ad hoc hypothesis
engâre-ye pad im
Fr.: hypothèse ad hoc
Addition of adjustments to a theory to save it from being falsified by compensating for anomalies not anticipated by the theory in its unmodified form. Theories that rely on continual, ad hoc adjustments are distrusted.
Fr.: hypothèse alternative
Fr.: synthèse d'ouverture
The method of combining the signals received by several smaller telescopes distributed over a very large area or baseline to provide the high angular resolution of a much large telescope.
Big Bang nucleosynthesis (BBN)
haste-handâyeš-e Meh Bâng, ~ Big Bang
Fr.: nucléosynthèse de Big Bang
The production of → light elements, roughly three minutes after the → Big Bang when the temperature of the → Universe dropped from 1032 K to approximately 109 K. In a short time interval → protons and → neutrons collided to produce → deuterium. Most of the deuterium then fused with other protons and neutrons to produce → helium and a small amount of → tritium. The element → lithium 7 could also arise form the coalescence of one tritium and two deuterium nuclei. According to the Big Bang nucleosynthesis theory, roughly 25% of the mass of the Universe consists of helium. It also predicts about 0.01% deuterium, and even smaller quantities of lithium. These predictions depend critically on the → baryon-photon ratio. Same as → primordial nucleosynthesis.
In biochemistry, the ability to produce organic compounds using energy contained in inorganic molecules. Chemosynthesis is similar to → photosynthesis. Instead of using light as an energy source to make food, energy or compounds from chemical reactions is used. Most chemosynthetic organisms are bacteria.
de Broglie hypothesis
engâre-ye de Broglie
Fr.: hypothèse de de Broglie
The suggestion by Louis de Broglie in 1924 whereby if → electromagnetic waves possess particle properties (→ particle nature), then it might be reasonable to suppose that material particles, such as → electrons, should possess wave properties (→ wave nature). The de Broglie hypothesis was based on the intuitive feeling that nature seems to have strong attachment to symmetry. In other words, if radiation has particle-like properties, then material particles should possess wave-like properties. At the time no direct experimental evidence was present for the validity of this suggestion. The first confirmation of de Broglie's hypothesis was provided by the → Davisson-Germer experiment. See also → wave-particle duality;
hasté andâyeš-e oskafti
Fr.: nucléosynthèse explosive
The explosive processes that are believed to occur in supernovae. Explosive carbon burning occurs at a temperature of about 2 × 109 degrees and produces the nuclei from neon to silicon. Explosive oxygen burning occurs near 4 × 109 degrees and produces nuclei between silicon and calcium in atomic weight.At higher temperatures, still heavier nuclei are produced.
giant impact hypothesis
engâre-ye barxord-e qulâsâ
Fr.: hypothèse de l'impact géant
A model for → Moon formation (initially put forward by
Hartmann and Davis, 1975,
Icarus 24, 504), according to which the → proto-Earth
suffered a collision with another → protoplanet
near the end of the → accretion process
that ejected material into a → circumterrestrial
disk, out of which the Moon formed. Also called
→ canonical model.
The giant impact hypothesis is the leading theory for lunar formation.
There are, however, some key observations that cannot be explained using this
model. First, the Moon is a large fraction of the mass of Earth (~ 1%) and it is
difficult to get enough mass into orbit to form such a massive Moon.
Second, the Moon has a similar bulk composition to the Earth, but
it is missing large amounts of more
→ volatile elements. The model does not properly
explain Moon's distinctive composition.
Finally, Earth and the Moon share virtually the same
→ isotopic ratios.
It is therefore expected that the body that hit the
Earth, often called → Theia,
would have had a different isotopic
ratio than the proto-Earth. In the canonical model, most of
the mass of the Moon comes from Theia and so the Moon should have a
different isotopic fingerprint than Earth, but it does not.
The type of impact that formed the Moon in the canonical model is
dictated by a very strong constraint, the → angular momentum
of the Earth-Moon system. It is
assumed that the angular momentum of the Earth-Moon system
immediately after the Moon formed was the same as it is today. This
assumption limits the velocity of the impact, the mass of the
impacting bodies, and the angle at which the two bodies
collided. It was found that only a grazing impact with a Mars-mass
impactor at near the escape velocity can put enough mass into orbit
to potentially form a lunar-mass Moon. This is why the canonical
model is such a specific type of impact.
However, the angular momentum of the Earth-Moon system could
have been reduced over time by competition between the
gravitational pull of Earth, the Moon and the Sun. Therefore,
the Moon-forming collision could have been much more energetic than
the canonical impact.
engâre (#), upâdâyan
A statement which is based on previous observations and which serves as a starting point for further investigation by which it may be proved or disproved. See also → theory, → model, → ad hoc hypothesis, → Kant-Laplace hypothesis, → arge number hypothesis, → nebular hypothesis, → null hypothesis, → statistical hypothesis, → statistical hypothesis testing.
Hypothesis, from M.Fr. hypothèse, from L.L. hypothesis, from Gk. hypothesis "base, basis of an argument, supposition," literally "a placing under," from → hypo- "under" + thesis "a placing, proposition," from root of tithenai "to place, put, set," didomi "I give;" from PIE base *dhe- "to put, to do;" cf. Mod.Pers. dâdan "to give," Mid.Pers. dâdan "to give," O.Pers./Av. dā- "to give, grant, yield," dadāiti "he gives;" Skt. dadáti "he gives;" L. dare "to give, offer," facere "to do, to make;" Rus. delat' "to do;" O.H.G. tuon, Ger. tun, O.E. don "to do."
Fr.: hypothèse de Kant-Laplace
The hypothesis of the origin of the solar system proposed first by Kant (1755) and later by Laplace (1796). According to this hypothesis, the solar system began as a nebula of tenuous gas. Particles collided and gradually, under the influence of gravitation, the condensing gas took the form of a disk. Larger bodies formed, moving in circular orbits around the central condensation (the Sun).
Named after the German prominent philosopher Immanuel Kant (1724-1804) and the French great mathematician, physicist, and astronomer Pierre-Simon Marquis de Laplace (1749-1827); → hypothesis.
large number hypothesis
engâre-ye adadhâ-ye bozorg
Fr.: hypothèse des grands nombres
The idea whereby the coincidence of various → large numbers would bear a profound sense as to the nature of physical laws and the Universe. Dirac suggested that the coincidence seen among various large numbers of different nature is not accidental but must point to a hitherto unknown theory linking the quantum mechanical origin of the Universe to the various cosmological parameters. As a consequence, some of the → fundamental constants cannot remain unchanged for ever. According to Dirac's hypothesis, atomic parameters cannot change with time and hence the → gravitational constant should vary inversely with time (G∝ 1/t). Dirac, P. A. M., 1937, Nature 139, 323; 1938, Proc. R. Soc. A165, 199.
Fr.: hypothèse nébulaire
The hypothesis first put forward in the 18-th century that the solar system formed from a primeval nebula around the Sun. Same as the → Kant-Laplace hypothesis.
no boundary hypothesis
engâre-ye giti bi karân-e âqâzin
Fr.: l'hypothèse de l'Univers sans limite initiale
The proposal whereby the → Universe would not have begun with a → singularity. Instead, the → Big Bang would be an ordinary point of → space-time. The proposal, advanced by James Hartle and Stephen Hawking (1983) results from an attempt to combine aspects of → general relativity and → quantum mechanics. Based on an imaginary time assumption, it predicts a closed Universe that would start at a single point, that can be compared to the North Pole of the Earth on a two-dimensional space. Before the → Planck era there was space, but the real time began with the Big Bang event. → Hartle-Hawking initial state.
The process by which → nuclear reactions at very high temperatures and pressures produce the various → chemical elements of the → periodic table, either in the → Big Bang or in stellar interiors. See also → primordial nucleosynthesis, → stellar nucleosynthesis, → explosive nucleosynthesis.
Fr.: hypothèse nulle
Statistics: The assumption of the absence of a particular pattern in a set of data. The null hypothesis, denoted by H0, is put forward to be rejected in order to support an → alternative hypothesis.
The process in green plants, algae, diatoms, and certain forms of bacteria by which carbohydrates are synthesized from carbon dioxide and water using light as an energy source. Most forms of photosynthesis release oxygen as a byproduct.
Fr.: nucléosynthèse primordiale
The formation of → chemical elements in the → early Universe, between about 0.01 seconds and 3 minutes after the → Big Bang, when the nuclei of primordial matter collided and fused with one another. Most of the → helium in the → Universe was created by this process. Same as → Big Bang nucleosynthesis
Fr.: synthèse spectrale
The process of computing line strengths in stellar spectra using an appropriate stellar atmosphere model, atomic and molecular data, and the numerical solution of the → radiative transfer equation at each point in the spectrum.
Fr.: hypothèse statistique
An assumed statement about the way a → random variable is distributed. A statistical hypothesis generally specifies the form of the → probability distribution or the values of the parameters of the distribution. The statement may be true or false. See also → null hypothesis.