Abelian group: A group of transformations in which the end result of a product of transformations does not depend upon the order in which they occur. In a non­ Abelian group, on the other hand, the order in which transformations take place is important. AdS/CFT correspondence: A symmetry that relates a theory of gravity in an anti­ de Sitter spacetime (AdS) to a conformalfield theory (CFT) without gravity on the bound­ ary of that spacetime. There is thus a deep connection between gravity and field theory, the exact nature of which is still being researched. annihilation: A process in which a particle and its antiparticle collide and destroy each other, thereby creating new particles. For example, an electron and an antielec­ tron can mutually annihilate to produce two photons. anomaly: An anomaly occurs in relativistic quantum field theory when a conservation law arising from a symmetry in the underlying theory is destroyed by quantum cor­ rections. The requirement that a theory must be free from anomalies imposes tight restrictions on its structure. anti-de Sitter spacetime: A highly symmetrical solution to the equations of gen­ eral relativity, which describes an empty Universe that possesses a negative cosmological constant. antiparticle: A particle and its antiparticle possess exactly the same rest mass, but otherwise have "mirror-reversed" properties. For example, a particle has an opposite electric charge to its antiparticle. Some particles, such as the photon, serve as their own antiparticle. asymptotic freedom: In quantum chromodynamics, the weakening of the color inter­ action between quarks and gluons as the distance between them decreases. baryon: A subatomic particle that is abound state of three quarks. Baryons have half-integer spin. Protons and neutrons are the most common type of baryon, though over one hundred different type baryons have been discovered in collider experiments. 287 Glossary Big Bang: Our expanding Universe is thought to have begun 13.7 billion years ago in an event called the Big Bang; immediately after the Big Bang, the Universe is thought to have been incredibly hot and dense. black hole: An object with a gravitational field that is so intense that nothing - not even light - can escape once the hole's event horizon has been crossed. boson: A particle with integer spin. Fundamental bosons include the photon, gluon, W boson, and Z boson, all of which are spin-1 particles; the graviton is a spin-2 boson. These fundamental bosons can thus be thought of as "particles of force." Another type of boson, the spin-O Higgs boson, is believed to be the mechanism by which fundamental particles acquire mass; the Higgs (as of April 2004) has not yet been observed in experiments. BPS state: A particular type of quantum state possessing supersymmetry, which has properties that can be uncovered by symmetry arguments. brane: A type of object in M-theory. A p-brane has p spatial dimensions; so a 0- brane is a particle, a 1-brane is astring, a 2-brane is a membrane, and so on. In some theories, our Universe may be a 3-brane embedded in a higher-dimensional space. branon: An extremely speculative quantum particle. If our Universe is a brane em­ bedded in higher dimensions, the brane can fluctuate in those dimensions. The rules of quantum mechanics tell us that such fluctuations would manifest themselves as par­ ticles called branons. bulk: In some theories, multidimensional branes are embedded in a higher­ dimensional space called the bulk. Gauge fields might be confined to the brane, whereas gravity would be free to propagate throughout the bulk. Calabi-Yau space: A particular type of space into which the six extra dimensions of superstring theory may undergo compactification. CERN: Conseil Europeen pour la Recherche Nucleaire - the European Organi­ zation for Nuclear Research - is the world's largest particle physics laboratory. It is located near Geneva, on the border between France and Switzerland. The Large Eleetron-Positron Collider at CERN closed in 2000, making way for the powerful Large Hadron Collider, which will become operational in 2007. charge: A quantity carried by a particle that determines its participation in an inter­ action process. For example, if a particle possesses electric charge then it participates in the electromagnetic interaction; if a particle possesses color charge then it participates in the strong interaction. chirality: The property of "handedness." The weak interaction is chiral: it distin­ guishes between a left-handed and a right-handedftrmion. 288 Glossary classical physics: Physics prior to the introduction of the quantum principle. In dassical physics, which incorporates Newtonian mechanics, quantities such energy are viewed as a continuum. In quantum mechanics, such quantities come in discrete amounts. Classical physics works extremely well in the everyday world; when physicists probe small distance scales, however, quantum mechanics must be used. closed string: In string theory, a loop of string that has no free ends. The graviton is interpreted to be a specific vibrational pattern of a dosed string. cold dark matter: Observations have revealed that halos of invisible matter sur­ round galaxies. This dark matter adds up to many times the mass of visible matter. The temperature of dark matter is a measure ofhow fast the particles in it are moving. Cold dark matter refers to partides moving much slower than the speed oflight. (Hot dark matter refers to partides that move at speeds dose to that oflight.) color: A type of charge carried by quarks and gluons. There are three different types of color charge, usually labelled as red, green, and blue. An iso la ted color charge is never seen in nature; we only ever observe color-neutral combinations of quarks. So a baryon is a combination of three quarks (one red, one green, one blue), and a meson is a combination of a quark and an antiquark (with a color and its anticolor). compactification: The extra spatial dimensions demanded by string theory are con­ ventionally thought to be curled up into unobservably small spaces (or compactified). confinement: A free isolated quark (or gluon) has never been observed. Instead, colored quarks and gluons are confined within colorless hadrons. There is as yet no exact proof that the theory of colored quark and gluon interactions - quantum chro­ modynamics - exhibits confinement; recently, however, physicists have proved that supersymmetric quantum chromodynamics possesses confinement. conformal field theory (CFT): A type of relativistic quantum field theory that looks the same at all energy scales. In string theory, the vibrations of strings can be described by a two-dimensional conformal field theory that lives on the surface of the string's world sheet. conformal symmetry: Conformal transformations are those that leave the angles between directions invariant. conservation law: When an interaction between partides takes place, the value of certain quantities (for example, electric charge, energy, and momentum) are the same before and after the interaction. The conservation laws are some of the most profound statements we can make about our Universe, and they ultimately derive from the symmetries of the Universe. cosmic ray: A high-energy partide - often a proton, or an atomic nudeus - that hits Earth's atmosphere from outer space. 289 Glossary cosmological constant: A term that can be added to the original equations of general relativity. Nowadays we interpret the term as the energy density of the vacuum, and calculating its value is one of the great challenges of M-theory. These theories imply that the cosmological constant is large; observation, though, teIls us it is small. coupling constant: A parameter that describes the strength of a given interaction. de Sitter spacetime: A highly symmetrical solution to the equations ofgeneral rela­ tivity, in which the Universe contains no matter but has a positive cosmological constant. Such a Universe expands at an exponentially increasing rate. dilaton: A hypothetical scalar field, which occurs naturally in Kaluza-Klein theory and other theories employing higher spatial dimensions. duality: A symmetry in which two seemingly different theories produce identical physical consequences. ekpyrotic scenario: A speculative theory in which our Universe began through the collision of two parallel branes. electromagnetic interaction: The interaction that occurs between objects due to their electric charge. The interaction is mediated by the exchange of photons. The relativistic quantum field theory of the interaction is called quantum electrodynamics. electron: A type of fundamental particle (a lepton). Electrons carry one unit of negative electric charge and possess a rest mass of 0.511 Me V. They make up the outer shells of atoms. electron Volt (eV): The energy gained by an electron when it is accelerated through an electrical potential of 1 Volt. The bin ding energy on an electron in an atom is of the order of 1 eV; this relatively low energy makes it easy to produce free electrons. electroweak interaction: At high energies, the electromagnetic interaction and the weak interaction can be described by a single electroweak theory. The theory of the electroweak interaction is a key part of the Standard Model. entropy: A measure of the amount of disorder in a physical system. When systems interact, the total entropy cannot decrease. equivalence principle: Over a small enough region of observation, it is impossible to distinguish between the effects of a gravitational field and those of accelerated mo­ tion. The equivalence principle means that all observers, no matter what their state of motion, can argue that they are at rest as long as they take an appropriate gravitational field into account. The equivalence principle was the foundation upon which Einstein constructed his theory ofgeneral relativity. event horizon: The area around a black hole at which the escape velo city is equal to the speed oflight.