What Schrödinger’s Cat Has in Common with Young Frankenstein

Posted Aug 13, 2011 by Nicole Yunger Halpern

Updated Aug 13, 2011 at 07:51 PM

In Mel Brooks’s 1974 film Young Frankenstein, the title character shows off his monster to what look like opera-goers. Late in the evening, satin rustles as the auditorium’s seats fill. Once the lights dim, the scientist has his creation perform simple tasks. The monster walks forward, walks backward, and tap dances in a top hat. (All right, not all the tasks are simple.) Disaster strikes when a light bulb explodes. Terrified, the creature wails and rushes at the audience.

Schrödinger’s cat is the top-hat-wearing monster of quantum mechanics.

Quantum mechanics is the physics of tiny systems, such as atoms and electrons. Many of its results seem counterintuitive. Whenever someone wants to illustrate quantum phenomena, he or she totes out Schrödinger’s cat and has it tap dance. Some retellings of the cat story resemble the original as much as Frankenstein’s monster resembles a Rockette. For anyone interested in the real McCoy, here’s an off-Broadway version.

Schrödinger’s cat originated in the 1935 article “The Present Situation in Quantum Mechanics.” Erwin Schrödinger, an Austrian Nobel laureate and a founder of quantum theory, wrote the article. He included the cat in a thought experiment, a puzzle that invites readers to consider, “What if?”

What if, Schrödinger wrote, you stick a cat in a sealed chamber? Along with the cat, include a radioactive atom, a radiation detector, and a vial of poisonous gas. The atom has a fifty-percent chance of decaying—of breaking into smaller pieces—in one hour. If the atom decays, the detector senses the decay. The detector triggers a mechanism that breaks the vial. Poisonous gas fills the chamber, and Kitty kicks the bucket. If the atom doesn’t decay, the detector doesn’t set off the mechanism. Kitty lives to meow another day.

An hour has passed. You place one hand on the knob, ready to open the chamber. How can we describe the cat’s aliveness or deadness?

Experience insists that the cat is alive or dead; we just don’t know which. Nuh-uh, counters quantum theory. The cat’s state depends on the atom’s state: The cat is “entangled” with the atom. Quantum mechanics governs the atom. Therefore, it governs the cat. The cat occupies a bizarre quantum state.

That state is called a “superposition.” We represent the superposition by a sum of mathematical objects. One object represents the “alive” state; one represents the “dead” state. The sum resembles this one:

½ (Atom whole & cat alive) + ½ (Atom decayed & cat dead).

Max Born proposed an interpretation for such a superposition: We have, upon opening the chamber, a fifty-percent chance of finding a living cat and a fifty-percent chance of finding that a dead cat. Born hit the nail on the head. If we performed Schrödinger’s experiment thousands of times and recorded the aliveness/deadness of thousands of cats, we’d mark “alive” about half the time and “dead” about half the time.

Here’s where the light bulb explodes.

Born’s interpretation doesn’t satisfy everyone. Some want to know how to describe the cat’s aliveness/deadness non-mathematically before we open the chamber. The descriptions I’ve encountered include: (1) The cat is alive and dead. (2) The cat is neither alive nor dead. (3) In one universe, the cat is alive. In another, “parallel,” universe, the cat is dead. (4) No non-mathematical description of the cat’s aliveness/deadness makes sense. (5) We shouldn’t concern ourselves with the cat’s aliveness or deadness, because we can’t test hypotheses about it.

Interpretation (5) might seem a cop-out. But, to test a hypothesis about the cat’s aliveness/deadness before we open the chamber, we’d have to examine the cat—we’d have to open the chamber. When we open the chamber, we create the situation described by Born: We find a corpse or we can imitate Gene Wilder. (Wilder played Dr. Frankenstein in Brooks’s film. As the monster wakes, Wilder shrieks the most delicious, melodramatic “It’s alive! It’s alive!” I’ve ever heard.) Either the mathematical object 100%(Atom whole & cat alive) or 100%(Atom decayed & cat dead) describes the system.

When we examine the cat, its aliveness/deadness changes. Physicists call this change “collapse.” The superposition state “collapses” onto the “live” state or the “dead” state.

What collapses the superposition state? A conscious person’s observation of the cat seems to do the trick. Should physical law reference consciousness? Does the cat have sufficient consciousness to collapse its state? Could a robot collapse the state?

Do these questions sound absurd?

Rumor has it that Schrödinger thought the whole thought experiment absurd. Cats aren’t like omelettes; although you can order a half-yolk-half-whites breakfast, cats don’t occupy half-dead-half-alive states. Since quantum mechanics implies that Schrödinger’s cat occupies a superposition state, and since a cat’s occupying a superposition state seems ridiculous, quantum mechanics can’t describe reality. So say the rumor mills about Schrödinger’s argument. Schrödinger did express dissatisfaction with quantum mechanics. But I haven’t found evidence in his 1935 article that he intended for his thought experiment to disprove quantum mechanics. A colleague reported that he revealed his intention in a letter. If I find the letter, check back for “Schrödinger’s Cat, Revealed! Part Deux.”

Part Un has covered entanglement, superpositions, Born’s Postulate, parallel universes, the Collapse Postulate, and Mel Brooks. Quite the symphony of ideas. The symphony impresses not least because the one-man-band (one-cat-band?) of Schrödinger’s paradox plays it all.

Perhaps because Schrödinger’s cat illustrates so many phenomena, it appears in many media. ThinkGeek.com sells a T-shirt whose front reads, “Schrödinger’s Cat is Dead” and whose back reads, “Schrödinger’s Cat is Alive.” Other companies offer similar clothing. John Gribbins entitled his books about quantum theory for nonscientists In Search of Schrödinger’s Cat and Schrödinger’s Kittens and the Search for Reality. On the front cover of David Griffiths’s textbook Introduction to Quantum Mechanics, a cat raises one paw; on the back, the cat slumps to the ground. Ordinary cats boss their owners; Schrödinger’s cat captivates physics enthusiasts.

With great power comes great responsibility: We must explain Schrödinger’s paradox accurately. Quantum theory contains enough mysteries. Please don’t compound the confusion by playing Frankenstein with the thought experiment.

And if you’ve never watched a Mel Brooks movie, run to Netflix.