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ultraviolet catastrophe to universal constant of gravitation.

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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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