[A~C] [D~H] [I~N] [O~T] [U~Z]
ultraviolet catastrophe
A shortcoming of the Rayleigh-Jeans formula, which attempted to describe the radiancy of a blackbody at
various frequencies of the electromagnetic spectrum. It was clearly wrong because as the frequency
increased, the radiancy increased without bound; something quite not observed; this was dubbed the
"ultraviolet catastrophe." It was later reconciled and explained by the introduction of the Planck radiation
law.
uncertainty principle (W. Heisenberg; 1927)
A principle, central to quantum mechanics, which states that two complementary parameters (such as
position and momentum, energy and time, or angular momentum and angular displacement) cannot both
be known to infinite accuracy; the more you know about one, the less you know about the other.
It can be illustrated in a fairly clear way as it relates to position vs. momentum: To see something (let's
say an electron), we have to fire photons at it; they bounce off and come back to us, so we can "see" it. If
you choose low-frequency photons, with a low energy, they do not impart much momentum to the
electron, but they give you a very fuzzy picture, so you have a higher uncertainty in position so that you
can have a higher certainty in momentum. On the other hand, if you were to fire very high-energy
photons (x-rays or gammas) at the electron, they would give you a very clear picture of where the
electron is (higher certainty in position), but would impart a great deal of momentum to the electron
(higher uncertainty in momentum).
In a more generalized sense, the uncertainty principle tells us that the act of observing changes the
observed in fundamental way.
uniformity principle (E.P. Hubble)
The principle that the laws of physics here and now are not different, at least qualitatively, from the laws
of physics in previous or future epochs of time, or elsewhere in the Universe. This principle was scoffed
at by the ancients who believed that the laws that governed the Earth and those that governed the
heavens were completely divorced; now it is used routinely in cosmology to describe the structure and
evolution of the Universe.
universal age paradox
Two of the most straightforward methods of calculating the age of the Universe -- through redshift
measurements, and through stellar evolution -- yield incompatible results. Recent (mid 1990s)
measurements of the distances of distant galaxies through the use of the Hubble Space Telescope
indicate an age much less than the ages of the oldest stars that we calculate through stellar evolution
theory. At present there is no conclusion to this paradox; a cosmological constant would rectify the
situation, but it's possible that the discrepancy will disappear with more accurate measurements of the age
of the Universe using both methods.
universal constant of gravitation; G
The constant of proportionality in Newton's law of universal gravitation and which plays an analogous role
in A. Einstein's general relativity. It is equal to 6.672 x 10-11 N m2/kg2.
van der Waals force (J.D. van der Waals)
Forces responsible for the non-ideal behavior of gases, and for the lattice energy of molecular crystals.
There are three causes: dipole-dipole interaction; dipole-induced dipole moments; and dispersion forces
arising because of small instantaneous dipoles in atoms.
wave-particle duality
The principle of quantum mechanics which implies that light (and, indeed, all other subatomic particles)
sometimes act like a wave, and sometime act like a particle, depending on the experiment you are
performing. For instance, low frequency electromagnetic radiation tends to act more like a wave than a
particle; high frequency electromagnetic radiation tends to act more like a particle than a wave.
weak equivalence principle; principle of uniqueness of freefall
The idea within general relativity that the worldline of a freefalling body is independent of its composition,
structure, or state. This principle, embraced by Newtonian mechanics and gravitation when Newton set
the inertial and gravitational masses equal to each other. This principle is incorporated into a stronger
version with the equivalence principle.
weber; Wb (after W. Weber, 1804-1891)
The derived SI unit of magnetic flux equal to the flux that, linking a circuit of one turn, produces in it an
electromotive force of 1 V as it is reduced to zero at a uniform rate in a period of 1 s; it thus has units of
V s.
Weiss constant
A characteristic constant dependent on the material, used in calculating the susceptibility of paramagnetic
materials.
See Curie-Weiss law.
Wiedemann-Franz law
The ratio of the thermal conductivity of any pure metal to its electrical conductivity is approximately
constant for any given temperature. This law holds fairly well except at low temperatures.
Wien displacement law
For a blackbody, the product of the wavelength corresponding to the maximum radiancy and the
thermodynamic temperature is a constant, the Wien displacement law constant. As a result, as the
temperature rises, the maximum of the radiant energy shifts toward the shorter wavelength (higher
frequency and energy) end of the spectrum.
Wien's displacement law constant, b
The constant of the Wien displacement law. It has the value 2.897 756 x 10-3 m K.
Woodward-Hoffmann rules
Rules governing the formation of products during certain types of organic reactions.
Young's experiment; double-slit experiment (T. Young; 1801)
A famous experiment which shows the wave nature of light (and indeed of other particles). Light is
passed from a small source onto an opaque screen with two thin slits. The light is refracted through these
slits and develops an interference pattern on the other side of the screen.
Zeeman effect; Zeeman line splitting (P. Zeeman; 1896)
The splitting of the lines in a spectrum when the source is exposed to a magnetic field.
