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Schroedinger's cat to Système Internationale d'Unités.


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 Schroedinger's cat (E. Schroedinger; 1935)

A thought experiment designed to illustrate the counterintuitive
and strange notions of reality that come along with quantum
mechanics.
A cat is sealed inside a closed box; the cat has ample air,
food, and water to survive an extended period. This box is
designed so that no information (i.e., sight, sound, etc.) can
pass into or out of the box  the cat is totally cut off from
your observations. Also inside the box with the poor kitty
(apparently Schroedinger was not too fond of felines) is a phial
of a gaseous poison, and an automatic hammer to break it, flooding
the box and killing the cat. The hammer is hooked up to a Geiger
counter; this counter is monitoring a radioactive sample and is
designed to trigger the hammer  killing the cat  should a
radioactive decay be detected. The sample is chosen so that
after, say, one hour, there stands a fiftyfifty chance of a decay
occurring.
The question is, what is the state of the cat after that one
hour has elapsed? The intuitive answer is that the cat is either
alive or dead, but you don't know which until you look. But it
is one of them. Quantum mechanics, on the other hands, says
that the wavefunction describing the cat is in a superposition of
states: the cat is, in fact, fifty per cent alive and fifty per
cent dead; it is both. Not until one looks and "collapses the
wavefunction" is the Universe forced to choose either a live cat
or a dead cat and not something in between.
This indicates that observation also seems to be an important
part of the scientific process  quite a departure from the
absolutely objective, deterministic way things used to be with
Newton.
 Schwarzschild radius

The radius r of the event horizon for a
Schwarzschild black hole of mass m is given by (in geometrized
units) r = 2 m. In conventional units,
r = 2 G m/c^{2}.

second; s

The fundamental SI unit of time, defined as the period of time
equal to the duration of 9 192 631 770 periods of the radiation
corresponding to the transition between two hyperfine levels of
the ground state of the cesium133 atom.
 siemens; S (after E.W. von Siemens, 18161892)

The derived SI unit of electrical conductance equal to the
conductance of an element that has a resistance of 1 O [ohm]; it
has units of O^{1}.
 sievert; Sv

The derived SI unit of dose equivalent, defined as the absorbed
dose of ionizing radiation multiplied by internationallyagreedupon
dimensionless weights, since different types of ionizing radiation
cause different types of damage in living tissue. The Sv, like
the Gy, has units of J/kg.
 sigma

See StefanBoltzmann constant.
 simultaneity principle

The principle that all frames of reference will have invariant
simultaneity; that is, two events perceived as simultaneous (i.e.,
having the same time coordinate) in one frame will be perceived as
simultaneous in all other frames. According to special
relativity, however, this is not the case; simultaneity is framedependent.
 singularity

The center of a black hole, where the curvature of spacetime is
maximal. At the singularity, the gravitational tides diverge;
no solid object can even theoretically survive hitting the
singularity. Although singularities generally predict
inconsistencies in theory, singularities within black holes do
not necessarily imply that general relativity is incomplete
so long as singularities are always surrounded by event
horizons.
A proper formulation of quantum gravity may well avoid
the classical singularity at the centers of black holes.
See cosmic censorship conjecture.
 Snell's law

See refraction law.
 speed of light (in vacuo); c

The speed at which electromagnetic radiation propagates in a
vacuum; it is defined as 299 792 458 m/s.
 spinorbit effect

An effect that causes atomic energy levels to be split because
electrons have intrinsic angular momentum (spin) in addition to
their extrinsic orbital angular momentum.
 standard quantum limit

The limit imposed on standard methods of measurement by the
uncertainty principle within quantum mechanics.
 static limit

The distance from a rotating black hole where no observer can
possibly remain at rest (with respect to the distant stars)
because of inertial frame dragging; this region is outside of the
event horizon, except at the poles where it meets the horizon at a
point. The region between the event horizon and the static limit
is called the ergosphere.
 StefanBoltzmann constant; sigma (Stefan, L. Boltzmann)

The constant of proportionality present in the StefanBoltzmann
law. It is equal to 5.6697 x 10^{8} W/m^{2}/K^{4}.
 StefanBoltzmann law (Stefan, L. Boltzmann)

The radiated power P (rate of emission of electromagnetic energy) of
a hot body is proportional to the radiating surface
area, A, and the fourth power of the
thermodynamic temperature, T. The constant of proportionality is the
StefanBoltzmann constant. Mathematically,
P = e sigma A T^{4},
where the efficiency rating e is called the emissivity of the
object.
 steradian; sr

The supplementary SI unit of solid angle defined as the solid
central angle of a sphere that encloses a surface on the sphere
equal to the square of the sphere's radius.
 SternGerlach experiment (O. Stern, W. Gerlach; 1922)

An experiment that demonstrates the features of spin (intrinsic
angular momentum) as a distinct entity apart from orbital angular
momentum.
 superconductivity

The phenomena by which, at sufficiently low temperatures, a
conductor can conduct charge with zero resistance. The current
theory for explaining superconductivity is the BCS theory.
 superfluidity

The phenomena by which, at sufficiently low temperatures, a fluid
can flow with zero viscosity. Its causes are associated with
superconductivity.
 superposition principle

The general idea that, when a number of influences are acting on a
system, the total influence on that system is merely the sum of
the individual influences; that is, influences governed by the
superposition principle add linearly. Some specific examples are:
 superposition principle of forces

The net force on a body is equal to the sum of the forces
impressed upon it.
 superposition principle of states

The resultant quantum mechnical wavefunction due to two or more
individual wavefunctions is the sum of the individual
wavefunctions.
 superposition principle of waves

The resultant wave function due to two or more individual wave
functions is the sum of the individual wave functions.
 Système Internationale d'Unités (SI)

The coherent and rationalized system of units, derived from the
m.k.s. system (which itself is derived from the metric system)
in common use in physics today.

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