What is quantum mechanics? Quantum mechanics is the leading cause of cat death, and also not the leading cause of cat death.
If a (lame) joke like that goes over your head, then you definitely need this list. Quantum mechanics reveals a fascinating set of physical laws and theories that explore such questions as "Wait, is the universe really made of string?" "Yo, dawg, what about time travel?" and everything in between. And everything inside. And around. And invisible. Don't worry if it all sounds confusing - it's only, like, the laws of nature and everything as we both know and don't know it. No big deal at all!
The good news is that we're all in this together. Come along on a journey through space and time (if you read that in the voice of Neil deGrasse Tyson, you are correct) to learn some quantum mechanics facts. Why, by the end of this quantum mechanics overview, you'll likely be able to deliver an explanation of quantum mechanics and a robust litany of quantum theory facts to your cat. Just, uh, leave the feline-themed experiments to Schrödinger.
So, what the heck is quantum mechanics? Well, it's a branch of physics, for one, so it falls largely under the umbrella of studying everything to do with our natural world, including space, time, and matter. Quantum mechanics deals specifically in particle physics, seeking to understand the motion of and interaction between the building blocks that comprise our universe (and perhaps others). It's been called "the science of the small," opposite to general relativity, which is known as "the science of the large."
Quantum mechanics also encompasses some freaky concepts, like particle superposition (being in more than one place at a time), string theory, multiple universes ("multiverses"), extra dimensions, and a whole galaxy of other theories to inspire an existential crisis about what's real and what it all means, man.
You can't talk about quantum mechanics without discussing Young's interference experiment, also known as the double-slit experiment. You can re-create this experiment yourself pretty easily: cut two slits in a piece of paper, shine a flashlight through the slits, and observe the linear patterns created. "Uh, great, what am I looking at?" you ask?
The above video does a better job of explaining the patterns than any number of words ever could. Basically, this experiment shows that light photons behave as both particle and wave, a property dubbed "wave-particle duality." This first observation of the duality of particles took place well before science established the field of quantum mechanics, and paved the way for things like the first photograph of light in both its particle and wave states.
In the wake of about 135 years of scientists trying to wrap their heads around the implications of Young's experiment, Austrian physicist Erwin Schrödinger devised a thought experiment that would further confuse everything, but also become one of the famous building blocks of the quantum superposition theory.
Rather than actually kill cats, Schrödinger presented for scientific pondering this bit of mental gymnastics: If there's a cat stuck in a booby-trapped box, only upon observation (i.e., when you open the box) can you ascertain if the cat is alive or dead. Which is to say, until the very moment of observation, both possibilities are true, so the cat is both alive and dead simultaneously. By imagining this bizarre cat torture, Schrödinger hoped to understand when particles leave a state of quantum superposition to become one thing or another.
Young's interference, Schrödinger's cat, and USB drives help to understand a concept called quantum superposition. At a high level, quantum superposition says that a thing can exist in all possible states until it's observed. Light is both a particle and a wave, a cat in a box is both alive and dead, and a USB drive is both up and down. It isn't until we observe something that it "fixes" on a state of being. Which leads us to binge-watching Netflix on a computer exponentially more powerful than anything we know today through a little project called "quantum computing."