NeAr Science Magazine

By Darragh McCann

In a 1935 review, Austrian physicist Erwin Schrödinger first posed the now-famous “Schrödinger’s Cat” thought experiment. He began by asking the reader to imagine a cat locked in a steel box. The box has an attached apparatus in which a radioactive sample has a 50/50 chance of decaying over the course of an hour. If the sample decays, it will be detected by a Geiger counter, which will in turn trigger the release of toxic cyanide gas into the box containing the cat. [1]

But what were his motivations for posing such absurd cruelty to animals? Firstly, it should be made clear that Schrödinger never suggested that anyone ought to actually carry out this experiment. He only meant it as a thought experiment, prefacing its description by noting that it was merely an example of a way in which one may extrapolate quantum mechanical uncertainty to imagine “quite ridiculous cases”. The remainder of his motivations behind the proposition lie in his philosophical beliefs, with his ultimate goal being to highlight the absurdity of the Copenhagen interpretation of quantum mechanics.

The Copenhagen interpretation is a philosophical conceptualisation of quantum physics, based on the body of research coming out of Copenhagen, Denmark, in the 1920s, led mainly by Niels Bohr, Werner Heisenberg, and Max Born. While this term describes a combination of many ideas, the main one Schrödinger took issue with was the notion that his quantum mechanical wave equations were merely probabilistic tools which did not describe any physical reality. [2]

The wavefunction was a mathematical representation derived by Schrödinger which accounted for the observed phenomena of both quantisation and wave-particle duality. Quantisation (the namesake of quantum physics) refers to the characteristic of particles having fixed properties, like how electrons in an atom occupy orbitals which can only have fixed energies. Wave-particle duality is an idea often attributed to Louis de Broglie, who suggested particles – which were thought to behave something like tiny marbles – could exhibit wave-like behaviour. While many had argued over whether photons acted like waves or marbles due to the conflicting evidence, de Broglie was the first to suggest they may act like both. To combine these concepts, the wavefunction describes particles as waves that can only have specific, quantised energies. The wavefunction described particle behaviour accurately, though nobody was quite sure how to physically interpret it. At first Schrödinger suggested particles could be thought of as just the region where the majority of the wavefunction lay (commonly used to visualise atomic orbitals in 3D), but this fails to account for many other observations. [3]

According to the Copenhagen interpretation, the wavefunction represents all possible states a particle can have. This was supported by Born’s 1926 discovery that the wavefunction could be used to directly determine the probabilities governing observed particle behaviour. To resolve the discrepancy between particles acting as waves of probability, yet having definitive properties once measured, the Copenhagen interpretation suggests that once observed, the wavefunction “collapses” into one of its possible states.

Entanglement is a quantum phenomenon in which two particles share a joint quantum state. This means that the properties of one particle are correlated with the other, no matter how far apart they are. For example, if two electrons are entangled in terms of spin, measuring one to be “up” instantaneously tells you the other is “down.” According to the Copenhagen interpretation, the particles exist in a superposition of all possible correlated states until one is measured, at which point the wavefunction collapses and the states of both particles become definite. [4]

Albert Einstein took issue with the probabilistic nature of the Copenhagen interpretation, famously saying in a letter to Born that God “does not play dice with the universe”. [5] He was dissatisfied with the idea of entanglement in particular as it predicted that for two entangled electrons at a large distance, the spin of one electron could in theory be determined instantaneously by measuring its entangled partner. If the spins exhibited superposition, information about the collapse of the measured electron’s wavefunction would have to travel instantaneously, violating Einstein’s postulation of a finite speed of light. This argument was outlined by Einstein, along with his colleagues Boris Podolsky and Nathan Rosen, becoming known as the “EPR paradox” (named after their initials). They suggested that the wavefunction must be incomplete due to the unintuitive nature of the results it produces. [6]

This is where the analogy of the cat again comes into play. Applying Born’s probabilistic interpretation, the cat can be described as a wavefunction, existing in two states of equal probability, one where the cat is dead, and another where it is alive. Schrödinger argued that the Copenhagen interpretation would suggest the radioactive decay remains in superposition until measured. This, he argues, would mean the cat would by extension be both dead and alive simultaneously until someone opened the box. Unfortunately for Schrödinger, his analogy has somewhat lost its intended meaning. Nowadays it is often used as an analogy to explain the Copenhagen interpretation, without noting that it was originally intended as a criticism of it. [1][7]

So who was right? EPR and Schrödinger, or the Copenhagen group? Intuitively you might be convinced by the EPR paradox or Schrödinger’s cat, but there is still no clear answer, though the many interpretations have been put to test. A famous example comes from Bell’s theorem, devised by Belfast-born physicist John Stewart Bell in 1964. Without getting into the nitty-gritty of the underlying mathematics, his theorem poses a method of testing whether particle properties are pre-determined, or are in superposition, and only determined at collapse. It states that if the quantum states of two entangled particles were not in superposition before measurement, performing a certain set of measurements will have a maximum possible outcome. However, certain quantum theories which consider superposition predict the outcome of these measurements would be higher than the maximum possible value derived by Bell. This theorem was put to the test by a group of researchers in the early 1970s, with the results violating Bell’s theorem, providing further evidence for superposition.  [8]

Additionally, in research published in January of this year, researchers at the University of Vienna were able to achieve observations of positional superposition (particles effectively being in two places at once) in sodium atoms at separations of 133 nm, the highest separation ever recorded for macroscopic particles. This brings us ever closer to observing superposition at macroscopic scales, which may eventually be used to test the EPR paradox. [9]

Ultimately there is no consensus on what is the correct interpretation of quantum mechanics. Perhaps physicist Richard Feynman put it best when he famously said: “I think I can safely say that nobody understands quantum mechanics.” [10]

References:

[1]: Schrödinger, E. (1935). Die gegenwärtige Situation in der Quantenmechanik. Naturwissenschaften, 23(48), 807–812. https://doi.org/10.1007/BF01491891

[2]: Faye, J. (2024). Copenhagen interpretation of quantum mechanics. In E. N. Zalta & U. Nodelman (Eds.), The Stanford Encyclopedia of Philosophy (Summer 2024 ed.). Retrieved March 12, 2026, from:  https://plato.stanford.edu/archives/sum2024/entries/qm-copenhagen/

[3]: Styer, D. F. (2000). Appendix A. In The strange world of quantum mechanics. Cambridge University Press. Retrieved March 13, 2026, from: https://www2.oberlin.edu/physics/dstyer/StrangeQM/history.html

[4]: NASA. What is the spooky science of quantum entanglement? Retrieved March 13, 2026, from: https://science.nasa.gov/what-is-the-spooky-science-of-quantum-entanglement/

[5]: St Mary’s University. (2014). Physics and Beyond: “God does not play dice”, What did Einstein mean? Retrieved March 13, 2026, from: https://www.stmarys.ac.uk/news/2014/physics-and-beyond-god-does-not-play-dice-what-did-einstein-mean

[6]: Einstein, A., Podolsky, B., & Rosen, N. (1935). Can quantum-mechanical description of physical reality be considered complete? Physical Review, 47(10), 777–780. https://doi.org/10.1103/PhysRev.47.777 

[7]: Baggot, J. (2025). The cat that wouldn’t die. aeon. Retrieved March 13, 2026, from: https://aeon.co/essays/no-Schrödingers-cat-is-not-alive-and-dead-at-the-same-time

[8]: Myrvold, Wayne, Genovese M., Shimony A. (2024). Bell’s Theorem. In E. N. Zalta & U. Nodelman (Eds.), The Stanford Encyclopedia of Philosophy (Spring 2024 ed.). Retrieved March 14, 2026, from: https://plato.stanford.edu/entries/bell-theorem/

[9]: Howarth, T. (2026). The world’s biggest Schrödinger’s cat just pushed quantum physics to the limit. BBC Science Focus. Retrieved March 14th, 2026, from:  https://www.sciencefocus.com/news/biggest-Schrödingers-cat[10]: Feynman, R. P. (1964). Probability and uncertainty (Messenger Lecture, Part 6). Cornell University.