| The laws list: U |
The laws list U
ultraviolet catastrophe to universal constant of
gravitation.
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U.
- 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.
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