 candela; cd

The fundamental SI unit of luminous intensity defined as the
luminous intensity in a given direction of a source that emits
monochromatic photons of frequency 540 x 10^{12} Hz and has a
radiant intensity in that direction of 1/683 W/sr.
 Carnot's theorem (S. Carnot)

The theorem which states that no engine operating between two
temperatures can be more efficient than a reversible engine.
 Casimir effect (Casimir)

A quantum mechanical effect, where two very large plates placed
close to each other will experience an attractive force, in the
absence of other forces. The cause is virtual
particleantiparticle pair creation in the vicinity of the plates. Also,
the speed of light will be increased in the region between the two
plates, in the direction perpendicular to them.
 causality principle

The principle that cause must always preceed effect. More
formally, if an event A ("the cause") somehow influences an event
B ("the effect") which occurs later in time, then event B cannot
in turn have an influence on event A. That is, event B
must occur at a later time t than event A, and further,
all frames must agree upon this ordering.
The principle is best illustrated with an example. Say that
event A constitutes a murderer making the decision to kill his
victim, and that event B is the murderer actually committing the
act. The principle of causality puts forth that the act of
murder cannot have an influence on the murderer's decision to
commit it. If the murderer were to somehow see himself committing
the act and change his mind, then a murder would have been
committed in the future without a prior cause (he changed his
mind). This represents a causality violation. Both time travel
and fasterthanlight travel both imply violations of causality,
which is why most physicists think they are impossible, or at
least impossible in the general sense.
 centrifugal pseudoforce

A pseudoforce that occurs when one is moving in
uniform circular motion. One feels a "force" directed outward
from the center of motion.
 Chandrasekhar limit (S. Chandrasekhar; 1930)

A limit which mandates that no white dwarf (a collapsed,
degenerate star) can be more massive than about 1.4 masses solar.
Any degenerate mass more massive must inevitably collapse into a
neutron star.
 Charles' law (J.A.C. Charles; c. 1787)

The volume of an ideal gas at constant pressure is proportional to
the thermodynamic temperature of that gas.
 Cherenkov [Cerenkov] radiation (P.A. Cherenkov)

Radiation emitted by a charged particle which is moving faster
than light in the medium through which it is travelling. No
particle can travel faster than light in vacuum, but the speed of
light in other media, such as water, glass, etc., are considerably
lower. Cherenkov radiation is the electromagnetic analogue of the
sonic boom, though Cherenkov radiation is a shockwave set up in
the electromagnetic field.
 chronology protection conjecture (S.W. Hawking)

The concept that the formation of any closed timelike curve will
automatically be destroyed by quantum fluctuations as soon as it
is formed. In other words, quantum fluctuations prevent time
machines from being created.
 Coanda effect

The effect that indicates that a fluid tends to flow along a
surface, rather than flow through free space.
 complementarity principle (N. Bohr)

The principle that a given system cannot exhibit both wavelike
behavior and particlelike behavior at the same time. That is,
certain experiments will reveal the wavelike nature of a system,
and certain experiments will reveal the particlelike nature of a
system, but no experiment will reveal both simultaneously.
 Compton effect (A.H. Compton; 1923)

An effect that demonstrates that photons (the quantum of
electromagnetic radiation) have momentum. A photon fired at a
stationary particle, such as an electron, will impart momentum to
the electron and, since its energy has been decreased, will
experience a corresponding decrease in frequency.
 conservation laws

A law which states that, in a closed system, the total quantity of
something will not increase or decrease, but remain exactly the
same; that is, its rate of change is zero. For physical
quantities, it states that something can neither be created nor
destroyed. Mathematically, if a scalar X is the quantity
considered, then
dX/dt = 0,
or, equivalently,
X = constant.
For a vector field F, the conservation law is
written as
div F = 0;
that is, the vector field F is divergencefree
everywhere (i.e., has no sources or sinks).
Some specific examples of conservation laws are:
 conservation of massenergy

The total massenergy of a closed system remains constant.
 conservation of electric charge

The total electric charge of a closed system remains constant.
 conservation of linear momentum

The total linear momentum of a closed system remains constant.
 conservation of angular momentum

The total angular momentum of a closed system remains constant.
There are several other laws that deal with particle physics,
such as conservation of baryon number, of strangeness, etc., which
are conserved in some fundamental interactions (such as the
electromagnetic interaction) but not others (such as the weak
interaction).
 constancy principle (A. Einstein)

One of the postulates of A. Einstein's special theory of
relativity, which puts forth that the speed of light in vacuum is measured as the same
speed to all observers, regardless of their relative motion. That
is, if I'm travelling at 0.9 c away from you, and fire a beam of
light in that direction, both you and I will independently measure
the speed of that beam as c.
One of the results of this postulate (one of the predictions
of special relativity) is that no massive particle can be
accelerated to (or beyond) lightspeed, and thus the speed of light
also represents the ultimate cosmic speed limit. Only massless
particles (collectively called luxons, including photons,
gravitons, and possibly neutrinos, should they prove to indeed be
massless) travel at lightspeed, and all other particles must
travel at slower speeds.
See tachyons, causality principle.
 equation of continuity

An equation which states that a fluid flowing
through a pipe flows at a rate which is inversely proportional to
the crosssectional area of the pipe. That is, if the pipe
constricts, the fluid flows faster; if it widens, the fluid flows
slower. It is in essence a restatement of the consevation of mass
during constant flow.
 Cooper pairs (L.N. Cooper; 1957)

See BCS theory.
 Copernican principle (N. Copernicus)

The idea, suggested by Copernicus, that the Sun, not the Earth, is
at the center of the Universe. We now know that neither idea
is correct (the Sun is not even located at the center of our
Galaxy, much less the Universe), but it set into effect a long
chain of demotions of Earth's and our place in the Universe, to
where it is now: On an unimpressive planet orbiting a mediocre
star in a corner of a typical galaxy, lost in the Universe.
 Coriolis pseudoforce (G. de Coriolis; 1835)

A pseudoforce which arises because of motion relative to a frame which is
itself rotating relative to second, inertial frame. The magnitude
of the Coriolis "force" is dependent on the speed of the object
relative to the noninertial frame, and the direction of the
"force" is orthogonal to the object's velocity.
 correspondence limit (N. Bohr)

The limit at which a more general theory reduces to a more
specialized theory when the conditions that the specialized theory
requires are taken away.
See correspondence principle.
 correspondence principle (N. Bohr)

The principle that when a new, more general theory is put forth,
it must reduce to the more specialized (and usually simpler)
theory under normal circumstances. There are correspondence
principles for general relativity to special relativity and
special relativity to Newtonian mechanics, but the most widely
known correspondence principle (and generally what is meant when
one says "correspondence principle") is that of quantum mechanics
to classical mechanics.
See correspondence limit.
 cosmic background radiation; primal glow

The background of radiation mostly in the frequency range 3 x
10^{11} to 3 x 10^{8} Hz discovered in space in 1965. It is believed
to be the cosmologically redshifted radiation released by the big
bang itself. Presently it has an energy density in empty space of
about 4 x 10^{14} J/m^{3}.
 cosmic censorship conjecture (R. Penrose, 1979)

The conjecture, so far totally undemonstrated within the context
of general relativity, that all singularities (with the possible
exception of the big bang singularity) are accompanied by event
horizons which completely surround them at all points in time.
That is, problematic issues with the singularity are rendered
irrelevant, since no information can ever escape from a black
hole's event horizon.
 cosmological constant; Lambda

The constant introduced to the Einstein
field equation, intended to admit static cosmological solutions. At
the time the current philosophical view was the steadystate model
of the Universe, where the Universe has been around for infinite time.
Early analysis of the field equation indicated that general relativity
allowed dynamic cosmological models only (ones that are either
contracting or expanding), but no static models. Einstein
introduced the most natural abberation to the field equation that
he could think of: the addition of a term proportional to the
spacetime metric tensor, g, with the constant of proportionality
being the cosmological constant:
G + Lambda g = 8 pi T.
Hubble's later discovery of the expansion of the Universe indicated
that the introduction of the cosmological constant was unnecessary;
had Einstein believed what his field equation was telling him, he
could have claimed the expansion of the Universe as perhaps the
greatest and most convincing prediction of general relativity;
he called this the "greatest blunder of my life."
 cosmological redshift

An effect where light emitted from a distant source appears
redshifted because of the expansion of spacetime itself.
Compare Doppler effect.
 coulomb; C (after C. de Coulomb, 17361806)

The derived SI unit of electric charge, defined as the amount of
charge transferred by a current of 1 A in a period of 1 s; it thus
has units of A s.
 Coulomb's law (C. de Coulomb)

The primary law for electrostatics, analogous to Newton's law of
universal gravitation. It states that the force between two point
charges is proportional to the algebraic product of their
respective charges as well as proportional to the inverse square
of the distance between them; mathematically,
F = 1/(4 pi epsilon_{0}) (q Q/r^{2}) e,
where q and Q are the strengths of the two charges, r is the
distance between the two, and e is a unit vector directed from the
test charge to the second.
 Curie constant; C (P. Curie)

A characteristic constant, dependent on the material in
question, which indicates the proportionality between its
susceptibility and its thermodynamic temperature.
 Curie's law (P. Curie)

The susceptibility, khi, of an isotropic paramagnetic substance
is related to its thermodynamic temperature T by the
equation
khi = C/T
See CurieWeiss law.
 CurieWeiss law (P. Curie, P.E. Weiss)

A more general form of Curie's law, which
states that the susceptibility, khi, of an paramagnetic substance
is related to its thermodynamic temperature T by the
equation
khi = C/T  W

The laws list
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Laws, rules, principles, effects, paradoxes, limits, constants, experiments, & thoughtexperiments in physics.

