Many of you have probably heard of the thought experiment known as Schrödinger's Cat. The basic premise of it is that you have a cat in a box. Also inside that box are a flask of poison gas, a radioactive element, and a detector hooked up to a small hammer to smash the flask. Now, if the radioactive element, well, radiates, the detector will detect it, smash the flask, and the poison will kill the cat. It is important to note that this experiment has never been performed, and actually performing it may get you labeled as a psychopath (or just someone who really, really hates cats). While the box is sealed, there is no way for an observed to know if the cat is alive or dead—that is, if the radioactive element has triggered the detector.

Artist's Rendition

The goal behind this experiment is the idea that, at some point during this experiment, the cat is both alive and dead—that is, a superposition of those states. Erwin Schrödinger himself came up with this scenario to demonstrate the ridiculousness of the Copenhagen interpretation of quantum mechanics, formulated by Niels Bohr and Werner Heisenberg in the mid-1920's, but, since then, it's been co-opted to explain the interpretation itself. So, let me get to the explaining.

In my last post on light I very briefly touched on wave-particle duality. Light, you see, is both a particle and a wave. You can model it as either, or observe it as either, and it works. But, until the point at which you observe it as one or the other, it is technically both. This same idea is applicable across all of quantum mechanics. Take the Heisenberg Uncertainty Principle:

In the common tongue, it reads that the more precisely you know the momentum (mass times velocity) of a particle, the less you must necessarily know about its position, and vis versa (the product of the degree of uncertainty of the momentum times the degree of uncertainty of the position must always be greater than or equal to a constant, which means lowering the uncertainty of one necessitates raising the uncertainty of the other). You can measure one pretty accurately, but you'll have no idea about the other. Just like with light, you can measure it as a wave, but you lose the ability to measure it as a particle (that is, until the system decoheres). Now, because of this uncertainty, and a lot of math (which I won't get into here), before you measure, say, a particle's position, it exists in a state of all possible positions for that particle, in a probability curve (which is described by the Schrödinger Equation Ψ). This curve represents all possible states of the system, and the system exists in a superposition of all those states. That is, until you measure the state. Once you do, it collapses the wave function, and the particle now exists in one distinct state from that superposition. At least, that is what the Copenhagen interpretation holds true.

Others at the time argued that the wave function (Ψ) described by the Schrödinger Equation was simply a probability distribution of the states of the system—that is, that the wave described the probability of finding the particle in that state. When you measured it, you weren't collapsing anything, the particle was already in that state, and you were just finding out about it. However, for this to hold, you would need hidden variables in the Equation, which the Copenhagen interpretation denied. In this interpretation, the measurement caused a change in the state of the function—moving it from that superposition—all states—to a single state. This was quite obviously hard to swallow for many physicists, including Schrödinger, who grew up on classical physics. This idea was absurd—a superposition of states, which measuring collapsed into a single one? Ridiculous. Hence the formulation of Schrödinger's eponymous cat thought experiment.

However, later advances in experiment and thought (in particular Bell's Theorem) caused physicists to lean more and more toward the Copenhagen interpretation, and today it's the most widely accepted view of quantum mechanics. Hence, the cat idea now being used to explain quantum mechanics (in particular the more-or-less canonical Copenhagen interpretation) rather than attempting to refute an interpretation of it. So, knowing that, let's not try to seal any cats in boxes with poison gasses and radioactive elements to try to understand quantum mechanics*. You'll either end up with a dead cat or a very upset one—and you don't want either.

* It wouldn't work anyway. In practice, the cat can observe itself. After all, it's not a single particle governed by the laws of quantum mechanics.