There are few thought experiments in science as famous as Schrödinger's Cat, even though most people couldn't explain it to you if they tried.
It's not that the implications of the thought experiment are opaque. In fact, the implications of the thought experiment are the one thing that almost everyone knows: that Schrödinger's Cat is both alive and dead at the same time.
But what does that even mean? What chain of logic could possibly lead to that kind of result?
Fortunately, you don't need a degree in physics to understand what Schrödinger was getting at with his thought experiment, and even Albert Einstein praised Schrödinger for devising such a simple illustration of some of the more confusing parts of quantum mechanics.
So, in short, don't worry. The Schrödinger's Cat thought experiment isn't nearly as complicated as many seem to believe, and properly understanding Schrödinger's Cat is an essential part of grasping the fundamental features of the bizarre quantum realm of physics.
Who was Erwin Schrödinger?
Erwin Schrödinger was a Nobel Prize-winning Austrian physicist who was instrumental in developing many of the fundamental aspects of quantum theory.
Other than his well-known thought experiment, Schrödinger is most famous for his wave equation, which is used to calculate the wave function of a quantum system at different points in time.
Even though he played such a large role in its formation, Schrödinger didn't always agree with his fellow quantum theorists. In fact, many of the ideas that they proposed for quantum mechanics sounded preposterous to Schrödinger, especially one of quantum mechanics' most famous features: superposition.
What is quantum superposition?
Quantum superposition is a feature of quantum mechanics where a particle can exist in more than one quantum state, and it is only when a particle is measured that its definite state can be determined.
Understandably, this adds a layer beneath physical reality that strikes many people as either counterintuitive or painfully obvious.
On the one hand, it hardly seems revolutionary to say that you can't determine a particle's state until you measure it. You can't determine your height until you measure it either, so what's the big deal?
The difference between the two is that you are a certain height, whether you measure it or not. If your height had the quantum property of superposition, you would not have a definite height at all prior to measurement.
Generally speaking, you would have an entirely even chance of being in any given measurable state, so if we restricted that to just the five-foot range, you would have a 1-in-12 chance of being five feet and one inch tall, five feet and two inches tall, and so on, but you wouldn't be any of those heights until we measured you.
This latter part cuts against our own lived experience since we never encounter something in our day-to-day lives that exist in such a superposition. When you descend in scale enough to be dealing with individual atoms and even smaller particles, not only is superposition possible, it's been verified time and again over the decades.
What is the Copenhagen interpretation?
The Copenhagen Interpretation of quantum mechanics isn't one thing specifically, but an assortment of ideas about quantum theory that are closely associated with two major founders of quantum mechanics, Neils Bohr and Werner Heisenberg.
What matters for us is the idea that Bohr postulated in the 1930s that a quantum particle and the instrument used to measure that particle do not act independently of each other, but rather become inextricably linked in the process of taking the measurement.
This has led to the common generalization that a particle "knows" that it is being watched and responds to the presence of an observer by defining its state so it can be measured.
This directly contradicts very basic principles of classical physics and logic, and it's what so flummoxed Schrödinger that he developed his famous thought experiment to show just how absurd that idea is.
What is the Schrödinger's Cat thought experiment?
In order to show that a particle can't be linked to the observer on a quantum level, Schrödinger devised the idea of a diabolical device in a box. Inside the box, there is Schrödinger's Cat, as we now know it, but there is also a Geiger counter wired to a hammer.
There is also a sealed glass bottle containing poison gas and a tiny amount of a radioactive substance. Quantumly, that substance can either decay or not decay at any given moment.
If the substance decays, the Geiger counter detects the radiation and triggers the hammer to break the glass bottle, releasing the gas into the box, which would in turn kill the cat. If the substance does not decay, nothing happens and the cat remains alive.
But, because of the principle of superposition, the substance can both decay and not decay, so the Geiger counter is both smashing the bottle and not smashing the bottle, and Schrödinger's cat is both alive and dead, all at the same time.
The Copenhagen interpretation would therefore imply that it isn't until the experiment is observed by opening the box that the quantum state of decay or not decay is decided, so it is only after opening the box that the true fate of the cat inside is settled.
How can a cat be alive and dead at the same time?
This question is exactly what Schrödinger was getting at with his thought experiment. The implications of the Copenhagen interpretation simply aren't logical when applied to his cat in a box.
The proposed outcome does not match our reality, and so Schrödinger and other opponents of the Copenhagen interpretation argued that it was straying away from science and entering the world of philosophy and metaphysics.
An important distinction that needs to be made is that Schrödinger was not saying that quantum superposition isn't real.
He was trying to illustrate that the human observers of the experiment are not the deciding factor, since any interaction with a particle in superposition by just about anything can count as an observation in the quantum sense.
Long before a human ever opens the box, the fate of Schrödinger's cat had already been decided by the Geiger counter.
Of the Copenhagen interpretation, Einstein, writing to Schrödinger in 1950, said;
this interpretation is, however, refuted, most elegantly by your system of radioactive atom + Geiger counter + amplifier + charge of gun powder + cat in a box, in which the [quantum wave-function] of the system contains the cat both alive and blown to bits. Is the state of the cat to be created only when a physicist investigates the situation at some definite time? Nobody really doubts that the presence or absence of the cat is something independent of the act of observation.
As Dr. Christopher Baird, an assistant professor of physics at West Texas A&M University writes: 'quantum state collapse is not driven just by conscious observers, and 'Schrodinger's Cat' was just a teaching tool invented to try to make this fact more obvious by reducing the observer-driven notion to absurdity. Unfortunately, many popular science writers in our day continue to propagate the misconception that a quantum state (and therefore reality itself) is determined by conscious observers."
So now you know the real story behind Schrödinger's cat, but don't worry, quantum mechanics is weird enough without having to resort to a feline multiverse.