Disclaimer: I Am Not A Physicist. All errors and inaccuracies are mine alone. Do not use this essay as a reference in your thesis paper. Please consult a professional physicist before using this essay to derive Maxwell’s equations. Do not derive while intoxicated. Designated derivers save lives.
The Schrödinger’s Cat thought experiment is one of those “quantum physics things” that practically everybody has heard of, but very few people really understand–it’s the kind of idea that makes a great headline if you just skim off the “weird” surface details and leave behind all the context that actually explains it. (It doesn’t help that a lot of the people teaching quantum physics find it weird and confusing, too.) But quantum physics is reality. We spend our whole lives in it–if it seems weird, that is not the fault of quantum physics!
So, if you find Schrödinger’s Cat confusing or bizarre, or if you’re not sure you fully understand it (or if you don’t have any idea what it’s about), we’re gonna fix that!
First, we gotta talk about superposition. Imagine a single atom of some radioactive material–let’s say, oh…potassium-40. Potassium-40 is an unstable isotope of potassium, meaning that eventually it will “decay” to a more stable configuration, releasing a bit of energy in the process–that’s radioactivity.
(Imagine one of those little rubber cup toys that you turn inside-out and then they pop back into shape after a few seconds. The rubber toy releases kinetic energy that propels it into the air, while the atom releases energy in the form of particles like neutrons or electrons.)
Now, according to old-fashioned classical physics, the exact moment the atom decays (radiates? radio-activates?) is fundamentally random, impossible to predict. However, we now know from quantum mechanics that the atom’s behavior isn’t random at all–it decays and doesn’t decay at every possible moment simultaneously. This is the phenomenon known as “superposition.”
I’m not going to go into the evidence here– maybe in a future post–for now, just take my word for it that scientists have conclusively determined, by experiment, that this really is a thing that happens, and that there really is no better way to describe what’s happening than to say the atom is in two (or more) different states simultaneously.
Schrödinger’s Cat starts with a single such atom. To keep things simple, we’ll imagine that instead of decaying continuously, the atom can only decay at specific points in time–exactly once per minute, say–and we’ll be conducting our thought experiment at one of these times. So the experiment starts with your watch’s minute hand ticking over, and the atom transitioning (the technical term is evolving) into a superposition of [decayed + not-decayed].
In our thought-experimental setup, we have a particle detector next to the atom that can tell whether or not it decayed. Now, since the atom is in a superposition of simultaneously decaying and not decaying, you may wonder what happens to the detector. Does it sense the decay, or not?
Luckily, the answer isn’t complicated. As far as we can tell, the laws that govern individual particles apply in exactly the same way to collections of multiple particles, no matter the size. So when the atom evolves into a superposition of [decayed + not-decayed], the sensor likewise evolves into a superposition of [sensing + not-sensing] the decay.
Wired up to the sensor is an explosive, and next to the explosive is a cat. As you might expect, when the minute hand ticks over and the atom and sensor go into their superpositions, the explosive also evolves into a superposition of [kablooie + not-kablooie], causing the cat to evolve into a superposition of [why? why does it have to be a cat? why does the cat have to explode?? WHY ARE THOUGHT EXPERIMENTS SO VIOLENT??? + napping].
The entire setup is inside a sealed box, so you have no way of knowing what’s going on inside until you open the lid. And what happens when you do open it? Well, naturally, you evolve into a superposition! Probably something like: [“what? what the f**k?? did they even think to run this by the ethics committee?! I’m gonna be sick!” + “awww, kitty!”].
So that’s Schrödinger’s Cat. That wasn’t so hard, was it? And now we’ve demystified Macroscopic Decoherence!
…all right, fine, I’ll give you some context.
Back in the bad old days, when people were first discovering quantum mechanics and had no idea what the results of their experiments actually meant, superposition confused the living daylights out of them. So much so, in fact, that when Schrödinger’s Cat was originally conceived, it was considered a paradox! You see, people kept experimenting with superposition, looking at their instruments, seeing a single experimental outcome (e.g. exploded decayed or not-decayed, never both), and thinking that this…mysterious phenomenon…went away when they looked at it. But that left open the question of exactly when the superposition disappears. Schrödinger’s Cat was intended as a critique of that question, but at the time nobody saw the obvious answer.1
Macroscopic Decoherence, also known as the “Many Worlds” theory, is the obvious answer. Those old, puzzled researchers kept trying to figure out why the superposition “went away,” forgetting–rather embarrassingly–that they themselves were made of particles, that they themselves could be subject to exactly the same laws, with no variation or exception, despite the enormous difference in scale. And so we ended up with a whole bunch of “collapse” theories that try to “explain” where and why the superposition “disappears,” and in the absence of any better ideas these theories gained so much traction that even today many people still believe in them and think that many-worlds is the “weird” or “improbable” theory.
Collapse theories have a lot of intuitive appeal, which helps explain why they’ve stuck around so long. Of course, so does the flat-Earth hypothesis! The Earth sure looks flat from where we’re standing, and the universe sure looks like only one thing happening at a time, rather than a zillion different possiblities all taking place at once. But, just like the flat-Earth hypothesis, this is an illusion of scale: different states within a superposition are less likely to interact the farther apart they are–and not just in space and time! (The technical term for this “distancing” is decoherence.) A large enough difference in charge, spin, momentum, or any other attribute can decohere a superposition, and that distance multiplies with each additional particle.
(Think of it sort of like walking in a random direction blindfolded while trying to hit a piñata: if it’s right up close you’ll probably find it, but if it’s thirty yards away you’d have to get very lucky to land a hit–and each additional piñata you have to find makes the task as a whole exponentially more difficult.)
You are made up of very, very many particles,[citation needed] and by the time your brain has noticed the results of a measurement, gazillions of those particles have evolved into very different superpositions. The only way you and an “other you” could ever possibly interact with each other is if you were already so similar that there was no detectable difference–every single particle (never mind neuron) in almost exactly the same state.
So these “many worlds” aren’t worlds you can go and visit, or even detect–they’re nothing like the “parallel dimensions” or “alternate universes” you often see in science fiction. We can’t directly verify their existence–but then, the same is true of superposition itself! And after all, we believe in lots of things we can’t see: we can’t directly observe quarks and atoms either, and you personally have probably never been to, say, the Red River Inn in Silt, Colorado–yet you’d be a fool to doubt their existence. The “many worlds” of Macroscopic Decoherence are implied by the fully general laws of physics we’ve pinned down by experiment. Unless and until new evidence radically overturns existing theory, there’s no reason to believe those other worlds aren’t every bit as real as our own.
…and now you can sound super smart in all those worlds by teaching your friends about “Macroscopic Decoherence!”
In Search of Secrets 
