Superposition is a principle of quantum theory that describes a challenging concept about the nature and behavior of matter and forces at the sub-atomic level. The principle of superposition claims that while we do not know what the state of any object is, it is actually in all possible states simultaneously, as long as we don't look to check. It is the measurement itself that causes the object to be limited to a single possibility.
In 1935, Erwin Schrodinger proposed an analogy to show how superposition would operate in the every day world: the somewhat cruel analogy of Schrodinger's cat. Here's Schrödinger's (theoretical) experiment: We place a living cat into a steel chamber, along with a device containing a vial of hydrocyanic acid. There is, in the chamber, a very small amount of a radioactive substance. If even a single atom of the substance decays during the test period, a relay mechanism will trip a hammer, which will, in turn, break the vial and kill the cat. The observer cannot know whether or not an atom of the substance has decayed, and consequently, cannot know whether the vial has been broken, the hydrocyanic acid released, and the cat killed. Since we cannot know, the cat is both dead and alive according to quantum law, in a superposition of states. It is only when we break open the box and learn the condition of the cat that the superposition is lost, and the cat becomes one or the other (dead or alive). This situation is sometimes called quantum indeterminacy or the observer's paradox: the observation or measurement itself affects an outcome, so that the outcome as such does not exist unless the measurement is made. (That is, there is no single outcome unless it is observed.)
Superposition is well illustrated by Thomas Young's double-slit experiment, developed in the early nineteenth century to prove that light consisted of waves. In fact, the noted physicist Richard Feynman claimed that the essentials of quantum mechanics could be grasped by an exploration of the implications of Young's experiment.
The Double-Slit Experiment
For this experiment, a beam of light is aimed at a barrier with two vertical slits. The light passes through the slits and the resulting pattern is recorded on a photographic plate. If one slit is covered, the pattern is what would be expected: a single line of light, aligned with whichever slit is open. Intuitively, one would expect that if both slits are open, the pattern of light will reflect that fact: two lines of light, aligned with the slits. In fact, however, what happens is that the photographic plate is entirely separated into multiple lines of lightness and darkness in varying degrees. What is being illustrated by this result is that interference is taking place between the waves/particles going through the slits, in what, seemingly, should be two non-crossing trajectories.
We would expect that if the beam of light particles or photons is slowed enough to ensure that individual photons are hitting the plate, there could be no interference and the pattern of light would be two lines of light, aligned with the slits. In fact, however, the resulting pattern still indicates interference, which means that, somehow, the single particles are interfering with themselves. This seems impossible: we expect that a single photon will go through one slit or the other, and will end up in one of two possible light line areas. But that is not what happens. As Feynman concluded, each photon not only goes through both slits, but simultaneously takes every possible trajectory en route to the target, not just in theory, but in fact.
In order to see how this might possibly occur, experiments have focused on tracking the paths of individual photons. What happens in this case is that the measurement in some way disrupts the photons' trajectories (in accordance with the uncertainty principle), and somehow, the results of the experiment become what would be predicted by classical physics: two bright lines on the photographic plate, aligned with the slits in the barrier. Cease the attempt to measure, however, and the pattern will again become multiple lines in varying degrees of lightness and darkness. Each photon moves simultaneously in a superposition of possible trajectories, and, furthermore, measurement of the trajectory causes the superposition of states to collapse to a single position.