In particle physics, the strong interaction
is the mechanism responsible for the strong
nuclear force (also called the strong force,
nuclear strong force, or colour force), and
is one of the four known fundamental interactions,
with the others being electromagnetism, the
weak interaction, and gravitation.
At the range of 10−15 m (1 femtometer),
the strong force is approximately 137 times
as strong as electromagnetism, a million times
as strong as the weak interaction, and 1038
times as strong as gravitation.
The strong nuclear force holds most ordinary
matter together because it confines quarks
into hadron particles such as the proton and
neutron.
In addition, the strong force binds neutrons
and protons to create atomic nuclei.
Most of the mass of a common proton or neutron
is the result of the strong force field energy;
the individual quarks provide only about 1%
of the mass of a proton.
The strong interaction is observable at two
ranges: on a larger scale (about 1 to 3 fm),
it is the force that binds protons and neutrons
(nucleons) together to form the nucleus of
an atom.
On the smaller scale (less than about 0.8
fm, the radius of a nucleon), it is the force
(carried by gluons) that holds quarks together
to form protons, neutrons, and other hadron
particles.
In the latter context, it is often known as
the color force.
The strong force inherently has such a high
strength that hadrons bound by the strong
force can produce new massive particles.
Thus, if hadrons are struck by high-energy
particles, they give rise to new hadrons instead
of emitting freely moving radiation (gluons).
This property of the strong force is called
color confinement, and it prevents the free
"emission" of the strong force: instead, in
practice, jets of massive particles are produced.
In the context of binding protons and neutrons
together to form atomic nuclei, the strong
interaction is called the nuclear force (or
residual strong force).
In this case, it is the residuum of the strong
interaction between the quarks that make up
the protons and neutrons.
As such, the residual strong interaction obeys
a quite different distance-dependent behavior
between nucleons, from when it is acting to
bind quarks within nucleons.
Differences in the binding energy of the nuclear
force between different nuclei power nuclear
fusion and nuclear fission.
Nuclear fusion accounts for most energy production
in the Sun and other stars.
Nuclear fission allows for decay of radioactive
elements and isotopes, although it is often
mediated by the weak interaction.
Artificially, the energy associated with the
nuclear force is partially released in nuclear
power and nuclear weapons, both in uranium
or plutonium-based fission weapons and in
fusion weapons like the hydrogen bomb.The
strong interaction is mediated by the exchange
of massless particles called gluons that act
between quarks, antiquarks, and other gluons.
Gluons are thought to interact with quarks
and other gluons by way of a type of charge
called color charge.
Color charge is analogous to electromagnetic
charge, but it comes in three types (±red,
±green, ±blue) rather than one, which results
in a different type of force, with different
rules of behavior.
These rules are detailed in the theory of
quantum chromodynamics (QCD), which is the
theory of quark-gluon interactions.
== History ==
Before the 1970s, physicists were uncertain
as to how the atomic nucleus was bound together.
It was known that the nucleus was composed
of protons and neutrons and that protons possessed
positive electric charge, while neutrons were
electrically neutral.
By the understanding of physics at that time,
positive charges would repel one another and
the positively charged protons should cause
the nucleus to fly apart.
However, this was never observed.
New physics was needed to explain this phenomenon.
A stronger attractive force was postulated
to explain how the atomic nucleus was bound
despite the protons' mutual electromagnetic
repulsion.
This hypothesized force was called the strong
force, which was believed to be a fundamental
force that acted on the protons and neutrons
that make up the nucleus.
It was later discovered that protons and neutrons
were not fundamental particles, but were made
up of constituent particles called quarks.
The strong attraction between nucleons was
the side-effect of a more fundamental force
that bound the quarks together into protons
and neutrons.
The theory of quantum chromodynamics explains
that quarks carry what is called a color charge,
although it has no relation to visible color.
Quarks with unlike color charge attract one
another as a result of the strong interaction,
and the particle that mediated this was called
the gluon.
== Behavior of the strong force ==
The word strong is used since the strong interaction
is the "strongest" of the four fundamental
forces.
At a distance of 1 femtometer (1 fm = 10−15
meters) or less, its strength is around 137
times that of the electromagnetic force, some
106 times as great as that of the weak force,
and about 1038 times that of gravitation.
The strong force is described by quantum chromodynamics
(QCD), a part of the standard model of particle
physics.
Mathematically, QCD is a non-Abelian gauge
theory based on a local (gauge) symmetry group
called SU(3).
The force carrier particle of the strong interaction
is the gluon, a massless boson.
Unlike the photon in electromagnetism, which
is neutral, the gluon carries a color charge.
Quarks and gluons are the only fundamental
particles that carry non-vanishing color charge,
and hence they participate in strong interactions
only with each other.
The strong force is the expression of the
gluon interaction with other quark and gluon
particles.
All quarks and gluons in QCD interact with
each other through the strong force.
The strength of interaction is parametrized
by the strong coupling constant.
This strength is modified by the gauge color
charge of the particle, a group theoretical
property.
The strong force acts between quarks.
Unlike all other forces (electromagnetic,
weak, and gravitational), the strong force
does not diminish in strength with increasing
distance between pairs of quarks.
After a limiting distance (about the size
of a hadron) has been reached, it remains
at a strength of about 10,000 newtons (N),
no matter how much farther the distance between
the quarks.
As the separation between the quarks grows,
the energy added to the pair creates new pairs
of matching quarks between the original two;
hence it is impossible to create separate
quarks.
The explanation is that the amount of work
done against a force of 10,000 newtons is
enough to create particle-antiparticle pairs
within a very short distance of that interaction.
The very energy added to the system required
to pull two quarks apart would create a pair
of new quarks that will pair up with the original
ones.
In QCD, this phenomenon is called color confinement;
as a result only hadrons, not individual free
quarks, can be observed.
The failure of all experiments that have searched
for free quarks is considered to be evidence
of this phenomenon.
The elementary quark and gluon particles involved
in a high energy collision are not directly
observable.
The interaction produces jets of newly created
hadrons that are observable.
Those hadrons are created, as a manifestation
of mass-energy equivalence, when sufficient
energy is deposited into a quark-quark bond,
as when a quark in one proton is struck by
a very fast quark of another impacting proton
during a particle accelerator experiment.
However, quark–gluon plasmas have been observed.
== Residual strong force ==
It is not the case that every quark in the
universe attracts every other quark in the
above distance independent manner.
Color confinement implies that the strong
force acts without distance-diminishment only
between pairs of quarks, and that in collections
of bound quarks (hadrons), the net color-charge
of the quarks essentially cancels out, resulting
in a limit of the action of the forces.
Collections of quarks (hadrons) therefore
appear nearly without color-charge, and the
strong force is therefore nearly absent between
those hadrons except that the cancellation
is not quite perfect.
A residual force remains (described below)
known as the residual strong force.
This residual force does diminish rapidly
with distance, and is thus very short-range
(effectively a few femtometers).
It manifests as a force between the "colorless"
hadrons, and is sometimes known as the strong
nuclear force or simply nuclear force.
The residual effect of the strong force is
called the nuclear force.
The nuclear force acts between hadrons, known
as mesons and baryons.
This "residual strong force", acting indirectly,
transmits gluons that form part of the virtual
π and ρ mesons, which, in turn, transmit
the force between nucleons that holds the
nucleus (beyond protium) together.
The residual strong force is thus a minor
residuum of the strong force that binds quarks
together into protons and neutrons.
This same force is much weaker between neutrons
and protons, because it is mostly neutralized
within them, in the same way that electromagnetic
forces between neutral atoms (van der Waals
forces) are much weaker than the electromagnetic
forces that hold electrons in association
with the nucleus, forming the atoms.Unlike
the strong force itself, the residual strong
force, does diminish in strength, and it in
fact diminishes rapidly with distance.
The decrease is approximately as a negative
exponential power of distance, though there
is no simple expression known for this; see
Yukawa potential.
The rapid decrease with distance of the attractive
residual force and the less-rapid decrease
of the repulsive electromagnetic force acting
between protons within a nucleus, causes the
instability of larger atomic nuclei, such
as all those with atomic numbers larger than
82 (the element lead).
Although the nuclear force is weaker than
strong interaction itself, it is still highly
energetic: transitions produce gamma rays.
The mass of nuclei is significantly different
from the masses of the individual nucleons.
This mass defect is due to the potential energy
associated with the nuclear force.
Differences between mass defects power nuclear
fusion and nuclear fission.
== Unification ==
The so-called Grand Unified Theories (GUT)
aim to describe the strong interaction and
the electroweak interaction as aspects of
a single force, similarly to how the electromagnetic
and weak interactions were unified by the
Glashow-Weinberg-Salam model into the electroweak
interaction.
The strong interaction has a property called
asymptotic freedom, wherein the strength of
the strong force diminishes at higher energies
(or temperatures).
The theorized energy where its strength becomes
equal to the electroweak interaction is the
grand unification energy.
However, no Grand Unified Theory has yet been
successfully formulated to describe this process,
and Grand Unification remains an unsolved
problem in physics.
If GUT is correct, after the Big Bang and
during the electroweak epoch of the universe,
the electroweak force separated from the strong
force.
Accordingly, a grand unification epoch is
hypothesized to have existed prior to this.
== See also ==
Nuclear binding energy
Color charge
Coupling constant
Nuclear physics
QCD matter
Quantum field theory and Gauge theory
Standard model of particle physics and Standard
Model (mathematical formulation)
Weak interaction, electromagnetism and gravity
Intermolecular force
Vortex
Yukawa interaction
