Wait, what is… Schrödinger’s cat?

Picture this: physicists locked in a room, arguing about quantum mechanics. Equations over a chalkboard, voices are being raised and then... in walks a cat. To prove their point, one of the physicists grabs the cat and puts the poor guy into a box containing a special device that releases poison at a random time. ‘See!’ the kitnapper argues, ‘It's now alive and dead!’. And so ends the tale of Schrödinger’s cat — now, thankfully the cat never really existed. But wait... what is it?

Quantum Mechanics: The Early Years

To understand what this all means, let’s travel back to the early 1900s. A German physicist named Max Planck had solved an issue called the blackbody radiation problem by proposing that light is absorbed in little “packets” of energy — quanta. He had treated light as tiny particles, rather than as a wave. This was confusing — light had been proved to be a wave 100 years before. Planck thought his solution was nothing more than a mathematical trick.

A few years later, a young Albert Einstein used Planck's theory to explain the photoelectric effect, where shining a light on a material induces a release of electrons. It was this that earned Einstein his only Nobel Prize — nope, he didn't get it for relativity! — and suggested that “quantum” ideas might show something more about nature.

And it didn't stop there. Niels Bohr applied the quantum ideas of Planck to the hydrogen atom and successfully predicted observations available at the time. Electrons no longer orbited randomly*: they had to occupy certain “positions” around the atomic nucleus — these are the electron energy levels.

Finally, Louis de Broglie proposed that particles, like electrons (read more here), could behave as waves. Thus kicked off the era of modern quantum mechanics, where matrix and wave mechanics emerged as mathematical tools to describe the quantum world. Don't worry too much about the details of these — just remember they are two ways of describing the same bizarre reality.

By the 1930s, the equations worked beautifully — but their meaning was a philosophical minefield. Physicists could describe quantum phenomena, but what did it all mean? This question sparks intense debates — and inspired one of the most famous thought experiments in science.

Schrödinger's Unlucky Cat

Physicists wanted to understand the implications of quantum mechanics. What did it say about reality? Bohr and Heisenberg thought the theory was complete, with all of its uncertainty and randomness baked into reality. Many physicists, Einstein included, published papers questioning this philosophy, known as the Copenhagen interpretation.

Erwin Schrödinger, the inventor of wave mechanics, certainly did not agree. To critique the interpretation, he devised his famous thought experiment. It goes something like this:

1. Place a cat in a box (hopefully, the cat likes this).

2. Place a vial of poison inside the box (the cat does not like this).

3. Connect the vial to a radiation detector aimed at a single, radioactive, particle.

4. Close the box.

5. If the particle decays, the detector breaks the vial. Bye-bye kitty cat.

6. If not, the cat remains unharmed.

Remember, radioactive decay is random. If the particle has a 50% of decaying in one hour, no-one has any way of knowing whether the cat is alive or dead util the box is opened. The cat's fate is entangled with the particle: what happens to one, affects the other.

Here's where things get strange (and highly concerning) for the cat. Quantum mechanics doesn't just say we don't know the cat's state — according to the equations, the cat is both alive and dead simultaneously until observed. Imagine explaining that to your vet!

Sound crazy? Schrödinger thought so too. He was clear: quantum mechanics is brilliant... for tiny particles. If you scale it up to macroscopic objects, it becomes absurd. Our everyday world doesn't have half-zombie cats.

Incoherent Cats

To understand this apparent paradox, a cat being both dead and alive, physicists talk about superposition. If you take a coin and lay it on a table**, it will show either heads or tails — it has a definite state. Now flip the coin into the air — while it's spinning, the coin is not one or the other, it can be thought of as both at once. A quantum particle in superposition is an endlessly spinning coin: it genuinely exists in multiple states simultaneously until something interacts with it — the coin has landed on the table.

Schrödinger took this idea to an extreme. The particle can be in a superposition of “decayed” and “not decayed”, but when the cat's fate depends on it, the whole cat becomes a part of the superposition. This is why the experiment is so odd — we never see everyday objects behaving like this.

So why don't we see cats in superposition? The answer is decoherence. Think of it like dropping a single drop of ink into a large fish tank of clear water. You can initially see the ink droplet clearly as it falls — but if you swirl the water, the droplet disperses, blending into the water until it looks clear.

Quantum states behave similarly. When a quantum system interacts with its environment (air molecules, photons, other particles), its quantum behaviour entangles with the environment. The “coherent” quantum effects of small particles cancel each other out in large systems, and leave the macroscopic world behaving as we expect. Just like ink dispersing in a tank, the quantum superposition doesn’t disappear — it spreads into the environment. This is why we don't see “superposed” cats, even though the maths says they could be.

Quantum Cats in Reality

Though we don't see quantum cats, it doesn't mean the principles present in the thought experiment aren’t useful. Superposition and entanglement are the key to both quantum computing and quantum cryptography. Quantum computers would work by accessing huge numbers of quantum states simultaneously, thus computing power can be exponentially large. However, current quantum computers are severely limited due to decoherence — so don’t be expecting a usable quantum computer in your house anytime soon.

Schrödinger’s cat also shows why interpretation matters so much. Quantum mechanics has many different interpretations, all of which give exactly the same results and are therefore no more correct that the others. But it does change the way we see things! The Copenhagen interpretation says the cat isn't really dead or alive until observed. The many-worlds interpretation suggests that in fact both outcomes occur — but these happen in separate branches of reality that never interact. It's still a hot topic of debate as to which interpretation is correct, but most physicists have a preference.

Summary

Quantum mechanics is deeply counterintuitive. It works amazingly in our calculations and labs, but scaling it up poses a huge problem — and this is what Schrödinger’s Cat highlights. This imaginary cat highlights that at the smallest level, it’s not worth applying common sense, and the universe is under no obligation to make sense.

The cat never existed (we hope), but it has taught us a lot about the strange and, frankly, absurd world of quantum mechanics.