One of the greatest contributors to the field of quantum mechanics is Erwin Schrödinger, an Austrian scientist who lived from 1887 to 1961.

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Perhaps his most famous contribution, the Schrödinger’s cat thought experiment, offered a new way of envisioning quantum superposition (check out my previous article on Thomas Young’s double-slit experiment for an in-depth explanation of the concept). In Schrödinger’s time, the main interpretation of quantum superposition was the Copenhagen interpretation, first put forth by Niels Bohr in 1920. This interpretation stated that quantum systems exist in all possible states at once until they are observed, which causes the system to collapse into a single reality. Essentially, the quantum system acts as a probability wave of all possible states and when it is observed, it acts as a particle and takes up a single state within the probability wave (1).

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Schrödinger was reluctant to accept this interpretation of quantum superposition and instead, he proposed his thought experiment to challenge it. In his thought experiment, a cat is placed in a sealed box with a small amount of a radioactive material for one hour. Also in the box is a geiger counter, a device which measures the decay of radioactive materials. The experiment relies on the fact that there is a 50% chance that the radioactive material will decay within the hour and be detected by the geiger counter. This would trigger a hammer to drop and break a flask containing hydrocyanic acid, killing the cat. There is also a 50% chance that the radioactive material will not decay within the hour and that the cat will live (2).

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Based on the Copenhagen interpretation of quantum superposition, until the box is opened, the cat is both alive and dead at the same time. Only when it is observed does the cat exist in a single state. Although quantum superposition can only occur with subatomic quantum systems and would not apply to something as large as a cat, Schrödinger used this thought experiment to argue that the Copenhagen interpretation must be inherently flawed. It goes against reason that a cat could be both alive and dead, and therefore the Copenhagen interpretation must have its limits (3, 4).Yet, the Copenhagen interpretation is still extremely popular among the scientific community, despite Schrödinger’s reservations about it.

Another popular interpretation is called the many-worlds theorem. It was first put forth by Hugh Everett III in 1957 and its supporters would argue that the cat is not both at once. Rather, the cat has branched off into two worlds: one in which it is alive and one in which it is dead. As a result, we live in a multiverse in which every possibility occurs somewhere. However, these worlds are separate from one another, which is the reason that we can only experience a single reality (5).

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There are many theories which aim to rationalize the actions of quantum systems, only a couple of which are discussed in this article. Schrödinger’s cat thought experiment continues to play an important role in making sense of these theories and placing the mind-boggling laws of subatomic systems into more practical terms.

Sources

1) “Quantum Superposition And Entanglement Explained.” *Clerro*, www.clerro.com/guide/491/quantum-superposition-and-entanglement-explained.

2) Clark, Josh. “How Quantum Suicide Works.” *HowStuffWorks Science*, 8 Mar. 2018, science.howstuffworks.com/innovation/science-questions/quantum-suicide.htm.

3) Andrew, Elise. “Schrödinger’s Cat: Explained.” *IFLScience*, 20 Mar. 2018, www.iflscience.com/physics/schrödinger’s-cat-explained/.

4) Merz, Theo. “Schrödinger’s Cat Explained.” *The Telegraph*, Telegraph Media Group, 12 Aug. 2013, www.telegraph.co.uk/technology/google/google-doodle/10237347/Schrodingers-Cat-explained.html.

5) Orzel, Chad. “What The Many-Worlds Interpretation of Quantum Physics Really Means.” *Forbes*, Forbes Magazine, 5 Jan. 2016, http://www.forbes.com/sites/chadorzel/2016/01/05/what-the-many-worlds-interpretation-of-quantum-physics-really-means/#3c07ca931102.

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