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. But what does the ubiquitous feline have to do with Einstein, Heisenberg, and quantum theory?
A brilliant scientist, Erwin Schrodinger was a Nobel Prize laureate, famous for creating the quantum model of atoms. He also thrust 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.
Classical vs Quantum Physics
Newton's classical laws of motion and mechanics taught us that we live in a clockwork universe. The physical world is entirely predictable (at least, in principle) with all events tightly bound by cause and effect.
For instance, the Moon's orbit is deterministic. Classical factors like gravity and inertia dictate its future position. With all the relevant data, we can know in advance exactly where the moon will be.
Causal determinism says there's an unbroken chain of events that goes all the way back to the Big Bang, and all the way forward to the end of the universe. Everything is connected.
But when quantum physicists began examining the universe at much smaller scales, they arrived at a different conclusion. They found that future events are unknowable, and can only be forecast in terms of probabilities.
If this quantum law played out at the classical level, we'd have big problems...
How is this possible when the rulebound classical world is literally made of quantum particles? Is everything Newton said suddenly wrong?
No, classical mechanics isn't wrong. Satellites won't fall out of orbit. The Golden Gate Bridge won't suddenly collapse. Seat belts won't inexplicably fail.
But that doesn't mean quantum mechanics is wrong, either. Quantum technologies are ticking along just fine, from atomic clocks, to quantum cryptography, to entanglement-enhanced microscopes.
The apparent conflict between these two domains just means that scale matters—and there are unknown factors we don't yet understand.
And so Schrodinger's Cat is merely a thought experiment that expresses this in a specific scenario. The point is we can't slap quantum interpretations onto the classical world and hope it all makes sense. It doesn't. In fact, it violates the hell out of classical sensibilities.
Instead, we have to accept that the quantum world (at the size of atoms and below) operates according to probabilistic laws. Meanwhile, the classical world (at the size of molecules and above) conforms to deterministic laws.
Now let's mix things up even more. Because quantum particles aren't always random in their behaviour. Sometimes they're completely predictable. And it all seems to hinge on whether or not we're measuring them.
The Double Slit Experiment
Here's a concrete example where we can see light behaving predictably or randomly based on what's occurring in its environment. Here's what you do:
- Fire individual photons of light at a barrier with two slits
- Measure the photons as they pass through the slits
- See two lines on the detector as the photons accrue
This is an intuitive result if we think of photons as discrete units. If you scaled everything up and threw darts at the slits, you would expect the same double slit pattern to emerge.
Now you tweak just one variable: stop measuring the photons in transit. You're only going to detect where the photons end up.
So you switch on your photon gun, leave the room and grab a cup of coffee. This is what you see when you get back.
What's this? Who the devil has been messing with your experiment?
The quantum overlords, that's who. Without real-time measurement, the photons of light switch from particle-like to wave-like behaviour. This is called wave-particle duality.
Quantum theory says that each photon travels as a wave of probabilities, embodying all possible routes to the detector screen. En route, the waves interact. They can combine to form peaks, or to cancel each other out, ultimately creating an interference pattern.
So far, so crazy. But why does light only play with itself when no-one's looking?
The Uncertainty Principle
The best explanation for the Double Slit experiment is the measurement-disturbance effect.
There are various ways to measure single photons in real-time, like photo-detectors, photomultipliers, or single-photon detectors. However, any kind of measurement device inherently disturbs the photons in transit.
At the quantum scale, taking a measurement means bouncing other quanta off your target. This gives you data on a photon's position, but in doing so imparts energy that changes its momentum.
The observer effect is often misunderstood to mean that a conscious observer can change quantum systems from afar. There's zero evidence for this. In physics, an observer is a quantum-scale measurement tool.
So humans can affect the quantum world with precise measuring technology, but not with our brains or our eyes. Unless you have laser eyes, because that changes everything.
Panic over, right? The observer effect is just an artefact!
Unfortunately, there's still a gap in the science. To date, experiments have found that the measurement-disturbance effect explains only half of the influence predicted by the Uncertainty Principle.
There is still a mystery factor directing the quantum world.
The Copenhagen Interpretation
In the 1920s, Bohr, Heisenberg, and Born brought us the Copenhagen Interpretation of quantum mechanics.
Collapse theories hold that quantum particles are intrinsically random, yet exist only mathematically while in superposition.
The Copenhagen Interpretation is conflicted when it comes to isolating what causes the wavefunction collapse. Heisenberg insisted on a sharp cut between a quantum system and an observer. Bohr said the collapse had to be local; some irreversible process within the system itself.
Einstein agreed the underlying maths was sound, but refused to accept the interpretation that nothing is real until it interacts with something else—locally or otherwise. He proposed there might be hidden variables to explain quantum phenomena; a concept that remains viable to this day.
Heisenberg's need for an outside observer just throws up more questions. How did the universe form without anything looking in? Is objective collapse is sometimes possible?
Whichever angle you take, the Copenhagen Interpretation won huge favour in the 1930s, and while Einstein's concerns were never fully addressed, it remains the most commonly taught view of quantum mechanics today.
The Many-Worlds Interpretation
Inspired by Schrodinger, Everett formally pushed back with an alternative explanation. And he only had to invent infinite universes to do so.
Everett imagined a universal wavefunction governing all possible realities. As the superpositions break down, they unravel from one another and continue to exist in separate universes.
If interpreted at the classical scale, the Many-Worlds interpretation implies you already died a near-infinite number of times before breakfast. The best and worst-case scenario (and everything in between) plays out in every moment of your life. Your proctologist is a famous musician in other material realms—and vice versa.
But let's not blend the rules between quantum and classical domains without good reason. Remember, scale matters.
Although the Many-Worlds Interpretation ditches the problem of the wavefunction collapse, it raises the new problem of near-infinite parallel worlds.
However, the MWI does offer some tantalising logic that cures us of quantum randomness, action at a distance, and the observer effect. It's a deterministic theory for a physical universe that also explains why the world can seem indeterministic.
Incidentally, the Many-Worlds theory is not to be confused with the Multiverse Hypothesis, which is the idea of other universes born of separate Big Bang events. Many Worlds is much, much crazier.
I promise Schrodinger's Cat is coming. But there is one more aspect of quantum theory causing physicists to cry themselves to sleep at night. It's called quantum entanglement.
"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 must therefore be incomplete.
The EPR Paradox shows that, despite the uncertainty principle, quantum theory still 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".
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 entirely real.
In 1964, Bell's maths showed the quantum world to 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 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.
I said there would be 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:
- A radioactive atom with a 50:50 probability of decaying within the hour
- A Geiger counter measuring the atom's radiation
- A hammer suspended over a flask of acid
- An unimpressed cat
Being a man of scientific rigour, Schrodinger's idea was to create a set of circumstances in which the cat's fate is entirely dependent on quantum probability.
After one hour, there are two possible states for Schrodinger's Cat:
- He's alive. The atom didn't decay, the hammer didn't fall, and the acid wasn't released.
- He's dead. The atom decayed, the hammer fell, and the acid was released. Sad face.
Heisenberg's Copenhagen Interpretation means that without a separate observer, the cat's life hangs in the balance. The atom is suspended in a state of quantum superposition, taking the hammer, the acid, and the cat along for the ride.
If Heisenberg is right, Schrodinger's Cat is neither dead nor alive. Its entire existence has become blurry and hypothetical until the system is observed.
Schrodinger argued such an idea was naïve and ridiculous. As part of the classical world, the cat would not simply fall into limbo until an observer determines whether it has 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.
But Schrodinger thought it was all too much. So he called out Heisenberg.
"This is bullshit," Schrodinger pointed out. Except he was Austrian, so he would have said: "Das ist Kuhscheiße."
Ok, here's what he really said:
Schrodinger wanted to illustrate how easy it is to arrive at absurd conclusions if we follow incomplete or inaccurate interpretations of quantum mechanics. It was ironic, then, when he admitted his own explanation would also "seem lunatic".
In 1952, Schrodinger made an early reference to a type of multiverse. He proposed that quantum superpositions are "not alternatives but all really happen simultaneously", inspiring Everett's formal Many-Worlds Interpretation a few years later.
Schrodinger and Everett both treated the wavefunction as mathematical theory and physical reality.
According to Many Worlds, the cat is alive in countless universes and dead in countless others. All probabilities actually occur as real events.
Does quantum randomness play out at the classical scale, let alone in the form of parallel universes? That's the problem facing physicists and philosophers today.
Confused? Disturbed? Horrified? Great. Welcome to the world of quantum theory.
Where does Schrodinger's Cat leave us? In practice, it's very difficult to maintain quantum indeterminacy for tiny fractions of a second, let alone for an hour while we wait to seal our hypothetical cat's fate.
If Schrodinger's Cat was real, the Geiger counter would interfere as a measurement device at the quantum scale. According to Bohr, so would any interaction between the radioactive atom and its quantum neighbours.
So don't get too hung up on Schrodinger's Cat. It's a hypothetical thought experiment we can't actually test. Physicists are apparently over it, while the rest of us mortals use it as a weird and wonderful entry point to quantum theory.