GoKawiilEvery time I write about particle physics, I encounter a moment of uncertainty about a quantity that, at first glance, ought to be clear. How many kinds of elementary particles should I say there are?
In experiments at the Large Hadron Collider, physicists smash together beams of protons, breaking them up into all possible elementary bits and pieces. Meanwhile, they have an incredibly accurate set of mathematical equations for describing these building blocks and all the ways they fit together. So, since the known particles of nature can be both empirically observed and theoretically described, you would think they could also be counted. But alas not. I knew that, for reasons we’ll see, the census is not so easy as it seems.
The Quanta Podcast We have a full tally of the elementary particles, but the deeper you go into that count, the more complicated — and weirder — the answer becomes. All episodes Your browser does not support the audio element. / APPLE SPOTIFY
So I recently emailed a few physicists to ask how each of them personally tallies nature’s fundamental constituents. The first indicator of just how complicated the issue is came in a reply from David Tong, the University of Cambridge physicist and textbook author, when we were scheduling a video call: “P.S. I think the true answer to your question is not an integer!”
We’ll get to that (it comes from a mysterious calculation from 2011), but let’s enter this rabbit hole from the top.
The known elementary particles and their interactions obey a set of equations called the Standard Model of particle physics. The Standard Model is a “quantum field theory,” a mathematical description of reality in which entities called quantum fields permeate the universe. Ripples moving through these fields are what we call elementary particles; some behave like matter, while others impart forces. The quantum fields and associated particles in the Standard Model underlie all known physical phenomena other than gravity, dark matter, and dark energy (all of which take unknown forms at a fundamental level).
In philosophy, “qualia” refers to the subjective qualities of our experience: what it’s like for Alice to see blue or for Bob to feel delighted. Qualia are “the ways things seem to us,” as the late philosopher Daniel Dennett put it. In these essays, our columnists follow their curiosity, and explore important but not necessarily answerable scientific questions.
In posters on classroom walls, the Standard Model displays 17 particles. There are 12 matter particles, or fermions: the electron, muon, and tau; three neutrinos; and six quarks. Each of them has a distinct set of sensitivities to various forces. There are also four force-carrying particles, or “bosons”: the photon (which imparts the electromagnetic force), the W and Z bosons (the weak force), and the gluon (the strong force). Finally, there’s the Higgs boson, a so-called scalar particle that’s neither matter nor force; rather, it imbues other particles with mass through its interactions with them.
Samuel Velasco/Quanta Magazine
It may just be this simple. “I think 17 is the right answer,” Melissa Franklin, a professor of particle physics at Harvard University, told me.
... continue reading