The Physics of the Universe - Difficult Topics Made Understandable


Introduction
 
Main Topics
 
Important Dates and Discoveries
 
Important Scientists
 
Cosmological Theories Through History
 
The Universe By Numbers
 
Glossary of Terms
 
A Few Random Facts
 
Sources

 
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Web-site designed by:
Luke Mastin
 

Glossary of Terms

Brief explanations and definitions of some of the most important concepts, laws, theories and technical terms used throughout this website. These are the same terms and definitions which are indicated by the green popup links within the main text of this website, but gathered together in one place, and listed in alphabetical order. Some (although by no means all) entries have links to pictures, glaned from various places on the Internet, which will appear in a popup window.

A   B   C   D   E   F   G   H   I   J   K   L   M   N   O   P   Q   R   S   T   U   V   W   X   Y   Z

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Abiogenesis:
The study of how life on Earth could have arisen from inanimate matter. It should not be confused with evolution (the study of how living things change over time), biogenesis (the process of lifeforms producing other lifeforms) or spontaneous generation (the obsolete theory of complex life originating from inanimate matter on an everyday basis).

Absolute Zero:
The lowest temperature possible, equivalent to -273.15°C (or 0° on the absolute Kelvin scale), at which point atoms cease to move altogether and molecular energy is minimal. The idea that it is impossible, through any physical process, to lower the temperature of a system to zero is known as the Third Law of Thermodynamics.

Accretion Disk:
Click for a picture
Accretion disk of a black hole
(Source: Internet Encyclopedia of Science: http://www.daviddarling.info/
encyclopedia/E/event_horizon.html
)
Diffuse material orbiting around a central body such as a protostar, a young star, a neutron star or a black hole. Gravity causes the material in the disc to spiral inwards towards the central body with great speed, and the gravitational forces acting on the material cause the emission of x-rays, radio waves or other electromagnetic radiation (known as quasars).

Alpha Particle (Alpha Decay):
Click for a picture
Alpha particle
(Source: Medical Pages: http://www.irradiation.co.uk/types.php)
A particle of 2 protons and 2 neutrons (essentially a helium nucleus) that is emitted by an unstable radioactive nucleus during radioactive decay. It is a relatively low-penetration particle due its comparatively low energy and high mass.

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.

Anthropic Principle:
The idea that the fundamental constants of physics and chemistry are just right (or “fine-tuned”) to allow the universe and life as we know it to exist, and indeed that the universe is only as it is because we are here to observe it. Thus, we find ourselves in the kind of universe, and on the kind of planet, where conditions are ripe for our form of life.

Antimatter:
Click for a picture
Pair production and pair annihilation of hydrogen and antihydrogen particles
(Source: Riken Research: http://www.rikenresearch.riken.jp/
eng/frontline/5444
)
A large accumulation of antiparticles - antiprotons, antineutrons and positrons (antielectrons) - which have opposite properties to normal particles (e.g. electrical charge), and which can come together to make antiatoms. When matter and antimatter meet, they self-destruct in a burst of high-energy photons or gamma rays. The laws of physics seem to predict a pretty much 50/50 mix of matter and antimatter, despite the observable universe apparently consisting almost entirely of matter, known as the “baryon asymmetry problem”.

Atom:
Click for a picture
Depiction of an atom of nitrogen (using the refined Bohr model)
(Source: Jefferson Lab: http://education.jlab.org/
qa/atom_model.html
)
The basic building block of all normal matter, consisting of a nucleus (which is itself composed of positively-charged protons and zero-charged neutrons) orbited by a cloud of negatively-charged electrons, so that the positive charge is exactly balanced by the negative charge and the atom as a whole is electrically neutral. Atoms range from about 32 to about 225 picometres in size (a picometre is a trillionth of a metre). A typical human hair is about 1 million carbon atoms in width.

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Beta Particles (Beta Decay):
Click for a picture
Beta particle
(Source: Medical Pages: http://www.irradiation.co.uk/types.php)
High-energy, high-speed electrons or positrons (antielectrons) emitted by some types of radioactive decay, when an unstable atomic nucleus with an excess of neutrons or protons undergoes beta decay (a process mediated by the weak nuclear force). The particles emitted are a form of ionizing radiation, also known as beta rays.

Big Bang:
Click for a picture
The Big Bang and the expansion of the universe
(Source: HETDEX: http://hetdex.org/dark_energy/index.php)
The huge “explosion” 13.7 billion years ago in which the universe (including all space, time and energy) is thought to have been created. According to this theory, the universe began in a super-dense, super-hot state and has been expanding and cooling ever since. The phrase was coined by Fred Hoyle during a 1949 radio broadcast.

Big Crunch:
Click for a picture
The expansion and contraction of a closed universe to a Big Crunch
(Source: How Stuff Works: http://science.howstuffworks.com/
big-crunch3.htm
)
One possible scenario for the ultimate fate of the universe, in which the gravity of the matter in the universe (providing that there is in fact a “critical mass”) will one day halt and reverse the universe’s expansion in a mirror image of the Big Bang, causing it to collapse into a black hole singularity. However, in the light of recent evidence for an accelerating universe, this is no longer considered the most likely outcome.

Black Body:
Click for a picture
Radiation emitted from a black body
(Source: Northern Arizona University Lectures: http://www4.nau.edu/meteorite/
Meteorite/Book-GlossaryB.html
)
An idealized object that absorbs all electromagnetic radiation that falls on it, without passing through and without reflection. The radiation emitted from a black body is mostly infrared light at room temperature, but as the temperature increases it starts to emit visible wavelengths, from red through to blue, and then ultraviolet light at very high temperatures.

Black Hole:
Click for a picture
Simulated black hole in front of the Milky Way
(Source: Space Time Travel: http://www.spacetimetravel.org/
galerie/galerie.html
- Credit: Ute Kraus)
The warped space-time remaining after the gravity of a massive body has caused it to shrink down to a point. It is a region of empty space with a point-like singularity at the centre and an event horizon at the outer edge. It is so dense that no normal matter or radiation can escape its gravitational field, so that nothing - not even light - can ever leave (hence its blackness). It is thought that most galaxies have a supermassive black hole at their heart.

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Classical Physics:
A general term used to describe the physics based on principles developed before the rise of general relativity and quantum mechanics, essentially physics as it had existed up to the early years of the 20th Century. It includes the mechanics of Galileo and Newton, the electrodynamics of Maxwell, the thermodynamics of Boyle and Kelvin, and usually even the special relativity of Einstein.

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.

Copernican Principle:
The idea that there is nothing special about our position in the universe, a generalized version of Nicolaus Copernicus’ recognition that the Earth is actually just a planet circling the Sun, and not vice versa.

Cosmic Microwave Background Radiation:
Click for a picture
WMAP colour-enhanced picture of cosmic background radiation
(Source: NASA / WMAP: http://map.gsfc.nasa.gov/
media/080997/index.html
)
Cosmic microwave background radiation (or CMB for short) is the “afterglow” of the Big Bang, a microwave radiation which still uniformly permeates all of space at a temperature of around -270°C (about 3° above absolute zero). It is considered to be the best evidence for the standard Big Bang model of the universe.

Cosmic Inflation:
Click for a picture
Graph of the expansion of the observable universe with inflation
(Source: Wikipedia: http://en.wikipedia.org/wiki/
Cosmic_inflation
)
The idea that, in the first split-second after the Big Bang, the universe underwent a fantastically fast (exponential) expansion driven by the vacuum of empty space. The theory was developed by Alan Guth in the early 1980s to explain certain problems and inconsistencies with the basic Big Bang theory, such as those related to the large-scale structure of the features of the universe, the “horizon problem”, the “flatness problem” and the “magnetic monopole problem”.

Cosmic Rays:
High speed, energetic particles (about 90% of which are protons) originating from space that impinge on Earth's atmosphere. Some are generated by our own Sun, some by supernovas, some by as yet unknown events in the farthest reaches of the visible universe. The term "ray" is a misnomer, as cosmic particles arrive individually, not in the form of a ray or beam of particles.

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.

Cosmological Principle:
Click for a picture
The cosmological principle is supported by pictures of different parts of the universe by the Hubble Space Telescope
(Source: Hubble Site: http://hubblesite.org/newscenter/
archive/releases/1996/01/
)
The starting point for the General Theory of Relativity and the Big Bang theory is that, that averaged over large distances, one part of the universe looks approximately like any other part, and that, viewed on sufficiently large distance scales, there are no preferred directions or preferred places in the universe. Stated in more technical terms, on large spatial scales, the universe is homogeneous and isotropic.

Critical Mass (Critical Density):
Click for a picture
Graph of how critical density affect the expansion of the universe
(Source: Northern Arizona University Lectures: http://www4.nau.edu/meteorite/
Meteorite/Book-GlossaryD.html
)
As applied to the universe as a whole, critical mass refers to the total required mass of matter in the universe which will allow the effects of gravity to overcome its continued outward expansion. If the universe contains more than the critical mass of matter, its gravity will eventually reverse the expansion, causing the universe to collapse back to what has become known as the Big Crunch. If, however, it contains insufficient matter, it will go on expanding forever. In the same way, critical density is that overall density of the matter in the universe which will just allow continued expansion.
In other contexts, critical mass is also used to refer to the amount of fissile material needed to sustain nuclear fission.

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Dark Energy:
Click for a picture
Estimated distribution of dark matter and dark energy in the universe
(Source: Wikipedia: http://en.wikipedia.org/wiki/Dark_energy)
An invisible, hypothetical form of energy with repulsive gravity that permeates all of space and that may explain recent observations that the universe appears to be expanding at an accelerating rate. In some models of cosmology, dark energy accounts for 74% of the total mass-energy of the universe. Its exact nature remains a mystery, although Einstein’s hypothesized “cosmological constant” is now considered a promising candidate.

Dark Matter:
Matter that gives out no light and does not interact with the electromagnetic force, but whose presence can be inferred from gravitational effects on visible matter. It is estimated that there may be between 6 and 7 times as much dark matter as normal, bright matter in the universe, although its exact nature remains a mystery.

Decoherence:
Click for a picture
Artist's impression of Schrödinger's Cat thought experiment, an example of decoherence
(Source: Wikipedia: http://en.wikipedia.org/
wiki/Shrodinger%27s_cat
)
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.

Density:
The mass of an object divided by its volume, a measure of how much it is compacted or crowded together (e.g. air is low in density, iron is high). Boyle’s Law dictates that a substance increases in density as its pressure is increased or as its temperature is decreased.

Dimensions:
Independent directions in space-time. We are familiar with the three dimensions of space (length, width and height, or east-west, north-south and up-down) and one of time (past-future), but superstring theory, for example, requires the universe to have ten dimensions.

DNA:
Click for a picture
Illustration and comparison of of RNA and DNA molecules
(Source: National Human Genome Research Institute: http://www.genome.gov/Pages/Hyperion/
DIR/VIP/Glossary/Illustration/rna.cfm
)
Deoxyribonucleic acid (DNA) molecules consist of two long intertwined polymers of nucleotides, with backbones made of sugars and phosphate groups joined by ester bonds, structured as the familiar double helix. DNA is responsible for the long-term storage of genetic information, and specifies the sequence of the amino acids within proteins. It is organized into structures called chromosomes, and contains the genetic instructions used in the development and functioning of all known living organisms and some viruses. The first accurate model of the structure of DNA was formulated by James Watson and Francis Crick in 1953. The genetic information from DNA is transmitted into the nucleus of cells by molecules of RNA, which controls certain chemical processes in the cell. Both DNA and RNA are considered essential building blocks of life.

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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.

Electric Field:
The field of force that surrounds an electric charge (in the same way as a magnetic field is the field of force that surrounds a magnet). Together, the electric and magnetic fields make up the electromagnetic field which underlies light and other electromagnetic waves, and changes in either field will induce changes in the other, as shown in the equations of James Clerk Maxwell.

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):
Click for a picture
The electromagnetic spectrum
(Source: Wikipedia: http://en.wikipedia.org/wiki/
Electromagnetic_spectrum
)
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.

Electron:
Click for a picture
Electron shell diagram for the element uranium (using the Bohr model of the atom)
(Source: Wikipedia: http://en.wikipedia.org/wiki/Wikipedia:
Picture_peer_review/Electron_shells
)
A negatively-charged sub-atomic particle. It is an indivisible, elementary particle, and is usually to be found orbiting the nucleus of an atom. Electrons in an atom (which exist in the same quantity as the number of protons in the nucleus of the particular atom, so that the overall electric charge is zero) are constrained to occupy certain discrete orbital positions or “shells” around the nucleus. Interactions between the electrons of different atoms play an essential role in chemical bonding and phenomena such as electricity, magnetism and thermal conductivity. The discovery of electrons is credited to the British physicist J. J. Thomson in 1897.

Element:
Click for a picture
Periodic Table of Elements
(Source: Wikipedia: http://en.wikipedia.org/wiki/Periodic_table)
A substance that cannot be reduced any further by chemical means. It is a pure chemical substance composed of atoms with the same atomic number (i.e. the same number of protons in its nucleus). There are 92 naturally occurring elements on Earth, and all chemical matter consists of these elements (although a further 25 have been discovered as products of artificial nuclear reactions). Elements with atomic numbers 83 or higher are inherently unstable, and undergo radioactive decay. The list of elements is usually shown in the form of a Periodic Table, in order of their atomic number (see box at right, or click ther source link for a more detailed interactive Periodic Table).

Elementary Particle:
Click for a picture
Standard Model of elementary particles
(Source: Wikipedia: http://en.wikipedia.org/wiki/
Elementary_particles
)
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:
Click for a picture
An entangled pair of particles can be seen to have complementary properties when measured
(Source: Universe Review: http://universe-review.ca/F13-atom.htm)
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.

Event Horizon:
Click for a picture
Event horizon of a black hole
(Source: Internet Encyclopedia of Science: http://www.daviddarling.info/
encyclopedia/E/event_horizon.html
)
A one-way boundary in space-time surrounding a black hole. Any matter or light that falls through the event horizon of a black hole can never leave, and any event inside the event horizon cannot affect an outside observer.

Exogenesis:
The hypothesis that life on Earth was transferred from elsewhere in the universe. A related but more limited concept is that of panspermia, the idea that "seeds" of life exist already all over the universe, and that life on Earth may have originated through these "seeds".

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).

Expanding Universe:
Click for a picture
Artist's impression of the "metric expansion" of the universe
(Source: Wikipedia: http://en.wikipedia.org/wiki/Big_Bang)
A universe which is constantly growing in size and in which the constituent parts (galaxies, clusters, etc) are flying ever further away from each other. Although contrary to the static universe which had been assumed throughout most of history, an expanding universe was confirmed by Edwin Hubble’s 1929 observations of the redshifts of distant Cepheid variable stars, and is consistent with most solutions to Albert Einstein’s general relativity field equations. It also suggests that, in the distant past, the universe was much smaller and ultimately had its beginning in a Big Bang type event.

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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.

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Galaxy:
Click for a picture
Examples of types of galaxies
(Source: Australia Telescope Outreach and Education: http://outreach.atnf.csiro.au/
education/senior/cosmicengine/
galaxy_formation.html
)
One of the basic building block of the universe, a galaxy is a massive system of stars, stellar remnants, gas, dust, and possibly a hypothetical substance known as dark matter, bound together by gravity. Galaxies may be anywhere from 1 to 100,000 light years across and are typically separated by millions of light years of intergalactic space. They are grouped into clusters, which in turn can form larger groups called superclusters and sheets or filaments. There are many different kinds of galaxy including spiral (like our own Milky Way galaxy), elliptical, ring, dwarf, lenticular and irregular. There are estimated to be over a hundred billion galaxies in the observable universe.

Gamma Ray:
Click for a picture
Gamma ray
(Source: Medical Pages: http://www.irradiation.co.uk/types.php)
A form of electromagnetic radiation produced by some kinds of radioactive decay. Gamma rays have the highest frequency and energy and the shortest wavelength in the electromagnetic spectrum, and penetrate matter more easily that either alpha particles or beta particles.

Gamma Ray Burst:
A narrow beam of intense electromagnetic radiation released during a supernova event, as a rapidly rotating, high-mass star collapses to form a black hole. They are the brightest events known to occur in the universe, and can last from milliseconds to several minutes (typically a few seconds). The initial burst is usually followed by a longer-lived 'afterglow' emitted at longer wavelengths (X-ray, ultraviolet, optical, infrared and radio).

Gas:
A state of matter consisting of a collection of particles (molecules, atoms, ions, electrons, etc) without a definite shape or volume, and that are in more or less random motion. A gas tends to have relatively low density and viscosity compared to the solid and liquid states of matter, expands and contracts greatly with changes in temperature or pressure (“compressible”), and diffuses readily, spreading and homogeneously distributing itself throughout any container.

General Theory of Relativity:
Click for a picture
General relativity predicts, among other things, the gravitational bending of light by massive bodies
(Source: Time Travel Research Center: http://www.zamandayolculuk.com/cetinbal/
HTMLdosya1/RelativityFile.htm
)
Sometimes known as the Theory of General Relativity, this was Albert Einstein’s refinement (published in 1916) of his earlier Special Theory of Relativity and Sir Isaac Newton’s much earlier Law of Universal Gravitation. The theory holds that acceleration and gravity are indistinguishable - the Principle of Equivalence - and describes gravity as a property of the geometry (more specifically a warpage) of space-time. Among other things, the theory predicts the existence of black holes, an expanding universe, time dilation, length contraction, gravitational light bending and the curvature of space-time. Although classical physics can be considered a good approximation for everyday purposes, the predictions of general relativity differ significantly from those of classical physics. They have become generally accepted in modern physics, however, and have been confirmed by all observations and experiments to date.

Geodesic:
Click for a picture
A geodesic is the shortest path between two points in curved space
(Source: Wikipedia: http://en.wikipedia.org/wiki/
Introduction_to_general_relativity
)
The shortest path between two points in curved space. It originally meant the shortest route between two points on the Earth's surface (namely a segment of a great circle) but, since its application in general relativity, it has come to mean the generalization of the notion of a straight line as applied to all curved spaces. In non-curved three-dimensional space, the geodesic is a straight line. In general relativity, a free falling body (on which only gravitational forces are acting) follows a geodesic in curved four-dimensional space-time.

Grand Unified Theory (or Unified Field Theory):
Also known as Grand Unification or GUT, this refers to any of several unified field theories that predict that at extremely high energies (such as occurred just after the Big Bang), the electromagnetic, weak nuclear, and strong nuclear forces are all fused into a single unified field. Thus far, physicists have only been able to merge electromagnetism and the weak nuclear force into the “electroweak force”. Beyond Grand Unification, there is also speculation that it may be possible to merge gravity with the other three gauge symmetries into a “theory of everything”.

Gravity (or Gravitational Force):
Click for a picture
Gravity causes space-time to curve around massive objects
(Source: Time Travel Research Center: http://www.zamandayolculuk.com/cetinbal/
HTMLdosya1/RelativityFile.htm
)
The force of attraction that exists between any two masses, whether they be stars, microscopic particles or any other bodies with mass. It is by far the weakest of the four fundamental forces (the others being the electromagnetic force, the strong nuclear force and the weak nuclear force), and yet, because it is a consistent force operating on all bodies with mass, it is instrumental in the formation of galaxies, stars, planets and black holes. It was approximately described by Sir Isaac Newton’s Law of Universal Gravitation in 1687, and more accurately described by Albert Einstein’s General Theory of Relativity in 1916.

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Half-Life:
A measure of the speed of radioactive decay of unstable, radioactive atoms. It is the time taken for half of the nuclei in a radioactive sample to disintegrate or decay. Half-lives can vary from a split-second to billions of years depending on the substance.

Hawking Radiation:
Click for a picture
Hawking radiation as particle pairs are created near a black hole
(Source: University of St Andrews: http://www.st-andrews.ac.uk/~ulf/fibre.html)
Random and featureless sub-atomic particles and thermal radiation predicted to be emitted by black holes due to quantum effects. Over long periods of time, as a black hole loses more matter through radiation than it gains through other means, it is therefore expected to dissipate, shrink and ultimately vanish.

Horizon:
Click for a picture
The horizon is the edge of the observable universe from a particular position
(Source: U. of Oregon Lectures: http://abyss.uoregon.edu/%7Ejs/lectures/early_univ.html)
The horizon of the universe is much like the horizon on Earth: it is the furthest that can be seen from a particular position. Because light has a finite speed and the universe has a finite age, we can only see objects whose light has had time to reach us since the Big Bang, so that the observable universe can be thought of as a bubble centred on the Earth.

Hubble’s Law:
Formulated by Edwin Hubble in 1929, the law states that the redshift in light coming from distant galaxies is proportional to their distance, so that every galaxy appears to be rushing away from us (or from any other point in the universe) with a speed that is directly proportionate to its distance from us. It is considered the first observational basis for an expanding universe (or the metric expansion of space), and the most often cited evidence in support of the Big Bang theory, and arguably one of the most important cosmological discoveries ever made.

Hydrostatic Equilibrium:
Click for a picture
Hydrostatic equilibrium
(Source: U. of Massachusetts: http://www.astro.umass.edu/%7Emyun/
teaching/a100/longlecture14.html
)
The state in which the force of gravitation working to crush a star is exactly balanced by the thermal pressure of its hot gas pushing outwards. It is the reason that stars in general do not implode or explode, and it also explains why the Earth's atmosphere does not collapse to a very thin layer on the ground.

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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.

Interference:
Click for a picture
Wave interference in Thomas Young's double-slit experiment
(Source: Wikipedia: http://en.wikipedia.org/wiki/Double-slit_experiments)
The ability of two waves passing through each other to mingle, reinforcing each other where crests coincide and cancelling each other out where crests and troughs coincide, similar to the way ripples in water interfere with each other. This results, for example, in an interference pattern of light and dark stripes on a screen illuminated by light from two sources.

Ion:
An atom or molecule that has been stripped of one or more of its orbiting electrons, thus giving it a net positive electric charge. Technically, an atom which gains an electron (thus giving it a net negative electric charge) is also a type of ion, known as an anion.

Isotope:
A possible form of an element, distinguishable from other isotopes of the same element by its differing mass, which is caused by a different number of neutrons in the nucleus (the number of protons, which gives the atomic number of the element, must be the same). Around 75% of isotopes are stable, while some are unstable or radioactive, and will decay over time into other elements.

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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.

Law of Universal Gravitation:
Click for a picture
Newton's Law of Universal Gravitation
(Source: Astronomy Notes: http://www.astronomynotes.com/
gravappl/s3.htm
)
Published by Sir Isaac Newton in 1687, and sometimes also known as the Universal Law of Gravity, this was the first formulation of the idea that all bodies with mass pull on each other across space. Newton observed that the force of gravity between two objects is proportional to the product of the two masses, and inversely proportional to the square of the distance between them. Although the theory has since been superseded by Albert Einstein's General Theory of Relativity, it predicts the movements of the Sun, the Moon and the planets to a high degree of accuracy and it continues to be used as an excellent approximation of the effects of gravity for everyday applications (relativity is only required when there is a need for extreme precision, or when dealing with the gravitation of very massive objects).

Length Contraction:
Click for a picture
At relativistic speeds, space “contracts” and time “dilates”
(Source: Time Travel Research Center: http://www.zamandayolculuk.com/cetinbal/
HTMLdosya1/RelativityFile.htm
)
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.

Life:
A difficult and contentious phenomenon to define, life is usually considered to be a characteristic of organisms that exhibit certain biological processes (such as chemical reactions or other events that results in a transformation), and that are capable of growth through metabolism and are capable of reproduction. The ability to ingest food and excrete waste are also sometimes considered requirements of life (e.g. bacteria are usually considered to be alive, whereas simpler viruses, which do not feed or excrete, are not).
The two distinguishing features of living systems are sometimes considered to be complexity and organization (negative entropy). Some organisms can communicate, and many can adapt to their environment through internally generated changes, although these are not universally considered prerequisites for life.

Light:
Technically, this refers to electromagnetic radiation of a wavelength that is visible to the human eye, although in the broader field of physics, it is sometimes used to refer to electromagnetic radiation of all wavelengths, whether visible or not. It exhibits “wave-particle duality” in that it can behave as both waves and particles (photons). Light travels at a constant speed of about 300,000 kilometres per second in a vacuum.

Light Year:
A convenient unit for measuring the large distances in the universe. It is the distance that light travels in one year which, given that light travels at 300,000 kilometres per second, works out to about 9,460,000,000 kilometres (9.46 trillion kilometres).

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Magnetic Field:
The field of force that surrounds a magnet (in the same way as an electric field is the field of force that surrounds an electric charge). Together, the magnetic and electric fields make up the electromagnetic field which underlies light and other electromagnetic waves, and changes in either field will induce changes in the other, as indicated by James Clerk Maxwell’s Equations of Electromagnetism.

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).

Molecule:
Click for a picture
Examples of organic molecules
(Source: U. of Wisconsin-Madison Lectures: http://newtraditions.chem.wisc.edu/
FPTS/fbform/fororgf.htm
)
A collection of atoms glued together by electromagnetic forces. A more formal definition might be: a sufficiently stable electrically neutral group of at least two atoms, in a definite arrangement, held together by very strong chemical bonds. A molecule may consist of atoms of the same chemical element (e.g. oxygen: O2) or of different elements (e.g. water: H2O). Organic molecules are those which include carbon, and the others are called inorganic.

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.

Multiverse (Parallel Universes):
Click for a picture
Parallel universes making up a multiverse
(Source: Mukto-Mona: http://www.mukto-mona.com/science/
physics/ParalellUniverse2003.pdf
)
A hypothetical set of multiple possible universes (including our own) which exist in parallel with each other. Our universe would then be just one of an enormous number of separate and distinct parallel universes, the vast majority of which would be dead and uninteresting, not having a set of physical laws which would allow the emergence of stars, planets and life.

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Neutrino:
Click for a picture
Super-Kamiokande, a neutrino observatory in Japan
(Source: Super-Kamiokande: http://www-sk.icrr.u-tokyo.ac.jp/sk/gallery/index-e.html)
A sub-atomic elementary particle with no electrical charge and very small mass that travels very close to the speed of light. They are created as a result of certain types of radioactive decay or nuclear reaction, such as the decay of a free neutron (i.e. one outside of a nucleus) into a proton and electron. Being electrically neutral and unaffected by the strong nuclear force or the electromagnetic force, neutrinos are able to pass through ordinary matter almost undisturbed and are therefore extremely difficult to detect, although when created in huge numbers they are capable of blowing a star apart in a supernova.

Neutron:
Click for a picture
The quark structure of a neutron
(Source: Wikipedia: http://en.wikipedia.org/wiki/Neutron)
One of the two main building blocks (along with the proton) of the nucleus at the centre of an atom. Neutrons have essentially the same mass as a proton (very slightly larger) but no electric charge, and are made up of one “up” quark and two “down” quarks. The number of neutrons in an atom determines the isotope of an element. Outside of a nucleus, they are unstable and disintegrate within about ten minutes.

Neutron Star:
A star that has shrunk under its own gravity during a supernova event, so that most of its material has been compressed into neutrons only (the protons and electrons have been crushed together until they merge, leaving only neutrons). Neutron stars are very hot, quite small (typically 20 to 30 kilometres in diameter), extremely dense, have a very high surface gravity and rotate very fast. A pulsar is a kind of highly-magnetized rapidly-rotating neutron star.

Newton’s Laws of Motion:
The three physical laws, published by Sir Isaac Newton in 1687, that form the basis for classical mechanics: 1) a body persists its state of rest or of uniform motion unless acted upon by an external unbalanced force; 2) force equals mass times acceleration; and 3) to every action there is an equal and opposite reaction.

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).

Nuclear Fission:
A nuclear reaction in which the nucleus of an atom splits into smaller parts, often producing free neutrons, lighter nuclei and photons (in the form of gamma rays). The process releases large amounts of energy, both as electromagnetic radiation and as kinetic energy of the resulting fragments.

Nuclear Fusion:
Click for a picture
The nuclear fusion process in the sun
(Source: U. of Berkely: http://cse.ssl.berkeley.edu/bmendez/
ay10/2002/notes/lec12.html
)
The welding together of two light nuclei to make a heavier nucleus, resulting in the liberation of nuclear energy. An example of this kind of nuclear reaction is the binding together of hydrogen nuclei in the core of the Sun to make helium. In larger, hotter stars, helium itself may fuse to produce heavier elements, a process which continues up the periodic table of elements as far as iron. The fusion of ultra-stable iron nuclei actually absorbs energy rather than releasing it, and so iron does not easily fuse to create heavier elements.

Nucleosynthesis:
Click for a picture
Sources of heavier elements (nucleosynthesis)
(Source: Northern Arizona University Lectures: http://www4.nau.edu/meteorite/
Meteorite/Book-GlossaryN.html
)
The process of creating new atomic nuclei from pre-existing protons and neutrons by a process of nuclear fusion. The primordial nucleons (hydrogen and helium) themselves were formed from the quark-gluon plasma in the first few minutes after the Big Bang, as it cooled to below ten million degrees, but nucleosynthesis of the heavier elements (including all carbon, oxygen, etc) occurs primarily in the nuclear fusion process within stars and supernovas.

Nucleus:
The tight cluster of nucleons (positively-charged protons and zero-charged neutrons, or just a single proton in the case of hydrogen) at the centre of an atom, containing more than 99.9% of the atom’s mass. The nucleus of a typical atom is about 100,000 smaller than the total size of the atom (depending on the individual atom).

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Oscillating Universe:
A cosmological model, in which the universe undergoes a potentially endless series of oscillations, each beginning with a Big Bang and ending with a Big Crunch. After the Big Bang, the universe expands for a while before the gravitational attraction of matter causes it to collapse back and undergo a “bounce”.

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Panspermia:
Click for a picture
Artist's depiction of life falling to Earth on a meteorite, according to the panspermia hypothesis
(Source: Internet Encyclopedia of Science: http://www.daviddarling.info/
encyclopedia/B/ballpans.html
)
The hypothesis that "seeds" of life exist already all over the universe, and that life on Earth may have originated through these "seeds", driven by a steady influx of cells or viruses arriving from space via comets. It is a more limited form of the related hypothesis of exogenesis, which also proposes that life on Earth was transferred from elsewhere in the universe, but makes no prediction about how widespread it may be.

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:
Click for a picture
Emission of electrons from a metal plate (photoelectric effect)
(Source: Northern Arizona University: http://www4.nau.edu/meteorite/
Meteorite/Book-GlossaryP.html
)
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.

Plasma:
Click for a picture
Plasma illustrated with a plasma lamp
(Source: Wikipedia: http://en.wikipedia.org/wiki/Plasma_(physics))
A partially ionized gas of ions and electrons, in which a certain proportion of the electrons are free rather than being bound to an atom or molecule. It has properties quite unlike those of solids, liquids or gases and is sometimes considered to be a distinct fourth state of matter. An example of plasma present at the Earth's surface is lightning.

Positron:
The antiparticle or antimatter counterpart of the electron. The positron, then, is an elementary particle with a positive electric charge, and the same mass and spin as an electron. The existence of positrons was first postulated in 1928 by Paul Dirac, and definitively discovered by Carl Anderson in 1932.

Primeval (or Primordial) Soup:
The theory of the origin of life on Earth first put forward by Alexander Oparin, whereby a “soup” of organic molecules could be created in a “reducing” oxygen-less atmosphere through the action of sunlight, creating the necessary building blocks for the evolution of life.

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).

Principle of Relativity:
Click for a picture
The principle of relativity says that the laws of physics are the same in all inertial systems
(Source: U. of Pacific Lectures: http://sol.sci.uop.edu/%7Ejfalward/
relativity/relativity.html
)
The idea, first expressed by Galileo Galilei in 1632 and also known as the principle of invariance, that the fundamental laws of physics are the same in all inertial frames and that, purely by observing the outcome of mechanical experiments, one cannot distinguish a state of rest from a state of constant velocity. Thus, all uniform motion is relative, and there is no absolute and well-defined state of rest.

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.

Prokaryotes and Eukaryotes:
Click for a picture
Diagrams of a eukaryotic and a prokaryotic cell
(Source: Windows to the Universe: http://www.windows2universe.org/earth/
Life/cell_organelles.html
)
Prokaryotes are primitive organisms that lack a cell nucleus or any other membrane-bound organelles. Most prokaryotes are single-celled (although some have multicellular stages in their life-cycles), and they are divided into two main domains, bacteria and archaea.
Eukaryotes, on the other hand, are organisms whose cells contain a nucleus and are organized into complex structures enclosed within membranes. Most living organisms (including all animals, plants, fungi and protists) are eukaryotes.

Proton:
Click for a picture
The quark structure of a proton
(Source: Wikipedia: http://en.wikipedia.org/wiki/Proton)
One of the two main building blocks (along with the neutron) of the nucleus at the centre of an atom. Protons carry a positive electrical charge, equal and opposite to that of electrons, and are made up of two “up” quarks and one “down” quark. The number of protons in an atom’s nucleus determines its atomic number and thus which chemical element it represents.

Pulsar:
Click for a picture
Magnetic field and radiation of a pulsar
(Source: Northern Arizona University: http://www4.nau.edu/meteorite/
Meteorite/Book-GlossaryP.html
)
A highly-magnetized rapidly-rotating neutron star that sweeps regular pulses of intense electromagnetic radiation (radio waves) around space like a lighthouse. The intervals between pulses are very regular, ranging from 1.4 milliseconds to 8.5 seconds depending on the rotation period of the star. A pulsar generally has a mass similar to our own Sun, but a diameter of only around 10 kilometres.

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Quantum:
The smallest chunk into which something can be divided in physics. Quantized phenomena are restricted to discrete values rather than to a continuous set of values. Some quanta take the form of elementary particles, such as photons which are the quanta of the electromagnetic field. Quanta are measured on the tiny Planck scale of the order of around 10-35 metres.

Quantum Electrodynamics:
Sometimes shortened to QED, it is essentially the theory of how light interacts with matter. More specifically, it deals with the interactions between electrons, positrons (antielectrons) and photons. It explains almost everything about the everyday world, from why the ground is solid to how a laser works to the chemistry of metabolism to the operation of computers.

Quantum Gravity (or Quantum Theory of Gravity):
A so-called “theory of everything” which combines the General Theory of Relativity (the theory of the very large, which describes one of the fundamental forces of nature, gravity) with quantum theory (the theory of the very small, which describes the other three fundamental forces, electromagnetism, the weak nuclear force and the strong nuclear force) into a unified theory. However, even the most promising candidates, like superstring theory and loop quantum gravity, still need to overcome major formal and conceptual problems, and this is still very much a work in progress.

Quantum State:
The set of characteristics describing the condition a quantum mechanical system is in. It can be described by a wave function or a complete set of quantum numbers (energy, angular momentum, spin, etc), although, when observed, the system is forced into a specific stationary "eigenstate". If a particle within a quantum system (such as an electron within an atom) moves from one quantum state to another, it does so instantaneously and in discontinuous steps (known as quantum leaps or jumps) without ever being in a state in between.

Quantum Theory (or Quantum Physics or Quantum Mechanics):
The physical theory of objects isolated from their surroundings. Because it is very difficult to isolate large objects, quantum theory (also known as quantum mechanics or quantum physics) is essentially a theory of the microscopic world of atoms and their constituents. Among its main principles are the dual wave-like and particle-like behaviour of matter and radiation (wave-particle duality), and the prediction of probabilities in situations where classical physics predicts certainties. Classical physics provides a good approximation to quantum physics for everyday purposes, typically in circumstances with large numbers of particles.

Quantum Tunnelling:
Click for a picture
Quantum tunnelling through a barrier
(Source: U. of Oregon Lectures: http://abyss.uoregon.edu/%7Ejs/ast123/lectures/lec06.html)
The quantum mechanical effect in which particles have a finite probability of crossing an energy barrier, or transitioning through an energy state normally forbidden to them by classical physics, due to the wave-like aspect of particles. The probability wave of a particle represents the probability of finding the particle in a certain location, and there is a finite probability that the particle is located on the other side of the barrier.

Quark:
A type of elementary particle which is the major constituent of matter. Quarks are never found on their own, only in groups of three within composite particles called hadrons (such as protons and neutrons). There are six different types (or “flavours”) of quarks - up, down, top, bottom, charm and strange - and each flavour comes in three “colours” - red, green or blue (although they have no colour in the normal sense, being much smaller than the wavelength of visible light). Quarks are the only particles in the standard model of particle physics to experience all four fundamental forces, and they have the properties of electric charge, colour charge, spin and mass.

Quasar:
Click for a picture
Quasar PG 0052+251 photographed by the Hubble Space Telescope
(Source: Hubble Space Telescope: http://www.spacetelescope.org/
images/html/opo9635a1.html
)
Short for QUAsi-StellAr Radio source, a quasar is an extremely powerful and distant active galactic nucleus (a compact region at the centre of a galaxy which has a much higher than normal luminosity). It derives most of its energy from very hot matter swirling into a central supermassive black hole, and can generate as much light as a hundred normal galaxies from a much smaller volume. It is one of the most powerful objects in the universe, and among the most distant things ever seen in space.

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Radioactivity (Radioactive Decay):
The disintegration of unstable heavy atomic nuclei into lighter, more stable, atomic nuclei, accompanied in the process by the emission of ionizing radiation (alpha particles, beta particles or gamma rays). This is a random process at the atomic level but, given a large number of similar atoms, the decay rate on average is predictable, and is usually measured by the half-life of the substance.

Redshift:
Click for a picture
Redshift and blueshift of moving objects
(Source: Astronomy Notes: http://www.astronomynotes.com/
light/s10.htm
)
The shifting of emitted electromagnetic radiation (such as visible light) towards the less energetic red end of the electromagnetic spectrum when a light source is moving away from the observer. This occurs as the wavelengths of light stretch as an object moves away (as opposed to being squashed by an approaching object), similar to the familiar Doppler effect on sound waves. Among other things, it can be used as a measure of the speed with which galaxies throughout the universe are moving away from us.

Relativity:
The theory, formulated essentially by Einstein’s theory has two main parts: the Special Theory of Relativity (or special relativity) which deals with objects in uniform motion, and the General Theory of Relativity (or general relativity) which deals with acclerating objects and gravity.

RNA and DNA:
Click for a picture
Illustration and comparison of of RNA and DNA molecules
(Source: National Human Genome Research Institute: http://www.genome.gov/Pages/Hyperion/
DIR/VIP/Glossary/Illustration/rna.cfm
)
Ribonucleic acid (RNA) is a type of single-stranded molecule that consists of a long chain of nucleotide units, each of which consists of a nitrogenous base, a ribose sugar and a phosphate. RNA transmits the genetic information from DNA into the nucleus of cells, and controls certain chemical processes in the cell. Both DNA and RNA are considered essential building blocks of life.

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Second Law of Thermodynamics:
The idea that entropy (the microscopic disorder of a body) can never decrease, but rather will tend to increase over time. In practice, this results in an inexorable tendency towards uniformity and away from patterns and structures, and means, for example, that heat always flows from a hot body to a cold one, and that differences in temperature, pressure and density tend to even out in an isolated physical system (or in the universe as a whole).

Simultaneity:
The idea, disproved by Einstein in his Special Theory of Relativity, that events that appear to happen at the same time for one person should appear to happen at the same time for everyone in the universe.

Singularity (or Gravitational Singularity):
Click for a picture
A gravitational singularity is hidden within a black hole
(Source: Northern Arizona University: http://www4.nau.edu/meteorite/
Meteorite/Book-GlossaryS.html
)
A region of space where the density of matter, or the curvature of space-time, becomes infinite and the concepts of space and time cease to have any meaning. At this point, the whole fabric of space-time ruptures and the precepts of Einstein’s General Theory of Relativity (and physics in general) break down and no longer apply, similar to the way in which a calculator returns an error when asked to divide by zero. According to general relativity, the Big Bang started with a singularity, and there is a singularity at the centre of a black hole.

Space-Time:
Click for a picture
The path taken by an object in both space and time is known as the space-time interval
(Source: Wikibooks: http://en.wikibooks.org/wiki/
Special_Relativity/Spacetime
)
Space-time (or spacetime or the spacetime continuum) is any mathematical model that combines space and time into a single construct. The fourth dimension of time is traditionally considered to be of a different sort than the three dimensions of space in that it can only go forwards and not back but, in Albert Einstein’s General Theory of Relativity, space and time are seen to be essentially the same thing and can therefore be treated as a single entity.

Special Theory of Relativity:
Albert Einstein’s first major theory, dating from 1905, special relativity builds on Galileo's more simplistic principle of relativity and relates what one person sees when looking at another person moving at constant speed relative to them. “Special” indicates that the theory restricts itself to observers in uniform or constant relative motion, a restriction Einstein addressed later in his General Theory of Relativity. The theory incorporates the principle that the speed of light is the same for all inertial observers, regardless of the state of motion of the source. Among other things, it reveals that the moving person appears to shrink in the direction of their motion (length contraction)and their time slows down (time dilation), effects which are ever more marked as speeds approach the speed of light. The theory also leads to some famous paradoxes like the so-called Time Travel Paradox and the Twin Paradox.

Speed of Light:
Click for a picture
The reasoning used by Rømer in 1675 to determine the speed of light
(Source: U. of Califormia Lectures: http://phyun5.ucr.edu/%7Ewudka/Physics7/Notes_www/node65.html)
In a vacuum, light travels at a speed of exactly 299,792,458 metres per second, or about 300,000 kilometres per second, a speed which remains constant irrespective of the speed of the source of the light or of the observer (one of the cornerstones of Albert Einstein’s Special Theory of Relativity). It is the term c in Einstein’s famous equation E = mc2.

Spin:
Click for a picture
Artist's representation of the spin and charge of an electron
(Source: Berkeley Lab: http://www.lbl.gov/Science-Articles/
Archive/sabl/2006/Jan/02-spin-drag.html
)
A fundamental property of sub-atomic elementary particles that means that behave as though they are spinning or rotating (although in reality they are not spinning at all). The concept has no direct analogue in the everyday world. Particles of spin ½ (e.g. electrons, positrons, neutrinos and quarks) make up all the matter in the universe, while particles with integer spin (0, 1 or 2) give rise to, or mediate, the forces operating between the matter particles (e.g. photons, gluons, W and Z bosons).

Star:
Click for a picture
The process of star formation
(Source: Spitzer Science Center: http://ssc.spitzer.caltech.edu/
documents/compendium/galsci/
)
A massive, luminous ball of gas or plasma, held together by its own gravity, that replenishes the heat it loses to space by means of nuclear energy generated in its core. Almost all of the elements heavier than hydrogen and helium were created by the nuclear fusion processes in stars. There are many different types of stars including binary stars, proto-stars, dwarf stars (like our nearest star which we call the Sun), supergiants, supernovas, neutron stars, pulsars, quasars, etc. There are a roughly estimated 10,000 billion billion stars (1022) in the observable universe.

Steady State Universe:
Click for a picture
In a steady state universe, overall density remains constant
(Source: Luke Mastin (own graphic)
A cosmological model developed by Fred Hoyle, Thomas Gold and Hermann Bondi in 1948 as the main alternative to the standard Big Bang theory of the universe. Steady state theory holds that the universe is expanding but that new matter and new galaxies are continuously created in order to maintain the perfect cosmological principle (the idea that, on the large scale, the universe is essentially homogenous and isotropic in both space and time), and therefore has no beginning and no end. The theory was quite popular in the 1950s and 1960s, but fell out of favour with the discovery of distant quasars and cosmic background radiation in the 1960s.

String:
An object with a one-dimensional spatial extent, length (unlike an elementary particle which is zero-dimensional, or point-like). According to string theory, the different fundamental particles of the standard model can be considered to be just different manifestations of one basic object, a string, with different vibrational modes. The characteristic length scale of strings is thought to be on the order of the Planck Length (about 10-35 metres, still too small to be visible in current physical laboratories), the scale at which the effects of quantum gravity are believed to become significant.
Cosmic string is a similar but separate concept which refers to one-dimensional topological defects, extremely thin but immensely dense, which are hypothesized to have formed as a result of phase changes soon after the Big Bang (analogous to the imperfections that form between crystal grains in solidifying liquids or the cracks that form when water freezes into ice). According to some theories, such cosmic strings grew as the universe expanded and were instrumental in the accretion of matter and the formation of galaxy clusters and large-scale structures in the universe.

String Theory (Superstring Theory):
Click for a picture
Artist's impression of the fundamental entities of superstring theory by Flavio Robles
(Source: Berkeley Lab: http://www.lbl.gov/Publications/Currents/
Archive/June-16-2000.html
)
A theory which postulates that the fundamental ingredients of the universe are tiny strings of matter (on the tiny scale of the Planck Length of around 10-35 metres) which vibrate in a space-time of ten dimensions. It is considered one of the most promising of the quantum gravity theories which hope to unite or unify quantum theory and the General Theory of Relativity, and apply to both large-scale structures and structures on the atomic scale.
Superstring theory (short for supersymmetric string theory) is a refinement of the more general theory of strings.

Strong Nuclear Force:
Also known as the strong interaction, this is the powerful but short-range force that holds protons and neutrons together in the nucleus of an atom despite the electromagnetic repulsion of same-charge particles, as well as holding together the constituent quarks which comprise neutrons and protons. It is one of the four fundamental forces of physics (along with the gravitational force, the electromagnetic force and weak nuclear force), and the most powerful, being 100 times the strength of the electromagnetic force, about 1013 times as great as that of the weak force and about 1038 times that of gravity.
The force is mediated by elementary particles called gluons which shuttle back and forth between the particles being operated on and "glue" the particles together. Unlike the other forces, the strength of the strong force between quarks becomes stronger with distance, acting like an unbreakable elastic thread. However, it only operates over a very small distance (less than the size of the nucleus), outside of which it fades away abruptly.

Supernova:
Click for a picture
A sequence of ultraviolet images of a supernova explosion (NASA, 2008)
(Source: National Geographic: http://news.nationalgeographic.com/
news/bigphotos/75772723.html
)
A cataclysmic explosion caused by the collapse of an old massive star which has used up all its fuel. For a short time, such an explosion may outshine an entire galaxy of a hundred billion ordinary stars. It leaves behind a cloud of brightly coloured gas called a nebula, and sometimes a highly compressed neutron star or even a black hole.

Superposition:
The ability in quantum theory of an object, such as an atom or sub-atomic particle, to be in more than one quantum state at the same time. For example, an object could technically be in more than one place simultaneously as a consequence of the wave-like character of microscopic particles.

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Time Dilation:
Click for a picture
At relativistic speeds, space “contracts” and time “dilates”
(Source: Time Travel Research Center: http://www.zamandayolculuk.com/cetinbal/
HTMLdosya1/RelativityFile.htm
)
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, another’s time (for example, an identical clock) appear to run slower. Thus, moving clocks run more slowly compared to stationary clocks and, the closer the speed of movement approaches to the speed of light, the greater the effect. Gravitational time dilation is a related phenomenon, whereby time passes more slowly the higher the local distortion of space-time due to gravity (such as near a black hole, for example).

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Uncertainty Principle:
Click for a picture
Heisenberg's microscope thought experiment to illustrate the effects of the uncertainty principle
(Source: U. of Kyushu Lectures: http://www.kutl.kyushu-u.ac.jp/
seminar/MicroWorld2_E/2Part1_E/
2P14_E/Heisenberg_QM_E.htm
)
The principle in quantum theory, formulated by Werner Heisenberg in 1926, which holds that the values of certain pairs of variables cannot BOTH be known exactly, so that the more precisely one variable is known, the less precisely the other can be known. For example, if the speed or momentum of a particle is known exactly, then its location must remain uncertain; if its location is known with certainty, then the particle’s speed or momentum cannot be known. Formulated another way, relating the unvertainties of energy and time, the uncertainty principle permits the existence of ultra-short-lived microscopic particles (virtual particles) in apparently empty space, which briefly blink into existence and blink out again.

Universe:
Click for a picture
Map of the main super-clusters of galaxies in an area covering about 7% of the observable universe
(Source: Daily Galaxy: http://www.dailygalaxy.com/my_weblog/
2008/01/map-of-the-univ.html
)
Everything that physically exists, including the entirety of space and time, all forms of matter, energy and momentum, and the physical laws and constants that govern them. The universe (or cosmos) is usually considered to have begun about 13.7 billion years ago in a gravitational singulary commonly known as the Big Bang, and has been expanding ever since. Some have speculated that this universe is just one of many disconnected universes, which are collectively denoted as the multiverse.

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Wave-Particle Duality:
The idea that light (and indeed all matter and energy) is both a wave and a particle, and that sometimes it behaves like a wave and sometimes it behaves like a particle. It is a central concept of quantum theory.

Weak Nuclear Force:
Also known as the weak interaction, it is one of the forces experienced by protons and neutrons in the nucleus of an atom, the other being the strong nuclear force. It is one of the four fundamental forces of physics (along with the gravitational force, the electromagnetic force and the strong nuclear force). It is called the weak force because it is about 1013 times weaker than the strong nuclear force and 1011 times weaker than the electromagnetic force, and it is also very short range in its effect.
The weak interaction is mediated by the exchange of heavy elementary particles known as W and Z bosons. It is responsible for radioactive beta decay (as it converts neutrons into protons) and for the production of neutrinos.

White Hole:
The theoretical time reversal of a black hole, which arises as a valid solution in general relativity. While a black hole acts as a vacuum, drawing in any matter that crosses its event horizon, a white hole acts as a source that ejects matter from its event horizon.

Wormhole:
Click for a picture
A wormhole is a theoretical "short-cut" between distant regions of space-time
(Source: Wikipedia: http://commons.
wikimedia.org/wiki/File:Worm3.jpg
)
A hypothetical “tunnel” through space-time that connects widely distant regions, thus providing a kind of short-cut through space-time. Although there is no observational evidence for wormholes, they are known to be valid solutions under the General Theory of Relativity.

 
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