23.1 The Four Fundamental Forces

The four fundamental forces are gravity, the electromagnetic force, the weak nuclear force, and the strong nuclear force.
A variety of particle accelerators have been used to explore the nature of subatomic particles and to test predictions of particle theories.

23.2 Quarks

There are three types of fundamental particles—leptons, quarks, and carrier particles.
Quarks come in six flavors and three colors and occur only in combinations that produce white.
Hadrons are thought to be composed of quarks, with baryons having three quarks and mesons having a quark and an antiquark.
Known particles can be divided into three major groups—leptons, hadrons, and carrier particles (gauge bosons).
All particles of matter have an antimatter counterpart that has the opposite charge and certain other quantum numbers. These matter–antimatter pairs are otherwise very similar but will annihilate when brought together.
The strong force is carried by eight proposed particles called gluons, which are intimately connected to a quantum number called color—their governing theory is thus called quantum chromodynamics (QCD). Taken together, QCD and the electroweak theory are widely accepted as the Standard Model of particle physics.

Attempts to show unification of the four forces are called Grand Unified Theories (GUTs) and have been partially successful, with connections proven between EM and weak forces in electroweak theory.
Unification of the strong force is expected at such high energies that it cannot be directly tested, but it may have observable consequences in the as-yet-unobserved decay of the proton. Although unification of forces is generally anticipated, much remains to be done to prove its validity.