How Many Elementary Particles Are There, Really?

E very 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.

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 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.

But every particle physicist, Franklin included, recognizes that there are caveats.

From 17, you can keep counting. Where you stop depends on your taste for complexity and mystery. The question of how many particles there are brings us to the edge of what’s known about the most basic levels of stuff.

There is one glaring problem with 17. To satisfy special relativity, each of the Standard Model’s matter fields supports both a particle and an “antiparticle,” which is identical to the particle except for having the opposite electric charge. This is what we popularly know as antimatter. So instead of 12 matter particles, there are really 24. Likewise, W bosons come in oppositely charged types known as W+ and W−. (This doesn’t happen to the Z bosons, photons, or gluons; they’re electrically neutral.)

Franklin excludes antiparticles from her census, she said, because mathematically they more or less mirror their particle versions. (Bizarrely, antiparticles are equivalent to particles moving backward in time, and vice versa.) Neither is possible without the other, so they shouldn’t be counted twice.

But I find that rationale unconvincing. Particles and antiparticles are undeniably distinct, even if they are secret twins. They can’t transform into each other (with the possible exception of neutrinos, which may or may not be their own antiparticles), and far from being functionally equivalent, they play totally different roles in reality. Matter is so dominant in our universe that any antimatter typically encounters matter quickly and annihilates. The reason for the cosmos’s matter-antimatter asymmetry is a major physics mystery.

Antiparticles bring the total up to 30.

But the notion that there’s only one gluon is another oversimplification. Really, the strong force is conveyed by eight gluons (and their associated fields), each possessing a distinct blend of charges known as “colors” and “anticolors.” The different gluons are impossible to distinguish experimentally, so Franklin, being an experimentalist, scoffed and shook her head when I asked if all eight should be tallied individually. Yet in the mathematical equations that define the Standard Model, the eight gluons are distinct from one another in the same way that the W and Z bosons differ. For consistency’s sake, we probably have to count all eight. So now we’re at 37.

Q…