A blackboard at CERN covered with theoretical physics equations by CERN theoretical physics fellow Alberto Ramos and physicist Antonio Gonzalez-Arroyo of the Universidad Autonoma de Madrid, photographed on April 19, 2016. Image credit: Dean Mouhtaropoulos/Getty Images
Bosons, leptons, hadrons, gluons—it seems like there’s a veritable zoo of subatomic particles, and you can be forgiven for occasionally mixing up your quarks and your squarks (yes, squarks are an actual thing, or at least an actual possible thing). The following list isn’t a complete catalogue of what’s out there; rather, it’s a kind of starter kit, a combination of the more important—and the more bizarre—particles that make up our universe. The list runs roughly in order from particles you learned about in high school physics class to more exotic entities that are, for now, little more than twinkles in theoretical physicists’ eyes.
1. ELECTRON: GIVER OF CHEMISTRY AND ELECTRICITY
While protons and neutrons (and their constituent quarks) give atoms their heft, it’s their entourage of much lighter electrons that determines how atoms come together to form molecules—in a word, it’s electrons that give us chemistry. (Think of a water molecule as two hydrogen atoms and an oxygen atom that have worked out a joint custody agreement for their 10 electron children.) Learning to manipulate electrons has been one of the greatest scientific triumphs in history. In the late 19th century, we learned to control the flow of electrons in wires—electricity! (Oddly, while electricity travels at light speed, the electrons themselves are only moving a couple of feet an hour.) A few decades later, we figured out how to fire a stream of electrons at a phosphorescent screen inside a vacuum tube—voila, television.
2. PHOTON: ELECTROMAGNETIC RADIATION CARRIER
The nature of light puzzled scientists and philosophers since ancient times. Some thinkers insisted that light behaved like a wave; others (most famously Isaac Newton) said light was made up of particles. In the early 20th century, Albert Einstein showed that Newton was on the right track, discovering that light is “quantized,” that is, made of discrete particles (even though it can behave like a wave, too). Unlike electrons and quarks (see below), photons have no “rest mass”—that is, they don’t weigh anything, in the everyday sense of the word. But photons still have energy. That energy turns out to be proportional to the frequency of the light, so that blue light (higher frequency) carries more energy per photon than red light (lower frequency). But photons carry more than just visible light; they convey all forms of electromagnetic radiation, including radio waves (with much lower frequencies than visible light) and x-rays (with much higher frequencies).
3. QUARK: YOU, ME, GOLF BALL, STAR, GALAXY
Quarks are what most of the actual, familiar stuff in the universe is made of—you and me, stars and planets, golf balls and galaxies. Quarks are drawn to one another through the so-called strong nuclear force, to form protons and neutrons, which make up the nuclei of atoms. (At least the visible parts. More on that later.) In fact, due to the peculiarities of the rules of quantum mechanics, they can only exist within these larger, composite beasts; we can never see a quark on its own. They come in six “flavors” (yup, another quantum mechanics thing): up, down, strange, charm, top, and bottom. Of these, the up and down quarks are the most stable, so it’s those two, in particular, that most “stuff” is made of (the others can exist only under more exotic conditions). First proposed in the 1960s, the quark model has since been confirmed by thousands of experiments, culminating in the discovery of the top quark at Fermilab in 1995.
4. NEUTRINO: ZIPPY, WITH A TEENY BIT OF MASS
Neutrinos are elusive, very lightweight particles that just barely interact with matter at all. They zip through matter so effortlessly that, for a long time, physicists wondered if they might have zero rest mass, like photons. First theorized by Wolfgang Pauli in 1930, they were detected in the 1950s—but it was only in the last couple of decades that physicists were able to show that neutrinos do, in fact, have a teeny amount of mass. (The 2015 Nobel Prize in Physics went to two physicists whose experiments helped to pin down some of the neutrino’s peculiar properties.) While tiny, neutrinos are also ubiquitous; some 100 trillion neutrinos, created in the center of the Sun (the closest major source), pass through your body each second. (And it doesn’t matter if it happens to be nighttime; the little particles zip right through the Earth as though it’s not even there.)
5. HIGGS BOSON: POTENTIAL MASS PROVIDER
Nicknamed the “God particle” by Leon Lederman back in 1993, the Higgs boson has become the most famous of all particles in the last few years. First postulated in the 1960s (by Peter Higgs as well as by several other physicists, working independently), it was finally snared at the Large Hadron Collider near Geneva in 2012. Why all the fuss over the Higgs? The particle had been the last piece of the so-called “Standard Model” of particle physics to show itself. The model, developed beginning in the 1960s, explains how all of the known forces operate, with the exception of gravity. The Higgs is believed to play a special role within this system, endowing the other particles with mass.
6. GRAVITON: LAST PIECE OF THE QUANTUM FIELD THEORY PUZZLE
The graviton (if it exists) would be a “force carrier,” like the photon. Photons “mediate” the force of electromagnetism; gravitons would do the same for gravity. (When a proton and an electron attract each other via electromagnetism, they exchange photons; similarly, two massive objects that attract each other via gravitation ought to be exchanging gravitons.) This would be a way of explaining the gravitational force purely in terms of quantum field theories—or, to put it more plainly, the graviton would connect gravitation and quantum theory, fulfilling a century-old quest. The problem is that gravity is by far the weakest of the known forces, and there’s no known way of building a detector that could actually snag the graviton. However, physicists know a fair bit about the properties that the graviton must have, if it’s out there. For example, it’s believed to be massless (like the photon), it should travel at the speed of light, and it has to be a “spin-two boson,” in the jargon of particle physics.
7. DARK MATTER PARTICLE: THE KEY TO MISSING MASS?
About 90 years ago, astronomers began to notice that there’s something funny about the way that galaxies move. It turns out that there isn’t enough visible matter in galaxies to account for their observed motion. And so astronomers and physicists have been struggling to explain the “dark matter” said to make up the missing mass. (In fact, there’s believed to be a lot more dark matter than ordinary matter, by a ratio of about five to one.) What might dark matter be made of? One possibility is that it’s made up of as-yet unknown fundamental particles, likely produced in the first moments after the big bang. A number of experiments are now underway in the hope of finding these particles.
8. TACHYON: CAUSE-AND-EFFECT MUDDLER (AND LIKELY NOT REAL)
Ever since Einstein put forward the first part of his theory of relativity, known as special relativity, we’ve known that nothing can move faster than light. (It’s okay to move at the speed of light, if you’re massless—like a photon.) Tachyons are hypothetical particles that always travel faster than light. Needless to say, they don’t mesh very well with what we know about the workings of the universe. But in the 1960s, some physicists found a loophole: As long as the particle was created above light speed and never traveled slower than light, it could theoretically exist. Despite this, tachyons very likely aren’t real. (There was a flurry of excitement in 2011, when scientists at a particle physics lab in Italy claimed that a certain kind of neutrino travelled slightly faster than light; they later admitted they had made a mistake.) If tachyons do exist, some people think they could be used to send signals into the past, making a muddle of cause-and-effect, and leading to famous conundrums such as the grandfather paradox. But most physicists say that in the unlikely event they do exist, this wouldn’t be a problem because tachyons aren’t supposed to interact with normal matter (like us) anyway.