Four Forces
We are all aware of many different forces. Tides, the wind,
electric motors, volcanic and chemical explosions, or human muscles,
can all exert force. However, physicists have found that all these
different kinds of force that we know are really versions of four
basic forces at work in the universe, each governing a different
aspect of how particles of matter interact. Two of these basic forces
(the strong and the weak
mentioned below) operate
entirely within atomic nuclei, and we are not normally aware
of them. The other two (electromagnetism
and gravity)
account for almost all the forces we come across in our everyday
lives.
Force-Carriers
How does force move from one object to another? It is easy to
imagine an object exerting a force when it touches another,
but there are many forces that are apparently exerted from a distance,
e.g. the gravitational force between the planets and the sun, or the
force that causes two magnetic poles to attract or repel each
other. How does one particle or object exert force on another that
is some distance away? In former times it was believed that force
could be transmitted mysteriously through space. Today we believe that
all the known forces are carried from one object to another by a
special group of particles. Some of these, such as photons, are well
known; while others, like gravitons, have yet to be discovered. In the
discussion below we mention the carrier that we believe acts for each
of the four forces. It is difficult to compare these basic forces
because they all have different strengths at different distances. The
relative strengths mentioned below apply only to particles fairly
close to each other, as in the nucleus of a small atom, where all four
forces can act simultaneously.
1. Inside the nucleus: the strong force
The most powerful of the forces, appropriately known as the
strong or nuclear force, binds together the protons and
neutrons (i.e. "nucleons") that constitute the nuclei of atoms. The
force is carried by particles called mesons, which are
constantly exchanged between the nucleons. This action apparently
holds them together in the nuclear unit.
However, recent experiments have proved that the force carried by
mesons is actually a secondary effect of the strong force
inside the protons and neutrons themselves! During the past
decades we have discovered that quarks are elementary particles,
and that they can join together in small groups to form protons and
neutrons (which are stable within nuclei), as well as a large number
of other short-lived particles. This new picture of nature's
fundamental building blocks now leads to a new quest: physicists want
to look at the basic strong force linking the quarks themselves within
these small groups. Because of their main function, the particles that
carry such a force have been aptly named gluons.
2. Between and inside atoms: electromagnetism
Next in strength comes the electromagnetic force, which has
only 1/14th the power of the strong force at this distance, but
its influence extends over a much greater span. This force acts between
any particles carrying an electric charge. It keeps (negatively
charged) electrons close to the (positive) nuclei of atoms, and so
governs chemical reactions. Carriers called photons transmit
this force between the particles involved. A stream of photons is what
we call electromagnetic radiation, e.g. sunlight, microwaves, cosmic
rays, X-rays, radio waves.
3. Radioactive decay: the weak force
Next comes the weak force, with 1/108 the power of
the strong force. It governs the radioactive decay of some elementary
particles. Physicists have recently found the carriers that transmit
this force, and have named them the W and Z
particles. They are much more massive than protons or neutrons,
and have extremely short lifetimes.
4. Shaping the Universe: gravity
At first sight, gravity seems to be the feeblest force in the
universe, with only 1/1046 the power of the strong
force. However, gravity maintains its effects over enormous distances
and is therefore the dominant influence throughout the known
cosmos. How is gravity transmitted? Scientists are looking for
particles (called gravitons) as well as for gravitational waves. This
is another area of the incredible, topsy-turvy world of subatomic and
cosmic physics that awaits further exploration.
Tying Them Together
Current research tries to show that even these "basic" forces are
really variations of a single universal force that appeared at the
time of the original "Big Bang". This idea is called the Grand Unified
Theory.
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