Fundamental interactions, also known as fundamental
forces or interactive forces, are modeled
in physics as patterns of relations in physical
systems, evolving over time, whose objects
appear not to be reducible to more basic entities.
There are four conventionally accepted fundamental
interactions—gravitational, electromagnetic,
strong nuclear, and weak nuclear—each understood
as the dynamics of a field. The gravitational
force is modeled as a continuous classical
field. Each of the other three is modeled
as a discrete quantum field, and exhibits
a measurable unit or elementary particle.
Gravitation and electromagnetism act over
potentially infinite distance—across the
universe—and mediate everyday phenomena
of human experience. The other two fields
act over minuscule, subatomic distances. Synthesizing
chemical elements via nuclear fusion within
stars and quasars, the strong binds the atomic
nucleus—the force released during nuclear
fission as in detonation of a nuclear bomb—whereas
the weak mediates radioactive decay.
Modest gravitational effects are conventionally
predicted via refinements of the theory of
universal gravitation. Yet the gravitational
phenomenon itself is conventionally explained
as a consequence of spacetime's dynamic geometry
"curving" in the vicinity of mass, and is
modeled by the general theory of relativity.
UG and GR comprise classical mechanics. As
energy alters spatial and temporal relations—effects
notable at vast energy levels—such relativistic
effects on space and time are modeled in relativistic
mechanics, whose relativity theory extends
classical mechanics via GR and special theory
of relativity.
Experimentally detected phenomena of elementary
particles were first modeled in quantum mechanics.
For predictive accuracy at high energy, however,
QM was set to SR, and yielded quantum field
theory, whose first quantized the electromagnetic
field, quantum electrodynamics. QED was reduced
along with the weak field to the QFT electroweak
theory. The strong field was modeled as quantum
chromodynamics. EWT together with QCD and
the Higgs mechanism—which models the phenomena
of some particles bearing mass—comprise
particle physics' Standard Model.
Theoretical physicists working beyond the
Standard Model seek to quantize the gravitational
field toward predictions that particle physicists
can experimentally confirm, thus yielding
acceptance of a theory of quantum gravity.
Other theorists seek to unite the electroweak
and strong fields within a Grand Unified Theory.
Yet all four fundamental interactions are
widely thought to align at an extremely minuscule
scale, although particle accelerators cannot
produce the massive energy levels to experimentally
probe at that Planck scale to experimentally
confirm such theories. Still, some theories,
principally string theory, seek both QG and
GUT within one framework, unifying all four
fundamental interactions along with mass production
within a theory of everything.
General relativity
In his 1687 theory, 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, or, if not
actually a force, to be instant interaction
among all objects. As conventionally interpreted,
Newton's theory of motion modeled a central
force without a communicating medium. Newton's
thus 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 filing space and
transmitting that force. Faraday conjectured
that ultimately, all forces unified into one.
In the early 1870s, James Clerk Maxwell unified
electricity and magnetism as effects of an
electromagnetic field whose third consequence
was light, traveling 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—were aligning
all phenomena and thereby holding valid the
Newtonian principle relativity or invariance.
Disfavoring hypotheses at unobservables, Einstein
discarded the aether, and aligned electrodynamics
with relativity by denying absolute space
and time, and stating relative space and time.
The two phenomena altered in the vicinity
of an object measured in to be motion—length
contraction and time dilation for the object
experienced to be in relative motion—Einstein's
principle special relativity, published in
1905.
Accepted as theory, too, special relativity
rendered Newton's theory of motion apparently
untenable, especially since Newtonian physics
postulated an object's mass to be constant.
A consequence of special relativity is mass
being a variant form of energy, condensed
into an object. By the equivalence principle,
published by Einstein in 1907, gravitation
is indistinguishable from acceleration, perhaps
two phenomena sharing a mechanism. That year,
Hermann Minkowski modeled special relativity
to a unification of space and time, 4D spacetime.
So stretching the three spatial dimensions
onto the single dimension of time's arrow,
Einstein arrived at general theory of relativity
in 1915. Einstein interpreted space as a substance,
Einstein aether, whose physical properties
receive motion from an object and transmit
it to other objects while modulating events'
unfolding. Equivalent to energy, mass contracts
space, which dilates time—events unfold
more slowly—establishing local tension.
The object relieves it in the likeness of
a free fall at light speed along the pathway
of least resistance, a straight line's equivalent
on the curved surface of 4D spacetime, a pathway
termed worldline.
Einstein abolished action at a distance by
theorizing a gravitational field—4D spacetime—that
waves while transmitting motion across the
universe a light speed. All objects always
travel at light speed in 4D spacetime. At
zero relative speed, an object is observed
to travel none through space, but age most
rapidly. That is, an object at relative rest
in 3D space exhibits its constant energy to
an observer by exhibiting top speed along
1D time flow. Conversely, at highest relative
speed, an object traverses 3D space at light
speed, yet is ageless, none of its constant
energy available to internal motion as flow
along 1D time. Whereas Newtonian inertia is
an idealized case of an object either keeping
rest or holding constant velocity by hypothetical
existence in a universe otherwise devoid of
matter, Einsteinian inertia is indistinguishable
from an object experiencing no acceleration
by existing in a gravitational field possibly
full of matter distributed uniformly. Conversely,
even massless energy manifests gravitation—which
is acceleration—on local objects by "curving"
the surface of 4D spacetime. Physicists renounced
belief that motion must be mediated by a force.
Standard Model
The electromagnetic, strong, and weak interactions
associate with elementary particles, whose
behaviors are modeled in quantum mechanics.
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. 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.
The electromagnetic interaction was modeled
with the weak interaction, whose force carriers
are W and Z bosons, traversing minuscule distance,
in electroweak theory. 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. 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. Predictions are usually made using
calculational approximation methods, although
such perturbation theory is inadequate to
model some experimental observations. 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. Some attempts
at GUTs hypothesize "shadow" particles, such
that every known matter particle associates
with an undiscovered force particle, and vice
versa, altogether supersymmetry. Other theorists
seek to quantize the gravitational field by
modeling behavior of its hypothetical force
carrier, the graviton and achieve quantum
gravity. One approach to QG is loop quantum
gravity. Still other theorists seek both QG
and GUT within one framework, reducing all
four fundamental interactions to a Theory
of Everything. 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 interaction
In the conceptual model of fundamental interactions,
matter consists of fermions, which carry properties
called charges and spin ±1⁄2. They attract
or repel each other by exchanging bosons.
The interaction of any pair of fermions in
perturbation theory can then be modeled 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. Since bosons carry one unit of angular
momentum, the fermion's spin direction will
flip from +1⁄2 to −1⁄2 during such an
exchange.
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 behavior 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 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 quantum mechanical view of the
fundamental forces other than gravity is that
particles of matter do not directly interact
with each other, but rather carry a charge,
and exchange virtual particles, 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
Gravitation
Gravitation is by far the weakest of the four
interactions. The weakness of gravity can
easily be demonstrated by suspending a pin
using a simple magnet. The magnet is able
to hold the pin against the gravitational
pull of the entire Earth.
Yet 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;
has an infinite range, like electromagnetism
but unlike strong and weak interaction;
cannot be absorbed, transformed, or shielded
against;
always attracts and never repels.
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, 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 was not the case — neglecting
the friction due to air resistance, and buoyancy
forces if an atmosphere is present all objects
accelerate toward the Earth at the same rate.
Isaac Newton's law of Universal Gravitation
was a good approximation of the behaviour
of gravitation. Our present-day understanding
of gravitation stems from Albert Einstein's
General Theory of Relativity of 1915, a more
accurate description of gravitation in terms
of the geometry of space-time.
Merging general relativity and quantum mechanics
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 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.
Electroweak interaction
Electromagnetism and weak interaction appear
to be very different at everyday low energies.
They can be modeled 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. Electromagnetic force and 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 is infinite-ranged like gravity,
but vastly stronger, 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 level, properties
of the chemical elements, including all chemical
bonding.
In a four kilogram jug of water there are
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
This is larger than what 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 cancelled 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 that it was 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 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 electro-magnetism on the very nature
of time and space.
In other work that departed from classical
electro-magnetism, Einstein also explained
the photoelectric effect by hypothesizing
that light was transmitted in quanta, 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 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 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 that
is 10−15 of 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 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 photon-like 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 electro-weak
unification.
Grand Unified Theories are proposals to show
that all of the fundamental interactions,
other than gravity, arise from a single interaction
with symmetries that break down at low energy
levels. GUTs predict relationships among constants
of nature that are unrelated in the SM. GUTs
also predict gauge coupling unification for
the relative strengths of the electromagnetic,
weak, and strong forces, a prediction verified
at the LEP 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 recent discovery that the expansion
of the universe is accelerating, 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.
See also
Standard Model
Strong interaction
Electroweak interaction
Weak interaction
Gravity
Quantum gravity
String Theory
Theory of Everything
Grand Unified Theory
Gauge coupling unification
Unified Field Theory
Quintessence, a hypothesized fifth force.
People: Isaac Newton, James Clerk Maxwell,
Albert Einstein, Richard Feynman, Sheldon
Glashow, Abdus Salam, Steven Weinberg, Gerardus
't Hooft, David Gross, Edward Witten, Howard
Georgi.
References
Notes
Bibliography
General:
Davies, Paul, The Forces of Nature, Cambridge
Univ. Press  2nd ed.
Feynman, Richard, The Character of Physical
Law, MIT Press, ISBN 0-262-56003-8 
Schumm, Bruce A., Deep Down Things, Johns
Hopkins University Press  While all interactions
are discussed, discussion is especially thorough
on the weak.
Weinberg, Steven, The First Three Minutes:
A Modern View of the Origin of the Universe,
Basic Books, ISBN 0-465-02437-8 
Weinberg, Steven, Dreams of a Final Theory,
Basic Books, ISBN 0-679-74408-8 
Texts:
Padmanabhan, T., After The First Three Minutes:
The Story of Our Universe, Cambridge Univ.
Press, ISBN 0-521-62972-1 
Perkins, Donald H., Introduction to High Energy
Physics, Cambridge Univ. Press, ISBN 0-521-62196-8 
Riazuddin. "Non-standard interactions". NCP
5th Particle Physics Sypnoisis 1: 1–25.
Retrieved March 19, 2011. 
