A joint Fermilab/SLAC publication
Header: What is a “particle”?

What is a “particle”?

06/02/16

Quantum physics says everything is made of particles, but what does that actually mean?

“Is he a dot or is he a speck? When he's underwater, does he get wet? Or does the water get him instead? Nobody knows.” —They Might Be Giants, “Particle Man”

We learn in school that matter is made of atoms and that atoms are made of smaller ingredients: protons, neutrons and electrons. Protons and neutrons are made of quarks, but electrons aren’t. As far as we can tell, quarks and electrons are fundamental particles, not built out of anything smaller.

It’s one thing to say everything is made of particles, but what is a particle? And what does it mean to say a particle is “fundamental”? What are particles made of, if they aren’t built out of smaller units?

“In the broadest sense, ‘particles’ are physical things that we can count,” says Greg Gbur, a science writer and physicist at the University of North Carolina in Charlotte. You can’t have half a quark or one-third of an electron. And all particles of a given type are precisely identical to each other: they don’t come in various colors or have little license plates that distinguish them. Any two electrons will produce the same result in a detector, and that’s what makes them fundamental: They don’t come in a variety pack.

It’s not just matter: light is also made of particles called photons. Most of the time, individual photons aren’t noticeable, but astronauts report seeing flashes of light even with their eyes closed, caused by a single gamma ray photon moving through the fluid inside the eyeball. Its interactions with particles inside creates blue-light photons known as Cherenkov light—enough to trigger the retina, which can “see” a single photon (though a lot more are needed to make an image of anything). 

Particle fields forever

That’s not the whole story, though: We may be able to count particles, but they can be created or destroyed, and even change type in some circumstances. During a type of nuclear reaction known as beta decay, a nucleus spits out an electron and a fundamental particle called an antineutrino while a neutron inside the nucleus changes into a proton. If an electron meets a positron at low velocities, they annihilate, leaving only gamma rays; at high velocities, the collision creates a whole slew of new particles.

Everyone has heard of Einstein’s famed E=mc2. Part of what that means is that making a particle requires energy proportional to its mass. Neutrinos, which are very low mass, are easy to make; electrons have a higher threshold, while heavy Higgs bosons need a huge amount of energy. Photons are easiest of all to make, because they don't have mass or electric charge, so there’s no energy threshold to overcome.

But it takes more than energy to make new particles. You can create photons by accelerating electrons through a magnetic field, but you can't make neutrinos or more electrons that way. The key is how those particles interact using the three fundamental quantum forces of nature: electromagnetism, the weak force and the strong force. However, those forces are also described using particles in quantum theory: electromagnetism is carried by photons, the weak force is governed by the W and Z bosons, and the strong force involves the gluons. 

All of these things are described together by an idea called “quantum field theory.” 

“Field theory encompasses quantum mechanics, and quantum mechanics encompasses the rest of physics,” says Anthony Zee, a physicist at the Kavli Institute for Theoretical Physics and the University of California, Santa Barbara. Zee, who has written several books on quantum field theory both for scientists and nonscientists, admits, “If you press a physicist to say what a field is, they’ll say a field is whatever a field does.”

Despite the vagueness of the concept, fields describe everything. Two electrons approach each other and they stir up the electromagnetic field, creating photons like ripples in a pond. Those photons then push the electrons apart.

What waves?

Waves are the best metaphor to understand particles and fields. Electrons, in addition to being particles, are simultaneously waves in the “electron field.” Quarks are waves in the “quark field” (and since there are six types of quark, there are six quark fields), and so forth. Photons are like water ripples: they can be big or small, violent or barely noticeable. The fields describing matter particles are more like waves on a guitar string. If you don’t pluck the string hard enough, you don’t get any sound at all: You need the threshold energy corresponding to an electron mass to make one. Enough energy, though, and you get the first harmonic, which is a clear note (for the string) or an electron (for the field). 

As a result of all this quantum thinking, it’s often unhelpful to think of particles as being like tiny balls.

“Photons [and matter particles] travel freely through space as a wave,” says Gbur, even though they can be counted as though they were balls. 

The metaphor isn’t perfect: The fields for electrons, electromagnetism and everything else fill all of space-time, rather than being like a one-dimensional string or two-dimensional pond surface. As Zee says, “What is waving when an electromagnetic wave goes through space? Nothing is waving! There doesn't need to be water like with a water wave.” 

And of course, we’re still left asking: If particles come from fields, are those fields themselves fundamental, or is there deeper physics involved? Until such time as theory comes up with something better, the particle description of matter and forces is something we can count on.