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rdinary matter is composed of protons, neutrons, and electrons, and at first

glance these particles seem enough to account for the structure of the universe

around us. Not all nuclides are stable, however, and neutrinos are needed for

beta decay to take place—indeed, without neutrinos the reaction sequences that power

the stars and that lead to the creation of elements heavier than hydrogen could not

occur. Furthermore, as discussed in Sec. 11.7, the electromagnetic interaction between

charged particles requires photons as its carrier, and the specifically nuclear interac-

tion between nucleons requires pions for the same purpose. Even so, only a few par-

ticles seem to be needed, all of them with clearly defined roles to play.

But things are not nearly so straightforward. Hundreds of other “elementary” particles

have been discovered, all of which decay rapidly after being created in high-energy

collisions between other particles. It has become clear that some of these particles

(called leptons) are more elementary than the others, and that the others (called

hadrons) are composites of a far smaller number of rather unusual particles called

quarks that have not been detected in isolation (and probably will never be).

13.1 INTERACTIONS AND PARTICLES

Which affects which

The four interactions we already know about—strong, electromagnetic, weak, and

gravitational—are apparently enough to account for all the physical processes and struc-

tures in the universe on all scales of size from atoms and nuclei to galaxies of stars.

The basic characteristics of these interactions are given in Table 13.1.

The list of fundamental interactions has changed over the years. Long ago, the strong

and weak interactions were unknown and it was not even clear that the gravity that

pulls things down to the earth, which we might call terrestrial gravity, is the same as

the gravity that holds the planets to their orbits around the sun. One of Newton’s great

accomplishments was to show that both terrestrial and astronomical gravity are the

same. Another notable unification was made by Maxwell when he demonstrated that

Elementary Particles 475

Table 13.1 The Four Fundamental Interactions. The graviton has not been experimentally detected as yet.

Relative Particles

Interaction Particles Affected Range Strength Exchanged Role in Universe

Quarks Gluons Holds quarks together to form

Strong  10 ^15 m 1 nucleons

Hadrons Mesons Holds nucleons together to form

atomic nuclei

Electromagnetic Charged particles   10 ^2 Photons Determines structures of atoms,

molecules, solids, and liquids; is

important factor in astronomical

universe

Weak Quarks and leptons  10 ^18 m  10 ^5 Intermediate Mediates transformations of

bosons quarks and leptons; helps

determine compositions of

atomic nuclei

Gravitational All   10 ^39 Gravitons Assembles matter into planets,

stars, and galaxies

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