Schrödinger’s Cat

Schrödinger’s [shroh-ding-erscat is a thought experiment in quantum physics, usually described as a paradox, devised by Austrian physicist Erwin Schrödinger in 1935. In the experiment, a cat is placed in a room that is separated from the outside world; a small amount of a radioactive element is in the room.

Within some time, say one hour, one of the atoms of the radioactive material may decay (because the material unstable), or it may not. If the material breaks down, it will release poisonous gas, which will kill the cat. The question now is: at the end of the hour, is the cat alive or dead?

Schrödinger says that as long as the door is closed, the cat could be dead or alive. There is no way to know until the door is opened. The problem is in that by opening the room, the person is interfering with the experiment. The person and the experiment have to be described with reference to each other. By looking at the experiment the person has influenced the experiment.

A famous physics theory (the Copenhagen interpretation) said that the cat was both dead and alive until its observation proved it to be one or the other (Superposition). The cat situation was first proposed by Schrödinger to actually demonstrate the foolishness of thinking about quantum states at macro levels.

Although the original ‘experiment’ was imaginary, similar principles have been researched and used in practical applications. In the course of developing this experiment, Schrödinger coined the term ‘entanglement’ (two particles which act in unison). He intended his thought experiment as a discussion of the EPR article—named after its authors Einstein, Podolsky, and Rosen—in 1935. The EPR article highlighted the strange nature of quantum entanglement, which is a characteristic of a quantum state that is a combination of the states of two systems (for example, two subatomic particles), that once interacted but were then separated and are not each in a definite state.

The Copenhagen interpretation implies that the state of the two systems undergoes collapse into a definite state when one of the systems is measured. Schrödinger and Einstein exchanged letters about Einstein’s EPR article, in the course of which Einstein pointed out that the state of an unstable keg of gunpowder will, after a while, contain a superposition of both exploded and unexploded states.

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