"Schrödinger's cat" is one of those phrases that gets wheeled out every now and again in conversation. Yet for those who have never been a student of physics, or at least heard Sheldon’s explanation on the Big Bang Theory, it may not be clear what it means. The short answer is that Schrödinger's cat is the name of a famous thought experiment first posited by the physicist, Erwin Schrodinger. The long answer and source of one of the most important debates in physics invite us to enter the bizarre domain of quantum mechanics to perhaps re-consider our assumptions about reality.
Erwin Schrödinger was born on August 12th, 1887 in Vienna, capital of Austria-Hungary. Erwin’s father, Rudolph Schrödinger, was a botanist and owner of an oilcloth factory. His mother, Georgine, was the daughter of Rudolph’s chemistry professor. An only child, Erwin was raised as a Lutheran and privately educated at home until the age of 11. Erwin excelled from an early age in language, math, and physics.
Schrödinger enrolled at the University of Vienna and studied physics. He was strongly influenced by fellow physicists Franz Exner, a pioneer of the field, and Fritz Hasenohrl, later Head of Theoretical Physics. Schrödinger was awarded his Ph.D. in 1910. He was drafted and sent to Italy as an artillery officer in 1914. Schrodinger was luckier than his colleague and mentor, Hasenohrl, who was killed by a grenade in 1915. Surviving the war, he held faculty positions at several universities before marrying Annemarie Bertel in 1920.
In 1921 Schrödinger became a professor at the University of Zurich, and it became clear that he had novel ideas in the area of atomic physics. In response to theories that the movement of electrons within an atom could be described as a wave, Schrödinger provided the necessary mathematical proof with his Wave Theory Equation of 1926. The spark of creativity was said to have come during a torrid affair with an old Viennese girlfriend.
Schrödinger's equation provided a robust description of the behavior and movement of atoms. By ascribing their movement to a probabilistic wave, the position and state of an atom could be treated statistically. Until a direct measurement was taken, the atom was said to be in a state of superposition. According to the Copenhagen Interpretation, this meant that it could be regarded as being in all locations at once. The key question for physicists, was Wave Theory merely a convenient mathematical tool or did it describe something physically real? Where was the atom, really?
In 1927 Schrödinger moved with his wife to the Friedrich Wilhelm University of Berlin. Here he rubbed elbows with Albert Einstein. While continuing to do important and innovative research there, he became increasingly alarmed by the growing anti-Semitic atmosphere in Germany. Following the example of Einstein, in 1937 he left Germany and moved to England, taking up a position at Oxford. Soon after he was awarded the 1933 Nobel Prize in Physics.
Despite his pivotal role in creating wave theory, Shrödinger was never comfortable with its implications. Like many physicists, he could not reconcile quantum uncertainty at the atomic level with our everyday experience of reality. In the world we see, people and objects have a definite place and state, whether we observe them or not. To highlight this contradiction, he formulated a thought experiment that involved a cat in a sealed box, a radioactive substance, and a Geiger counter to measure the substance's decay. During one hour, there would be a 50% chance the radioactive material would decay and emit a single atom. If this happened, the Geiger counter would detect it and activate a relay that would cause a hammer to smash a flask of acid. The acid would be released into the box, killing the cat. The question was, is the cat alive or dead?
From a quantum mechanical perspective, the cat is said to be in a superimposed state, with an equal probability of being either dead or alive. The state of the cat is only determined when it is measured or observed. This seemed absurd, Schrödinger wrote, as surely the cat must be one or the other, whether an observer decides to take a look or not. Yet this is exactly how atoms appeared to behave. By linking the fate of the cat to the fluctuations of atomic nuclei, Shrödinger was demonstrating the central problem of the Copenhagen Interpretation of quantum mechanics.
Needless to say, when asked what he thought about the Copenhagen Interpretation of his Wave Theory, Shrödinger said: "I don't like it, and I'm sorry I ever had anything to do with it." Schrodinger would go on to argue for years with his contemporaries about the meaning of his own experiment.
Popular rumor is that at some point during the 1930’s Schrödinger may have actually owned a cat named Milton at his home in Oxford. Thankfully, despite providing some inspiration to his owner, there is no evidence that Schrödinger's cat was ever put in a box with a flask of acid.
Schrödinger died on January 4th, 1961 in the same city in which he was born, Vienna. By the time of his death, both he and his theoretical feline had left an indelible mark on the science of quantum mechanics. While many great minds contributed to developing wave theory and the concept of indeterminacy, only one will forever be known as the father of quantum mechanics, and that is Erwin Schrödinger.
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