What is the Schrödinger’s cat thought experiment?

The Schrödinger’s cat thought experiment is one of the most famous paradoxes in quantum mechanics, conceived by the physicist Erwin Schrödinger in 1935. It was proposed as a critique of the “Copenhagen interpretation” of quantum mechanics, which was the dominant interpretation of quantum mechanics at the time.

The Copenhagen Interpretation and Quantum Superposition

Before we dive into the thought experiment, let’s briefly touch on the Copenhagen interpretation and the concept of superposition. The Copenhagen interpretation, named after the home city of one of quantum theory’s founding fathers, Niels Bohr, proposes that a quantum system remains in superposition until it interacts with, or is observed by, the external world. At that moment—commonly known as the measurement problem—the system “collapses” into one of its many possible states.

Superposition, meanwhile, is a fundamental principle of quantum mechanics that holds that any two (or more) quantum states can be added together, or “superposed,” and the result will be another valid quantum state. The superposition principle allows quantum systems to be in multiple states simultaneously. For example, an electron in an atom can simultaneously be in different positions, or a photon can be in multiple polarization states at once.

Schrödinger’s Cat: The Thought Experiment

Now let’s move onto the experiment itself. Schrödinger proposed a scenario where a cat is placed in a sealed box with a radioactive sample, a Geiger counter, a hammer, and a small flask of hydrocyanic acid. Here’s how it works:

The radioactive sample has a 50% chance of decaying within one hour. If it decays, it will emit a particle that will be detected by the Geiger counter, which will then trigger the hammer to break the flask of hydrocyanic acid. This will release the poison and kill the cat.

If the radioactive sample does not decay within that hour, the Geiger counter will not be triggered, the hammer will not break the flask, the poison will not be released, and the cat will remain alive.

The conundrum, according to quantum mechanics and the idea of superposition, is that until one opens the box to check, the cat is both alive and dead at the same time. The act of opening the box to observe the cat “collapses” the superposition into one of the possible states. In the Copenhagen interpretation, the cat is both dead and alive until observed.

Why Schrödinger Proposed This Scenario

Schrödinger, however, was uncomfortable with this interpretation. His thought experiment was not intended to illustrate the possibility of cats being both alive and dead but to critique what he saw as the absurdity of the existing interpretation of quantum mechanics. Schrödinger argued that it was not plausible to have macroscopic systems like cats in a superposition of states—his scenario was designed to highlight this absurdity.

Further Implications and Interpretations

The Schrödinger’s cat thought experiment has inspired numerous discussions and alternative interpretations about the nature of quantum systems, measurements, and reality itself. For instance, the Many Worlds interpretation proposes that both outcomes occur, but in different, parallel universes: in one universe, the cat is alive; in another, it’s dead.

Regardless of the interpretation, Schrödinger’s cat has proven a useful tool for provoking thought and debate about the philosophical questions raised by quantum mechanics, especially as they relate to the macroscopic world we experience daily.

Today, more than 80 years after Schrödinger’s famous thought experiment, we still don’t have definitive answers to many of the questions he raised about the measurement problem and the concept of superposition. While quantum mechanics has been

extraordinarily successful in predicting experimental results, the deeper meaning of its strange features—superposition, entanglement, wave-particle duality—is still a matter of ongoing debate.

However, it’s important to remember that thought experiments like Schrödinger’s cat are models that help us understand and question complex theories. They are not meant to depict real situations but to challenge our assumptions and stretch our understanding. In this regard, Schrödinger’s cat has certainly fulfilled its purpose. It remains one of the most popular illustrations of the enigmatic world of quantum physics and continues to stimulate discussions about the interpretation of quantum mechanics.