Explainer: The particle zoo

Various subatomic particle types are compared to how they oddly relate to animals in a zoo
Sep 8, 2008 — 12:00 am EST

An atom was once considered the smallest unit of matter, but even smaller particles have taken the atom's place as the littlest bits that exist in the universe. This model offers a simplistic view of protons and neutrons in an atom’s nucleus, with electrons orbiting about them.


Particles are the building blocks of matter, and matter makes up everything you can see. The Earth and moon are matter. So is your body, your computer’s screen, even the air you breathe. Which means they’re all made of particles. Lots and lots of particles, of all different kinds, stuck together.

Atoms, which used to be considered the smallest unit of matter, are made from particles too. Just how small is an atom? That’s a tricky question, since different atoms have different sizes and atoms are mostly empty space. But here’s one way to think about it: Let’s say you wanted to fill up a baseball-sized bowl with gold atoms. You’d need roughly twice as many of these atoms as it would take to fill an Earth-sized bowl with baseballs.

Particles that are even smaller than an atom are called “subatomic.” The main subatomic particles that make up atoms are protons, electrons and neutrons. But some of these particles are also made of even smaller particles. Protons and neutrons, for example, are made of subatomic particles called “quarks.” There are six kinds of quarks, each with a weird name: up, down, strange, charm, top and bottom.

Dozens of types of subatomic particle exist, and scientists suspect there may be still more to discover. When a new type emerges, scientists tend to give them pretty odd-sounding names. To date, we’ve got bosons, fermions, leptons, muons, pions, neutrinos, photons, gluons, and gravitons.

Neutrinos are unusually weird because they have almost no mass and they fly through space at almost the speed of light. Three types exist: muon neutrinos, electron neutrinos and tau neutrinos.

And the strangest particle of all: the tachyon. It’s considered “hypothetical,” which means it might not even exist. If it does, it can go faster than the speed of light and travel back in time.

No wonder some physicists refer to these — the smallest inhabitants of our universe — as their “particle zoo.”

Power Words

atom    The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and neutrally charged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.

atomic    Having to do with atoms, the smallest possible unit that makes up a chemical element.

electron  A negatively charged particle; the carrier of electricity within solids.

gluon    A subatomic particle believed to bind other particles together.

hypothesis  A proposed explanation for a phenomenon. In science, a hypothesis is an idea that hasn’t yet been rigorously tested. Once a hypothesis has been extensively tested and is generally accepted to be the accurate explanation for an observation, it becomes a scientific theory. Ideas that are based on a hypothesis are referred to as hypothetical.

matter  Something which occupies space and has mass. Anything with matter will weigh something on Earth.

 neutrino  A subatomic particle with a mass close to zero. Neutrinos rarely react with normal matter. Three kinds of neutrinos are known.

neutron  A subatomic particle carrying no electric charge that is one of the basic pieces of matter. Neutrons belong to the family of particles known as hadrons.

photon  A particle representing the smallest possible amount of light or other electromagnetic radiation.

proton  A subatomic particle that is one of the basic building blocks of the atoms that make up matter. Protons belong to the family of particles known as hadrons.

quarks   A family of subatomic particles that each carries a fractional electric charge. Quarks are building blocks of particles called hadrons. Quarks come in types, or “flavors,” known as: up, down, strange, charm, top and bottom.

subatomic    Anything smaller than an atom, which is the smallest bit of matter that has all the properties of whatever chemical element it is (like hydrogen, iron or calcium).

theory  (in science)  A description of some aspect of the natural world based on extensive observations, tests and reason. A theory can also be a way of organizing a broad body of knowledge that applies in a broad range of circumstances to explain what will happen. Unlike the common definition of theory, a theory in science is not just a hunch. Ideas or conclusions that are based on a theory — and not yet on firm data or observations — are referred to as theoretical.

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