(Redirected from Fundamental force)
A 'fundamental interaction' or 'fundamental force' is a mechanism by which particles interact with each other, and which cannot be explained in terms of another interaction. Every observed
physical phenomenon can be explained by these interactions. The apparent irreducible nature of these interactions leads physicists to study the properties of these forces in great detail. In modern physics, there are four fundamental interactions (forces):
gravitation,
electromagnetism, the
weak interaction, and the
strong interaction. Their magnitude and behavior vary greatly, as described in the table below.
{| class="wikitable"
|-
! Interaction !! Current Theory !! Mediators !! Relative Strength[1] !! Long-Distance Behavior !! Range(m)
|-
| Strong || Quantum chromodynamics
(QCD) || gluons || 1038 ||
(see discussion below) || 10-15
|-
| Electromagnetic || Quantum electrodynamics
(QED) || photons || 1036 || || infinite
|-
| Weak || Electroweak Theory || W and Z bosons || 1025 || || 10-18
|-
| Gravitation || General Relativity
(GR) || gravitons || 1 || || infinite
|}
The modern quantum mechanical view of the three fundamental forces (all except 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 are the mediators of the interaction of
electric charges; and
gluons are the mediators of the interaction of
color charges.
The interactions
Gravitation
Main articles: Gravitation
''Gravitation'' is by far the weakest interaction, but at long distances is the most important force. There are two reasons why gravity's strength relative to other forces become important at long distances. The first is that gravity has an infinite range like electromagnetism. The second reason why gravity is important at long distances is because all masses are positive and therefore gravity's interaction can not be screened like in electromagnetism. Thus large bodies such as planets, stars and galaxies dominantly feel gravitational forces. In comparison, the total electric charge of these bodies is zero because half of all charges are negative. In addition, unlike the other interactions, gravity acts universally on all matter. There are no objects that lack a gravitational "charge".
Because of its long range, gravity is responsible for such large-scale phenomena as the structure of galaxies,
black holes and the
expansion of the universe, as well as more elementary astronomical phenomena like the
orbits of
planets, and everyday experience: objects fall; heavy objects act as if they were glued to the ground; people are limited in how high they can jump.
Gravitation was the first kind of interaction which was described by a mathematical theory. In ancient times,
Aristotle theorized that objects of different masses fall at different rates. During the
Scientific Revolution,
Galileo Galilei experimentally determined that this was not the case — if friction due to air resistance is neglected, all objects accelerate toward the ground at the same rate.
Isaac Newton's
law of Universal Gravitation (
1687) was a good approximation of the general behaviour of gravity. In
1915,
Albert Einstein completed the
General Theory of Relativity, a more accurate description of gravity in terms of the
geometry of
space-time.
An area of active research today involves merging the theories of general relativity and
quantum mechanics into a more general theory of
quantum gravity. It is widely believed that in a theory of quantum gravity, gravity would be mediated by a massless spin 2 particle which is known as the
graviton. Gravitons are hypothetical particles not yet observed.
Although general relativity appears to present an accurate theory of gravity in the non-quantum mechanical limit, there are a number of alternate theories of gravity. Those under any serious consideration 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.
Electromagnetism
Main articles: Electromagnetism
''Electromagnetism'' is the force that acts between
electrically charged particles. This phenomenon includes the
electrostatic force, acting between charges at rest, and the combined effect of
electric and
magnetic forces acting between charges moving relative to each other.
Electromagnetism is also an infinite-ranged force, but it is much stronger than gravity, and therefore describes almost all phenomena of our everyday experience, ranging from
lasers and
radios to the structure of
atoms and
metals to phenomena such as
friction and
rainbows.
Electrical and magnetic phenomena have been observed since ancient times, but it was only in the 1800s that scientists discovered that electricity and magnetism are two aspects of the same fundamental interaction. By
1864,
Maxwell's equations had rigorously quantified the unified phenomenon. In
1905, Einstein's theory of
special relativity resolved the issue of the constancy of the speed of light, and explained the
photoelectric effect by theorizing that light was transmitted in quanta, which we now call
photons. Starting around 1927,
Paul Dirac unified quantum mechanics with the relativistic theory of
electromagnetism; the theory of
quantum electrodynamics was completed in the
1940s.
Weak interaction
Main articles: Weak interaction
The ''weak interaction'' or ''weak nuclear force'' is responsible for some phenomena at the scale of the atomic nucleus, such as
beta decay. Electromagnetism and the weak force are theoretically understood to be two aspects of a unified
electroweak interaction — this realization was the first step toward the unified theory known as the
Standard Model. In electroweak theory, the carriers of the weak force are massive
gauge bosons called the
W and Z bosons. The weak interaction is the only known interaction in which
parity is not conserved; it is left-right asymmetric. It even breaks
CP symmetry.
However, it does conserve
CPT.
Strong interaction
Main articles: Strong interaction
The strong interaction is the most complicated force because it takes on several different behaviors depending on the distance that is being tested. At distances larger than 10
femtometers, the strong force is incredibly weakly interacting, which is why it wasn't hypothesized to exist until the beginning of the 20th century. When protons and neutrons were discovered to be the constituents of the nucleus, it was necessary to postulate that there was an additional force that was stronger than electricity and magnetism so that the protons would be bound together in a 10
-15 fraction of the volume of an atom.
Hideki Yukawa postulated the existence of a particle with a mass of 100 MeV to explain this force. The
pion was discovered in 1947 and ushered in the era of nuclear physics. An extremely complicated theory of the strongly interacting particles, known as
hadrons, was developed. Hundreds of
hadrons were discovered from the 1940s to 1960s.
In 1973
David Gross,
Frank Wilczek, and
David Politzer proposed
asymptotic freedom as the theory of the strong force and put forth
quantum chromodynamics or QCD, as a force mediated by
gluons that act between particles that carry "color charge",
quarks and gluons. A characteristic of the strong interaction is that
gluons interact with each other.
Current developments
The
Standard Model is a theory of three fundamental forces — electromagnetism, weak interactions and strong interactions; however, these three forces are not tied together.
Howard Georgi and
Sheldon Glashow discovered that the Standard Model particles can arise from a single interaction, known as a
grand unified theory. Grand unified theories predict relationships between otherwise unrelated constants of nature in the Standard Model.
Gauge coupling unification is the prediction from grand unified theories for the relative strengths of the electromagnetic, weak and strong forces and this prediction was verified at
LEP in 1991 for
supersymmetric theories.
Currently, there is no complete theory of
quantum gravity. The leading candidate for a framework to fit quantum gravity is
string theory.
In theories
Beyond the Standard Model, there are frequently
fifth forces and the search for these forces is an on-going line of experimental research in physics. In
supersymmetric theories, there are particles that only acquire their masses through supersymmetry breaking effects and these particles, known as
moduli can mediate new forces. Another possible motivation for new forces is related to the accelerating expansion of the universe. The most concrete examples of new forces from the cosmological expansion result from modifications of
General Relativity.
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 the proposed fifth (or fourth if the '
electroweak' unification allowed) force.
★ ''People'':
Isaac Newton,
James Clerk Maxwell,
Albert Einstein,
Sheldon Glashow,
Abdus Salam,
Steven Weinberg,
Gerardus 't Hooft,
David Gross,
Edward Witten,
Howard Georgi
Notes
1. Approximate. The exact strengths depend on the particles and energies involved.
References
★ Feynman, Richard P. (1967). ''The Character of Physical Law''. MIT Press. ISBN 0-262-56003-8
★ Weinberg, S. (1993). ''The First Three Minutes: A Modern View of the Origin of the Universe''. Basic Books. ISBN 0-465-02437-8
★ Weinberg, S. (1994). ''Dreams of a Final Theory''. Vintage Books USA. ISBN 0-679-74408-8
★ Padmanabhan, T. (1998). ''After The First Three Minutes: The Story of Our Universe''. Cambridge University Press. ISBN 0-521-62972-1
★ Perkins, Donald H. (2000). ''Introduction to High Energy Physics''. Cambridge University Press. ISBN 0-521-62196-8
العربية
中国
Français
Deutsch
Ελληνική
हिन्दी
Italiano
日本語
Português
Русский
Español
