Schrödinger’s cat is a thought experiment in quantum mechanics that demonstrates a paradox of quantum superposition. Quantum superposition states that any two or more quantum states can be added together to form a valid quantum state, while at the same time, every quantum state can be represented as the sum of two or more distinct states. In quantum superposition, a quantum system such as an atom or photon can exist as a combination of multiple states, corresponding with different possible outcomes. In the Schrödinger’s cat experiment, a hypothetical cat can be considered both alive and dead at the same time as a result of subatomic events that may or may not occur. The experiment was a result of a conversation Erwin Schrödinger had with Albert Einstein in 1935.
There is a related theory, from which the Schrödinger’s cat experiment arose. That theory is called the Copenhagen Interpretation, which states that until a quantum system interacts with, or is observed by the external world, it remains in superposition. When the system is observed, it collapses into one or another of the possible wave particle states. Schrödinger, in his correspondence with Einstein about his experiment, acknowledged it was useful in illustrating the absurdity of quantum mechanics. Many physicist’s today however, take the experiment as a quite literal example of quantum mechanic’s complex nature.
The experiment in Schrödinger’s own words: “One can even set up quite ridiculous cases. A cat is penned up in a steel chamber, along with the following device (which must be secured against direct interference by the cat): in a Geiger counter, there is a tiny bit of radioactive substance, so small, that perhaps in the course of the hour one of the atoms decays, but also, with equal probability, perhaps none; if it happens, the counter tube discharges and through a relay releases a hammer that shatters a small flask of hydrocyanic acid. If one has left this entire system to itself for an hour, one would say that the cat still lives if meanwhile no atom has decayed. The first atomic decay would have poisoned it. The psi-function of the entire system would express this by having in it the living and dead cat (pardon the expression) mixed or smeared out in equal parts.
It is typical of these cases that an indeterminacy originally restricted to the atomic domain becomes transformed into macroscopic indeterminacy, which can then be resolved by direct observation. That prevents us from so naïvely accepting as valid a “blurred model” for representing reality. In itself, it would not embody anything unclear or contradictory. There is a difference between a shaky or out-of-focus photograph and a snapshot of clouds and fog banks.”
One interpretation of this experiment is that while the box is closed, the system exists in a superposition of the states undecayed nucleus / alive cat, and decayed nucleus / dead cat, simultaneously. Only once the box is opened and an observation is made does the wave function collapse into one state or the other. Schrödinger’s experiment poses the question: ”when does a quantum system stop existing as a superposition of states and become one or the other?” It also demonstrates how these microscopic quantum states dictate the larger scope of our observable universe.
Our human experience would suggest that we can not be in an alive and dead or other state at once, though the experiment would suggest one can and is. Does the cat’s existence at all, require an external observer? In response to this absurd experiment, Einstein wrote: “You are the only contemporary physicist, besides Laue, who sees that one cannot get around the assumption of reality, if only one is honest. Most of them simply do not see what sort of risky game they are playing with reality—reality as something independent of what is experimentally established. [. . .] Nobody really doubts that the presence or absence of the cat is something independent of the act of observation.”
In the many-worlds interpretation of quantum mechanics, observation is not a special process. Rather, both the alive and dead cats persist after the experiment, though they are in decoherent states from each other. That would mean, there is a universe in which an observer finds a dead cat, and also one in which an observer finds an alive cat, though these universes have no communication or effective interaction with each other. With opening the box, an observer’s state becomes entangled with and corresponds to that of the cat. These mechanisms of quantum decoherence are the same that produce what is known as consistent histories. Decoherence prevents us from observing the other state. For example, in this theory, your car may have started this morning, but in another state, in another universe, it also did not, though you have no way to communicate with the state of yourself whose car wouldn’t start.
Such split realities and superimposed states have been achieved with photons when cooled to near absolute zero. A beryllium ion has also been trapped in a superimposed state. Experiments of this nature have been proposed for the flu virus and bacterium. In quantum computing, several qubits achieve a superimposed state of both 0 and 1 simultaneously.