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Glossary of Selected Terms
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GLOSSARY OF SELECTED TERMS ABSOLUTE SPACE: According to Newtonian physics, the geometric arena where physical phenomena take place. Its properties are independent of the state of motion of observers. ABSOLUTE TIME: According to Newtonian physics, absolute time flows at the same precise rate, independent of the state of motion of observers. ACTION AT A DISTANCE: The assumption, essential in Newtonian physics, that bodies may influence each other without mutual contact, as in the case of the gravitational attraction between the Sun and the planets. AETHER (Aristotelian): The material substance of heavenly bodies above the lunary sphere. AETHER (Electromagnetic): The medium believed by nineteenthcentury physicists to support the propagation of electromagnetic waves. ATOMISM: A doctrine originated by pre-Socratic philosophy in ancient Greece where the Universe is composed of small, indivisible components called atoms. BINDING ENERGY: The energy associated with the binding, by the action of an attractive force, of two or more components in a physical system. It is also the energy released by the system when its bonds are broken by the action of an external agent, or by some intrinsic instability of the system. BLACKBODY: A perfect absorber of radiation. Kirchhoff showed that the interior of a hollow cavity could mimic a perfect blackbody. In order to study the nature of the radiation produced by heating the walls of the cavity, Kirchhoff made a small hole in one of the walls. The radiation is known as blackbody radiation and is uniquely determined by the temperature of the blackbody. CALORIC HYPOTHESIS: The hypothesis that heat is an invisible fluid that is transferred between bodies on contact. The caloric hypothesis was abandoned in 1789, after the detailed studies of Benjamin Thompson. CENTRIPETAL FORCE: A force acting in the direction of the center of motion. COMPLEMENTARITY PRINCIPLE: Introduced by Bohr, it states that wave and particle are complementary and mutually exclusive ways of representing quantum objects. COSMIC RAYS: Showers of high-energy particles that pour from outer space into our atmosphere. COSMOGONY: The study of the origin of the Universe. COSMOLOGICAL CONSTANT: A parameter, introduced by Einstein in 1917, to guarantee the stability of his finite and static model of the Universe. COSMOLOGICAL PRINCIPLE: Introduced by Einstein, it states that the Universe is on average the same everywhere and in all directions. Mathematically, it translates into saying that the Universe is homogeneous and isotropic. This principle was extended in 1948 by the proponents of the steady-state model to include also no change in time, the so-called “perfect cosmological principle.” COSMOLOGY: The study of the physical properties and evolution of the Universe. CREATION MYTH: A myth concerned with the creation of the world. CRITICAL DENSITY: The energy density that determines if the Universe will expand forever or if it will contract into a big crunch. Its value is roughly 10229 g/cm3. CURVATURE RADIUS: The time-dependent parameter that determines the relative distance between two observers in homogeneous and isotropic cosmologies. DECOUPLING: According to the big-bang model, decoupling is the event that took place when atoms formed and photons broke loose from their interactions with protons and electrons, being thus free to roam the Universe. These photons presently have a blackbody spectrum at approximately three absolute degrees, the socalled cosmic background radiation detected by Penzias and Wilson in 1965. 314 GLOSSARY OF SELECTED TERMS [44.204.164.147] Project MUSE (2024-03-19 04:37 GMT) DEISM: The belief in a God who created the world and its natural laws, but who does not interfere with the world after that. DOPPLER EFFECT: The effect where waves from a source in motion relative to an observer have their wavelengths changed. For a source approaching an observer, the wavelength decreases, while for a source receding from an observer, the wavelength increases. ELECTROMAGNETIC RADIATION: The radiation emitted by accelerated electric charges. ELECTRON: The negatively charged elementary particle found in atoms. ENTROPY: A quantitative measure of the degree of disorder of a physical system. According to the second law of thermodynamics, the entropy of an isolated system cannot decrease. EPICYCLE: A mathematical device developed in ancient Greece to model celestial orbits. It consists of a circle to which a celestial body is attached, which has a center revolving around another larger circle. EQUIVALENCE PRINCIPLE: Introduced by Einstein, it states that the effects of gravity can be mocked by an accelerated motion. FIELD: A physical effect that exists in a region of space. The effect is a manifestation of one or more of...