We will now revisit the difficulties involved in the measurement problem through a famous thought experiment devised by Erwin Schrödinger in 1935 to illustrate a point in quantum mechanics about the nature of wave particles.

A cat is placed in a steel box along with a Geiger counter, a vial of poison, a hammer, and a radioactive substance that has a 50% chance of decaying in an hour. Being a random process it is impossible to predict when the radioactive substance will decay, but the Geiger counter detects when it does decay, triggering the hammer to release the poison and subsequently killing the cat.

An observer does not know whether the cat is alive or dead until the box is opened—because the cat’s fate is intrinsically connected to whether or not the atom has decayed; the atom exists in a state known as a superposition—both decayed and not decayed at the same time. In Schrödinger’s own words the cat would be “living and dead … in equal parts” until it is observed.

In other words, until the box is opened, the cat’s state is completely unknown and therefore, until the system collapses the superposition into one configuration, the cat would exist in some zombie superposition state of being both alive and dead.

A Benjamin Schwartz cartoon from the March 30th New Yorker. The vet in the cartoon uses the “good news – bad news” formula to explain things to Schrödinger: the good news is that the cat is alive; the bad news is that the cat is dead!

Of course, it is impossible for an organism to simultaneously be alive and dead and quantum superposition does not work for large objects such as cats. Schrödinger used his thought experiment to illustrate the oddities associated with the measurement issue in quantum mechanics and his opposition to the Copenhagen Interpretation which he thought was inherently flawed. But the Copenhagen interpretation has endured and most physicists believe in its legitimacy.

According to Wikipedia, “Since Schrödinger’s time, other interpretations of the mathematics of quantum mechanics have been advanced by physicists, some of which regard the ‘alive and dead’ cat superposition as quite real. Intended as a critique of the Copenhagen interpretation (the prevailing orthodoxy in 1935), the Schrödinger’s cat thought experiment remains a defining touchstone for modern interpretations of quantum mechanics. Physicists often use the way each interpretation deals with Schrödinger’s cat as a way of illustrating and comparing the particular features, strengths, and weaknesses of each interpretation.”

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