the consequences brought by human expanded experience, and inspect the impact it had on language. After our intuitive notions of space and time were shattered by relativity, quantum theory has brought its own set of reformations some of which necessitated an outright abandonment of some fundamental concepts about the world. The Planck scale has turned out to be a world so different from what classical physics explains that it may well be considered to be governed by its own set of laws. Despite its enormous predictive success, for a long time no one had an idea how to interpret the physical significance of the theory. The Schrödinger equation had an oracle like status until Bohm suggested treating the modulus of the square of the wave function as the representation of the probability of finding the quantum system in some particular state upon measurement. These early difficulties in attributing meaning to the quantum model signified a dawn of a conceptual crisis.
The abandonment of any determinate properties existing at all prior to measurement was forced upon us as a necessary consequence of the desire of making any sense of the predicted phenomena. This alien property of the Planck scale referred to as the complementarity principle has emerged from countless and consistent results of experiments performed on quantum systems. A quantum entity behaves both as a wave and a particle at the same time. This duality is based on concepts which are self exclusive in classical physics - something is either a wave or a particle, but not both. On the other hand concepts such as position and momentum which are inseparable in the classical (non quantum) scale turn out to be impossible to coexist simultaneously at the Plank level. An electron for instance simply does not possess the property of a definite position and equally definite momentum at the same time. These incompatibilities of various attributes at the Plank scale have been rigorously formalized by Heisenberg in what became one of the most celebrated and experimentally confirmed properties of the quantum world – the uncertainty principle.
The conceptual crisis deepened as the rate of new experience surpassed the scope of most preexisting concepts. Only mathematics had kept pace with the exponential expansion of unique experiences in this human scientific endeavor. Generating concepts clearly defined and potentially infinite in scope due to its symbolic nature, mathematics works as the litmus paper of consistency in observation. It has been among other things the ultimate conceptual generator. Its formal structure and rigor make it the indispensable tool for systematic and consistent accommodation of new phenomena to newly provided sets of abstractions. Due to its limitless capacity it contains a record of the entire human experience and makes possible unambiguous communication, albeit abstract, to facilitate further inquiry.
Nevertheless human curiosity demands concepts which satisfy intuition, and no abstraction no matter how precisely defined, will suffice if it does not produce some comprehensible mental picture. This naturally has become increasingly difficult with the discovery of the exotic nature of realms not accessible to direct observation and governed by laws which defy common sense.
In spite of these difficulties scientist still try to aid imagination with concepts “closer to home”. In an attempt to bring closer to reason the fuzzy world of superimposed states, which defy the law of excluded middle in logic, Heisenberg borrows the Aristotelian concept of potentia in order to give a more intuitive picture of nature before measurement. So it is rather possibilities or tendencies that remain objective in the quantum view of reality.
My belief, of a puzzled student of physics and philosophy is that even if mathematics will persevere in maintaining a well defined and conceptually rigorous picture of reality, it will necessarily not be for the appreciation of any human individual, and this picture will ultimately fade away into incomprehensible heights of abstraction. As a consequence of this Bohr’s wish for physics to offer a language which could be “unambiguously communicated in common human language” seems as an impossible task.
Werner Heisenberg, Physics and Philosophy (Penguin Classics, 2000)
Nick Herbert, Quantum Reality – Beyond the New Physics (Anchor Books,1987)
Peter Kosso, An Introduction to the Philosophy of Physics (1998)
David Miller, Phil2011 Philosophy of Modern Physics lecture notes (Sem. II, 2007).
The argument is problematic since Berkeley goes from stating that since perceptions or, using Kant’s terminology which will be adopted in this essay, phenomena is all we can have access to, nuomena necessarily do not exist. That is to say Berkeley makes an unjustified leap from an agnostic framework by his own definition to that of an anti realist. He clearly wishes to have the cake and eat it too
 For the purpose of this essay a definition of dogmatic realism will be adopted from Heisenberg’s Philosophy and Physics. He defines it as a form of objectivating reality. According to him we objectivate reality or in particular a statement about reality when we claim that its content does not depend on the conditions under which it can be verified. Dogmatic realism claims that there are no statements about reality which cannot be objectivated. Philosophy and Physics, Werner Heisenberg (2000) p.43
 The Copenhagen interpretation which laid the fundament for quantum theory and which was the target of the EPR argument professed to be exclusively a theory of phenomena.
 The original EPR experiment concerned two momentum correlated electrons, but for reasons of clarity the conceptually simpler version of the experiment proposed by David Bohm involving polarization correlated photons (Nick Herbert, 1987, p.201) will be adopted in this paper.
 David Miller, Phil2011 Philosophy of Modern Physics lecture notes (Sem. II, 2007), Part2 Physics, p.3.