The great physicist Niels Henrik David Bohr was born in Copenhagen, Denmark on the 7th of October, 1885. His father was a professor of physiology at the University of Copenhagen and he was also a staunch Lutheran while his mother was from a “wealthy Jewish family of bankers and parliamentarians.”
In 1903, Bohr became an undergraduate at the University of Copenhagen where he initially studied philosophy and mathematics, before he made the change to study physics in 1905. And in 1911, he completed his doctorate under the tutelage of the physicist Christian Christiansen.
Then he travelled to England for his post-doctorate, and there, in Trinity College, Cambridge, he worked with Sir J. J. Thompson who discovered the electron and in the University of Manchester, he worked with Ernest Rutherford who discovered the modern atomic structure.
In England, Bohr carried out experiments with these two great physicists and this must have sharpened and furthered his insight about physics and about the atomic world.
He returned to Copenhagen in 1912 and he became an assistant professor at the University of Copenhagen. He also got married to Margrethe Nørlund and they had six children, out of which two died and four “went on to lead successful lives”.
One of the children that “went on to lead successful lives”, Aage Niels Bohr, became “a very successful physicist like his father, winning the Nobel Prize in Physics in 1975.”
In 1913, Niels Bohr building on Rutherford’s atomic structure published his own model of the atomic structure. He showed that the atom consists of a positively charged nucleus with negatively charged electrons revolving in discrete or stationary orbits around the nucleus.
With this new idea of discrete or stationary orbits in the atom, Niels Bohr went ahead to inform us that light is absorbed or radiated from the atom when an electron jumps or transits from one stationary orbit to another.
Bohr’s atomic model became the basis of quantum mechanics and also the reason why he was awarded the Nobel prize in physics in 1922, which according to the citation was “for his services in the investigation of the structure of atoms and of the radiation emanating from them”.
He became a full professor in 1916 and he built upon this his idea in subsequent years, and in 1921, with the assistance of the Danish government and the Carlsberg Foundation, he founded the Institute of Theoretical Physics in Copenhagen.
So, in the 1920s, the Institute of Theoretical Physics in Copenhagen became the focal point of the development of quantum mechanics and it drew the attention of many physicists of that time who visited the institute.
Also, it was in the 1920s that Niels Bohr’s philosophical view of quantum mechanics and of physics took shape. And because he was at the opposite end of the belief in realism, he entered into some fierce intellectual debates with Einstein who held on to realism.
It is at this point that we now have to look at the legacy of Niels Bohr who established the complementarity principle to be the accepted principle of quantum mechanics.
As it became clear to everyone in the 1920s that a new field of physics called quantum mechanics has been born based on the pioneering works of Max Planck (who first established the discrete nature of the atomic world), Albert Einstein (who showed us the discrete nature of light), and Bohr (who presented the discrete basis of the atomic structure), physicists were puzzled by what quantum mechanics implied for the way we do physics.
It became clear that atomic entities such as the electron and the proton can behave like particles and also like waves. And the same can be said of light, which can also behave like a particle and also like a wave. It became as though there is no longer any such things as “objective reality”.
This latter viewpoint was what Bohr held and this did not go down well with Albert Einstein who felt that an objective reality exists. Einstein would always contend that an electron would always exist somewhere whether we are observing it or not.
But to Bohr, observation was what was fundamental. Prior to observation, the electron would exist in an indeterminate or probabilistic state and that it was observation that collapsed the electron to a position or scatters it as wave, as the case may be.
Bohr made it clear that it was only the nature of the experiment or observation that determines what we will see about the electron. Thus, there is no “objective reality” out there.
Einstein argued with Bohr about all of this which relates to the philosophical implications of quantum mechanics. Einstein had helped establish quantum mechanics but he would not accept it philosophical implications which seemed to deny the existence of an “objective reality”.
But Bohr would and this is what draws the line between these two great physicists and at the same time shows us the most far-reaching legacy of Niels Bohr.
In his denial of an “objective reality”, Bohr is quoted to have said that:
“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.”
And also that “everything we call real is made of things that cannot be regarded as real.” However, the most stark quote that shows Bohr’s philosophy about quantum mechanics is that:
“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.”
To Bohr, “there is no quantum world” only abstract descriptions. Bohr was a dedicated skeptic of reason, unlike Einstein who would say that “the universe is comprehensible.”
The acceptance by Bohr and other physicists that supported the Copenhagen Interpretation of the limited ability to know or of the power of reason and that denied “objective reality” came as a shock to Einstein who was more of a traditional scientist who held on to the classical belief in an “objective reality” that exists regardless or independent of observation.
Bohr was a positivist with Ernst Mach and some of the British philosophers such as Immanuel Kant, David Hume and John Locke who had placed limits on reason and in our ability to have the knowledge of “things in themselves”.
Bohr made clear his position about the purpose of physics when he said that: “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.”
This is a complete deviation from realism and from the classical tradition of physics. It is a deviation from Newton’s legacy which assures us that the laws of the universe, whatever they are, are knowable and that they can be clearly stated as he did for the physical laws of the universe.
But quantum mechanics, as Bohr and his camp led it, was challenging the ground philosophy of physics and the existence of “objective reality”. In accepting that physics is about what we can say about nature, Bohr proposed the complementarity principle to show that the entities which we observe are both particles and waves.
According to Bohr, the paradox of quantum mechanics was final and irresolvable. However, Einstein disagreed, and in a letter to Max Born, he said that:
“Quantum mechanics is very impressive. But an inner voice tells me that it is not yet the real thing. The theory produces a good deal but hardly brings us closer to the secret of the Old One. I am at all events convinced that He does not play dice.”
It is good to see the two views: Bohr accepted that the paradox of quantum mechanics was without explanation; it was just how it is. But Einstein did not accept that the paradox was without explanation; he felt that there was a/some hidden variable(s).
He felt that there was something else about nature that quantum mechanics does not reveal which if revealed will make us see why the paradox exists in the first place or rather only seems to exist.
Bohr’s complementarity principle thus stands as one of his legacies for physics and it was borne out of a deep and honest skepticism for reason and for “objective reality”. According to Bohr, it was the final say and not a temporary or ad hoc provision. The reverse which would be the position of Einstein.
I have made the comparisons between Bohr’s views about quantum mechanics and those of Einstein so that you can see clearly the legacy of Bohr. This is because the legacies of these two great physicists are inter-twined as till the end of their lives, they were brought into intellectual debates about the nature of quantum mechanics and about reality.
Some physicists accept that Bohr’s views triumph over those of Einstein as was already evident in the Solvay Conference of 1927 where Bohr showed that Einstein was wrong about the incompleteness of quantum mechanics.
But that would be a hurried conclusion as post-modern physics has arrived to show that quantum mechanics is really incomplete, (and in fact wrong, to be more honest about the situation). Post-modern shows that there is an underlying principle in the atomic world that is responsible for the observed paradox and not the complementarity principle.
I would come back to this in this article but I won’t dwell much on it since I have done so in some of my scientific posts already. Also, I just want to focus more on the legacy of Niels Bohr without referring much to the position of post-modern physics.
To Bohr, quantum mechanics is complete. But to Einstein, it wasn’t. This contrary views by these two great physicists have gone a long way to shape how we do physics today, even as physicists still debate about the nature of quantum mechanics and what it is trying to tell us about the universe.
In so many ways, Niels Bohr represented the break away from the philosophy of classical physics, and in a way, I agree that this was needed, but we had to be right about it.
In this regard, Bohr, Heisenberg and others were skeptical about the capacity of language to exactly represent reality. They felt that the language of physics had developed only to explain classical objects or concepts.
They felt that we can only understand our observations using classical concepts and that these concepts may not really represent or capture the reality under observation.
In a way, they are saying that language is just necessary for us to make sense of our experiments and observations and not really to represent reality. Niels Bohr and his camp were, in my opinion, the first to realize, though unknowingly or rather inappropriately, the limits of physical science.
The real break was not to be from the classical way of describing nature but from the physical way of describing nature. The whole doubt that the application and the limit of language brought into the philosophy of quantum mechanics were to expose to us the limit of physical science and not of the classical way of describing nature.
Niels Bohr and the Copenhagen philosophers thought that it was the reverse and they did not even consider to explicitly question physical science. Quantum mechanics raised doubts about physical science and not the classical way of describing nature. There is a difference between the two.
Let me make clear what I mean by the difference between physical science and the classical way of describing the universe. To better understand this, you can see the distinction as between the physical way and the classical way of describing the universe.
The physical way of describing the universe is more of an epistemological term than the classical way of describing the universe which is more about conceptualization and history. Now, in a scientific theory, you can remove the physical way while still preserving the classical way of understanding the universe.
This is what post-modern physics has done. For example, in classical physics, we have the concept of energy which is physically described. Now, if we describe the concept of energy metaphysically, it ceases to be a physical concept while it still remains a classical concept, at least historically.
What quantum mechanics or rather atomic physics really requested was a metaphysical way of describing the classical concepts that still applied to it. Classical concepts such as energy, force, momentum etc. still apply to quantum mechanics or atomic physics, but we needed a metaphysical description of these entities in order to see how to go beyond the complementarity principle.
It is this move by post-modern physics that shows us that the principle of non-inertia is the core principle that underlies the paradoxical nature of quantum mechanics. This is against the complementarity principle which is just the acceptance of this paradox and not an explanation of it.
Thus, with the long-awaited explanation of this paradox, we find the restoration of “objective reality” into physics. In post-modern physics, we are forced to explain and to resolve the wave-particle paradox of quantum mechanics.
We now have to agree that quantum mechanics was not complete after all or rather it was not the final theory as the new principle of non-inertia has emerged from relativity and not from quantum mechanics.
Nevertheless, we have Bohr to thank for his introduction of stationary orbits into physics which still finds need even in post-modern physics and also for his important position as a reminder that we should always take caution even if we disapprove of his skepticism in our path towards understanding reality.
And finally, we must never forget the man Bohr and we must never forget that his instrumentalist view of quantum mechanics which gives precedence to observation and experiment above any underlying reality was borne out of deep thinking about the nature of quantum mechanics, for which he said that “anyone who is not shocked by quantum mechanics has not understood it.”
Until next time,
I will be here.
– M. V. Echa