The Curious Tale of Schrodinger's Cat Quiz

Answer: Because it is a truly random process

It turns out that it's hard to devise something that's truly random. Even seemingly random things, like the "random number generator" built into your computer (or FunTrivia's), often have an underlying pattern -- they're pseudo-random. For a paradox like that of Schrödinger's cat -- which relies on the outside observer not knowing or predicting what's happening inside the box until it's been opened -- we really do need something random to drive the action.

The radioactive decay of an atom is one of the few truly random processes in nature: we can predict the decay of a large number of atoms pretty well (it's governed by their half-life, the length of time after which we expect half the atoms in a sample to have decayed), but we can't predict at what moment any single atom will decay.

Answer: A Geiger counter

Geiger counters, named for physicist Hans Geiger, detect ionizing radiation -- that is, radiation with enough energy to strip an electron from an atom or a molecule. The design is simple: a typical Geiger counter is a metal tube filled with an inert gas like helium.

A thin wire runs along the center of the tube, and it carries a high voltage relative to the tube walls. When a particle of ionizing radiation passes through the tube, it ionizes an atom of the gas -- and the newly freed components of that atom ionize other atoms in a cascade.

The gas is thus briefly able to conduct electricity, leading to a current on the wire. This current might rotate a needle on a dial or register as a distinctive click, depending on the device's design. A Geiger counter is a magnificently sensitive instrument, which is why Schrödinger pressed it into service for his imaginary experiment; in his set-up, a single count would release a hammer, which in turn would smash the vial of poisonous gas.

Answer: The probability of observing each possible outcome

A wave function is a mathematical way of describing a physical system, so named because, in quantum mechanics, it helps to think of physical objects as waves. Wave functions, which are really vectors, can be constructed by adding up the perpendicular unit vectors for each possible state of the system, with each state vector multiplied by a complex coefficient. Squaring that coefficient gives the probability of measuring the system in the corresponding state.

If you find this confusing, you're in very good company. The paradox of Schrödinger's cat is designed to illuminate the gaps in this idea: what does a wave function really signify physically, and just what is involved in a measurement? Continue on, and you'll see that physicists disagree on these matters even today.

Answer: A superposition of states

The idea here is that -- until we somehow force the universe's hand -- a physical system simultaneously exists in all the states that are available to it. This is why, when discussing atomic structure, we no longer talk about an electron having some well-defined orbit like a planet does; instead, we talk about an electron "cloud," because the electron is everywhere that it's allowed to be.

This is strange enough behavior for electrons; for a cat, it's much harder to swallow!

Answer: Entanglement

Entanglement may seem a simple concept, but it gives rise to some very strange behavior. In this case, for example, entanglement forces a classical entity (the cat) to behave in a distinctly quantum way. In other scenarios, it can lead to things like the measurement of one particle affecting the behavior of its entangled partner -- even when the two are light years apart, a phenomenon Albert Einstein famously mocked as "spooky action at a distance." Entanglement is the basis of the emerging field of quantum cryptography, where researchers honor Schrödinger's feline by calling certain flavors of entanglement "cat states."

Answer: When it is observed

The cat's status is described by a wave function showing equal parts liveness and deadness. When the cat's state is measured, only one of these possibilities is observed, and the wave function collapses to just that one outcome: another measurement, taken immediately afterward, will yield the same result.

The Copenhagen interpretation is powerful, but leaves some important issues unsettled -- particularly the question of just what "measurement" means. Can the cat (or even the Geiger counter or the decaying atom) observe itself and collapse its own wave function? Or is the only observer worth mentioning the one standing ready to open the box -- and if so, what makes him or her special?

Answer: Many-worlds interpretation

First formulated in 1957 by Hugh Everett, the many-worlds interpretation holds that every possible outcome of a quantum process really does happen -- but each in its own universe, decoupled from each other and not affecting each other in any way. In this interpretation, the observer isn't special -- he or she is just one of a number of identical observers in newly separated universes -- so these embarrassing observation-related paradoxes just fall away. Plus, the idea of a (perhaps infinitely) branching multiverse makes for excellent science fiction.

Answer: Ensemble

"Ensemble" is French for "together," and in physics it refers to a large collection of identical experiments -- temperature baths, engines, cats in boxes, whatever. The key here is that the laws of statistics have greater predictive power for larger numbers of measurements.

For example, it's easy to predict the outcome of 1000 coin tosses -- 500 heads and 500 tails -- and be right to within less than a percent (a couple of flips). It's much harder to predict the outcome of ten flips to that level of precision. Proponents of the ensemble interpretation argue that it's meaningless to talk about this experiment as applied to a single cat, and that's why you see apparent contradictions.

When you're talking about 100,000 cats, you avoid the messy situation of having a single cat be half-dead and half-alive.

Answer: Quantum decoherence

Classical physics -- the physics developed by the likes of Galileo, Newton, and Maxwell -- does a great job of explaining most of the physics we see on an everyday basis. But when you start looking more closely, classical physics begins to break down: we need general relativity to explain Mercury's orbit, for example, and quantum mechanics to explain everything from the workings of atoms to the light emitted by stars. Quantum decoherence explains how, on large scales, quantum behavior gives way to something that can be approximated by classical laws. Through interaction with its environment, the system essentially loses information, and the different components of its wave function no longer interfere with each other.

Answer: A thought experiment

A thought experiment (also called a gedanken experiment, from the German) is a more rigorous way of asking, "Hey, what if ...?" Like Schrödinger's cat, it can be used to explore the quirks or inconsistencies of a theory. Or a thought experiment might help you to formulate the theory in the first place.

For example, Albert Einstein worked through many of the concepts in his theory of special relativity by conducting thought experiments: imagining an observer traveling alongside a beam of light, for example, or in an elevator orbiting the earth in free fall.