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The laws list S
Schroedinger's cat to Syst�me Internationale d'Unit�s.
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S.
- 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.
-
- 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.
-
- 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.
- Syst�me Internationale d'Unit�s (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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