Schrödinger's Cat Paradox

The Quantum Revolution

In the early days of the quantum revolution, many prominent scientists, including Albert Einstein himself, were still concerned about how the highly mathematical quantum model of reality related to the physical universe. Newton’s time-tested model made life simple. Particles could be modeled like billiard balls – real, solid objects. However, the quantum world was a great deal more complicated.

The quantum model of reality explains particles in terms of probability using wave equations. Instead of a particle being a bouncy ball, it is a described in terms of a superposition of waves that represent its probable states. Erwin Schrödinger complained in his 1935 paper that “the classical concept of state becomes lost,” citing the famous Heisenberg uncertainty principle. To highlight this problem, he posed his now legendary “cat paradox.”

Schrödinger’s Cat

Schrödinger purposed to put a fictional, but no doubt loveable, cat into a box shielded from any outside interference. Inside the box, he also places a radioactive substance, “so small,” says Schrödinger, “that perhaps in the course of an hour one of the atoms decays, but also, with equal probability, perhaps none.” Finally, he inserts a mechanism consisting of a Geiger counter connected to a bottle of poison. If the counter detects the radioactive decay of the atom, the mechanism breaks the flask of poison and poor kitty dies. If the counter does not detect the decay, then the cat lives to the great joy of feline lovers everywhere.

What is the point of this experiment? As Schrödinger explains, the quantum function describing the state of the system would “express this by having in it the living and dead cat (pardon the expression) mixed or smeared out in equal parts.” Quantum theory is agreeable on the atomic level or smaller, but what Schrödinger was trying to show is that once a microscopic system is magnified to have results on a larger, cat-sized scale, the results are unacceptable.

Interpretations and Implications

Schrödinger’s famous paradox has been answered with a variety of interpretations. The leaders of the quantum revolution produced the Copenhagen interpretation, which, in simplest terms, claims that the state of the system (including the cat) is not fixed until it is measured. In over-simplified terms, the act of opening the box to check on the cat determines whether the cat is alive or dead. Alternatively, science fiction fans love Everett’s “many worlds model” which DeWitt says reflects “a continual splitting of the universe into a multitude of mutually unobservable but equally real worlds.” According to this interpretation, one might wonder if there is a universe where the flipped coins always land on heads and physicists wonder why.

This paradox also illustrates a larger issue. For everyday tasks, the Newtonian model works well, but it fails when applied to atoms or massive astrophysical objects. The Quantum model explains particle physics with ease (if you’re a particle physicist with a fancy computer to do the math), but it simply fails to function in the world of cats, people, and stars. General Relativity faces the same challenge. Its area of expertise relates to astrophysical problems, but it is useless for anything smaller.

All of our physical models are bound to their respective scale of application. One of science’s greatest challenges is finding a model that can explain quarks as well as it explains cats.

Resources:

DeWitt, B. S., and N. Graham (eds): 1973, The Many-Worlds Interpretation of Quantum Mechanics (Princeton University Press, Princeton).

Schrodinger, Erwin. The Present Situation in Quantum Mechanics: A Translation of Schrödinger’s “Cat Paradox Paper”. Translator: John D. Trimmer. Proceedings of the American Philosophical Society, 124, 323-38.

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