In fact, Bohr had already convinced Heisenberg that his uncertainty principle was a special case of a more general idea that Bohr had been expounding for some time – a thread of Ariadne that would guide travelers lost through the quantum world; a principle of great and general import named the principle of complementarity.
Complementarity arose naturally for Bohr after the strange discoveries of subatomic particles revealed a world that was fundamentally probabilistic. The positions of subatomic particles could not be assigned with definite certainty but only with statistical odds. This was a complete break with Newtonian classical physics where particles had a definite trajectory, a place in the world order that could be predicted with complete certainty if one had the right measurements and mathematics at hand. In 1925, working at Bohr’s theoretical physics institute in Copenhagen, Heisenberg was Bohr’s most important protégé had invented quantum theory when he was only twenty-four. Two years later came uncertainty; Heisenberg grasped that foundational truth about the physical world when Bohr was away on a skiing trip in Norway and Heisenberg was taking a walk at night in the park behind the institute.
When Bohr came back he was unhappy with the paper Heisenberg had written, partly because he thought the younger man seemed to echo his own ideas, but more understandably because Bohr – a man who was exasperatingly famous for going through a dozen drafts of a scientific paper and several drafts of even private letters – thought Heisenberg had not expressed himself clearly enough. The 42-year-old kept working on the 26-year-old until the latter admitted that “the uncertainty relations were just a special case of the more general complementarity principle.”
So what was this complementarity principle? Simply put, it was the observation that there are many truths about the world and many ways of seeing it. These truths might appear divergent or contradictory, but they are all equally essential in representing the true nature of reality; they are complementary. As Bohr famously put it, “The opposite of a big truth is also a big truth”. Complementarity provided a way to reconcile the paradoxes that seemed to bedevil quantum theory’s interpretation of reality.
The central scientific paradox was what is called wave-particle duality. In 1803, the British polymath Thomas Young had proposed that light, contrary to Isaac Newton’s view of it, consists of waves; an experiment like diffraction makes this wave nature clear. A hundred years later, in 1905, Einstein proposed that light in fact consists of particles, an idea he invoked in order to explain the photoelectric effect and which won him a Nobel Prize; these particles were later called photons. Soon it was found through other experiments that all subatomic particles and not just photons could display wave and particle behavior. In 1924, the French physicist and aristocrat Louis de Broglie saw a way through the impasse when he came up with a simple equation that related the momentum of a particle – a particle property – inversely to its wavelength – a wave property.
In spite of de Broglie’s insight, particles clearly don’t look like waves and waves don’t look like particles in real life. In fact the very names seem to put them at odds with one another. It was Bohr who saw both the problem and the solution. Particles and waves both exist and are equally valid and essential ways of interpreting the quantum world. Depending on what experiment you do you might see one or the other and never both, but they are not contradictory, they are complementary. Most crucially, you simply cannot make sense of reality without having both in hand. It was a powerful insight that cut through the complexities of intuition and language; it was not too different in principle from other counterintuitive truths that science has uncovered, for instance the truth that both lighter and heavier bodies fall at the same rate. Complementarity rationalized opposing tendencies of the physical world and indicated that they were one. It was what had made Bohr subsume the opposing quantities in Heisenberg’s uncertainty principle under the same rubric.
Complementarity was also pregnant with far more general interpretation. The most effective application of it to human affairs in Bohr’s hands was the problem posed by nuclear weapons. Even before the bomb had been used on Hiroshima, Bohr saw deeper and further than anyone else that the very fact that nuclear weapons are so enormously destructive might make them the most potent force for peace that the world has ever seen, simply because statesmen will realize that nobody can truly “win” a nuclear war if everyone has them. “We are in a completely new situation that cannot be resolved by war”, Bohr said. The complementarity of the bomb continues to keep the peace through deterrence.
Another noteworthy example was a speech delivered by Bohr in 1938 to the International Congress of Anthropological and Ethnological Sciences at Kronberg Castle in Denmark. Apologizing at the outset for presuming to speak about a topic on which he was not an expert, Bohr proceeded to provide a succinct summary of complementarity in the context of atomic physics. Turning to biology, he then made the perspicacious observation – still the subject of considerable debate – that reason and instinct which might appear to be opposed to each other are complementary, both providing a complete picture of a sentient being. Bohr then came to the crux of the matter, pointing out that complementarity is of the essence when people are judging other cultures which might seem divergent from their own but which turn out to be different and equally productive ways of looking at the world. As Bohr eloquently put it: “Each culture represents a harmonious balance of traditional conventions by means of which latent potentialities of human life can unfold themselves in a way which reveals to us new aspects of its unlimited richness and variety.” Bohr believed that contact between cultures can go a long way in not just dispelling biases but in mutually enriching both parties: “A more or less intimate contact between different human societies can lead to a gradual fusion of traditions, giving birth to a quite new culture”. This is as clear an appeal for internationalism and mutual understanding that one can think of; if everyone had understood complementarity, maybe we might have had less fascism, imperialism and genocide. The final goal of complementary views of societies, as Bohr pointed out powerfully in the same lecture, isn’t different from the goal of science as a whole – it is “the gradual removal of prejudices”.
As we approach what seem to be novel problems in the 21st century, Bohr’s complementarity is a message in a bottle from one fraught world to another, telling us that seeing these new problems through the lens of an old principle can be most rewarding. We seem to live in a time when many see social and political problems through a binary, black-or-white, zero sum lens. Either my viewpoint is right or yours, but not both. Complementarity bridges that division. For instance consider the problem of individualism vs communalism, a divide that also hints at the cultural divide Bohr spoke about, in this case largely an Eastern vs Western divide. Western society is fiercely individualistic; self-interests guide people’s lives and most people don’t want others to tell them that they should live their lives for others. Meanwhile, Eastern and some European societies are much more communal; community interests often override self-interest and individuals are told that their self-development should take a backseat to the development of their community and society. Bohr’s complementarity tells us that this divergent view should not exist. Communal and individualistic views are both essential for looking at the world and building a more productive society; in fact one can gain self-knowledge and wisdom by working for a community, and likewise a community can be improved when people engage in individualistic self-improvement that helps everyone.
There are other problems for which complementarity provides a potential solution. I will speak mostly for the United States since that’s where I live, but these problems are in fact global. Consider the problem of immigration, one to which Bohr’s 1938 address is directly applicable. People criticized as “globalists” think that unfettered immigration is a net good. The opposing camp thinks that preserving a nation’s culture is important, and too much or too rapid immigration will weaken this culture. But complementarity tells us that nationalism is in fact strengthened when immigrants work together for the common good of the country. At the same time, immigrants should put their country first and prioritize work that will strengthen their nation’s economy, military and social institutions. We are global citizens, but we are also shaped by evolution and culture to take care of our immediate own. The opposite of a big truth is also a big truth.
Even scientific debates like the nature (genes) vs nurture (environment) conundrum can benefit from complementary views. People criticized as biological “essentialists” believe that genes dictate a lot of an individual’s physical and psychological makeup while the opposing “nurture” camp believes that much of the effect of genes can be changed by the environment. But complementarity says that just like the joint wave-particle view of reality, individuals are whole and complete, and this wholeness arises from a combination of genes and environment. In that sense, how much of a person’s mental and physical constitution we can control by either manipulating their genes or their environment is almost irrelevant. What’s relevant is the basic understanding in the first place that both matter; even if both camps agree with this baseline, they would already be talking a lot more with each other.
A third application of complementarity to international affairs, one which stems directly from Bohr’s view of the complementarity of the bomb, is to the relationship between United States and China, a relationship which will likely be the single-most important geopolitical determinant of the 21st century. China clearly has an autocratic regime that is not likely to yield to demands for more democratic behavior, both internally and externally, anytime soon. This has led to many in the United States to regard China as an implacable foe, almost a second Soviet Union. An important consequence of this view has been to see almost every technological development in the two countries, from gene editing to artificial intelligence to new weapons, as a contest.
But irrespective of the moral wisdom of engaging in this contest, complementarity tells us that such contests are likely to lead to the mutual ruin of both China and the United States, and by extension the rest of the world; for the same reason that any arms race would hollow out countries’ coffers and ramp up the specter of mutual annihilation. The reason is simple: both computer code and the physics of nuclear weapons are products of the fundamental laws of science and technology discovered or invented by human minds. Both can be divined and implemented by any country with smart scientists and engineers, which basically means any developing or developed country. An arms race in AI between China and the United States, for instance, would be as futile and dangerous as was the nuclear arms race between the United States and the former Soviet Union. Both countries would be fooling themselves if they think they can write better computer code and keep it secret for a long time. In that sense the fact that computer code, just like nuclear fission, is essentially a discovery of the human mind poses inconvenient truths for both countries. Whether we like it or not, we need to realize the complementarity of artificial intelligence akin to how we grudgingly realized the complementarity of the bomb: we need to realize that AI is powerful, that it is dangerous, that its secrets cannot stay secret for very long, that there are no real defenses against it, that the very dangers of AI cry for peaceful solutions to AI, and therefore that mutual cooperation between China and the United States under an umbrella of an international organization like the United Nations would be the only solution to avoid mutual cyber-destruction. China might be autocratic, but it has self-interests and wouldn’t want to see its own ruin. In the end, harmony between the United States and China might not be forced by bridging the moral divide between the two countries’ social and political systems: it would be forced by the very laws of science and technology.
Without oversimplifying the issue, it’s clear to me that Bohr’s complementarity provides a mediating middle ground for almost any other social or political issue I can think of; it’s not so much that it offers a solution but that it will compel each side to see the importance of the other side’s argument in providing a complete view of reality that cannot be provided by either viewpoint by itself. Pro-life or pro-choice? One can respect both the life of an unborn child and the life of the mother; the two are complementary. Socialism or capitalism? One can certainly have a mixed market economy – of the kind found in Niels Bohr’s home country for instance – that would give us the benefits of both. Climate legislation or rapid economic growth? One can create jobs related to new climate technologies that will result in economic growth. Science or religion? They address complementary aspects of the world, Stephen Jay Gould’s “non-overlapping magisteria”.
If we accept the idea of complementarity, we are in essence accepting the validity of all ways of looking at the world, and not just one. This does not mean that all ways are equally right – we can’t accept the germ theory of disease and the “theory” of diseases as a punishment from God on equal terms – but it is precisely through placing them on a level playing field and letting them play out their logical flow that we can even know how much of which view is right. In addition, Bohr realized that the world is indeed gray, that even flawed visions may contain snatches of truth that should be acknowledged as potential building blocks in our view of reality. But ultimately, Bohr’s plea for complementarity was a plea for what he called an “open world”, an ideal that for him was the highest that the peoples of the world could aspire to, an ideal that arose naturally from the democratic republic of science. If we accept complementarity, we automatically become open to examining every single approach to a problem, every way of parsing reality. Most importantly, we become open to true, unfettered communication with our fellow human beings, a tentative but lasting step toward Bohr’s – and science’s – “gradual removal of prejudices”. That seems like an important message for today.
First published on 3 Quarks Daily.