Reproduced from here.
Mighty stick figures don’t tell the whole story. Of course, they don’t, I might add. The equation I linked to last time, and which is depicted below, comes closer to that.
So, how does one even think about solving such a mountain of symbols?
The answer lies in symmetries, i.e. physical or mathematical features of a system (observed or intrinsic) which remain unchanged under some transformation of the system.
I’d assume that not even the hyperlink above really helps you understand that sentence. I know it didn’t with me, when I was given this explanation. Here’s about as much as I can write: Equations like this describe physical systems. If a physical quantity, something, anything really, stays the same under a given set of cases, we may call it a symmetry for the given set of cases within a given physical system. If you remember your school physics, then energy would be a symmetry for the given set of school physics cases. When that isn’t quite what you observe, the symmetry is “broken” and you have to dig a little deeper.
That’s another text for another day.
When confronted with an insane equation, we look for symmetries within the framework of the physical reality the equation describes. Using these, we can normally assume weird stuff, which makes the equation easier to handle, and we find out things like this: Atoms are made of quarks. We predict it; we prove it. End of (a usually rather long) story.
Don’t arrest me on the word “equation”, by the way. I’m too lazy to type out “non-linear system of oodles of coupled partial differential equations”, which is what nature mostly requires, and what that thing up there, despite its one-equation-appearance, really is. No, I’ll happily call it an equation. It’s enough that I hid in the closet crying for a week or two when first confronted with QED. You don’t have to re-live that part. It wasn’t pretty.
Climbing out of the closet again, I plowed on. Stuff that isn’t made of quarks is called leptons. Those are electrons and a host of other “point particles”, i.e. we safely assume them to not have any physical extension. Good. Now, what in the world would a point particle even be and “point” as opposed to what? You know, according to what I told you last time, the only spatial extension in atoms and things that make any sense are space-related probabilities. Empty Rutherfordian atoms and all that. Pointing out that point particles are to be treated like points seems pointless. As it were.
It isn’t. Yes, I have stopped crying since.
Anyway, it turns out that quarks and leptons make up matter. They interact by means of four forces of nature, each of which has a so-called gauge boson connected to it. Gauge bosons are imaginary particles, existing only for the shortest of times. The one for gravity, the graviton, needs a couple of extra spatial dimensions to be feasible as an idea, much less existing.
Yes, I just wrote that.
Perhaps, I have started crying again, too. And stopped again. There’s no point, you know. Either you’re a physicist or you aren’t. I’m a physicist. Simple as that.
Speaking about those forces of nature, they’re quite the bunch. They listen to names like weak and strong force as well as electromagnetic and gravitational force. We know a bit about the first three by means of a wonderfully absurd structure called quantum physics, of which particle physics is an aspect. General relativity deals with the fourth one. You can read all about them here, but what struck my fancy as a student was just how desperate experimentalists seemed to want to get to high enough energies to test the idea of “uniting the forces”.
There exists a non-military subset of humanity, which indulges in a subculture, where this makes sense.
See, we know that at high enough energies, say towards the beginning of the universe, the weak and electromagnetic forces essentially are the same thing. There are strong indications that at even higher energies the electroweakmagnetic force or whatever it is called, will unify with the strong force, and then eventually we would talk about gravity, too.
I will admit that my studies drifted into other things at this point. Still doing physics, and still staying interested, but you know, there was a threshold over which I didn’t want to go. My specialization would become atmosphere physics, more or less because particle physics had made me cry and rejoice at the same time. There may be hardier people than me out there, and there are, but I couldn’t continue with it. I mean, do you have any idea just how much more advanced theory is compared to experimentation in particle physics? No? Well, let’s talk about just how we experiment on things like this next time, shall we? What does it really mean to “reach higher energies”?
…to be continued here.
 Here’s an example of a symmetry breaking: Something that moves will slow down because friction warms up both it and the surface it moves along – and the air it moves through, too – if it moves through air, that is. Could be something moving through empty space – but you know what I mean, right? The energy for all that miniscule warming up of things around the something has to come from somewhere. It comes from the energy inherent in the motion.