There are few thought experiments in science as famous as Schrödinger’s cat, 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 only thing almost everyone knows: that Schrödinger’s cat is alive and dead at the same time.
But what does that even mean? What logic chain could possibly lead to such a result?
Fortunately, you don’t need a physics degree to understand what Schrödinger was getting at with his thought experiment, and even Albert Einstein praised Schrödinger for developing such a simple illustration of some of the more confusing parts of quantum mechanics.
So in short, don’t worry. The Schrodinger’s cat thought experiment is not nearly as complicated as many seem to believe, and a proper understanding of Schrodinger’s cat is an essential part of understanding the fundamental features of the bizarre quantum realm of physics.
Who was Erwin Schrodinger?
Erwin Schrödinger was a Nobel Prize-winning Austrian physicist who was instrumental in developing many fundamental aspects of quantum theory.
In addition to his well-known thought experiment, Schrödinger is best known for his wave equation, which is used to calculate the wave function of a quantum system at different points in time.
Despite playing such a large role in their creation, Schrödinger did not always agree with his colleagues in quantum theory. In fact, many of the ideas they proposed for quantum mechanics sounded absurd to Schrödinger, particularly one of the most famous properties of quantum mechanics: superposition.
What is quantum superposition?
Quantum superposition is a feature of quantum mechanics in which a particle can exist in more than one quantum state, and only when a particle is measured can its definitive state be determined.
Understandably, this adds a layer beneath physical reality that seems either counterintuitive or painfully obvious to many people.
On the one hand, it hardly seems revolutionary to say that you can’t determine the state of a particle until you measure it. You also can’t determine your size until you measure it, so what’s the big deal?
The difference between the two is that you are a definite size whether you measure it or not. If your height had the quantum property of superposition, you wouldn’t have a specific height at all before the measurement.
In general, you would have an entirely equal chance of being in any given measurable condition. So if we narrowed that down to the 5′ range, you would have a 1 in 12 chance of being 5′ tall. five feet two inches tall and so on, but you wouldn’t have any of those sizes until we measured you.
This last part cuts against our own lived experience, since we never encounter anything that exists in such a superposition in our daily lives. When you scale down to deal with individual atoms and even smaller particles, not only is superposition possible, this has been verified time and time again over the decades.
What is the Copenhagen Interpretation?
The Copenhagen interpretation of quantum mechanics is nothing special, but rather a collection of ideas on quantum theory closely associated with two great founders of quantum mechanics, Neils Bohr and Werner Heisenberg.
What matters to us is the idea, posited by Bohr in the 1930s, that a quantum particle and the instrument used to measure that particle do not act independently of each other, but are inextricably linked in the process of measurement.
This has led to the common generalization that a particle “knows” that it is being observed and responds to the presence of an observer by defining its state so that it can be measured.
This directly contradicts the fundamental principles of classical physics and logic, and this so baffled Schrödinger that he developed his famous thought experiment to show how absurd this idea is.
What is Schrödinger’s cat thought experiment?
To show that a particle cannot be linked to the observer at the quantum level, Schrödinger came up with the idea of a diabolical device in a box. Inside the box is Schrödinger’s cat as we know it today, but 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. Quantum-wise, this substance can either decay or not decay at any point in time.
As the substance breaks down, the Geiger counter detects the radiation and triggers the hammer to shatter the glass bottle, releasing the gas into the box, which in turn would kill the cat. If the substance doesn’t break down, nothing happens and the cat stays alive.
But due to the principle of superposition, the substance can both decay and not decay, so the Geiger counter smashes the bottle and doesn’t smash the bottle, and Schrodinger’s cat is alive and dead at the same time.
The Copenhagen interpretation would therefore imply that it is only upon observing the experiment by opening the box that the quantum state of decay or non-decay is decided, so only after the box is opened is the true fate of the cat inside clarified.
How can a cat be alive and dead at the same time?
This is exactly the question Schrödinger aimed at with his thought experiment. The implications of the Copenhagen interpretation just don’t make sense when applied to your cat in a box.
The proposed result does not correspond to our reality, and so Schrödinger and other opponents of the Copenhagen interpretation argued that it moved away from science and entered the world of philosophy and metaphysics.
An important distinction to make is that Schrödinger was not say that quantum superposition is not real.
He tried to illustrate that the human observers of the experiment are not the decisive factor, since any interaction with a particle in superposition with just about anything can be considered an observation in the quantum sense.
Long before a person opens the box, the fate of Schrödinger’s cat has already been decided on the Geiger counter.
Einstein wrote to Schrödinger about the Copenhagen interpretation in 1950:
however, this interpretation is most elegantly refuted by your system of radioactive atom + geiger counter + amplifier + charge of gunpowder + cat in a box, in which the [quantum wave-function] of the system contains the cat both alive and in pieces. Should the state of the cat only arise when a physicist examines the situation at a certain point in time? No one really doubts that the cat’s presence or absence is something independent of the act of observation.
like dr Christopher Baird, assistant professor of physics at West Texas A&M University, writes: “The collapse of the quantum state is not only driven by conscious observers, and “Schrödinger’s cat” was just a teaching device invented to make this fact more obvious by reducing the observer-controlled imagination to absurdity. Unfortunately, many popular science writers of our time continue to propagate the misconception that a quantum state (and thus reality itself) is determined by conscious observers.”
So now you know the real story behind Schrodinger’s cat, but don’t worry, quantum mechanics is weird enough without resorting to a feline multiverse.