In this blog post, we’ll examine the Copenhagen interpretation of quantum mechanics and the significance of various interpretations through Shinichiro Tomonaga’s “The Photon Trial.”
A defendant charged with burglary stands before a judge. The defendant entered an empty house through an open window and was caught red-handed inside the room. Traces of the crime remain on the wall opposite the window, and the trial is centered precisely on those traces. As the title suggests, the defendant in this trial is none other than the “photon,” a particle of light. The central issue of the trial is which of the two open windows in the house the photon entered through. When the prosecutor asks the defendant which window it entered, the photon claims that it passed through both windows simultaneously. The prosecutor counters that this is a preposterous claim, not even worth considering. The defense attorney then begins his argument to prove that the photon did indeed pass through both windows at the same time.
This unique story, which puts a photon on trial, is “The Photon Trial,” featured in Shin’ichirō Tomonaga’s ‘The World View of Quantum Mechanics’. The author, Shin’ichirō Tomonaga, is a physicist who was awarded the 1965 Nobel Prize in Physics for his significant contributions to the development of quantum electrodynamics (QED) through the Tomonaga–Schwinger theory. I find his approach to explaining quantum mechanics through this intriguing narrative format so impressive that I have chosen it as the starting point for this article.
Let’s return to the trial and listen to the defense attorney’s argument. Suppose, as the prosecutor stated, that someone is watching the left window and sees a photon pass through it. In this case, since a photon is a single particle, it seems contradictory to claim that it also passed through the right window at the same time. Therefore, we can acknowledge that if someone is observing the windows, the photon can pass through only one of the two windows. However, can we conclude that “the photon cannot pass through both windows simultaneously even when no one is observing them” based solely on the fact that “when the windows are being observed, the photon passes through only one window”?
In response, the defense attorney proposes two experiments. The first is an experiment in which a police officer directly observes which window the photon passes through from beside the windows and then checks the marks left on the wall. The second is an experiment in which the photon is allowed to pass while no one is observing the windows, and the marks left on the wall are then examined. In the first experiment, the police officer observed the windows and found that the marks on the wall were identical to those that would appear if one window were closed and the other left open. This demonstrates that the photon passed through only one of the two windows. However, in the second experiment—where no one observed which window the photon passed through—a pattern completely different from the first experiment appeared. Furthermore, this pattern matched the one found at the actual crime scene. Ultimately, the trial concludes with results supporting the claim that the photon passed through both windows simultaneously.
The key point Tomonaga sought to convey through this “photon trial” is the relationship between “observation” and “physical quantities.” The act of “observation”—monitoring the windows—influences the photon’s behavior, that is, the physical quantities, leading to different outcomes. When observation takes place, the photon is described as passing through only one window, but when no observation occurs, it is described as passing through both windows simultaneously. So how can we draw such a conclusion even though we cannot directly observe the photon passing through both windows simultaneously?
The reason is that, theoretically, when we assume the photon passes through both windows simultaneously, we can calculate the traces it would leave on the wall, and the results of those calculations match the data obtained from actual experiments. This explanation illustrates the representative way of thinking behind the “Copenhagen Interpretation,” which was developed primarily by Niels Bohr.
In 1927, the physics community was grappling with various phenomena that could not be explained by classical mechanics alone, as established by Newton. Against this backdrop, at the Fifth Solvay Conference—which brought together world-renowned physicists—Niels Bohr presented a new interpretation of quantum mechanics. The core of this interpretation—later known as the “Copenhagen interpretation”—consists of two main points. First, the state of a particle is expressed as a superposition of wave functions representing probabilities. Second, according to Heisenberg’s uncertainty principle, the act of observation cannot be separated from the physical quantity, and the measurement process itself influences both the state of the system and the outcome.
The Copenhagen interpretation caused a major stir in the physics community at the time. Although some aspects were difficult to accept intuitively, it received widespread support from many physicists because it explained and predicted various experimental results with great accuracy. Einstein, on the other hand, strongly opposed this probability-based interpretation, as epitomized by his famous statement, “God does not play dice.” Schrödinger also devised the famous thought experiment known as “Schrödinger’s Cat” to criticize the Copenhagen interpretation. This thought experiment posits a scenario in which a cat is placed inside a completely sealed box along with a device that releases poison gas with a certain probability. Until the box is opened, it is impossible to determine whether the cat is alive or dead.
According to the Copenhagen interpretation, before the box is opened, the cat’s state is described as a superposition of the two states—“alive” and “dead”—existing simultaneously. In other words, the cat’s state is expressed as a superposition of two wave functions and is determined as a single state the moment observation occurs. Can we truly accept that the cat’s states of being alive and dead coexisted simultaneously until the box was opened? Intuitively, it is difficult to accept the claim that the cat’s state is determined only when someone observes it. Schrödinger pointed out precisely this issue, criticizing the Copenhagen interpretation for equating the mathematical model of superposition too closely with actual reality.
Amid opposition to the Copenhagen interpretation, various interpretations of quantum mechanics emerged. Notable examples include the “Ensemble Interpretation” supported by Einstein and others, the “Many-Worlds Interpretation” proposed by Hugh Everett III, and the “Seoul Interpretation” proposed by South Korean physicist Professor Jang Hoe-ik.
The Many-Worlds Interpretation holds that different states do not exist in superposition within a single system; rather, each possible outcome is realized simultaneously in a separate universe. For example, it posits that at any given moment, universes in which the cat is alive and universes in which it is dead each exist simultaneously, and opening the box is explained as the process of experiencing one of those worlds. There is no interaction between these universes that causes them to influence one another. Although initially regarded as a highly radical claim, the Many-Worlds Interpretation is still considered one of the most actively researched interpretations of quantum mechanics today, alongside the Copenhagen interpretation.
Another interpretation, the ensemble interpretation, is an approach that seeks to understand quantum mechanics from a statistical perspective. This interpretation explains that it is not that a single cat exists in both a living and a dead state simultaneously, but rather that when the same experiment is repeated a very large number of times, the results show that half of the entire population is alive and half is dead. Building on this perspective, Einstein proposed the “hidden variable theory,” suggesting that there must be some factor not yet revealed by current theory.
Although there are various interpretations of quantum mechanics, no clear conclusion has yet been reached regarding which interpretation most accurately describes nature. However, the reason the Copenhagen interpretation has long been the most widely accepted is not due to philosophical persuasion but to its experimental success. Bohr persuaded scientists through experimental results rather than philosophical debate, and indeed, the Copenhagen interpretation has successfully explained countless experimental results and provided accurate predictions. As a result, in modern physics, the accuracy with which a theory explains and predicts experimental results has become a more important criterion than the philosophical merits of a particular interpretation. Nevertheless, the question of how to interpret quantum mechanics remains a major topic in the philosophy of science, and perhaps the cat is still posing new questions to us from inside that dark box.
