What is Schrodinger's Cat?
Schrodinger's Cat is a hypothetical thought experiment created in 1935 by a man who loved physics and despised cats.
Now, in this thought experiment, there's a 50/50 chance that Schrodinger's Cat will be fatally poisoned. So rather than make him cute and pettable, let's think of him as a grumpy arsehole of a cat. Just look at him.
Here's documented evidence of Schrodinger's Cat literally stealing candy from a baby.
And here he is attempting to convince the baby's mother that vaccines cause autism.
Like I say. Total arsehole.
Schrodinger's Cat Explained
Schrodinger's felinicidal fantasies stemmed from quantum theory, an emerging study of physics birthed by Max Planck in 1900. Planck said that when it comes to miniscule quantum scales, the classical laws of physics no longer apply. This created a big headache for physicists.
Not only did Planck declare that Newton's laws don't apply at the quantum level, he also said the new rules that apply to the quantum world are stupid and illogical. My words, not his.
Part of the problem is determinism: the notion that the outcome of any event is preordained by the events that come before. The moon's orbit is deterministic because we can calculate the gravity and inertia already at play, and use those numbers to make reliable predictions of its future orbit.
Before quantum theory, all signs pointed to us living in a clockwork universe.
But the quantum world is different. Sometimes it's deterministic—and sometimes it's probabilistic.
Probabilities are shown in real numbers ranging from 0 to 1, where 0 means an event is impossible and 1 means an event is certain (ie, determined). The classic example of probability is a coin toss, where the likelihood of a coin landing heads is 0.5.
So while classical physics has us concluding the entire universe is a cascade of deterministic outcomes, quantum physics has us concluding the universe can sometimes succumb to random probabilistic influences. And there's the rub.
How can the universe be both probabilistic and deterministic when the two notions conflict on a fundamental level?
Physicists still to debate the puzzle to this day, with various theories and experiments attempting to explain it. Take the Double Slit experiment...
The Double Slit Experiment
The Double Slit experiment shows how elementary particles behave either deterministically or probabilistically depending on whether or not you're measuring them.
Here's what you do: fire individual photons of light at a barrier containing two slits. Most of the photons smack into the barrier. A few pass through one of two identical slits with a 0.5 probability. Here's the top-down view:
A measurement device placed above the barrier reports which slit the photons go through, while the detector screen confirms this as a probability distribution of two equal lines:
So far, so great. The experiment shows a population of photons behaving in a deterministic way, following the rule that two equal slits produce a probability distribution of 0.5.
Now you tweak just one variable. You take away the measurement device monitoring the slits. Your only data comes from the detector screen, which produces a retrospective analysis of the photon paths.
So 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.
What's this? Who the devil has been messing with your experiment?
The quantum overlords, that's who. Simply by removing the real-time measurement of the photon paths, their behaviour switched from deterministic to probabilistic. Here's how we visualise that:
What Does The Double Slit Experiment Mean?
The Double Slit experiment is an example of quantum uncertainty, introduced simply by the act of measurement. Let's drill into this.
In the first instance, photons being monitored in real-time took the form of pre-determined waves, making them particle-like in behaviour. It's almost as if they know you're watching.
But when you stop measuring the photon paths, they take the form of probabilistic waves. All possible paths that each photon can take are played out as wave functions; a mathematical equation which describe the probable location of each photon. This produces a much broader probability distribution, described as an interference pattern.
It's as if your paranoid particle became a groovy, carefree wave.
Recent Double Slit experiments have sought to hone in on the effect of a measuring device placed at the barrier. After all, this was the only boundary condition that changed. They found that different degrees of measurement created different degrees of interference.
But why does the act of measurement produce quantum uncertainty at all?
It's partially explained by the slit measurement device introducing new particles to the experiment, which interact with the photons and thereby help determine their paths. This was how Werner Heisenberg visualised it when he created his Uncertainty Principle:
The Uncertainty Principle: It's impossible to know both the position AND the momentum of a quantum particle at the same time without affecting it. By measuring one trait, you automatically change the other and determine its path.
However, more recent experiments have found the measurement-disturbance only accounts for about half of the mathematical uncertainty produced. There's another factor at play.
What is this mystery factor? How does the act of observation disturb the fundamental behaviour of light? It's as if the path of a photon is probabilistic until an observer forces it down a deterministic path.
And it's not just light that react this way. The Double Slit experiment is often performed today with electrons, showing that matter can also break the deterministic laws of physics.
Does this mean the unobserved universe isn't real, or exists in potentially infinite places at once, until it comes into view? Is our reality like a computer game, where nothing is rendered until the player observes it?
The Uncertainty Principle Changes Everything
Heisenberg's Uncertainty Principle is such a head-scratcher that quantum physicists are still grappling with its implications today.
The broader conclusion will affect how all of us think about our reality. If elementary particles switch inexplicably between deterministic and probabilistic behaviours, what does this mean for macroscopic entities composed of these building blocks?
Here are two of the most popular hypotheses that attempt to explain the nature of quantum uncertainty:
#1 The Copenhagen Interpretation: Nothing Exists Until We Observe It
The oldest proposal comes from Niels Bohr and Werner Heisenberg. They suggested that quantum particles don't materially exist until we measure them. Instead, they exist only theoretically as waves of possibilities, known as "superpositions", which collapse into a single definitive outcome on observation.
Albert Einstein agreed that the maths behind the Copenhagen Interpretation was sound. But he refused to accept the conclusion:
Since reality itself is made up of quantum particles, then nothing is real until we observe it.
Despite Einstein's indictment, the Copenhagen Interpretation is one of the most commonly taught explanations of quantum uncertainty today.
But there is another possible explanation, said to be favoured by the majority of quantum physicists. And its core tenet is the exact opposite of the Copenhagen Interpretation.
#2 The Many-Worlds Interpretation: All Possibilities Exist in an Infinite Multiverse
Hugh Everett proposed that superposition is objectively real. All possible quantum events exist simultaneously in alternate universes.
Since you're made of quantum building blocks, the Many-Worlds Interpretation implies you died an infinite number of times today before breakfast. It also means you're a famous musician in other universes—and famous musicians have mundane alternative lives.
This extraordinary hypothesis was favoured by Stephen Hawking and Richard Feynman. While the Many-World theory is superb fodder for science fiction, physicists believe there's no way these universes could interact. But we could one day detect them.
Before we get to Schrodinger's Cat, there's one more bizarre aspect of quantum theory causing physicists to cry themselves to sleep at night.
Amid the quantum hullabaloo of the 1930s, Albert Einstein, Boris Podolsky, and Nathan Rosen published the EPR Paradox. Their thought experiment was supposed to illustrate how quantum mechanics violates classical physics, and therefore the theory was incomplete.
According to quantum law, the EPR Paradox pointed out that photons can still share a wave function long after they've been separated.
In other words, quantum particles can become invisibly connected and "talk" to each other instantaneously, even when separated by vast distances. Einstein derided this as "spooky action at a distance".
In his Theory of Relativity, Einstein had already established that nothing can travel faster than the speed of light. And that included the simultaneous exchange of dinner plans between photons on different sides of the planet. Something had to be missing.
When Erwin Schrodinger read the EPR paper, he wrote a letter to Einstein, coining the phrase "quantum entanglement". Schrodinger then published a paper of his own, saying that entanglement is "the characteristic trait of quantum mechanics, the one that enforces its entire departure from classical lines of thought".
Einstein and Schrodinger agreed that quantum entanglement was crazy, and that something was amiss with quantum theory. Neither man would live to see quantum entanglement proven mathematically and experimentally over the next forty years.
In 1964, John Stewart Bell provided a mathematical framework to dissect the EPR Paradox in terms of "hidden variables". His work led to experimental proof of quantum entanglement between photons, neutrinos, electrons, and even buckyballs (cages of sixty interlinked carbon atoms).
In fact, Einstein's "spooky action" is so well mastered today that it has 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. Schrodinger's Cat, to be precise. A cat created to undermine the Copenhagen Interpretation, which states that quantum objects aren't real until we observe them.
In his thought experiment, Schrodinger imagined a cat inside a lead box. Beside the cat there's a hammer suspended over a vial of poison. And triggering the hammer to fall is a single atom of radioactive material that has a 50/50 chance of decaying in the next hour.
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 has a truly random chance of being dead or alive.
Next, you wait an hour before checking in on the feline. At this point, there are two possible states for Schrodinger's Cat:
- He's alive. The atom didn't decay, the hammer didn't fall, and the poison wasn't released. Hooray.
- He's dead. The atom decayed, the hammer fell, and the poison was released. Sad face.
Quantum uncertainty suggests that, without an observer, the cat's life hangs in the balance. The radioactive atom is suspended in superposition of decay and non-decay, the outcome of which determines the cat's fate.
If we apply the Copenhagen Interpretation, Schrodinger's cat is both dead and alive in a hypothetical realm of potential. But neither state is real.
It's only when you look inside the box that you break the superposition, the wave function collapses, and the cat becomes dead or alive.
We all know it's nonsense to declare that something "becomes dead". It's just bad grammar. But that's the whole problem with quantum theory, isn't it? It breaks all our comfortable rules.
Schrodinger thought so too. He said the cat doesn't stop existing just because we're not observing it. He took a classical object, stuck it in a quantum-controlled setting, and called out Niels Bohr.
"This is bullshit," Schrodinger pointed out. Except he was Austrian, so he would have said: "Das ist Kuhscheiße."
Everett's Many-Worlds Interpretation, which came just a few years before Schrodinger's death, aligned far better with his logic. Both said there is no wave function collapse. Everett's idea means that after an hour in the box, the cat is alive in countless universes and dead in countless others.
Collapse or no collapse, modern experiments into quantum uncertainty suggest the cat's fate doesn't reside entirely in the eyes of a conscious observer. Any interaction with other quantum particles—even the cat itself—can determine the outcome. Opening the box just makes it real for you, but that doesn't mean it didn't already happen.
If you're an animal lover, the notion of an infinite number of dead cats probably doesn't feel so great. So feel free to consider an alternative victim in your quantum-murder fantasy. Someone who might actually deserve to be in the box, like a narcissistic megalomaniac?
That's the spirit.