This article discusses why interpretations of quantum mechanics, especially the Copenhagen interpretation, are misunderstood by the public and scientific students. The lack of understanding of the wave-particle duality and complementarity concepts, as well as the neglect of philosophical aspects in physics education, are pointed out as key factors, and the importance of education to address these misconceptions is emphasized.
The 21st century is arguably the age of quantum mechanics. Quantum mechanics is the theoretical basis for technologies that have a huge impact on society, such as explaining how semiconductors work. As a result, physicists are introducing quantum mechanics in public lectures and columns, and many science students are studying quantum mechanics from their undergraduate programs.
Whether it’s in a general physics textbook or a popular science column, the Copenhagen interpretation is often mentioned when introducing quantum mechanics. Why is this “interpretation” mentioned? Because physical theories are composed of mathematical structures and semantics. Since a physical theory is a description of the world, it must be clear how the symbols in its mathematical structure translate to physical quantities in reality, and how the operations between symbols translate to interactions in reality. In classical mechanics, interpretation was not discussed much. This is because its premises, the absolute interpretation of space-time and the causal interpretation of events, fit well with everyday experience and are naturally accepted. However, since quantum mechanics is far from empirical intuition, the interpretation of its mathematical structure has become important, and physicists often refer to the Copenhagen interpretation, which is accepted as the mainstream interpretation, when introducing quantum mechanics.
When it comes to understanding physical theories, interpreting the mathematical results is just as important as understanding the mathematical structure. Understanding the physical implications is important, especially in science lectures that cover quantum mechanics at a liberal arts level. However, the interpretation of quantum mechanics is misunderstood by the public and many science students. In this context, we are referring to the mainstream interpretation of quantum mechanics, the Copenhagen interpretation. In Korea and abroad, many studies and tools have been developed to measure the misunderstanding and understanding of quantum mechanics concepts, such as the Quantum Mechanics Concept Inventory (QMCI) (Falk, 2004) and the Quantum Physics Conceptual Survey (QPCS) (Wuttiprom et al., 2009). In Korea, Lim et al. (2012) used the QPCS developed by Wuttiprom et al. (2009) to investigate the understanding of basic concepts of quantum mechanics among physics majors, and found that the mean of the correct answer rate of items was 56.5%, the standard deviation was 22.2%, and the lowest was 9.7%, especially for questions asking about physical concepts. This suggests that many people find the interpretation of quantum mechanics very difficult and misunderstood. In this article, we will discuss why many people misunderstand the interpretation of quantum mechanics and how to overcome this problem. In particular, we will focus on the concepts of wave-particle duality and complementarity, which are two of the biggest controversies in the philosophy of quantum mechanics and the most frequently misunderstood concepts among college students. It should be noted that this article does not argue for or against complementarity.
According to Bohr, a conceptual framework is complementary if it satisfies the following four conditions 1. they describe different properties, 2. together they provide a complete description of the object, 3. they have exclusive meanings, and 4. they are applied diachronically, not synchronically. The particle and wave models of light are complementary because they imply different properties of light, both models are necessary to explain all phenomena, both models have equal status, and both models cannot be applied simultaneously to a phenomenon.
The Copenhagen Interpretation interprets physical objects as having “complementary” wave-particle duality. Suppose we observe particle-like behavior in the first experiment and wave-like behavior in the second experiment for a physical object. This forces us to concede that the object has at least both wave and particle properties, which we call wave-particle duality. The question is how to interpret the contradictory experimental results. If one adopts a realist interpretation, one is faced with the paradoxical situation that something that was literally a particle must change into a wave, and one must be able to describe the mechanism of this causal process. Bohr argued that this paradox can be resolved by adopting a complementary framework. According to Bohr, the world itself has a complementary framework. In fact, Bohr believed that every concept has a complementary counterpart. Wave and particle are complementary pairs of concepts, and we observe wave and particle by differentiating the phenomena we observe, not by actually switching between wave and particle states of light.
There are two major misconceptions that often arise in people’s understanding of the complementary wave-particle duality. The first misconception is that Light is both a wave and a particle, but when humans observe it, it appears as either a wave or a particle. This misconception is consistent with the expression that when we observe one of its properties, the other disappears. The analogy is that light is like a coin that has both heads and tails, but when we look at it, we can only see one side, and due to the limitations of our cognitive abilities, we can only observe one property at a time. However, Bohr’s complementarity is not this interpretation. Within the complementarity framework, the particle model and the wave model are not publicly incompatible.
Many people still think that a definite physical quantity of light must be defined, and they want to know a definite answer to the question of what light is. The wrong answer to this question leads to a second misconception: That we can’t know whether light is a wave or a particle until we make an observation. Unlike the previous misconception, this one doesn’t claim that light is both a wave and a particle, but the problem is that it’s not well established by complementarity in the first place. This discussion extends outside the complementarity framework. The complementarity framework of the Copenhagen interpretation is the world itself, and the properties and meanings of objects cannot be well defined prior to observation. Thus, according to the Copenhagen interpretation, it is not that we cannot know whether light is a wave or a particle prior to observation, but that we cannot have any discussion about light prior to observation.
So far, we have discussed the complementary wave-particle duality and two misconceptions about it. We will now discuss the factors that contribute to these misconceptions in two aspects: the nature of the complementarity duality and the way modern physics is taught. The factors are as follows.
First, complementarity does not resolve the logical difficulties that arise when quantum mechanics is expressed in classical language. While the complementary framework is, according to Bohr, the world itself, complementarity is undeniably a metaphysical tool that tries to avoid the paradoxes that arise from duality by attributing opposing properties of wave and particle to a single object, light. In other words, the logical situation that unfolds in complementarity is one that is not found in the macroscopic world. Complementarity describes different properties, which together cannot be fully explained, exclusive, and publicly applicable. People have difficulty understanding the Copenhagen Interpretation because it is structurally well-established but conflicts with basic logic.
Second, complementarity is not only at odds with logic, but also with common ideas about how scientific theories develop. According to Karl Popper, scientific theories should be disprovable, and when inconsistencies between existing theories and new experiments arise, the old theory should be revised to a new theory that can explain the new experiments. Einstein proposed the photon hypothesis to explain the photoelectric effect, but paradoxically, the photon hypothesis failed to explain the wave-like nature of particles described by existing theories, and no scientific theory has emerged that explains both the wave-like and particle nature of particles. However, since complementarity is just a logical framework invented by Bohr, it cannot be disproved, so it is difficult to obtain the status of a scientific theory. Therefore, there is still no theory that explains both the wave and particle nature of particles. However, conventional wisdom leads people to naturally assume that there is an advanced theory that explains both wave and particle nature. The conflict between this idea and quantum mechanics is what leads people to believe that light is like a coin.
Third, modern physics education asks various mathematical questions within a theoretical framework, but does not raise conceptual questions about the framework itself. For example, the quantum mechanics section of one general physics textbook mentions duality only in the phrase “light has the properties of both waves and particles,” and none of the 90 or so exercises in the text asks students to define the concept of duality. This level of mention does nothing to help students understand the Copenhagen interpretation. The consequences of this pedagogical approach are also evident in the quantum mechanics comprehension survey, where out of 25 questions with an average correct answer rate of 56.5%, the four questions asking for the concept of particle-wave duality had a 22.6% correct answer rate, and the two questions asking for the concept of the uncertainty principle had a 30. In particular, the four questions that asked about the concept of wave-particle duality were analyzed based on the response concentration index and the percentage of correct answers, and it was found that the probability of having a wrong concept was relatively high. This problem is also found in Mashhadi’s (1996) survey of undergraduate students’ conceptions of the wave-particle duality. Taken together, it can be argued that a physics education approach that weakly addresses philosophical aspects increases the probability of misconceptions about quantum mechanics.
To summarize the discussion so far First, we reviewed the concept of complementarity as coined by Bohr and clarified what complementary duality is, which is the goal of the Copenhagen interpretation. We then examined the misconceptions that this interpretation of duality raises, and presented the factors that contribute to these misconceptions. The first factor, that complementarity conflicts with conventional logic, and the second, that complementarity also conflicts with the idea of how scientific theories develop, are related to the nature of complementary duality. The third factor, that physics education is lacking in philosophical coverage, is related to the modern way physics is taught.
What can be done to address the misunderstanding of duality? The most urgent need is for education to emphasize the interpretation of quantum mechanics. We’ve only discussed physics textbooks and surveys of university students’ understanding of the concept, but the Copenhagen interpretation is not always clearly communicated in science lectures. In some cases, lectures that are simplified for the public may even lead to misunderstandings of complementarity, such as the proverbial analogy of light as an elephant and humans as blind men. While pithy analogies are appealing because they make it difficult to describe the logical structure of complementarity in terms of conventional logic, they are undesirable because they obscure the essence of the interpretation. In line with Bohr’s statement that a complementary property of truth is intelligibility, I believe that while we may never know the true interpretation of quantum mechanics, we should at least have a clear idea of what the existing interpretations claim. I hope that philosophical interpretations of quantum mechanics will be given more prominence in education and research.
