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High Energy Physics

Terms in this set (34)

Angular Momentum
A measure of the momentum of a body in rotational motion about its centre of mass. Technically, the angular momentum of a body is equal to the mass of the body multiplied by the cross product of the position vector of the particle with its velocity vector. The angular momentum of a system is the sum of the angular momenta of its constituent particles, and this total is conserved unless acted on by an outside force.
Complementarity
The idea in quantum theory that items can be separately analyzed as having several contradictory, and apparently mutually exclusive, properties. For example, the wave-particle duality of light, where light can either behave as a particle or as wave, but not simultaneously as both.
Cosmological Constant
A term added by Albert Einstein as a modification to his original theory of general relativity, in order to balance the attractive force of gravity and achieve a static or stationary universe. It represents the possibility that there is a density and pressure associated with apparently empty space, and that the overall mass-energy of the universe is actually much greater than currently estimated. Once dismissed as just a mathematical "fix", it has been revived in recent years with the discovery of the apparent acceleration of the expansion of the universe.
Decoherence
The process by which bodies and quantum systems lose some of their more unusual quantum properties (e.g. superposition, or the ability to appear in different places simultaneously) as they interact with their environments. When a particle decoheres, its probability wave collapses, any quantum superpositions disappear and it settles into its observed state under classical physics.
Electric Charge
A property of microscopic particles, which may be either positive (e.g. protons) or negative (e.g. electrons). Particles with the same charge repel each other, and particles with opposite charges attract each other. The field of force that surrounds an electric charge is called an electric field, and a river of charged particles flowing through a conductor is called an electric current.
Electromagnetic Force (or Electromagnetism)
The force that an electromagnetic field exerts on electrically charged particles. It is one of the four fundamental forces of physics (along with the gravitational force and the strong and weak nuclear forces), and the one responsible for most of the forces we experience in our daily lives. The electromagnetic forces acting between the electrically charged protons and electrons inside atoms and between atoms are essentially responsible for gluing together all ordinary matter.
Although hugely stronger (1042 times) than the force of gravity, it is a less dominant force on larger scales because the attractive and repulsive interactions tend to cancel each other out. Like gravity, the electromagnetic force is subject to an inverse-square law, and its strength is inversely proportional to the square of the distance between the particles. The force is mediated or operated by the exchange of photons between the particles. The 'electrostatic force' is one aspect of the electromagnetic force, which arises when two charged particles are static (i.e. not in motion).
Electromagnetic Radiation (or Electromagnetic Waves)
A wave that travels though space at the speed of light, consisting of an electrical field that periodically grows and dies, alternating with a magnetic field that periodically dies and grows. Electromagnetic waves carry energy and momentum, which may be imparted when it interacts with matter.
In order of increasing frequency, the electromagnetic spectrum includes radio waves, microwaves, terahertz radiation, infrared radiation, visible light, ultraviolet radiation, x-rays and gamma rays.
Elementary Particle
A particle with no substructure (i.e. not made up of smaller particles) and which is therefore one of the basic building blocks of the universe from which all other particles are made. Quarks, electons, neutrinos, photons, muons and gluons (along with their respective antiparticles) are all elementary particles; protons and neutrons (which are made up of quarks) are not.
Energy
Sometimes defined as the ability to do work or to cause change, energy is notoriously difficult to define. In accordance with the Law of Conservation of Energy, energy can never be created or destroyed but it can be changed into different forms, including kinetic, potential, thermal, gravitational, sound, light, elastic and electromagnetic. The standard scientific unit of energy is the Joule.
Entanglement
The phenomenon in quantum theory whereby particles that interact with each other become permanently dependent on each other's quantum states and properties, to the extent that they lose their individuality and in many ways behave as a single entity. At some level, entangled particles appear to "know" each other's states and properties.
Entropy
A measure of the disorder of a system and of its constituent molecules. More specifically, in thermodynamics it is a measure of the unavailability of a system's energy to do work. The Second Law of Thermodynamics embodies the idea that entropy can never decrease, but rather will tend to increase over time, approaching a maximum value as it reaches thermal equilibrium. A classic example of increasing entropy is ice melting in water until both reach a common temperature.
Exotic Particle:
A kind of theoretical particle said to exist by some theories of modern physics, whose alleged properties are extremely unusual. Examples include tachyons (particles that always travels faster than the speed of light), WIMPs (weakly interacting massive particles which do not interact with electromagnetism or the strong nuclear force), axions (particles with no electric charge, very small mass and very low interaction with the strong and weak forces) and neutrinos (particles that travel close to the speed of light, lack an electric charge and are able to pass through ordinary matter almost undisturbed).
Fundamental (or Elementary) Forces
There are four basic forces of physics that are believed to underlie all phenomena in the universe. Listed in order of strength they are: the strong nuclear force, the electromagnetic force, the weak nuclear force and the gravitational force (or gravity). It is thought likely that, in extremely high energy conditions such as occurred near the beginning of the Big Bang, the four fundamental forces of nature are actually unified in a single theoretical framework (known as the Grand Unified Theory).
According to quantum field theory, the forces between particles are mediated by other particles, and the fundamental forces can be described by the exchange of virtual force-carrying particles: the strong nuclear force mediated by gluons; the electromagnetic force by photons; the weak nuclear force by W and Z bosons; and gravity by hypothetical gravitons.
Inertia
The natural tendency (as defined in Sir Isaac Newton's First Law of Motion of 1687) of objects to resist changes in their state of motion. Therefore, a body at rest tends to stay at rest and, once set in motion, a body tends to stay moving at a constant speed in a straight line (or along a geodesic in curved space) unless acted on by an outside force. An example of an inertial force is centrifugal force, which in reality is just due to a body trying to continue in a straight line while constrained to move along a curved path.
Inertial Frame (or Inertial System)
A reference frame in which the observers are not subject to any accelerating force. An inertial frame is a frame of reference in which a body remains at rest or moves with constant linear velocity unless acted upon by outside forces (as stipulated by Sir Isaac Newton's First Law of Motion, Force = Mass × Acceleration). Any frame of reference that moves with constant velocity relative to an inertial system is itself an inertial system.
Law of Conservation of Energy
Also known as the First Law of Thermodynamics, this is the principle that energy can never be created or destroyed, only converted from one form to another (e.g. the chemical energy of gasoline can be converted into the energy of motion of a car). The total amount of energy in an isolated system (or in the universe as a whole) therefore remains constant.
Length Contraction
The phenomenon, predicted by Albert Einstein's Special and General Theories of Relativity, whereby, from the relative context of one observer's frame of reference, space or length appears to decrease as the relative velocities increase.
Magnetic Monopole
A hypothetical particle that is a magnet with only one pole, and which therefore has a net magnetic charge. Although the existence of monopoles is indicated by both classical theory and quantum theory (and predicted by recent string theories and grand unified theories), there is still no observational evidence for their physical existence.
Mass
A measure of the amount of matter in a body. It can also be seen as a measure of a body's inertia or resistence to change in motion, or the degree of acceleration a body acquires when subject to a force (bodies with greater mass are accelerated less by the same force and have greater inertia). Mass is often confused with weight, which is the strength of the gravitational pull on the object (and therefore how heavy it is in a particular gravitational situation), although, in everyday situations, the weight of an object is proportional to its mass.
Mass-Energy Equivalence
The concept that any mass has an associated energy, and that, conversly, any energy has an associated mass. In Einstein's Special Theory of Relativity, this relationship is expressed in the famous mass-energy equivalence formula, E = mc2, where E = total energy, m = mass and c = the speed of light in a vacuum. Given that c is a very large number, it becomes apparent that mass is in fact a very concentrated form of energy.
Matter
Anything that has both mass and volume (i.e. takes up space). Matter is what atoms and molecules are made of, and it exists in four states or phases: solid, liquid, gas and plasma (although other phases, such as Bose-Einstein condensates, also exist).
Momentum
A measure of how much effort is required to stop a body, defined as the body's mass multiplied by its velocity. Thus, a large heavy body (e.g. a train) going relatively slowly may have more momentum than a smaller body going very fast (e.g. a racing car). The Law of Conservation of Momentum rules that the total momentum of an isolated system (one in which no net external force acts on the system) does not change.
Nonlocality
The rather spooky ability of objects in quantum theory to apparently instantaneously know about each other's quantum state, even when separated by large distances, in apparent contravention of the principle of locality (the idea that distant objects cannot have direct influence on one another, and that an object is influenced directly only by its immediate surroundings).
Pauli Exclusion Principle
The prohibition on two identical fermions from sharing the same quantum state simultaneously. Among other implications it stops electrons (which are a kind of fermion) from piling on top of each other, thereby explaining the existence of different types of atoms and the whole variety of the universe around us.
Photoelectric Effect
The phenomenon in which, when a metallic surface is exposed to electromagnetic radiation above a certain threshold frequency (typically visible light and x-rays), the light is absorbed and electrons are emitted. The discovery of the effect is usually attributed to Heinrich Hertz in 1887, and study of it (particularly by Albert Einstein) led to important steps in understanding the quantum nature of light and electrons and in formulating the concept of wave-particle duality.
Photon
A particle (or quantum) of light or other electromagnetic radiation, which has no intrinsic mass and can therefore travel at the speed of light. It is an elementary particle and the basic unit of light, and effectively carries the effects of the electromagnetic force. The modern concept of the photon as exhibiting both wave and particle properties was developed gradually by Albert Einstein and others.
Planck Constant
The proportionality constant (h) which provides the relation between the energy (E) of a photon and the frequency (v) of its associated electromagnetic wave in the so-called Planck Relation E = hv. It is essentially used to describe the sizes of individual quanta in quantum mechanics. Its value depends on the units used for energy and frequency, but it is a very small number (with energy measured in Joules, it is of the order of 6.626 × 10-34 J·s).
Planck Energy
The super-high energy (approximately 1.22 × 1019 GeV) at which gravity becomes comparable in strength to the other fundamental forces, and at which the quantum effects of gravity become important.
Planck Length
The fantastically tiny length scale (approximately 1.6 × 10-35 metres) at which gravity becomes comparable in strength to the other fundamental forces. It is the scale at which classical ideas about gravity and space-time cease to be valid, and quantum effects dominate.
Planck Temperature
The temperature of the universe at 1 Planck Time after the Big Bang, approximately equal to 1.4 × 1032°C.
Planck Time
The time it would take a photon travelling at the speed of light to cross a distance equal to the Planck Length. This is the "quantum of time", the smallest measurement of time that has any meaning, and is approximately equal to 10-43 seconds.
Planck Units
"Natural units" of measurement (i.e. designed so that certain fundamental physical constants are normalized to 1), named after the German physicist Max Planck who first proposed them in 1899. They were an attempt to eliminate all arbitrariness from the system of units, and to help simplify many complex equations in modern physics. Among the most important are the Planck Energy, the Planck Length, the Planck Time and the Planck Temperature.
Principle of Equivalence
The idea that no experiment can distinguish the acceleration due to gravity from the inertial acceleration due to a change of velocity (or acceleration).
Probability Wave (or Wave Function)
A description of the probability that a particle in a particular state will be measured to have a given position and momentum. Thus, a particle (an electron, photon or any other kind of particle), when not being measured or located, takes the form of a field or wave of probable locations, some being more probable or likely than others.
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