Today we discuss the philosophy of Niels Bohr and some of its influences, such as pragmatism. We will explore these by looking at some Bohr quotes.
[Please note that this may not exactly match the audio. However, there should be no significant differences.].
Welcome to episode seven of the Metaphysics of Physics podcast. I am Ashna, your host and guide through the hallowed halls of the philosophy of science. Thanks for tuning in!
With this show, we are fighting for a more rational world, mostly by looking through the lens of the philosophy of science. We raise awareness of issues within the philosophy of science and present alternative and rational approaches.
You can find all the episodes, transcripts and subscription options on the website at metaphysicsofphysics.com.
Today we will be taking a look at the philosophy of Niels Bohr. We will be making the case for three central elements of Bohr’s philosophy and we will use quotes to show that he did indeed hold to these tenets.
If you are interested in further readings, links to the sources from which many of these quotes are taken have been provided in the show notes.
So, what are these three central tenets of Bohr’s philosophy?
Firstly, the rejection of reality and objective facts.
Secondly, a rejection of the Law of Identity.
And thirdly, that Bohr ascribed to a kind of acausality, that is a rejection of causality. This might seem a consequence of the second and arguably it is. If you reject the Law of Identity, it is no surprise that you might also reject causality.
What do these tenets point to? Which philosophical influence or influences might Bohr then be said to have suffered from?
Let’s begin by looking at his background.
Bohr is considered one of the foremost and founding figures of modern physics. He is best known for his contributions to quantum theory and his work on the structure of atoms, for which he won the Nobel Prize in physics in 1922.
He was born in 1885 and died 1962, aged 77. He became interested in physics at a young age and acquired a doctorate in physics in 1911 from the University of Copenhagen, at the relatively young age of 26.
It should be noted that his father, Christian Bohr, was a friend of the well-known philosopher Harald Høffding. Christian would invite Høffding to the Bohr household and Niels would observe and take part in many philosophical discussions with this philosopher.
Høffding was heavily influenced by Kant and later became a positivist. Positivism holds that knowledge begins with sensory experience and Bohr most certainly agreed with this.
These early experiences with Høffding seems to have sparked an intense interest in philosophy. Furthermore, Høffding was not his only philosophical influence.
Bohr was a philosophical eclectic and was influenced by various other philosophers, such as the pragmatist William James.
As we shall see, the Neo-Kantian philosophies he was exposed to influenced his physics for the worst.
His early work was on the measurement of surface tension of liquids using oscillating fluid jets, for which he won a prize offered by the Copenhagen Academy of Sciences while still a student. He also got published by the Royal Society of London for related work.
While he was researching material for his doctoral thesis on the electron theory of metals, he came across Max Planck’s early work on quantum theory. There he read about how light was quantized into packets. After this, his work became a lot more abstract and eventually positively unworldly!
In 1912, Bohr met Ernest Rutherford, the discoverer of the atomic nucleus. Rutherford also worked on an early atomic model, for which he received the 1908 Nobel Prize. Bohr went on to make his own atomic model, under the guidance of Rutherford.
Bohr combined Rutherford’s description of the nucleus and Planck’s quantum theories into his own atomic model. For this, he received the Nobel Prize in 1922.
Bohr’s atomic model is considered one of his most significant contributions to modern physics. It shows the atom consisting of a positively charged nucleus with negatively charged electrons traveling around it in separate circular orbits with discrete radii. And that the electrons can transition between orbits by emitting or absorbing energy equal to the quanta of light.
His theoretical work was also used to help understand how nuclear fission works and was used in the first attempts to split uranium in the 1930s.
Bohr developed a concept of “complementarity” which would prove to be perhaps one of his most irrational and revealing philosophical notions. This concept held that there was seldom a single way to describe something and that often, seemingly contradictory, mutually-exclusive descriptions had to be embraced.
The most well-known and influential examples of complementarity at work is that of the particle-wave duality of light and other subatomic “particles”.
It is unfortunate that complementarity would come to lie at the heart of quantum mechanics. As we shall see, it is useful in revealing the deep irrationality of quantum mechanics and its philosophical influences.
Now, let’s look at some Bohr quotes that point to the pillars of his philosophy, the first one being, the rejection of reality and objective facts.
Bohr’s Rejection of Reality and Objectivity
“Isolated material particles are abstractions, their properties being definable and observable only through their interaction with other systems.” 
As we shall see, Bohr does not think that physical reality has any place in physics. He believes that knowledge is the process of forming relationships between abstractions, not the study of the concrete physical world.
“From the very beginning, however, one was not unprepared in this domain to come upon failure of the forms of perception adapted to our ordinary sense impressions.” 
Here, this domain refers to the atomic theory and Bohr admits that he was not unprepared for the alleged failure of the forms of perception. What could have lead one to be prepared for this? Philosophy.
“There is no quantum world. There is only an abstract quantum physical description. It is wrong to think that the task of physics is to find out how nature is. Physics concerns what we can say about Nature.” 
So, in other words, we cannot know physical reality, only abstract descriptions. Physics is not the study of reality as it is, but simply a pragmatic description that explains what we observe.
An abstract description of what? The next quote gives us a clue.
“We must be clear that when it comes to atoms, language can be used only as in poetry. The poet, too, is not nearly so concerned with describing facts as with creating images and establishing mental connections.” 
Here Bohr means that language can not be used as a precise method of the description of reality. The purpose of words, of concepts, of ideas is not to describe reality, but to describe and relate “images” and “mental connections”.
In other words, the purpose of language, of concepts, is not to study physical reality, but to study and relate abstractions.
So when we asked, “An abstract description of what?” Of, what? Nothing? Or perhaps abstract descriptions of sense perceptions. But sense perceptions of what? We shall return to this in a little bit.
Are these abstractions to be considered objective? Bohr does not think so.
“I consider those developments in physics during the last decades, which have shown how problematical such concepts as objective and subjective are, a great liberation of thought.” 
Bohr does not consider it worthwhile discussing whether or not an idea is objective or subjective. This is not too surprising given that one of Bohr’s many philosophical influences was, as we shall see, pragmatism.
You might ask whether there was any justification for Bohr saying these kinds of things. It is certainly widely asserted that Bohr and his peers were forced to reluctantly accept the implications of quantum theory.
But there can be no physical justification for claiming that reality is unknowable.
Why then did Bohr adopt this view? Is it because the quantum world really is “inflicted with a kind of vagueness”? Or, is it because of his philosophical influences?
“The situation which we meet here is characterized by the fact that we are apparently forced to choose between two mutually contradictory conceptions of the propagation of light. One, the idea of light waves, the other, the corpuscular [particle] view of the of the theory of light quanta, each conception expressing fundamental aspects of our experience. As we shall see in the following, this apparent dilemma marks a particular limitation of our forms of perception which is bound up with the quantum of action.” 
Are we forced to choose between contradictory conceptions of light? No. Objectivity, the recognition of reality as it really is, requires us to identify the fact that it can be only one of those and to attempt to identify which one is the case.
Far from it being a dilemma, it is a requirement of objectivity and identifying reality. To ignore this requirement is to reduce the concept of light to a floating abstraction and to make it impossible to identify its true nature.
Nor is it a limit of our “forms of perception” as such. Nothing that exists can exist with a contradictory nature. To be is to be something, free of contradictions.
Far from being a “limitation” of our “forms of perception” this is a necessary fact which everything, including perception, must adhere to.
“An independent reality in the ordinary physical sense can neither be ascribed to the phenomena or to the agencies of observation. After all, the concept of observation is so far arbitrary as it depends upon which objects are included in the system to be observed. Ultimately, every observation can, of course, be reduced to our sense perceptions.” 
Here Bohr advocates the idea that observation is not the observation of real entities in the physical world, but of floating sense perceptions. In his view, science is not the study of external physical reality, but the description of sense perceptions.
Sense perceptions of what? Reality does not begin with sense perception. To perceive is to perceive something.
Our senses do not distort that which they observe. Neither do our senses create their own content. But, Bohr frequently spoke as if he believed that our senses create their own content or distort that which they do observe.