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Understanding Schrödinger’s' cat

Everyone has heard of the most famous thought experiment in history, Schrödinger’s Cat. This experiment entails a cat that is put into a box with a harmful radioactive substance. After closing the box, the cat can be thought of as both dead and alive. Its living state is a mere probability. But what exactly makes it so special and significant?

Erwin Schrödinger was one of the most influential physicists of the 1920s, paving a new path for physics - a new era of discoveries: quantum physics. In 1926, Schrödinger used Einstein's theories alongside those of two other incredible physicists, Louis de Broglie and Max Planck, to create the Schrödinger wave equation, H𝚿 = E𝚿. This equation is able to describe the momentum, position, and time, as well as other more complicated terms (such as spin) of a quantum system. Quantum systems are systems that follow the rules of quantum mechanics; microscopic objects do not follow the same physics as large objects! For example, electrons do not behave in the same way as footballs.

A fundamental principle that Schrödinger used to create this new equation was wave-particle duality. Scientists for many years did not know whether a particle, such as an electron, was a wave (forms of energy) or an object that contains mass. It turns out that particles act as both waves and objects with mass! If a photon (a particle of light) is travelling down a path, it will travel as a wave, but when it hits an object such as a detector, it will behave as a particle. 

This dual behaviour was famously demonstrated in the double-slit experiment, first performed by Thomas Young in 1801 to show the wave-like behaviour of light. However, it wasn’t fully understood and accepted until the work of Louis de Broglie in 1924, who extended wave-particle duality to all particles, including electrons.

The double-slit experiment only requires a dark room, a monochromatic light source (one colour), a barrier with two vertical slits, and a flat white sheet. You would then place the barrier with two slits at an equal distance from both the light source and the white sheet. Turning on the light, you would find that photons travel through the two slits, yet they produce more than two lines on the white sheet; they could produce three or four, or even seven, depending on how narrow you make the slits. But why is that?

Superposition is the ability of a quantum system, such as a photon, to occupy multiple different states until measured. In the case of the double-slit experiment, each photon is in a superposition of travelling through both slits at the same time, forming something known as an interference pattern. However, when we try to measure which slit each wave goes through, this superposition collapses, and the photons no longer act as waves but rather as particles, leading to only two rays on the white sheet.

The significance of the Schrödinger equation, alongside wave-particle duality and superposition, helps physicists describe how quantum systems function over time. Scientists are now able to think of electrons and other particles not as physical objects, but as probabilistic regions of space where waves or particles occupy at a given moment in time.

The Schrödinger cat analogy was first made to demonstrate that comparing quantum mechanics to classical mechanics is not doable. Schrödinger believed that they both operate completely differently. The notion that the cat is both dead and alive can work when speaking about microscopic objects such as atoms. When referring to large objects such as a cat, quantum mechanics cannot hold true due to a term known as decoherence, which is when a quantum system is changed by its interactions with the physical environment.

This leads to the question of why quantum physics is not able to be modelled with classical physics. Why do microscopic objects operate differently from macroscopic objects?

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The Hidden Physics Behind Mass

The Higgs mechanism is responsible for particles gaining mass by interacting with a field known as the Higgs field that permeates all of space like a dense syrup-like substance. The more a particle interacts with the Higgs field, the heavier it becomes. Imagine particles travelling through this field and being influenced by it in a way that hinders their movement and gives them mass.

In the field of particle physics known as the Standard Model various particles play roles in governing different forces; W and Z bosons oversee the weak nuclear force (involved in radioactive decay) photons transmit the electromagnetic force (responsible for light) and gluons manage the strong nuclear force (essential for nucleus stability). The significance of the Higgs field lies in its ability to give mass to W and Z bosons. Without this aspect these particles would exhibit behaviours inconsistent with our observations. The finding of the Higgs boson, in 2012 was significant as it validated the process by which particles get their mass through this mechanism.