I realized that my mental model of a simple three-quark proton was deeply (or simply) wrong.
For an explanation using more showing and less telling: Checking what's inside a proton
You’ve heard the famous statement that “a proton is made from two up quarks and a down quark”. But in this basic article, and this somewhat more advanced one, and in a recent post where I went into some details about what we know about proton structure, I’ve claimed to you that protons are chock full of particles, most of which carry a tiny fraction of the proton’s energy, and most of which are gluons, along with a substantial number of of quarks and antiquarks.
What I want to do in this article is show you evidence that the statements made about proton structure in this post are true. After all, why should you have to take my word for such things? Let’s look at some LHC data, and see how it confirms these notions.
I recently flipped through the "Cartoon Guide to Physics", expecting an easy-to-understand rehash of ideas I was long familiar with; and that's what I got - right up to the last few pages, where I was presented with a fairly fundamental concept that's been absent from the popular science media I've enjoyed over the years. (Specifically, that the uncertainty principle, when expressed as linking energy and time, explains what electromagnetic fields actually /are/, as the propensity for virtual photons of various strengths to happen.) I find myself happy to try to integrate this new understanding - and at least mildly disturbed that I'd been missing it for so long, and with an increased curiosity about how I might find any other such gaps in my understanding of how the universe works.
So: what's the biggest, or most surprising, or most interesting concept /you/ have learned of, after you'd already gotten a handle on the basics?