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In the late 1920s the Austrian physicist Erwin Schroedinger came up with an ingenious thought experiment. His proposed experiment was to see if you can kill a cat without looking at it and without the ASPCA [1] running you down like those crazed mobs in the bad horror movies. Just kidding; actually his experiment was to prove that the field of Quantum Mechanics, which he himself had helped to pioneer, was in fact completely ludicrous. He was like the Cheshire Cat of the physics world: "We're all mad here. I'm mad. You're mad." (Except he didn't smile constantly and couldn't disappear.) The experiment consisted of placing three things in a sealed box: a cat, a vial of poisonous gas, and a radioactive mineral. The experiment is set up so that two conditions are true: 1) if the radioactive mineral decays it will release the gas in some way and thus kill the cat, and 2) there is a 50/50 chance of the mineral decaying in the limited time the experiment takes up. According to the Schroedinger Wave Form Equation (developed by none other than Spiro Agnew, um, that is, Erwin Schroedinger) and the theories that go along with it, you can not determine what will happen, only the probability of a certain event occurring. Strangely, this event does not actually happen until you observe it. Let's say you shoot one photon at a photographic plate divided into two regions. There is a 50% chance of it hitting section A, and a 50% chance of it hitting section B. Until you develop and look at the plate, these are the probabilities, and both exist at the same time. When you look at the plate, one of two things happen, depending on what school of Quantum Physics you belong to. There is the Copenhagen Interpretation (so named because that was Einstein's brand of chewing tobacco), which states that when you look at the plate, the wave form will "collapse" and the probability of the photon hitting section A will "jump" to one, while the probability of the photon hitting section B goes to zero. There is also the Many Worlds Interpretation. Here, when you look at the plate, the universe splits into two parallel universes, one where the photon hits A and one where it hits B. The Many Worlds Interpretation is the basis for the popular television show "Melrose Place" (or is it 90210?). Because the wave form collapses or the universe splits when the system is observed, not when the event occurs, you can not tell whether the cat is alive or dead before you look in the box. Therefore, this deceptively complex cat manages to be both alive and dead at the same time, until you actually look in the box. This defies all common logic and obviously must be wrong. This is why Schroedinger created this paradox: to prove how stupid our explanations for the sub-atomic realm seem, let alone our explanations for the normal realm! There is, however, a way around this problem. If you could look at the object without a single particle hitting it, then the wave form wouldn't collapse or the universe wouldn't split. While this seems impossible, using a device known as an interferometer and a series of light polarizers it becomes a possibility. You can conclude that something is in a location by the fact that a photon does not exhibit interference. Not only that, but it also makes thousands of julienne fries [2]. This is all thanks to the wave-particle duality of light, which is a different story altogether [3]. This proves a very important point: if man concentrates his entire brain power on one task, eventually he will get too bored, pop open a beer and watch the Jets lose. [1] ASPCA, American Society for the Prevention of Cruelty to Animals, a fun American concept. [2] A popular American infomercial-like term. Used often in cheesy commercials for Ginsu knives and the like. [3] Because one path of the interferometer is blocked, the beam splitter which usually recombines the two beams split at the entrance of the interferometer acts differently. Even if only one photon is sent through the system, and it doesn't hit the object, interference will not occur. This means a photon detector set up in a region that usually shows canceling interference now has a 50-50 chance of being hit, proving that something is blocking the path. For information on how to do it with better than 50% chances, see the article in October 1996's Scientific American, "Quantum Seeing in the Dark" by Paul Kwiat, Harald Weinfurter, and Anton Zeilinger.