The subject of Quantum Mechanics has to do with the very small. That’s what all famous physicists of yesteryear kept spouting, but they’re all dead (R.I.P in peace).
I tend to agree, as it’s only at those length scales that quantum effects become noticeable. But everything’s made up of the quantum (or so we think, what’s a Quantum anyway? It’s not in the ingredients list on this gum thingy!), so to some extent, quantum mechanics describes all the world around us.
Quantum effects are all around us, but since we’re talking about quantum mechanics, of course, these phenomena are all subtle, and it’s a miracle anyone ever discovered them (though not really, it just takes an inquiring mind), like the color of things, the bonding of molecules, and so on. It’s when trying to describe objects existing at those scales that we run into the fundamental derivations of Quantum Mechanics.
A quanta is a discrete package, something with a sense of the finite. Things we consider discrete are whole, as in a whole bagel, or a whole donut, as opposed to half a bagel, or no donut.
Particles were generally conceived of as particulate matter, as in the model of gasses being composed of little bullets bouncing around in a container. Well, it works pretty well for things like that, because gasses are made of particles of molecules or single elements.
But what about light? We can see from experiments that light clearly has wave-like characteristics. I mean, ffs, it diffracts like waves do, but in 3D, and if you look at certain experiments from a top-down perspective, like in the Double Slit Experiment, we see that light interferes with itself like how a similarly set up Double Slit Experiment done in a tank of water would have water waves interfere with itself.
But then Einstein came along and tried explaining the Photoelectric Effect, and we can see that his contribution actually describes nature pretty well. He used the idea of discrete energy, first introduced by Planck before him, who was trying to figure out how Black Bodies behaved.
Turns out, you can set up a Double Slit Experiment with a twist; instead of using a broad detection scheme, like a photographic plate, you set up a small area detector that supposedly detects the intensity of light being shot at a certain area at a time. We see that actually, instead of a continuous input of wave-like radiation being transmitted, there are spikes of incoming radiation. Almost like they were particles coming in instead.
It’s true, light is composed of particles that Einstein coined photons, and it’s just how it is. There’s some other conclusive proof of this, but originally it was Einstein’s Nobel Prize winning paper on the photoelectric effect that gave verification of this reality. It effectively ushered in the Quantum Era.
But then what’s with all the interference and diffraction around here? It doesn’t make sense to talk about a single particle “diffracting” or especially not “interfering with itself”. But it’s still what happens.
Once you notice (with your mind’s eye) that light is made of photons, we can interpret intensity as concentration, or really, a probability that a photon will hit. Like in the Double Slit Experiment, what’s actually being interfered with is the probability of photon paths or in other words, the probability of a reality a photon exists wherever you’re looking. More technically, it’s the particle’s respective Wave Functions interfering with one another.
Might sound weird but that’s quantum mechanics for you.
The history of quantum mechanics doesn’t end there, it gets weirder and weirder with the Stern-Gerlach Experiment on the spin of quantum particles, and the Compton Scattering Experiment just for starters. Wikipedia done good when they composed a timeline of quantum mechanics here. In Uni, we’ve barely scratched the surface, but already, the insights in here are real (in the sense that “it just got real”, ya see?).