The laws list
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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 fifty-fifty 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/c2.
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 cesium-133 atom.
siemens; S (after E.W. von Siemens, 1816-1892)
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 internationally-agreed-upon 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 Stefan-Boltzmann 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 frame-dependent.
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.
spin-orbit 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.
Stefan-Boltzmann constant; sigma (Stefan, L. Boltzmann)
The constant of proportionality present in the Stefan-Boltzmann law. It is equal to 5.6697 x 10-8 W/m2/K4.
Stefan-Boltzmann 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 Stefan-Boltzmann constant. Mathematically,
P = e sigma A T4,
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.
Stern-Gerlach 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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