Schrodinger’s cat is a thought experiment which was devised by you guessed it, Erwin Schrodinger. It was based around a paper called EPR, so called because it was written by Einstein, Podolsky and Rosen.

EPR, pointed out the strange nature of quantum superposition (the combination of all possible states of a system), and was developed to challenge the Copenhagen Interpretation, which states that the superposition or wave function collapses on the moment of observation.

Now to show the incompleteness of quantum mechanics Schrodinger developed a thought experiment, which applied quantum mechanics to a cat. The purpose of the experiment was to impose a superposition from a subatomic particle to a macro (large scale) system, by coupling the particle with the cat.

If we put a cat in a sealed box where the cat’s life depends upon the state of a particle, according to the Copenhagen interpretation it meant that the cat was in superposition, it is both dead and alive until the box is opened.

Schrodinger did this to show the strangeness of Quantum Mechanics and as a counter argument to the Copenhagen interpretation.

This is what Schrodinger proposed:

“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 which 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 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 naively 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.”

See the video below for a detailed explination of the thought experiment:

The point of this thought of experiment was to try to find out when does a system stop being described in the quantum sense and start being described in the classical sense. It was also performed to highlight the apparent paradox that we cannot believe a cat is both dead and alive, yet the theory says it is!

Now this theory brought a lot of other interpretations and arguments I will discuss just a few:

The Copenhagen interpretation, countered Schrodinger’s argument with the following, they stated that the collapse of the wavefunction is caused when you open the box, at the time they argued that the observation takes place then the particle hits the Geiger counter hence collapsing the wavefunction before the box is open.

However more work has been done recently on the Copenhagen interpretation and the results we get from it are extremely accurate, what needs to be understood is what constitutes a measurement, is the mere consciousness of the cat enough to collapse the wavefunction?

Objective collapse theories means that the wavefunction collapses spontaneously, before the box is opened. It does this when a time, mass, temperature reach a certain threshold. Hence the cat would have settled into a definite state before it was observed.

This theory requires a slight change but has big implications as it requires the wavefunction to collapse by the passage of time. So this means that the state is determined by the one before is and the one before that one etc etc, all the way back to very beginning of time. It’s state was pre-determined from the beginning of the universe, wow and we thought we had free will!

Everett's many-worlds interpretation is pretty much in the title. Everett suggested that the observation of a system is not a “special process”. That in fact both the dead and alive states exist but are decoherent. So when the box is opened two worlds are created one in which the cat is alive and one in which it is dead.

Since they are decoherent there can be no communication between the two. When an observer opens the box they become entangles with the cat and hence you observe a single state with no interference from the other. See video below for more info on this interpretation.

One of the main difficulties is distinguishing this interpretation from the Copenhagen interpretation as there is no communication between the separate worlds. However there was a thought experiment thought up by Max Tegmark, however I will discuss this in another hub.

Even though it was a purely theoretical thought experiment applications of the principles have been found in quantum computing, namely the state of the cat matches that of the qubit where they are all in superposition i.e. all 1’s and 0’s. For more information check out some of the great books listed. Also for more quantum physics experiemts check out more in the series.

• Quantum Physics - A central point for this series
• Quantum Physics - Sub Atomic particles
• Quantum Physics - How Small is Small? How Fast is Fast
• Quantum Physics - Double Slit Experiment
• Quantum Physics - The Bell Inequality Experiments
• Quantum physics - Quantum Suicide
• Quantum Physics - Popper's Experiment
• Quantum physics - Erwin Schrodinger
• Quantum Physics - Niels Bohr
• Quantum Physics - Werner Heisenberg