What is Schrödinger’s Cat?

History

Schrödinger's Cat is an iconic thought experiment first proposed by Austrian physicist Erwin Schrödinger in 1935. This concept has been crucial in illuminating the absurd implications that can result from minor differences in interpretations of quantum mechanics.

Schrödinger is often viewed as a pivotal figure in the development of quantum theory, particularly for his wave equation that described how quantum systems evolve over time.

His Schrödinger's Cat thought experiment was originally formulated as a critique of the concept of superposition required by some quantum theories. However, in popular media, it is often misconstrued as an illustration of an inherent property of quantum mechanics.

What is Quantum Mechanics?

As the name implies, Quantum Mechanics involves "quanta"—indivisible units of energy that particles can possess. Here's a quick refresher:

Quantum mechanics is the branch of physics that deals with the behavior of matter and light at the smallest scales, typically at the level of atoms and subatomic particles. The concept of indivisible units of energy—quanta—challenges the classical notion of a continuous spectrum of energy.

The field also introduces the concept of entanglement, where particles become correlated in such a way that the state of one instantaneously influences the state of another, regardless of the distance separating them. Quantum mechanics is fundamental to understanding phenomena at the nanoscale, and it has led to technological advancements such as quantum computing, lasers, and semiconductors. It is a pillar of modern physics, alongside general relativity, and is essential for explaining the behavior of particles in both isolated systems and in interactions with electromagnetic forces.

The Thought Experiment

A thought experiment is a mental exercise to imagine the physical implications of mathematical descriptions. This thought experiment involves the implications of atomic superposition, which we'll explain below. You'll also be able to conduct a version of the experiment yourself with a Schrödinger's Cat Blind Box.

Premise

Imagine a sealed box. Inside of this box is a cat, a glass vial of poison, a Geiger counter, and a radioactive atom.

Trigger

These objects are set up in a way so that if the radioactive atom decays, the Geiger counter will detect the emitted particle. There is no way of knowing when—or if—the atom will decay while the box is sealed.

Upon detection, the Geiger counter triggers the glass vial to be smashed, often by letting it fall or releasing a hammer upon it. When the glass smashes, the poison is released and kills the cat.

Observation

You're outside of the box and can only learn the current state of the contents by opening it. Until your observation, some theories in quantum mechanics say that the radioactive atom is considered in a quantum superposition of both states, decayed and undecayed, simultaneously.

Since the cat's state is tied to the state of the atom, it suggests that the cat must also be in superposition of its states, alive and dead, simultaneously until observed.

Can something be in a superposition of alive & dead, or does this prove the concept is fundamentally absurd?

Explanation

Some tend to get caught up in specific details about the cat, the box, etc. To avoid this, it helps to think of the items involved in the experiment in general terms of the function they serve:

• Sealed Box — Something to prevent any observation of the components of the experiment.
• Radioactive atom — Something theorized to exist in a superposition of two states simultaneously until observed. It can only change states one time.
• Geiger counter — A method of detecting changes in the state of the "atom".
• Cat — Something that can be in one of two states, but not both. It can only change states one time.
• Vial of poison — Something to switch the "cat" from one state to the other when the "atom" is detected to change states.

So the Geiger counter and vial of poison together can be considered a method of changing the state of the "cat" based on the state of the "atom"—a translator from the microscopic to the macroscopic.

In these terms, it may be clearer what Schrödinger is trying to say. By creating a hypothetical—but not impossible—scenario where the state of something that seemingly cannot be in a superposition is directly tied to the state of something that is theorized to exist in a superposition, he is poking a big hole in the concept of superposition.

The thought experiment shows that there are real-world implications to these quantum theories that cannot be ignored.