Schrodinger's Cat

Austria: 1935
Schrödinger intended his thought experiment as a discussion of the   EPR article—named after its authors   Einstein,   Podolsky, and   Rosen—in 1935. [1 ]   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   collapses<span class="apple-converted-space" style="color:rgb(255,255,255);font-family:Arial;line-height:19.1875px;">  <span style="color:rgb(255,255,255);font-family:Arial;line-height:19.1875px;">into a definite state when one of the systems is <span class="apple-converted-space" style="color:rgb(255,255,255);font-family:Arial;line-height:19.1875px;">  measured<span style="color:rgb(255,255,255);font-family:Arial;line-height:19.1875px;">. Schrödinger and Einstein exchanged letters about Einstein's EPR article, in the course of which Einstein pointed out that the state of an <span class="apple-converted-space" style="color:rgb(255,255,255);font-family:Arial;line-height:19.1875px;">  unstable<span class="apple-converted-space" style="color:rgb(255,255,255);font-family:Arial;line-height:19.1875px;">  <span style="color:rgb(255,255,255);font-family:Arial;line-height:19.1875px;">keg of <span class="apple-converted-space" style="color:rgb(255,255,255);font-family:Arial;line-height:19.1875px;">  gunpowder<span class="apple-converted-space" style="color:rgb(255,255,255);font-family:Arial;line-height:19.1875px;">  <span style="color:rgb(255,255,255);font-family:Arial;line-height:19.1875px;">will, after a while, contain a <span class="apple-converted-space" style="color:rgb(255,255,255);font-family:Arial;line-height:19.1875px;">  superposition<span class="apple-converted-space" style="color:rgb(255,255,255);font-family:Arial;line-height:19.1875px;">  <span style="color:rgb(255,255,255);font-family:Arial;line-height:19.1875px;">of both exploded and unexploded states.

To further illustrate, Schrödinger describes how one could, in principle, transpose the superposition of an atom to large-scale systems. He proposed a scenario with a cat in a sealed box, wherein the cat's life or death depended on the state of a subatomic particle. According to Schrödinger, the Copenhagen interpretation implies that <span class="apple-converted-space" style="line-height:14.4pt;font-size:10pt;color:white;font-family:Arial;">  the cat remains both alive and dead<span class="apple-converted-space" style="line-height:14.4pt;font-size:10pt;color:white;font-family:Arial;">  (to the universe outside the box) until the box is opened.

The thought-experiment goes as follows:

One can even set up quite ridiculous cases. A cat i s penned up in a steel chamber, along with the following device (which must be secured against direct interference by the cat): in a Gieger 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 psi-function  of the entire system would express this by having in it the living and dead cat 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.

Before being bronzed for his over-ambition, Escher was sent out to the collect the cat.