A 75-year-old feline with more than 9 lives

By Paul Halpern

Seventy-five years ago, in November 1935, Austrian physicist Erwin Schrödinger unleashed a monstrous feline on the world. In one of the most famous thought experiments ever conceived, he envisioned a situation in which a perfectly ordinary cat would become a half-living, half-dead creature.

Placed in a closed box with a radioactive substance that has a 50 percent chance of decaying in a given time, and poisoned only if the substance happens to decay, the cat teeters, zombielike, between life and death. Only after the experimenter opens the box and observes the situation does the cat shed its ambiguous state and become either alive or dead.

The logic behind the cat experiment has to do with quantum measurement. Physicists believe that on a subatomic scale, there are particles that maintain a mixture of possibilities of certain physical properties until they are observed. By connecting a minute quantum event - radioactive decay - to something relatively large and tangible - a cat - Schrödinger showed that quantum uncertainty would lead to strange juxtapositions of otherwise clear-cut situations.

How could a tabby be purring and perished at the same time? If its existence were tied to a quantum event, it could indeed be both at once until the moment of revelation.

As it turned out, Schrödinger himself would soon be confronted by real-life ambiguities. After the Nazis annexed his native land, and despite earlier opposition to them, he stated his support for the new regime in an effort to keep his job. (He had declined an earlier offer to work at Princeton, which would have helped him avoid the dilemma altogether.) Eventually, though, he was forced to flee Austria and ended up in neutral Ireland. Unlike other physicists who lined up on either side during World War II, Schrödinger appreciated the chance to be on the sidelines and continue his theoretical work.

Historians love to contemplate "what if" questions. Sometimes, even the tiniest change might lead to huge political implications: Think of the 2000 presidential election, in which only a very small percentage of votes in Florida determined the outcome. It is often instructive to consider "parallel realities" in which different historical events unfolded, and to wonder what the world would be like as a result.

Many scientists have increasingly embraced the idea of alternative histories as a way of resolving the paradox of Schrödinger's cat in a more satisfactory way. According to an influential 1957 paper by graduate student Hugh Everett, each time a subatomic event such as the release of radiation occurs, reality bifurcates. This yields multiple paths representing all possible outcomes.

This startling notion, called the many-worlds interpretation, implies that Schrödinger's cat survives in one branch of the universe and dies in the other. The instant this split occurs, any observers monitoring the scene divide, too. One version of the spectators witnesses the survival of the cat, and the other records its death - each group wholly unaware of the others' experience. Because there are myriad quantum events, this means there are innumerable copies of the universe, each with subtle differences.

Everett believed these splits would permit a kind of "quantum immortality." Each time a living creature passes away in one reality, a parallel version carries on in another. He thought that the maze of possibilities would allow death to be indefinitely postponed. Hence, Schrödinger's cat, by continuing its existence along at least one strand of the tangle of choices at each junction, would have an inexhaustible supply of lives.

Lifetimes are traditionally considered linear - steady passages from birth to death. Everett's conclusion was therefore a radical rethinking of existence. The concept of parallel realities has seeped into popular culture, yielding, for example, an alternative version of how the crew of Star Trek's Enterprise began its journey.

If you happen to be reading this piece online, you are experience something like parallel worlds yourself. Hypertext and the Web have rendered our online reading experiences an almost endless maze of possibilities. Clicking on a succession of links, each of us navigates through a distinct landscape. Our narratives branch off with each decision we make.

Schrödinger's ambiguous cat, a mixture of contradictory possibilities, seems more than ever an emblem of our increasingly fragmented paths through the world. As our experiences diverge, our visions of the same events have become as disparate as the contrast between a cozy kitten and a feline phantom. The purring creature persists as a perfect symbol of burgeoning complexity and unresolved contradictions.