The Editors of Encyclopædia Britannica
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Alternative Titles: basic force, fundamental force
RELATED TOPICS
atom
electromagnetism
electroweak theory
matter
physical science
quantum field theory
strong force
Martinus J.G. Veltman
weak force
Yukawa Hideki
Fundamental interaction, in physics, any of the four basic forces—gravitational, electromagnetic, strong, and weak—that govern how objects or particles interact and how certain particles decay. All the known forces of nature can be traced to these fundamental interactions. The fundamental interactions are characterized on the basis of the following four criteria: the types of particles that experience the force, the relative strength of the force, the range over which the force is effective, and the nature of the particles that mediate the force.
Gravitation and electromagnetism were recognized long before
the discovery of the strong and weak forces because their effects on ordinary
objects are readily observed. The gravitational force, described systematically
by Isaac Newton in the 17th century, acts between all objects having mass; it
causes apples to fall from trees and determines the orbits of the planets
around the Sun. The electromagnetic force, given scientific definition by James
Clerk Maxwell in the 19th century, is responsible for the repulsion of like and
the attraction of unlike electric charges; it also explains the chemical
behaviour of matter and the properties of light. The strong and weak forces
were discovered by physicists in the 20th century when they finally probed into
the core of the atom. The strong force acts between quarks, the constituents of
all subatomic particles, including protons and neutrons. The residual effects
of the strong force bind the protons and neutrons of the atomic nucleus
together in spite of the intense repulsion of the positively charged protons
for each other. The weak force manifests itself in certain forms of radioactive
decay and in the nuclear reactions that fuel the Sun and other stars. Electrons
are among the elementary subatomic particles that experience the weak force but
not the strong force.
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subatomic particle: The basic forces and their messenger
particles
The previous section of this article presented an overview
of the basic issues in particle physics, including the four fundamental
interactions that affect all of matter. In this section the four interactions,
or basic forces, are treated in greater detail. Each force is described on the
basis of the following characteristics: (1) the property of matter on which
each force acts; (2) the...
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The four forces are often described according to their
relative strengths. The strong force is regarded as the most powerful force in
nature. It is followed in descending order by the electromagnetic, weak, and
gravitational forces. Despite its strength, the strong force does not manifest
itself in the macroscopic universe because of its extremely limited range. It
is confined to an operating distance of about 10−15 metre—about the diameter of
a proton. When two particles that are sensitive to the strong force pass within
this distance, the probability that they will interact is high. The range of
the weak force is even shorter. Particles affected by this force must pass
within 10−17 metre of one another to interact, and the probability that they
will do so is low even at that distance unless the particles have high
energies. By contrast, the gravitational and electromagnetic forces operate at
an infinite range. That is to say, gravity acts between all objects of the
universe, no matter how far apart they are, and an electromagnetic wave, such
as the light from a distant star, travels undiminished through space until it
encounters some particle capable of absorbing it.
SIMILAR TOPICS
mesic atom
stress
nuclear reaction
inertial force
equilibrium
d’Alembert’s principle
equivalence principle
radioactivity
monatomic gas
spin
For years physicists have sought to show that the four basic
forces are simply different manifestations of the same fundamental force. The
most successful attempt at such a unification is the electroweak theory,
proposed during the late 1960s by Steven Weinberg, Abdus Salam, and Sheldon Lee
Glashow. This theory, which incorporates quantum electrodynamics (the quantum
field theory of electromagnetism), treats the electromagnetic and weak forces
as two aspects of a more-basic electroweak force that is transmitted by four
carrier particles, the so-called gauge bosons. One of these carrier particles
is the photon of electromagnetism, while the other three—the electrically
charged W+ and W− particles and the neutral Z0 particle—are associated with the
weak force. Unlike the photon, these weak gauge bosons are massive, and it is
the mass of these carrier particles that severely limits the effective range of
the weak force.
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