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Fundamental Interaction
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Fundamental Interactions, also known as fundamental forces, are the interactions in
physical systems that do not appear to be reducible to more basic interactions. There are four
conventionally accepted fundamental interactions—gravitational, electromagnetic, strong
nuclear, and weak nuclear. Each one is understood as the dynamics of a field. The
gravitational force is modelled as a continuous classical field. The other three are each
modelled as discrete quantum fields, and exhibit a measurable unit or elementary particle.
The two nuclear interactions produce strong forces at minuscule, subatomic distances. The
strong nuclear interaction is responsible for the binding of atomic nuclei. The weak nuclear
interaction also acts on the nucleus, mediating radioactive decay. Electromagnetism and
gravity produce significant forces at macroscopic scales where the effects can be seen
directly in every day life. Electrical and magnetic fields tend to cancel each other out when
large collections of objects are considered, so over the largest distances (on the scale of
planets and galaxies), gravity tends to be the dominant force.
Theoretical physicists working beyond the Standard Model seek to quantize the gravitational
field toward predictions that particle physicists can experimentally confirm, thus yielding
acceptance to a theory of quantum gravity (QG). (Phenomena suitable to model as a fifth
force—perhaps an added gravitational effect—remain widely disputed.) Other theorists seek
to unite the electroweak and strong fields within a Grand Unified Theory (GUT). While all
four fundamental interactions are widely thought to align on a highly minuscule scale,
particle accelerators cannot produce the massive energy levels required to experimentally
probe at that Planck scale (which would experimentally confirm such theories.) Yet some
theories, such as the 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).
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General relativity
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 the early 1870s, 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.
Disfavouring hypotheses at unobservables, Albert 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 to be in motion—
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length contraction and time dilation for the object experienced to be in relative motion—
Einstein's principle special relativity, published in 1905.
Special relativity was accepted as a theory too. It 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 modelled special relativity to a unification of
space and time, 4D spacetime. Stretching the three spatial dimensions onto the single
dimension of time's arrow, Einstein arrived at the 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 at 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 its 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.
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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.
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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.
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