Physicists talk a lot about fundamental particles. But what do we mean by fundamental?
The Ancient Greek philosopher Democritus thought the world was composed of fundamental indivisible objects, constantly in motion. He called these objects “atoms”, and believed they could never be created or destroyed, with every other phenomenon explained by different types of interlocking atoms.
The things we call atoms today aren’t really like this, as you probably know. Atoms aren’t indivisible: their electrons can be split from their nuclei, and with more energy their nuclei can be split into protons and neutrons. More energy yet, and protons and neutrons can in turn be split into quarks. Still, at this point you might wonder: could quarks be Democritus’s atoms?
In a word, no. Nonetheless, quarks are, as far as we know, fundamental particles. As it turns out, our “fundamental” is very different from Democritus’s. Our fundamental particles can transform.
Think about beta decay. You might be used to thinking of it in terms of protons and neutrons: an unstable neutron decays, becoming a proton, an electron, and an (electron-anti-)neutrino. You might think that when the neutron decays, it literally “decays”, falling apart into smaller pieces.
But when you look at the quarks, the neutron’s smallest pieces, that isn’t the picture at all. In beta decay, a down quark in the neutron changes, turning into an up quark and an unstable W boson. The W boson then decays into an electron and a neutrino, while the up quark becomes part of the new proton. Even looking at the most fundamental particles we know, Democritus’s picture of unchanging atoms just isn’t true.
Could there be some even lower level of reality that works the way Democritus imagined? It’s not impossible. But the key insight of modern particle physics is that there doesn’t need to be.
As far as we know, up quarks and down quarks are both fundamental. Neither is “made of” the other, or “made of” anything else. But they also aren’t little round indestructible balls. They’re manifestations of quantum fields, “ripples” that slosh from one sort to another in complicated ways.
When we ask which particles are fundamental, we’re asking what quantum fields we need to describe reality. We’re asking for the simplest explanation, the simplest mathematical model, that’s consistent with everything we could observe. So “fundamental” doesn’t end up meaning indivisible, or unchanging. It’s fundamental like an axiom: used to derive the rest.
I feel the term ‘fundamental particle’ is slightly misleading in quantum field theory, as it isn’t the particle that is fundamental, but the quantum field, and a particle is simply an excitation of said quantum field.
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“It’s not impossible. But the key insight of modern particle physics is that there doesn’t need to be.”
I have yet to meet anyone who has dealt with fundamental physics who thinks that there isn’t some deeper “within the Standard Model” layer out there, although you are absolutely correct that we don’t need to seem to need one.
Do you, at a gut instinct level, think that there is probably a deeper explanation that could be reductionist relative to the couple dozen experimentally measured constants of the Standard Model with 17 kinds of particles (not including parity, color charge, and antiparticle variations) from which the Standard Model could be derived?
To be clear, this is very much a separate question: whether or not there needs to be a deeper BSM theory, there doesn’t need to be a “Democritus-style indestructible particles” theory. (I’m not even sure the latter can be formulated consistently but I chose to err on the side of vagueness since I’m not certain.)
On a gut level, I absolutely think that there should be some deeper theory behind the SM. I recognize that this feeling mostly comes from an untrustworthy instinct (“nature just shouldn’t be like that!”), but I think there are justifications for it as well. The SM (plus gravity) seems to genuinely not work once you zoom in far enough (Landau pole of U(1), non-renormalizability of quantum gravity) and while I can’t say 100% that this means we need a better theory (c.f. my posts over the last year about not understanding EFT), smarter people than I seem to believe it does.
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