In this blog post, we’ll examine the background of quantum mechanics, the main points of the Copenhagen interpretation and the many-worlds interpretation, and how these two interpretations view “observation,” and then reflect on the purpose of quantum mechanics.
Long ago in the history of physics, Max Planck published a paper and experimental results based on his blackbody radiation experiments, stating that “all energy consists of very small packets with discrete values.” This shook the foundations of the prevailing view among physicists at the time, who had believed that all phenomena in the macroscopic world could be explained by Newtonian mechanics. From this point onward, as measurement equipment advanced and physicists gained the ability to peer into the microscopic world, they published various experimental results that revealed the limitations of Newtonian mechanics. These results, which could not be explained by existing theories, soon led to the recognition of the need for a new framework of mechanics—and thus, “quantum mechanics” was born. Quantum mechanics soon solidified its position by successfully providing answers to natural phenomena that could not be resolved by existing physics, such as Einstein’s explanation of the photoelectric effect and Bohr’s model of the hydrogen atom spectrum. Furthermore, through the efforts of countless physicists and developments such as the transformation theory proposed by Paul Dirac and Pascual Jordan, it established itself as a distinct field of study.
However, this merely signified the completion of the mathematical “form”—the tool used to describe the content of quantum mechanics—and did not imply that it had been established from the perspective of physical interpretation. Even now, decades later, conflicts of opinion continue among prominent physicists regarding the “interpretation” of quantum mechanical phenomena. From the “Copenhagen interpretation”—often referred to as the mainstream interpretation—to the “many-worlds interpretation,” which emerged in response to the limitations of the former, and various other interpretations, these different approaches continue to complement one another by addressing the gaps that no single interpretation can fully explain. At a quantum mechanics conference held in Austria in the past, a poll was conducted among 33 world-renowned physicists asking which interpretation they supported; the Copenhagen interpretation received 42%, the information-theoretic interpretation 24%, and the Many-Worlds Interpretation 18%. Given that no single interpretation surpassed 50% in this poll, it seems clear that every interpretation has its own limitations. In this article, I will briefly introduce the “Copenhagen interpretation” and the “many-worlds interpretation”—which account for a significant portion of the field and hold opposing views on the act of “observation”—and discuss their limitations. The “information-theoretic interpretation” is excluded from this discussion because it views quantum mechanics from a perspective somewhat different from the other two.
The basic framework of the Copenhagen interpretation, proposed by Bohr and Heisenberg, is as follows: “As long as the observer does not apply measuring equipment, the electron within an atom moves according to the laws of causality. However, when attempting to describe the electron in a fixed position and time within spacetime, discrete interactions intervene, causing it to deviate from causality, and the observer has no choice but to rely on quantum probabilities.” Newtonian mechanics states that particles observed in the macroscopic world possess specific, well-defined values for position and momentum when they move. This means that in the macroscopic world, where observers—that is, humans—exist, position and momentum are fixed. However, in the microscopic world, an electron in motion exists in a state where these physical quantities cannot be definitively determined as a single value. Rather, various different states coexist simultaneously. For example, if there are three possible states—1, 2, and 3—that an electron can occupy, the electron exists in all three states simultaneously before it is observed. Mathematically, this can be expressed as the sum of the wave functions corresponding to 1, 2, and 3. When an observer in the macroscopic world observes the electron, the electron is reduced to a single state among the many possible states, determined by the probability associated with that state. In other words, the act of “observation” changes the state of the electron. For instance, the moment the electron is determined to be in state 1, the possibility of it existing in states 2 and 3 simultaneously disappears. This is what the Copenhagen interpretation refers to as the “collapse of the wave function.” Consequently, the act of “observation” holds immense significance in the Copenhagen interpretation.
Famous physicists such as Einstein refuted the Copenhagen interpretation, citing various thought experiments as evidence. Among these, the best-known is “Schrödinger’s Cat.” Inside a sealed box is a cat and a device capable of releasing poison depending on the quantum-mechanically determined state of a particle. Whether the cat inside the box is alive or dead after a certain amount of time—which is itself a state—is determined only at the moment the lid is opened and the cat is observed. This thought experiment illustrates that applying the act of “observation,” which holds great significance in the microscopic world, to the macroscopic world leads to strange conclusions; as a result, it has prompted deep reflection on the meaning of observation in quantum mechanics. This is where the “many-worlds interpretation” begins.
Everett III, formerly of Princeton University, refuted the claims of the Copenhagen interpretation, arguing that the probability distribution of a particle does not disappear through the act of observation, but rather that multiple possible worlds existed from the very beginning. In other words, a particle’s position is not determined to a single value through observation; rather, multiple parallel worlds already exist, and we simply observe one of them. The states we do not directly observe are unfolding in other possible worlds. For example, if our observation yields state 2, the other states do not disappear at that moment—they are unfolding in other worlds, and we simply cannot observe them.
However, the Many-Worlds Interpretation clearly has its problems. If an event that could occur happens in all possible worlds, what meaning does the probability associated with each state even have? Everett attempted to resolve this by simply multiplying each state by a weight. However, what it means for one world to be more important than another remains a subject of debate.
Considering these two interpretations—each with its own significance and limitations—I tend to favor the Copenhagen interpretation. If we go back to the origins of quantum mechanics, we see that it was ultimately born to explain phenomena in the microscopic world. In other words, since it was created to unravel phenomena through observation and the resulting facts, the observer’s act of observation holds such significant meaning. In the actual observation of particles, in order to observe the state of a single particle, we must exert a certain degree of influence on it. In physics, this is called a “perturbation.” In other words, observing a particle’s state involves inducing a slight change in the particle and then observing its response. Although the connection between inducing a change in the particle and the collapse of the wave function in mathematical terms is somewhat ambiguous, they agree in that both alter the original state. This means that the act of observation itself is physically significant.
Quantum mechanics has moved away from the deterministic mindset of classical Newtonian mechanics and adopted a probabilistic approach. How many people could easily accept the idea that the position of an apple right in front of them changes with varying probabilities every time they look at it? It is true that both the Copenhagen interpretation and the Many-Worlds interpretation have their own significance and limitations when it comes to helping people—who are accustomed to the definite states of objects—understand the phenomena occurring in the microscopic world. I am an engineering student studying how to create new technologies by utilizing scientific phenomena and the results obtained from observing them. The development of quantum mechanics has provided us with tools to interpret phenomena in the microscopic world, and this has directly led to technologies that are useful to people. MRI (Magnetic Resonance Imaging) equipment, which has made a significant contribution to modern medicine, was developed by applying the NMR (Nuclear Magnetic Resonance) phenomenon from quantum mechanics; this technology is used not only in spectroscopy—which determines molecular structures—but even in oil exploration. Based on this, I believe that the interpretation better suited to explaining phenomena—which I consider to be the purpose of science—will hold greater significance.
