In this blog post, we examine the problems of superposition, observation, and interpretation raised by Schrödinger’s cat, and deeply explore why this thought experiment remains a core question in quantum mechanics to this day.
A cat is locked inside a steel box. Inside the box, alongside the cat, is a flask containing toxic hydrogen cyanide. If this flask breaks, the cat will die from the hydrogen cyanide. This flask is designed to break automatically if a radioactive atom decays, meaning the cat’s fate depends entirely on whether the radioactive atom decays or not. So what will become of the cat?
Naturally, many people think the cat has a 50% chance of being dead and a 50% chance of being alive. Most people provide their own reasoning, answering either ‘the cat must be alive’ or ‘the cat must be dead’. This answer, which might sound almost too obvious, holds profound significance in quantum mechanics. Quantum mechanics deals with structures at the atomic scale and the movement of electrons, which are even smaller, leading to phenomena that defy everyday intuition. The above question is one such example. I aim to explain how this question was posed, what meaning it holds in quantum mechanics, and why it is such an important problem, so that readers can understand. This requires an introduction to the fundamental concepts of quantum mechanics. Therefore, I will first introduce the Schrödinger equation, the Copenhagen interpretation, and the related debates among scientists before addressing the question.
First, let us examine how Schrödinger’s wave equation emerged and what it signifies. Until the 19th century, before quantum mechanics research gained momentum, classical physics—based on Newtonian mechanics and electromagnetism—had systematically established various theories and rules. However, as the 20th century dawned, the framework of classical mechanics, once considered perfect, began to crumble. This was because numerous phenomena and experimental results emerged that classical mechanics simply could not explain. Many scientists grappled with explaining these, and in the process, de Broglie discovered that all matter possesses both particle-like and wave-like properties simultaneously. Thus, by the early 20th century, the dual nature of matter (particle + wave) was accepted.
At this time, Schrödinger questioned how electrons actually move within matter. According to the atomic theory presented by Bohr in 1913, electrons exist around the nucleus, orbiting in fixed orbits, and at this time, the electrons take the form of standing waves. The circumference of an electron’s orbit must precisely match its wave period. If electrons behaved like waves, their orbits would be expected to spread out across the entire orbital plane. Conversely, if electrons behaved like rigid particles, they would move along fixed circular paths, like planets orbiting the sun.
Schrödinger, inspired by de Broglie’s proposal of the wave nature of matter, interpreted the electron as a three-dimensional wave. He resolved to mathematically describe how the electron actually moves. Immersed solely in this problem in a remote mountain cabin, he finally completed and published an equation in 1926 that represents the probability of an electron, possessing wave properties, existing at a specific location. Solving this equation yields the wave function, which allows prediction of not only the electron’s position but also various mechanical properties like momentum and kinetic energy. Furthermore, by applying the concept of duality to all matter, including atoms, he earned the honor of being recognized as the founder of quantum mechanics.
Schrödinger’s wave mechanics, which interpreted matter—including electrons—from a wave perspective, gained widespread support among physicists. However, some scholars, including Heisenberg, sought to analyze electrons using the mathematical tool of matrices rather than waves. Bohr also reached different conclusions while partially accepting Schrödinger’s ideas.
When the observed substance is fixed as either a particle or a wave through observation, Schrödinger explained this as the wave function “collapsing.” The wave function contains the probability of the substance existing as a wave and the probability of it existing as a particle. When an observer measures light, the light’s properties as either a particle or a wave are determined, causing the probabilistic meaning to vanish. When the wave function collapses in this way, the measured object exists in one form or the other. This is not because the measured object changes its own properties, but because the substance inherently possesses both particle and wave nature simultaneously. Bohr further argued that the very act of measuring light determines what form it will take. In other words, observing a particular substance acts like a switch, selecting its final state; before observation, the system’s state can only be described probabilistically. Thus, before observation, light or particles simultaneously possess wave-like and particle-like properties, but the moment observation begins, a specific property is determined. This interpretation by Bohr is known as the “Copenhagen interpretation.”
Einstein and Schrödinger strongly opposed Bohr’s claims. Einstein, in particular, felt great discomfort with the concept that an existing particle could be governed by probability. At this point, the reader must not confuse one thing. Schrödinger also introduced a wave function incorporating probability concepts, but this was a method of describing the electron by calculating the probability of its existence at specific locations. That is, the electron possesses duality as a particle that behaves wave-like. However, Bohr believed probability determined whether a specific substance would behave like a particle or like a wave. This logic is quite ambiguous, akin to saying, “There could be two eggs or three eggs, and the number is determined by probability.”
Schrödinger devised a hypothetical scenario to counter Bohr’s ambiguous probability theory. This is the very question introduced at the beginning of this article, and this thought experiment is called “Schrödinger’s Cat.” The premise that the cat’s life or death is determined the moment the box is opened is analogous to the situation where light simultaneously possesses wave and particle states, and then, depending on the observer’s choice, the wave function collapses and only one property is selected. Schrödinger pointed out that this abstract explanation could not be applied to a real animal. A cat is clearly either alive or dead; it cannot exist in a mixed state of both. He viewed Bohr’s interpretation as merely a simplifying shorthand to discuss phenomena at a deeper level. In fact, applying the Copenhagen interpretation literally leads to the paradox that we cannot even assert the existence of any object in the universe.
Through this thought experiment, Schrödinger effectively exposed the inherent contradiction in Bohr’s claim. As a result, Bohr’s interpretation lost the support of many, while the wave equation and wave mechanics proposed by Schrödinger gained greater solidity. This became a crucial foundation of modern quantum mechanics. However, the Schrödinger equation also has limitations. Solving this equation to obtain the wave function requires a very high level of mathematical skill. For a single electron, the necessary variables can be mathematically expressed with precision, allowing a solution to be found. But for multi-electron atoms with two or more electrons, it is extremely difficult to mathematically express the attractive and repulsive forces between multiple electrons with precision. This is considered a major limitation of the Schrödinger equation. To overcome this, numerous scientists dedicated themselves to research. They eventually solved the problem by focusing on one electron among many and utilizing ideas such as the remaining electrons forming new electron shells. This enhanced the equation’s completeness and made a significant contribution to the development of quantum mechanics.
In the early 20th century, many scientists were trapped within the framework of existing physical laws and failed to explain new experimental results. In contrast, Schrödinger, unconstrained by conventional wisdom, viewed phenomena from a new perspective and created an equation describing the probability of an electron’s existence at a specific location. Although Bohr later argued against his theory, Schrödinger pointed out the contradictions in Bohr’s interpretation through the cat thought experiment, thereby further enhancing the credibility of his own theory. Quantum mechanics achieved remarkable progress based on such debates and reflections, and is now deeply embedded in many areas of daily life. Various new technologies, including semiconductors and other electronic devices, magnetic levitation trains, and superconductors, operate based on the physical laws of the microscopic world, such as the dual nature of matter. Their potential for development is also limitless. Through this article, I hope readers will develop at least a little interest in quantum mechanics and begin to pay attention to the new world quantum mechanics will create in the future.
