In physics, the fundamental interactions,
also known as fundamental forces, are the
interactions that do not appear to be reducible
to more basic interactions.
There are four fundamental interactions known
to exist: the gravitational and electromagnetic
interactions, which produce significant long-range
forces whose effects can be seen directly
in everyday life, and the strong and weak
interactions, which produce forces at minuscule,
subatomic distances and govern nuclear interactions.
Some scientists hypothesize that a fifth force
might exist, but the hypotheses remain speculative.
Each of the known fundamental interactions
can be described mathematically as a field.
The gravitational force is attributed to the
curvature of spacetime, described by Einstein's
general theory of relativity.
The other three are discrete quantum fields,
and their interactions are mediated by elementary
particles described by the Standard Model
of particle physics.
Within the Standard Model, the strong interaction
is carried by a particle called the gluon,
and is responsible for quarks binding together
to form hadrons, such as protons and neutrons.
As a residual effect, it creates the nuclear
force that binds the latter particles to form
atomic nuclei.
The weak interaction is carried by particles
called W and Z bosons, and also acts on the
nucleus of atoms, mediating radioactive decay.
The electromagnetic force, carried by the
photon, creates electric and magnetic fields,
which are responsible for the attraction between
orbital electrons and atomic nuclei which
holds atoms together, as well as chemical
bonding and electromagnetic waves, including
visible light, and forms the basis for electrical
technology.
Although the electromagnetic force is far
stronger than gravity, it tends to cancel
itself out within large objects, so over large
distances (on the scale of planets and galaxies),
gravity tends to be the dominant force.
Many theoretical physicists believe these
fundamental forces to be related and to become
unified into a single force at very high energies
on a minuscule scale, the Planck scale, but
particle accelerators cannot produce the enormous
energies required to experimentally probe
this.
Devising a common theoretical framework that
would explain the relation between the forces
in a single theory is perhaps the greatest
goal of today's theoretical physicists.
The weak and electromagnetic forces have already
been unified with the electroweak theory of
Sheldon Glashow, Abdus Salam, and Steven Weinberg
for which they received the 1979 Nobel Prize
in physics.
Progress is currently being made in uniting
the electroweak and strong fields within what
is called a Grand Unified Theory (GUT).
A bigger challenge is to find a way to quantize
the gravitational field, resulting in a theory
of quantum gravity (QG) which would unite
gravity in a common theoretical framework
with the other three forces.
Some theories, notably string theory, seek
both QG and GUT within one framework, unifying
all four fundamental interactions along with
mass generation within a theory of everything
(ToE).
== History ==
=== 
Classical theory ===
In his 1687 theory, Isaac Newton postulated
space as an infinite and unalterable physical
structure existing before, within, and around
all objects while their states and relations
unfold at a constant pace everywhere, thus
absolute space and time.
Inferring that all objects bearing mass approach
at a constant rate, but collide by impact
proportional to their masses, Newton inferred
that matter exhibits an attractive force.
His law of universal gravitation mathematically
stated it to span the entire universe instantly
(despite absolute time), or, if not actually
a force, to be instant interaction among all
objects (despite absolute space.)
As conventionally interpreted, Newton's theory
of motion modelled a central force without
a communicating medium.
Thus Newton's theory violated the first principle
of mechanical philosophy, as stated by Descartes,
No action at a distance.
Conversely, during the 1820s, when explaining
magnetism, Michael Faraday inferred a field
filling space and transmitting that force.
Faraday conjectured that ultimately, all forces
unified into one.
In 1873, James Clerk Maxwell unified electricity
and magnetism as effects of an electromagnetic
field whose third consequence was light, travelling
at constant speed in a vacuum.
The electromagnetic field theory contradicted
predictions of Newton's theory of motion,
unless physical states of the luminiferous
aether—presumed to fill all space whether
within matter or in a vacuum and to manifest
the electromagnetic field—aligned all phenomena
and thereby held valid the Newtonian principle
relativity or invariance.
=== The Standard Model ===
The Standard Model of particle physics was
developed throughout the latter half of the
20th century.
In the Standard Model, the electromagnetic,
strong, and weak interactions associate with
elementary particles, whose behaviours are
modelled in quantum mechanics (QM).
For predictive success with QM's probabilistic
outcomes, particle physics conventionally
models QM events across a field set to special
relativity, altogether relativistic quantum
field theory (QFT).
Force particles, called gauge bosons—force
carriers or messenger particles of underlying
fields—interact with matter particles, called
fermions.
Everyday matter is atoms, composed of three
fermion types: up-quarks and down-quarks constituting,
as well as electrons orbiting, the atom's
nucleus.
Atoms interact, form molecules, and manifest
further properties through electromagnetic
interactions among their electrons absorbing
and emitting photons, the electromagnetic
field's force carrier, which if unimpeded
traverse potentially infinite distance.
Electromagnetism's QFT is quantum electrodynamics
(QED).
The electromagnetic interaction was modelled
with the weak interaction, whose force carriers
are W and Z bosons, traversing the minuscule
distance, in electroweak theory (EWT).
Electroweak interaction would operate at such
high temperatures as soon after the presumed
Big Bang, but, as the early universe cooled,
split into electromagnetic and weak interactions.
The strong interaction, whose force carrier
is the gluon, traversing minuscule distance
among quarks, is modeled in quantum chromodynamics
(QCD).
EWT, QCD, and the Higgs mechanism, whereby
the Higgs field manifests Higgs bosons that
interact with some quantum particles and thereby
endow those particles with mass comprise particle
physics' Standard Model (SM).
Predictions are usually made using calculational
approximation methods, although such perturbation
theory is inadequate to model some experimental
observations (for instance bound states and
solitons.)
Still, physicists widely accept the Standard
Model as science's most experimentally confirmed
theory.
Beyond the Standard Model, some theorists
work to unite the electroweak and strong interactions
within a Grand Unified Theory (GUT).
Some attempts at GUTs hypothesize "shadow"
particles, such that every known matter particle
associates with an undiscovered force particle,
and vice versa, altogether supersymmetry (SUSY).
Other theorists seek to quantize the gravitational
field by the modelling behaviour of its hypothetical
force carrier, the graviton and achieve quantum
gravity (QG).
One approach to QG is loop quantum gravity
(LQG).
Still other theorists seek both QG and GUT
within one framework, reducing all four fundamental
interactions to a Theory of Everything (ToE).
The most prevalent aim at a ToE is string
theory, although to model matter particles,
it added SUSY to force particles—and so,
strictly speaking, became superstring theory.
Multiple, seemingly disparate superstring
theories were unified on a backbone, M-theory.
Theories beyond the Standard Model remain
highly speculative, lacking great experimental
support.
== Overview of the fundamental interactions
==
In the conceptual model of fundamental interactions,
matter consists of fermions, which carry properties
called charges and spin ±​1⁄2 (intrinsic
angular momentum ±​ħ⁄2, where ħ is
the reduced Planck constant).
They attract or repel each other by exchanging
bosons.
The interaction of any pair of fermions in
perturbation theory can then be modelled thus:
Two fermions go in → interaction by boson
exchange → Two changed fermions go out.The
exchange of bosons always carries energy and
momentum between the fermions, thereby changing
their speed and direction.
The exchange may also transport a charge between
the fermions, changing the charges of the
fermions in the process (e.g., turn them from
one type of fermion to another).
Since bosons carry one unit of angular momentum,
the fermion's spin direction will flip from
+​1⁄2 to −​1⁄2 (or vice versa) during
such an exchange (in units of the reduced
Planck's constant).
Because an interaction results in fermions
attracting and repelling each other, an older
term for "interaction" is force.
According to the present understanding, there
are four fundamental interactions or forces:
gravitation, electromagnetism, the weak interaction,
and the strong interaction.
Their magnitude and behaviour vary greatly,
as described in the table below.
Modern physics attempts to explain every observed
physical phenomenon by these fundamental interactions.
Moreover, reducing the number of different
interaction types is seen as desirable.
Two cases in point are the unification of:
Electric and magnetic force into electromagnetism;
The electromagnetic interaction and the weak
interaction into the electroweak interaction;
see below.Both magnitude ("relative strength")
and "range", as given in the table, are meaningful
only within a rather complex theoretical framework.
It should also be noted that the table below
lists properties of a conceptual scheme that
is still the subject of ongoing research.
The modern (perturbative) quantum mechanical
view of the fundamental forces other than
gravity is that particles of matter (fermions)
do not directly interact with each other,
but rather carry a charge, and exchange virtual
particles (gauge bosons), which are the interaction
carriers or force mediators.
For example, photons mediate the interaction
of electric charges, and gluons mediate the
interaction of color charges.
== The interactions ==
=== 
Gravity ===
Gravitation is by far the weakest of the four
interactions at the atomic scale, where electromagnetic
interactions dominate.
But the idea that the weakness of gravity
can easily be demonstrated by suspending a
pin using a simple magnet (such as a refrigerator
magnet) is fundamentally flawed.
The only reason the magnet is able to hold
the pin against the gravitational pull of
the entire Earth is due to its relative proximity.
There is clearly a short distance of separation
between magnet and pin where a breaking point
is reached, and due to the large mass of Earth
this distance is disappointingly small.
Thus gravitation is very important for macroscopic
objects and over macroscopic distances for
the following reasons.
Gravitation:
Is the only interaction that acts on all particles
having mass, energy and/or momentum
Has an infinite range, like electromagnetism
but unlike strong and weak interaction
Cannot be absorbed, transformed, or shielded
against
Always attracts and never repels (see function
of geodesic equation in general relativity)Even
though electromagnetism is far stronger than
gravitation, electrostatic attraction is not
relevant for large celestial bodies, such
as planets, stars, and galaxies, simply because
such bodies contain equal numbers of protons
and electrons and so have a net electric charge
of zero.
Nothing "cancels" gravity, since it is only
attractive, unlike electric forces which can
be attractive or repulsive.
On the other hand, all objects having mass
are subject to the gravitational force, which
only attracts.
Therefore, only gravitation matters on the
large-scale structure of the universe.
The long range of gravitation makes it responsible
for such large-scale phenomena as the structure
of galaxies and black holes and it retards
the expansion of the universe.
Gravitation also explains astronomical phenomena
on more modest scales, such as planetary orbits,
as well as everyday experience: objects fall;
heavy objects act as if they were glued to
the ground, and animals can only jump so high.
Gravitation was the first interaction to be
described mathematically.
In ancient times, Aristotle hypothesized that
objects of different masses fall at different
rates.
During the Scientific Revolution, Galileo
Galilei experimentally determined that this
hypothesis was wrong under certain circumstances
— neglecting the friction due to air resistance,
and buoyancy forces if an atmosphere is present
(e.g. the case of a dropped air-filled balloon
vs a water-filled balloon) all objects accelerate
toward the Earth at the same rate.
Isaac Newton's law of Universal Gravitation
(1687) was a good approximation of the behaviour
of gravitation.
Our present-day understanding of gravitation
stems from Einstein's General Theory of Relativity
of 1915, a more accurate (especially for cosmological
masses and distances) description of gravitation
in terms of the geometry of spacetime.
Merging general relativity and quantum mechanics
(or quantum field theory) into a more general
theory of quantum gravity is an area of active
research.
It is hypothesized that gravitation is mediated
by a massless spin-2 particle called the graviton.
Although general relativity has been experimentally
confirmed (at least for weak fields) on all
but the smallest scales, there are rival theories
of gravitation.
Those taken seriously by the physics community
all reduce to general relativity in some limit,
and the focus of observational work is to
establish limitations on what deviations from
general relativity are possible.
Proposed extra dimensions could explain why
the gravity force is so weak.
=== Electroweak interaction ===
Electromagnetism and weak interaction appear
to be very different at everyday low energies.
They can be modelled using two different theories.
However, above unification energy, on the
order of 100 GeV, they would merge into a
single electroweak force.
Electroweak theory is very important for modern
cosmology, particularly on how the universe
evolved.
This is because shortly after the Big Bang,
the temperature was approximately above 1015
K, the electromagnetic force and the weak
force were merged into a combined electroweak
force.
For contributions to the unification of the
weak and electromagnetic interaction between
elementary particles, Abdus Salam, Sheldon
Glashow and Steven Weinberg were awarded the
Nobel Prize in Physics in 1979.
==== Electromagnetism ====
Electromagnetism is the force that acts between
electrically charged particles.
This phenomenon includes the electrostatic
force acting between charged particles at
rest, and the combined effect of electric
and magnetic forces acting between charged
particles moving relative to each other.
Electromagnetism has infinite range like gravity,
but is vastly stronger than it, and therefore
describes a number of macroscopic phenomena
of everyday experience such as friction, rainbows,
lightning, and all human-made devices using
electric current, such as television, lasers,
and computers.
Electromagnetism fundamentally determines
all macroscopic, and many atomic levels, properties
of the chemical elements, including all chemical
bonding.
In a four kilogram (~1 gallon) jug of water
there are
4000
g
H
2
O
⋅
1
mol
H
2
O
18
g
H
2
O
⋅
10
mol
e
−
1
mol
H
2
O
⋅
96
,
000
C
1
mol
e
−
=
2.1
×
10
8
C
{\displaystyle 4000\ {\mbox{g}}\,H_{2}O\cdot
{\frac {1\ {\mbox{mol}}\,H_{2}O}{18\ {\mbox{g}}\,H_{2}O}}\cdot
{\frac {10\ {\mbox{mol}}\,e^{-}}{1\ {\mbox{mol}}\,H_{2}O}}\cdot
{\frac {96,000\ {\mbox{C}}\,}{1\ {\mbox{mol}}\,e^{-}}}=2.1\times
10^{8}C\ \,\ }
of total electron charge.
Thus, if we place two such jugs a meter apart,
the electrons in one of the jugs repel those
in the other jug with a force of
1
4
π
ε
0
(
2.1
×
10
8
C
)
2
(
1
m
)
2
=
4.1
×
10
26
N
.
{\displaystyle {1 \over 4\pi \varepsilon _{0}}{\frac
{(2.1\times 10^{8}C)^{2}}{(1m)^{2}}}=4.1\times
10^{26}N.}
This force is larger than the planet Earth
would weigh if weighed on another Earth.
The atomic nuclei in one jug also repel those
in the other with the same force.
However, these repulsive forces are canceled
by the attraction of the electrons in jug
A with the nuclei in jug B and the attraction
of the nuclei in jug A with the electrons
in jug B, resulting in no net force.
Electromagnetic forces are tremendously stronger
than gravity but cancel out so that for large
bodies gravity dominates.
Electrical and magnetic phenomena have been
observed since ancient times, but it was only
in the 19th century James Clerk Maxwell discovered
that electricity and magnetism are two aspects
of the same fundamental interaction.
By 1864, Maxwell's equations had rigorously
quantified this unified interaction.
Maxwell's theory, restated using vector calculus,
is the classical theory of electromagnetism,
suitable for most technological purposes.
The constant speed of light in a vacuum (customarily
described with a lowercase letter "c") can
be derived from Maxwell's equations, which
are consistent with the theory of special
relativity.
Einstein's 1905 theory of special relativity,
however, which flows from the observation
that the speed of light is constant no matter
how fast the observer is moving, showed that
the theoretical result implied by Maxwell's
equations has profound implications far beyond
electromagnetism on the very nature of time
and space.
In another work that departed from classical
electro-magnetism, Einstein also explained
the photoelectric effect by utilizing Max
Planck's discovery that light was transmitted
in 'quanta' of specific energy content based
on the frequency, which we now call photons.
Starting around 1927, Paul Dirac combined
quantum mechanics with the relativistic theory
of electromagnetism.
Further work in the 1940s, by Richard Feynman,
Freeman Dyson, Julian Schwinger, and Sin-Itiro
Tomonaga, completed this theory, which is
now called quantum electrodynamics, the revised
theory of electromagnetism.
Quantum electrodynamics and quantum mechanics
provide a theoretical basis for electromagnetic
behavior such as quantum tunneling, in which
a certain percentage of electrically charged
particles move in ways that would be impossible
under the classical electromagnetic theory,
that is necessary for everyday electronic
devices such as transistors to function.
==== Weak interaction ====
The weak interaction or weak nuclear force
is responsible for some nuclear phenomena
such as beta decay.
Electromagnetism and the weak force are now
understood to be two aspects of a unified
electroweak interaction — this discovery
was the first step toward the unified theory
known as the Standard Model.
In the theory of the electroweak interaction,
the carriers of the weak force are the massive
gauge bosons called the W and Z bosons.
The weak interaction is the only known interaction
which does not conserve parity; it is left-right
asymmetric.
The weak interaction even violates CP symmetry
but does conserve CPT.
=== Strong interaction ===
The strong interaction, or strong nuclear
force, is the most complicated interaction,
mainly because of the way it varies with distance.
At distances greater than 10 femtometers,
the strong force is practically unobservable.
Moreover, it holds only inside the atomic
nucleus.
After the nucleus was discovered in 1908,
it was clear that a new force, today known
as the nuclear force, was needed to overcome
the electrostatic repulsion, a manifestation
of electromagnetism, of the positively charged
protons.
Otherwise, the nucleus could not exist.
Moreover, the force had to be strong enough
to squeeze the protons into a volume whose
diameter is about 10−15 m, much smaller
than that of the entire atom.
From the short range of this force, Hideki
Yukawa predicted that it was associated with
a massive particle, whose mass is approximately
100 MeV.
The 1947 discovery of the pion ushered in
the modern era of particle physics.
Hundreds of hadrons were discovered from the
1940s to 1960s, and an extremely complicated
theory of hadrons as strongly interacting
particles was developed.
Most notably:
The pions were understood to be oscillations
of vacuum condensates;
Jun John Sakurai proposed the rho and omega
vector bosons to be force carrying particles
for approximate symmetries of isospin and
hypercharge;
Geoffrey Chew, Edward K. Burdett and Steven
Frautschi grouped the heavier hadrons into
families that could be understood as vibrational
and rotational excitations of strings.While
each of these approaches offered deep insights,
no approach led directly to a fundamental
theory.
Murray Gell-Mann along with George Zweig first
proposed fractionally charged quarks in 1961.
Throughout the 1960s, different authors considered
theories similar to the modern fundamental
theory of quantum chromodynamics (QCD) as
simple models for the interactions of quarks.
The first to hypothesize the gluons of QCD
were Moo-Young Han and Yoichiro Nambu, who
introduced the quark color charge and hypothesized
that it might be associated with a force-carrying
field.
At that time, however, it was difficult to
see how such a model could permanently confine
quarks.
Han and Nambu also assigned each quark color
an integer electrical charge, so that the
quarks were fractionally charged only on average,
and they did not expect the quarks in their
model to be permanently confined.
In 1971, Murray Gell-Mann and Harald Fritzsch
proposed that the Han/Nambu color gauge field
was the correct theory of the short-distance
interactions of fractionally charged quarks.
A little later, David Gross, Frank Wilczek,
and David Politzer discovered that this theory
had the property of asymptotic freedom, allowing
them to make contact with experimental evidence.
They concluded that QCD was the complete theory
of the strong interactions, correct at all
distance scales.
The discovery of asymptotic freedom led most
physicists to accept QCD since it became clear
that even the long-distance properties of
the strong interactions could be consistent
with experiment if the quarks are permanently
confined.
Assuming that quarks are confined, Mikhail
Shifman, Arkady Vainshtein and Valentine Zakharov
were able to compute the properties of many
low-lying hadrons directly from QCD, with
only a few extra parameters to describe the
vacuum.
In 1980, Kenneth G. Wilson published computer
calculations based on the first principles
of QCD, establishing, to a level of confidence
tantamount to certainty, that QCD will confine
quarks.
Since then, QCD has been the established theory
of the strong interactions.
QCD is a theory of fractionally charged quarks
interacting by means of 8 bosonic particles
called gluons.
The gluons interact with each other, not just
with the quarks, and at long distances the
lines of force collimate into strings.
In this way, the mathematical theory of QCD
not only explains how quarks interact over
short distances but also the string-like behavior,
discovered by Chew and Frautschi, which they
manifest over longer distances.
=== Beyond the Standard Model ===
Numerous theoretical efforts have been made
to systematize the existing four fundamental
interactions on the model of electroweak unification.
Grand Unified Theories (GUTs) are proposals
to show that the three fundamental interactions
described by the Standard Model are all different
manifestations of a single interaction with
symmetries that break down and create separate
interactions below some extremely high level
of energy.
GUTs are also expected to predict some of
the relationships between constants of nature
that the Standard Model treats as unrelated,
as well as predicting gauge coupling unification
for the relative strengths of the electromagnetic,
weak, and strong forces (this was, for example,
verified at the Large Electron–Positron
Collider in 1991 for supersymmetric theories).Theories
of everything, which integrate GUTs with a
quantum gravity theory face a greater barrier,
because no quantum gravity theories, which
include string theory, loop quantum gravity,
and twistor theory, have secured wide acceptance.
Some theories look for a graviton to complete
the Standard Model list of force-carrying
particles, while others, like loop quantum
gravity, emphasize the possibility that time-space
itself may have a quantum aspect to it.
Some theories beyond the Standard Model include
a hypothetical fifth force, and the search
for such a force is an ongoing line of experimental
research in physics.
In supersymmetric theories, there are particles
that acquire their masses only through supersymmetry
breaking effects and these particles, known
as moduli can mediate new forces.
Another reason to look for new forces is the
discovery that the expansion of the universe
is accelerating (also known as dark energy),
giving rise to a need to explain a nonzero
cosmological constant, and possibly to other
modifications of general relativity.
Fifth forces have also been suggested to explain
phenomena such as CP violations, dark matter,
and dark flow.
In December 2015, two observations in the
ATLAS and CMS detectors at the Large Hadron
Collider hinted at the existence of a new
particle six times heavier than the Higgs
boson.
However, after obtaining more experimental
data, the anomaly appeared to not be significant.
== See also ==
Quintessence, a hypothesized fifth force.
Gerardus 't Hooft
Edward Witten
Howard Georgi
