What is Schrodinger's Cat? The Thought Experiment Explained

What is Schrodinger's Cat?

Schrodinger's Cat is a hypothetical thought experiment created in 1935 by a man who loved physics and hated cats.

A brilliant scientist, Erwin Schrodinger is also famous for winning a Nobel Prize, and thrusting physics onto biologists in his highly influential book, What is Life?

But you're here for the cat. So let's see how this oddball thought experiment works and what it really says about quantum theory.


Schrodinger's Cat is an Arsehole

You may already know that there's a 50/50 chance that Schrodinger's Cat will be fatally poisoned. So rather than make him cute and pettable, I made him a grumpy arsehole of a cat.

Schrodinger's Cat Comic

Here he is literally stealing candy from a baby.

Cartoon of Schrodinger's Cat stealing candy from a baby

And here he is trying to convince the baby's mother that vaccines cause autism. Like I say. Total arsehole.

Cartoon of Schrodinger's Cat posing as a doctor

Classical vs Quantum Physics

Before we can appreciate Schrodinger's Cat, we need to get to grips with the conflict between classical and quantum physics. Let's take this chronologically.

For instance, the Moon's orbit is deterministic. Various contributing factors like gravity and inertia can be used to make successful predictions about its future position. If we had all the relevant data, we could know in advance whether the moon will be struck by an asteroid, or crash into the Earth, or explode into fondue.

Determinism explained

Causal determinism says the future is a foregone conclusion, dictated by antecedent events and the laws of nature.

Determinism says there's an unbroken chain of events that go all the way back to the Big Bang, and all the way forward to the end of the universe. Everything is causally connected and therefore predictable—at least in principle.

For 200 years, many observations of the physical world fit with Newton's laws, giving determinism a very solid grounding with physicists.

Cue quantum theory and its examination of the universe at much, much smaller scales.

It all started in 1900, when Planck realised that energy is quantized in discrete packets. Planck's constant, which defines a photon's energy, would fundamentally violate classical law.

Once Planck opened the door to this new reality, a flurry of excitable physicists burst through.

As new principles of physics emerged, the maths kept saying the same thing. While classical events are deterministic, quantum events are probabilistic.

That's a lot to unpack. And our intuitive response can lead us down some dodgy alleys.

For instance, if we compare a quantum system to a horse race, then the act of watching the race is what determines the winner. Alternatively, not watching causes all kinds of chaos: the race doesn't occur, and/or all the horses win, and/or all the horses explode into fondue. This is a problem.

Probability explained

Probability means the future can't be predicted with certainty; any given outcome can only be assigned a mathematical chance of happening.

The deeper we contemplate probability, the more troubling it becomes. If our classical world behaved this randomly, then hitting the brake pedal wouldn't necessarily cause our car to stop. (This isn't even touching on the idea of an observer-determined reality. This is just classic Newton.)

Yet our classical world is entirely constructed of quantum particles. How can reality exist in this conflicted state?

Schrodinger's felinicidal fantasy was an expression of all these frustrations and more. He put a classical object (a cat) alongside a quantum object (an atom) to highlight the paradox of these interdependent systems following fundamentally different rules.

But before we see Schrodinger's Cat explained, let's look at an experiment that shows the quantum world behaving randomly.

The Double Slit Experiment

The famous Double Slit experiment shows how quantum particles can behave either deterministically or probabilistically depending on whether they're being measured or observed.

Here's what you do:

  • Fire photons of light at a barrier with two equal slits
  • Most of the photons smack into the barrier
  • The rest are measured as they pass through the slits
  • Two lines accumulate on the photon detector

Here's the top-down view.

The Double Slit experiment shows photons behaving as deterministic particles when they're being measured

The Double Slit experiment shows photons behaving as deterministic particles when they're being measured.

This is a nice predictable result. Let's see that detector screen front on. It shows a population of photons behaving like deterministic particles.

Two equal lines show the 50/50 distribution of light, explained by photons behaving as particles

Two equal lines produce an equal distribution of light, explained by photons behaving as particles.

Now you tweak just one variable: remove the measurement device at the slits. You won't know which slit the photons pass through, only where they land on the detector screen.

You switch on your photon gun, then leave the room and grab a cup of coffee. This is what you see when you get back.

Multiple lines show the wider distribution of photons behaving like waves

An interference pattern.

What's this? Who the devil has been messing with your experiment?

The quantum overlords, that's who. Without directly measuring the photon paths, they switched from behaving like deterministic particles to probabilistic waves. This is wave-particle duality.

Illustration of the Double Slit Experiment: without observation, photon waves produce a broad probability distribution

The Double Slit experiment shows photons behaving as probabilistic waves when they're not being measured.

Each photon travels as a wave of probabilities, embodying all the possible routes it can physically take. Each wave is then refracted by the slits, allowing it to interact with itself to create an interference pattern on the detector.

So the photons are being a little bit naughty here. But why does light only play with itself when no-one's looking?

It's partly explained by a confounding variable called the measurement-disturbance effect. Your measurement device emits particles which bounce off the photons in order to track their position. In doing so, it changes the system, and you get a different result.

Indeed, less precise forms of measurement produce weaker degrees of interference. This is how the act of observation can change the outcome of an experiment. It's about the one thing that physicists and psychologists have in common.

Panic over, right? The measurement effect is just an artefact.

But it's not enough. In the strongest case, the measurement-disturbance effect only explains half of the uncertainty. There's still a mystery factor making the quantum world appear fickle.

And it's not just photons of light that demonstrate this observer-dependent duality. The Double Slit experiment takes the same course with electrons, showing that physical matter can also break free from determinism for no apparent reason.

The Copenhagen Interpretation

To explain these observations, Bohr and Heisenberg brought us the Copenhagen Interpretation. For Heisenberg, it rested on a sharp distinction between a system and its observer. For Bohr, the subjective observer was irrelevant; it came down to some irreversible process within the system itself.

Collapse theories make two claims:

  • Quantum events are genuinely random and probabilistic
  • Reality unfolds according to some kind of action at a distance

Einstein agreed the underlying maths was sound. But he refused to accept the interpretation that nothing is real until we observe it.

Albert Einstein cartoon: I like to think the moon is there, even if I am not looking at it

"I like to think the moon is there, even if I am not looking at it." - Albert Einstein

It's creepy as hell. But the bigger problem is Heisenberg's observer effect. How did the universe form in the first place without any observers? Is objective collapse sometimes possible?

There are still gaps in this argument waiting to be filled. As a result, the Copenhagen Interpretation remains one of the most commonly taught views of quantum mechanics today.

The Many-Worlds Interpretation

Building on the earlier work of Schrodinger, Everett formalised an alternative explanation that allows the quantum world to be rigidly deterministic. And he only had to invent infinite universes to do so.

Everett's interpretation means we already died an infinite number of times before breakfast. Your proctologist is a famous musician in other material realms. And vice versa.

Cartoon of Kanye West in a parallel universe

I have no idea how to caption this.

While the Many-Worlds Interpretation ditches the problem of the wavefunction collapse, it raises the new problem of parallel worlds. Still, it's the second most popular interpretation of quantum mechanics today.

Quantum Entanglement

There's one more aspect of quantum theory causing physicists to cry themselves to sleep at night.

"Quantum entanglement is the characteristic trait of quantum mechanics, the one that enforces its entire departure from classical lines of thought." - Erwin Schrodinger

Amid the quantum hullabaloo of the 1930s, Einstein, Podolsky, and Rosen published the EPR Paradox. Their thought experiment was designed to show how quantum theory was still terribly silly and therefore woefully incomplete.

The EPR Paradox shows that quantum theory allows us to measure the state of a photon without directly disturbing it. How? By taking the measurement from its entangled twin that lives very far away.

Einstein hated this idea because it violated the local realism view of determinism. He derided it as "spooky action at a distance".

Quantum entanglement explained

Quantum entanglement violates Einstein's idea that nothing can travel faster than the speed of light, including the simultaneous exchange of dinner plans between photons on different sides of the planet.

When Schrodinger read about the EPR Paradox, he wrote to Einstein suggesting the phrase quantum entanglement. Both agreed it was a crazy hypothetical implication of quantum theory. And yet it turned out to be completely real.

In 1964, Bell did the maths to show how our world may indeed be non-local, with interactions too far apart in space, and too close together in time, to be connected by signals moving at the speed of light.

Bell's work inspired many experiments which proved quantum entanglement between photons, neutrinos, electrons, buckyballs, and even small diamonds. Indeed, entanglement now has practical applications in cryptography and microscopy, and work is underway to develop an ultrasecure quantum internet.

What is Schrodinger's Cat?

I promised you a cat. Not just any cat, but one that's created and destroyed in order to undermine the Copenhagen Interpretation.

In his thought experiment, Schrodinger created a closed system of:

  • An angry cat inside a lead box
  • A hammer suspended over a vial of poison
  • A Geiger counter
  • A radioactive atom with a 50/50 probability of decaying
Schrodinger's Cat Cartoon

The initial conditions of the Schrodinger's Cat thought experiment.

This may look like a convoluted setup. But being a man of scientific rigour, Schrodinger's idea was to create a set of circumstances in which the cat's fate is probabilistic and therefore unknowable.

There are two possible states for Schrodinger's Cat:

  1. He's alive. The atom didn't decay, the hammer didn't fall, and the poison wasn't released. Hooray.
  2. He's dead. The atom decayed, the hammer fell, and the poison was released. Sad face.
If the radioactive atom decays, Schrodinger's Cat is doomed

If the radioactive atom decays, Schrodinger's Cat is doomed.

Schrodinger's Cat: The Copenhagen Interpretation

Heisenberg's view means that, without an observer intruding on this closed system, the cat's life hangs in the balance. The atom is suspended in a state of quantum superposition, taking the hammer, the poison, and the cat along for the ride.

Schrodinger's cat is neither dead nor alive. Until the wavefunction collapses, his entire existence has become hypothetical.
The Copenhagen Interpretation says that Schrodinger's cat is neither dead nor alive; he has simply ceased to exist

The Copenhagen Interpretation says that Schrodinger's Cat is neither dead nor alive; he has simply ceased to exist.

While Heisenberg believed that only an outside observer could collapse the superposition, Bohr looked to some objective element within the system.

Either way, this triggers the wavefunction to collapse, revealing whether the atom has officially decayed, and whether the cat has officially become dead.

We all know it's nonsense to declare that something has become dead. It's just bad grammar. That's the whole problem with quantum theory, isn't it? It breaks all our comfortable rules.

Schrodinger thought so too. As Einstein once bemoaned, reality doesn't stop existing just because we're not looking at it. So he took a classical object (a cat), tied it to a quantum particle (a radioactive atom), and called out Heisenberg.

"This is bullshit," Schrodinger pointed out. Except he was Austrian, so he would have said: "Das ist Kuhscheiße."

Schrodinger wanted to illustrate how easy it is to arrive at absurd conclusions if we follow incomplete theories of quantum mechanics. However, somewhat ironically, he did admit that his own alternative explanation would also "seem lunatic".

Schrodinger's Cat: The Many-Worlds Interpretation

In 1952, Schrodinger made an early reference to the multiverse. He proposed that quantum superpositions are "not alternatives but all really happen simultaneously". This is what inspired Everett's Many-Worlds Interpretation a few years later.

Schrodinger and Everett both treated the wavefunction as mathematical theory and physical reality.

According to Many Worlds, there's no need for the wavefunction to collapse and deliver a single outcome. After an hour in the box, the cat is alive in countless universes and dead in countless others. All theoretical probabilities play out as real events.

The Many Worlds Interpretation says Schrodinger's Cat is both dead and alive in infinite parallel universes

The Many Worlds Interpretation says Schrodinger's Cat is both dead and alive in infinite parallel universes.

Horrified? Confused? Disturbed? Great. Welcome to the world of quantum theory.

Final Thoughts

While we're still talking about it almost 90 years on, Schrodinger's Cat is not a great critique of quantum theory. The whole wait-an-hour scenario is misleading because it's actually very difficult to maintain quantum indeterminacy even for brief periods.

For instance, the Geiger counter is a valid observer. Even Schrodinger's Cat himself is an observer. Our nosing in the box an hour later makes no difference to the cat's fate.

So don't get too hung up on Schrodinger's Cat. Physicists are over it. For the rest of us mortals, this colourful scenario holds no magical answers, but can serve as an entry point to learning more about quantum theory.

Lastly, I didn't enjoy drawing the dead cat. And an infinite number of dead cats really isn't my jam. So how about putting a different character in our quantum-murder fantasy?

Schrodinger's Trump Cartoon

That's the spirit.

Becky Casale Author Bio

Becky Casale is a science blogger based in Auckland. If you like her content, please share it with your friends. If you don't like it, why not punish your enemies by sharing it with them?