"Erwin Schrödinger is going through airport security when an official asks to check his bag. After opening the bag, the official is appalled and shouts, "Sir, did you know there is a dead cat in your bag!" And Schrödinger calmly replies, "Well now there is."
If you're asking, "Who is Schrödinger? And why does he have a dead cat in his bag?" you probably missed the punch line of the joke. Don't worrywe'll get there, but before we investigate Schrödinger's "cat in a box" quantumblurringmindboggling thought experiment, I want you to take a deep breath and plug your nosebecause we're about to dive into some deep quantum mechanics.
Photograph of Toby the cat copyright Kevin Steele.

Quantum mechanics is a branch of physics that scientists use to describe the behaviors of small particles (like electrons). But unlike classical physicswhich describes the behaviors of big objects like baseballs and rocketsquantum mechanics doesn't deal in nice exact answers. Instead, it deals in probabilities. For example, if I asked the question, "Where is Suzy?" classical mechanics would predict, "Suzy is on the couch," while quantum mechanics would tell us that "Suzy is probably on the couch, but she might also be in the bathroom, or walking in the garden, and there is a small but nonzero chance that Suzy is currently enjoying tea on the far side of the moon."
So if scientists can use classical mechanics to predict the exact trajectory of a NASA spacecraft headed for Mars, why can't they use quantum mechanics to predict something as simple as the location of an electron?
Warning: This is where things start to get weird...and a little disturbing. There is an inherent indeterminacy embedded in quantum mechanics that prevents scientists from predicting variables like position and momentum exactly. But what causes this indeterminacy? Ask Einstein and he would say indeterminacy is a reflection of our own ignorance. But ask Niels Bohr and he would argue that particles don't have finite positions or momentums until they are measured by an observer, at which point the act of measurement itself forces the particle to "take a stand" and choose a state. Pascual Jordan, one of the fathers of quantum mechanics, put it this way: "Observations not only disturb what is to be measured, they produce it...we compel (the particle) to assume a definite position."
This is the underlying concept of the "cat in the bag" joke. Erwin Schrödinger, one of the masterminds of quantum theory, devised a thought experiment purely to highlight the absurdity of Bohr's interpretation. In his thought experiment, Schrödinger sets up a scene where a cat is placed in a box with one of Bohr's "indecisive particles," but the life or death of that cat depends upon which state the particle chooses. If the particle chooses one state (let's call it state A), the cat lives, but if the particle chooses the other state (state B), poisonous gas is released into the box and the cat dies. Applying Bohr's view of indeterminacy to this situation, the particle doesn't have a definite state and exists as a sort of hybridboth A and B at oncethat physicists call a "superposition." But think about what this means for the cat. If the particle exists in a superposition of states A and B, the cat also must exist in a superposition of two states, dead and alive. But as soon as an observer opens the box (or bag), the sheer act of observation "compels" the particle to exist in either A or B, and thus the cat must be either dead or alive. This situation is rather awkward if the first observer is oblivious to the experimentlike airport security.
Why would Bohr advocate something that sounds so ridiculous? And why would Schrödinger's derisive analogy become the poster child of quantum theory? Even today there is no consensus on a "right" interpretation of quantum theory. (For more on this debate, visit NOVA's physics blog, The Nature of Reality.) However, experimental results confirm that measurements (like peeking inside Schrödinger's bag) can affect and even determine the state of quantum systems. Quantum particles behave like they "know" when they're being watched, and adjust their behavior accordingly, like a group of mischievous youngsters keenly aware of an adult presence in the room.
Perhaps the "measurement problem," as it is called, is not all that strangeit merely seems so because we lack the words to explain it. If a quantum particle tried to convey to us what it feels like to be a quantum particle, it would be like a cube explaining to a square what it feels like to be 3D. The only language that bridges the two different worldsquantum and classical, 2D and 3Dis mathematics. And mathematically, we can describe this strange quantum world incredibly accurately using probability.
Happily for cats, today Schrödinger's thought experiment is used only as a mascot for quantum theory and not as a standard of thought. Theorists are still working to explain the measurement problem with fresh interpretations of quantum mechanics that could resolve the apparent paradox. So cats everywhere are safe...at least for now.