8 Subatomic Particles You Should Know

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.


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.


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).


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.


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.)


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.


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.


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.


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.

Dean Mouhtaropoulos/Getty Images
Essential Science
What Is a Scientific Theory?
Dean Mouhtaropoulos/Getty Images
Dean Mouhtaropoulos/Getty Images

In casual conversation, people often use the word theory to mean "hunch" or "guess": If you see the same man riding the northbound bus every morning, you might theorize that he has a job in the north end of the city; if you forget to put the bread in the breadbox and discover chunks have been taken out of it the next morning, you might theorize that you have mice in your kitchen.

In science, a theory is a stronger assertion. Typically, it's a claim about the relationship between various facts; a way of providing a concise explanation for what's been observed. The American Museum of Natural History puts it this way: "A theory is a well-substantiated explanation of an aspect of the natural world that can incorporate laws, hypotheses and facts."

For example, Newton's theory of gravity—also known as his law of universal gravitation—says that every object, anywhere in the universe, responds to the force of gravity in the same way. Observational data from the Moon's motion around the Earth, the motion of Jupiter's moons around Jupiter, and the downward fall of a dropped hammer are all consistent with Newton's theory. So Newton's theory provides a concise way of summarizing what we know about the motion of these objects—indeed, of any object responding to the force of gravity.

A scientific theory "organizes experience," James Robert Brown, a philosopher of science at the University of Toronto, tells Mental Floss. "It puts it into some kind of systematic form."


A theory's ability to account for already known facts lays a solid foundation for its acceptance. Let's take a closer look at Newton's theory of gravity as an example.

In the late 17th century, the planets were known to move in elliptical orbits around the Sun, but no one had a clear idea of why the orbits had to be shaped like ellipses. Similarly, the movement of falling objects had been well understood since the work of Galileo a half-century earlier; the Italian scientist had worked out a mathematical formula that describes how the speed of a falling object increases over time. Newton's great breakthrough was to tie all of this together. According to legend, his moment of insight came as he gazed upon a falling apple in his native Lincolnshire.

In Newton's theory, every object is attracted to every other object with a force that’s proportional to the masses of the objects, but inversely proportional to the square of the distance between them. This is known as an “inverse square” law. For example, if the distance between the Sun and the Earth were doubled, the gravitational attraction between the Earth and the Sun would be cut to one-quarter of its current strength. Newton, using his theories and a bit of calculus, was able to show that the gravitational force between the Sun and the planets as they move through space meant that orbits had to be elliptical.

Newton's theory is powerful because it explains so much: the falling apple, the motion of the Moon around the Earth, and the motion of all of the planets—and even comets—around the Sun. All of it now made sense.


A theory gains even more support if it predicts new, observable phenomena. The English astronomer Edmond Halley used Newton's theory of gravity to calculate the orbit of the comet that now bears his name. Taking into account the gravitational pull of the Sun, Jupiter, and Saturn, in 1705, he predicted that the comet, which had last been seen in 1682, would return in 1758. Sure enough, it did, reappearing in December of that year. (Unfortunately, Halley didn't live to see it; he died in 1742.) The predicted return of Halley's Comet, Brown says, was "a spectacular triumph" of Newton's theory.

In the early 20th century, Newton's theory of gravity would itself be superseded—as physicists put it—by Einstein's, known as general relativity. (Where Newton envisioned gravity as a force acting between objects, Einstein described gravity as the result of a curving or warping of space itself.) General relativity was able to explain certain phenomena that Newton's theory couldn't account for, such as an anomaly in the orbit of Mercury, which slowly rotates—the technical term for this is "precession"—so that while each loop the planet takes around the Sun is an ellipse, over the years Mercury traces out a spiral path similar to one you may have made as a kid on a Spirograph.

Significantly, Einstein’s theory also made predictions that differed from Newton's. One was the idea that gravity can bend starlight, which was spectacularly confirmed during a solar eclipse in 1919 (and made Einstein an overnight celebrity). Nearly 100 years later, in 2016, the discovery of gravitational waves confirmed yet another prediction. In the century between, at least eight predictions of Einstein's theory have been confirmed.


And yet physicists believe that Einstein's theory will one day give way to a new, more complete theory. It already seems to conflict with quantum mechanics, the theory that provides our best description of the subatomic world. The way the two theories describe the world is very different. General relativity describes the universe as containing particles with definite positions and speeds, moving about in response to gravitational fields that permeate all of space. Quantum mechanics, in contrast, yields only the probability that each particle will be found in some particular location at some particular time.

What would a "unified theory of physics"—one that combines quantum mechanics and Einstein's theory of gravity—look like? Presumably it would combine the explanatory power of both theories, allowing scientists to make sense of both the very large and the very small in the universe.


Let's shift from physics to biology for a moment. It is precisely because of its vast explanatory power that biologists hold Darwin's theory of evolution—which allows scientists to make sense of data from genetics, physiology, biochemistry, paleontology, biogeography, and many other fields—in such high esteem. As the biologist Theodosius Dobzhansky put it in an influential essay in 1973, "Nothing in biology makes sense except in the light of evolution."

Interestingly, the word evolution can be used to refer to both a theory and a fact—something Darwin himself realized. "Darwin, when he was talking about evolution, distinguished between the fact of evolution and the theory of evolution," Brown says. "The fact of evolution was that species had, in fact, evolved [i.e. changed over time]—and he had all sorts of evidence for this. The theory of evolution is an attempt to explain this evolutionary process." The explanation that Darwin eventually came up with was the idea of natural selection—roughly, the idea that an organism's offspring will vary, and that those offspring with more favorable traits will be more likely to survive, thus passing those traits on to the next generation.


Many theories are rock-solid: Scientists have just as much confidence in the theories of relativity, quantum mechanics, evolution, plate tectonics, and thermodynamics as they do in the statement that the Earth revolves around the Sun.

Other theories, closer to the cutting-edge of current research, are more tentative, like string theory (the idea that everything in the universe is made up of tiny, vibrating strings or loops of pure energy) or the various multiverse theories (the idea that our entire universe is just one of many). String theory and multiverse theories remain controversial because of the lack of direct experimental evidence for them, and some critics claim that multiverse theories aren't even testable in principle. They argue that there's no conceivable experiment that one could perform that would reveal the existence of these other universes.

Sometimes more than one theory is put forward to explain observations of natural phenomena; these theories might be said to "compete," with scientists judging which one provides the best explanation for the observations.

"That's how it should ideally work," Brown says. "You put forward your theory, I put forward my theory; we accumulate a lot of evidence. Eventually, one of our theories might prove to obviously be better than the other, over some period of time. At that point, the losing theory sort of falls away. And the winning theory will probably fight battles in the future."

Farrin Abbott, SLAC/Flickr // CC BY-NC-SA 2.0
An Ancient Book Blasted with High-Powered X-Rays Reveals Text Erased Centuries Ago
Farrin Abbott, SLAC/Flickr // CC BY-NC-SA 2.0
Farrin Abbott, SLAC/Flickr // CC BY-NC-SA 2.0

A book of 10th-century psalms recovered from St. Catherine’s Monastery on Egypt's Sinai Peninsula is an impressive artifact in itself. But the scientists studying this text at the U.S. Department of Energy's SLAC National Accelerator Laboratory at Stanford University were less interested in the surface text than in what was hidden beneath it. As Gizmodo reports, the researchers were able to identify the remains of an ancient Greek medical text on the parchment using high-powered x-rays.

Unlike the Large Hadron Collider in Switzerland, the Stanford Synchrotron Radiation Lightsource (SSRL) used by the scientists is a much simpler and more common type of particle accelerator. In the SSRL, electrons accelerate to just below the speed of light while tracing a many-sided polygon. Using magnets to manipulate the electrons' path, the researchers can produce x-ray beams powerful enough to reveal the hidden histories of ancient documents.

Scanning an ancient text.
Mike Toth, R.B. Toth Associates, Flickr // CC BY-NC-SA 2.0

In the case of the 10th-century psalms, the team discovered that the same pages had held an entirely different text written five centuries earlier. The writing was a transcription of the words of the prominent Greek physician Galen, who lived from 130 CE to around 210 CE. His words were recorded on the pages in the ancient Syriac language by an unknown writer a few hundred years after Galen's death.

Several centuries after those words were transcribed, the ink was scraped off by someone else to make room for the psalms. The original text is no longer visible to the naked eye, but by blasting the parchment with x-rays, the scientists can see where the older writing had once marked the page. You can see it below—it's the writing in green.

X-ray scan of ancient text.
University of Manchester, SLAC National Accelerator Laboratory, Flickr // CC BY-NC-SA 2.0

Now that the researchers know the hidden text is there, their next step will be uncovering as many words as possible. They plan to do this by scanning the book in its entirety, a process that will take 10 hours for each of the 26 pages. Once they've been scanned and studied, the digital files will be shared online.

Particle accelerators are just one tool scientists use to decipher messages that were erased centuries ago. Recently, conservationists at the Library of Congress used multispectral imaging, a method that bounces different wavelengths of light off a page, to reveal the pigments of an old Alexander Hamilton letter someone had scrubbed out.

[h/t Gizmodo]


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