Physics Glossary

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Action-at-a-distance: A description of a force, such as Newton's law of gravity, in which two separated bodies are said to directly exert forces on each other. In the modern description, the bodies produce a gravitational field, which in turn exerts forces on the two bodies. See gravitational field.

Alpha decay: The disintegration of an atomic nucleus, in which the final products are an alpha particle and a nucleus with two fewer protons and two fewer neutrons than the original.

Alpha particle: A particle consisting of two protons and two neutrons bound together. The nucleus of a helium atom is an alpha particle.

Antibaryon: The antiparticle of a baryon.

Antimatter: Matter that is not composed of protons, neutrons, and electrons, like the matter we see around us, but is instead composed of antiprotons, antineutrons, and positrons (the antiparticles of electrons).

Antineutrino: The antiparticle of a neutrino.

Antineutron: The antiparticle of a neutron. A neutron and antineutron both have the same mass and zero electric charge, but can be differentiated by their interactions: a neutron and an antineutron can annihilate into gamma rays, while two neutrons cannot.

Antiparticle: For every known type of particle, there exists an antiparticle with exactly the same mass, but with the opposite electric charge. When a particle and its antiparticle come together, they can always annihilate to form gamma rays. The antiparticle of an electrically neutral particle is sometimes the same as the original particle (e.g., photons) and sometimes it is distinct (e.g., neutrons).

Antiproton: The antiparticle of the proton.

Antiquark: The antiparticle of the quark.

Asymptotic freedom: A force of interaction between particles is said to be asymptotically free if it becomes weaker as the energy of the interacting particles increases. Empirically the force between quarks in a proton or neutron is found to be asymptotically free, a feature that can be explained by assuming that the force is described by a Yang-Milis theory.

Baryogenesis: The hypothetical process by which the early universe acquired a large positive baryon number.

Baryon number: The total number of baryons in a system, minus the number of antibaryons, is called the baryon number of the system. In particle physics experiments the total baryon number of a system is always found to be conserved, but our theories predict that baryon number would not be conserved at the extraordinarily high temperatures that prevailed in the early universe.

Baryon: Protons and neutrons, which comprise most of the mass of ordinary matter, as well as a number of short-lived particles such as the lambda, sigma and delta, are called baryons. These particles have in common the fact that they are each composed of three quarks.

Beta decay: The disintegration of an atomic nucleus, in which an electron (which was historically called a beta particle) and an antineutrino are emitted. Since the electron carries away one unit of negative charge, the final nucleus has a charge one greater than the initial nucleus.

Big crunch: If the universe has a mass density exceeding the critical mass density, then gravity will eventually reverse the expansion, causing the universe to re-collapse into what is often called the big crunch. See also closed universe.

Big-bang nucleosynthesis: The process, which took place between one second and 3-4 minutes after the beginning, in which the protons and neutrons of the primordial soup condensed to form the lightest atomic nuclei: deuterium, helium-3, helium-4, and lithium-7. See isotope and lithium.

Blackbody radiation: If a closed box made of any material is heated to a uniform temperature, the interior will become filled with electromagnetic radiation (photons) with an intensity and spectrum determined by the temperature alone, independent of the composition of the box. Radiation with this intensity and spectrum is called blackbody, or thermal radiation. The intensity and spectrum are determined by the criterion of thermal equilibrium; i.e., only far this intensity and spectrum will the absorption and emission of photons by the walls be in balance for each wavelength, so the intensity for each wavelength can be independent of time. The universe today appears to be permeated with blackbody radiation at a temperature of 2.73°K--the cosmic background radiation-which we interpret as a remnant afterglow of the heat of the early universe.

Black hole: An object with such a strong gravitational field that even light cannot escape. Matter can fall into a black hole, but according to classical physics no matter or energy can leave it. (Hawking has used quantum theory to show that black boles emit blackbody radiation, but the effect is significant only for black boles much smaller than those that are expected to form by the collapse of stars, which have masses of several solar masses or more.)

Blue-shift: If a star or galaxy is moving towards us, the radiation from the star or galaxy appears shifted towards shorter wavelengths, or towards the blue end of the spectrum. See Doppler shift.

Bottom: A flavour of quark. See flavour.

Bubble: The false vacuum decays in a manner similar to the way water boils, farming bubbles of normal matter in the midst of the false vacuum, just as bubbles of steam farm in the midst of water heated past its boiling point. (The bubbles that form when the false vacuum decays should not be confused with false vacuum bubbles, which have false vacuum rather than normal matter on the inside.)

Chaotic inflationary universe theory: A version of the inflationary universe theory, proposed by Andrei Linde in 1983, for which the energy density diagram far the fields driving inflation can be as simple as a bowl, with a unique minimum at the centre. If the initial randomly chosen value of the fields corresponds to a point high up the hill on the side of the bowl, then sufficient inflation can occur as the fields roll towards the state of minimum energy density.

Charmed: A flavour of quark. See flavour.

Charmonium: A bound state consisting of a charmed quark and a charmed antiquark. A major impetus for the quark theory was the discovery in 1974 of the J/ Ψ particle, a particle whose properties closely matched the predictions for charmonium.

Closed universe: A homogeneous, isotropic universe is said to be temporally closed if gravity is strong enough to eventually reverse the expansion, causing the universe to re-collapse. It is said to be spatially closed if gravity is strong enough to curve the space back on itself, forming a finite volume with no boundary. Triangles would contain more than 180°, the circumference of a circle would be less than n times the diameter, and a traveller intending to travel in a straight line would eventually find herself back at her starting point. If Einstein's cosmological constant is zero, as is frequently assumed, then a universe which is temporally closed is also spatially closed, and vice versa.

Colour: Each flavour of quark can exist in three variations, called colours, usually labelled as red, green, and blue. The colour of a quark has no relation to its visual appearance, but the word colour is used because there are three variations, in analogy with the three primary colours. Measurable properties of the quarks, such as electric charge and mass, depend on the flavour but not the colour, but the colour is responsible for the interactions that bind the quarks together (see Yang-Milis theories). Individual quarks cannot exist independently, but are forever confined within baryons or mesons, each of which is colourless. Baryons achieve colourlessness by being composed of three quarks, one of each colour, while mesons achieve colourlessness by pairing each coloured quark with its corresponding antiquark.

Confinement: The property of quarks which implies that they cannot exist as free particles, but are forever bound into protons, neutrons, etc. See colour.

Cosmic background radiation: See blackbody radiation.

Cosmic distance ladder: A method for estimating distances to the remote galaxies by using a sequence of techniques. Each technique in the sequence is calibrated by the previous techniques, and extends the range of measurement to greater distances.

Cosmic strings: Microscopically thin, spaghetti-like objects which, according to some theories of elementary particles, could form randomly during a phase transition in the early universe. Cosmic strings could provide the seeds for structure formation in the universe, as an alternative to the possibility that the seeds originated as quantum fluctuations during inflation.

Cosmological constant problem: The puzzle of why the cosmological constant has a value which is either zero, or in any case roughly 120 orders of magnitude or smaller than the value that particle theorists would expect. Particle theorists interpret the cosmological constant as a measure of the energy density of the vacuum, which they expect to be large because of the complexity of the vacuum. See vacuum.

Cosmological Principle: A term introduced by E.A. Milne in 1933 to describe the assumption that the universe is both homogenous and isotropic.

Cosmological term: See cosmological constant.

Critical mass density: If the cosmological constant is assumed to vanish, then the critical mass density is that density which puts the universe just on the border between eternal expansion (open universe) and eventual collapse (closed universe).

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