The Schrodinger's Cat Experiment

The boundary between classical and quantum world: The Schrodinger's Cat experiment.

In the classical world, objects can be well defined in terms of their speed and position: when you are in a car  you can easily tell where exactly you are and what speed you are traveling with. 

In the sub-atomic world, where the theory of quantum mechanics applies, things are quite different.  According to Heisenberg's Uncertainty principle the position and the speed of a particle (such as an electron, proton or a nucleus) can not be defined at the same time. Instead particles can be in more than one place at the same time. 

The mathematical formalism for this concept is based on the Probability Density Function (PDF) (the Normal (gaussian) distribution used in Statistics is an example of a PDF). Assigning a PDF to a particle we can say that the particle is everywhere in space and there is a certain probability to find it at a given place.

If we take this concept a step further, we end up with the theory of Superposition:  The position (or the state) of a particle is made up of the sum of all the possible positions (or states) and each of those positions (or states) comes with a certain probability.

Let us give an example. We know that an electron can rotate (spin) either clockwise or anti-clockwise. At certain conditions and according to the superposition theory of quantum mechanics, the state of its rotation is made up of 70% probability of clockwise spinning and 30% probability of anti-clockwise spinning.

The interesting point is that when we setup an experiment to measure the spin of an electron, then, by the time of measurement, one of its possible states collapses (or-in other words-gets probability zero), whereas the remaining state is what we measure finally (or-in other words-it gets probability 100%).  The idea is that a system is a mixture of all the possible states that it can be and when an observation is made only one of its states survives (which we eventually measure). 

But if this is the case for the sub-atomic world, it is definitely not the case (or at least doesn't seem so!) in the world that we feel in our daily lives. Where is the boundary then, between Quantum and Classical world?

This is what Schrodinger tried to demonstrate back in 1935 with his famous (imaginary) experiment.

In brief, the experiment involved a cat placed in a box so that it doesn't interact with anything from the outside world. Together with the cat there is a radioactive source, a Geiger-Miller counter (which detects the radiation emitted by the source) and also a bottle with toxic gas. The bottle is connected to the Geiger-Miller counter is such a way (the detailed mechanical connection is irrelevant here) that when, the counter detects radiation, it triggers a device, which eventually breaks the bottle which in turn releases the toxic gas killing the cat.

We know that the radioactive decay is totally random process:  for a given time interval, there is 50% probability that the radioactive source will decay and 50% probability that it will not. Taken into account the concepts developed at the beginning, the state of the radioactive source is a mixture of "half a decayed state and half of an un-decayed state". It then follows that, as the experiment is setup, the cat is in a state of "half dead and half alive" !!. Until you open the box, no one can tell the state of the cat and, until then, the cat is alive and non-alive at the same time.

At this point one should understand that this is not an issue that we can not predict the status of the cat nor that half of the cat is dead and half of the cat is alive. The cat is simply the superposition of the living and deceased state. 

Schrodinger's experiment demonstrated that there is not a fine line between quantum and classical physics. Research is still going on to deeply understand, interpret and -why not- expand the theory of quantum mechanics.

One can find more information on quantum mechanics and Schrodinger's experiment on the web. 

A very good and reliable source the Physics World magazine published by the Institute of Physics:

http://physicsworld.com/cws/search?page=1&query=%22Schrodinger%22