Introduction

The most fundamental constituents of our universe are the elementary, or fundamental particles. They are the "building blocks" that make up everything in the universe. Though their most basic nature is still being debated, most of these particles have well-documented, experimentally verified properties. Listed here are the most important of those properties. However, many aspects of cosmology are still being developed, and additional elementary particles of higher mass may be discovered in the near future with the next generation of particle accelerators.

The Structure of Matter

The elementary particles known today are divided into three families. The constituents of these families, as stated above, make up all the matter in the universe. All "everyday" matter is made up of atoms, once thought to be the smallest division of matter. However, atoms have a substructure in their own right - they are made of a negatively charged electron cloud surrounding a positively-charged nucleus. The nucleus in turn can be divided still further into positively charged protons and neutral neutrons. A proton is made of two up-quarks and a down-quark, and a neutron is made of two down-quarks and an up-quark. Their charges can be easily determined from the tables below - electrons have electric charge -1, hence the electron cloud's overall negative charge. Protons are made of two up-quarks of electric charge 2/3 each, and one down-quark of electric charge -1/3; these charges add together to produce an overall charge of 1. Neutrons consist of two down-quarks of -1/3 each and one up-quark of 2/3; these cancel out to an overall charge of 0. Thus, the atom itself is neutral.

The standard pictorial depiction of the atomic model has the electrons in fixed positions, orbiting the nucleus as planets orbit the sun. However, quantum physics has shown this is an oversimplified model - in actuality, the electrons do not occupy a defined position, but instead have associated wave-functions that determine the probabilities of finding an electron at a certain position. These probabilities are the most precisely the electron's position can be measured. (These principles are discussed more fully in The Uncertainty Principle.)

The Family Tables

The following tables summarize most of the properties of the various particles belonging to the three families of subatomic particles. The properties listed here, along with the name of the particle, are: mass, electric charge, strong charge, weak charge. The strong charges of quarks have been creatively designated red, green, and blue by physicists; they do not, however, actually have color - they are too small for light waves to bounce off. A particle's mass determines how it responds to gravity, the electric charge determines its response to electromagnetism, its strong charge determines its response to the strong force, and its weak charge determines its response to the strong force. For advanced notes, click here.

Family 1

Family 2

Family 3

The Messenger Particles

This topic is discussed more fully in The Four Fundamental Forces; however, a discussion of fundamental particles would be incomplete without a brief mention of the messenger particles associated with each force. Three of these particles have been confirmed experimentally; the graviton has not yet been found, but calculations show that its mass should be zero. Weak gauge boson come in two types with different masses.

Unanswered Questions

These tables leave a number of questions unanswered. Scientists do not know for sure if these are the only fundamental particles, or if there are other, more massive particles as yet inaccessible to our particle accelerators. Also on the list of unknowns is the reason the particles are divided into three families and the reasons behind the individual particles' properties. It seems strange to many people that the electric, strong, and weak charges conform to such regular patterns while the particles' masses vary so widely. Answers to these questions may soon come in the form of string theory, a new theoretical framework being developed and tested by physicists. Though still in its early stages and incompletely understood, many believe string theory to be the long-sought "theory of everything" that will explain all our questions about our universe. More information is available in Introduction to String Theory.

Note: The weak charges given here more accurately represent the "third component" of weak isospin. The "right-handed" components of these particles are not listed, but their only difference is their lack of weak charge. Go back.