Absolute Zero Lowest temperature attainable. As a body is cooled, its atoms move more and more sluggishly. At absolute zero, equivalent to -273.15 on the Celsius scale, they cease to move altogether. (In fact, this is not entirely true since the Heisenberg uncertainty principle produces a residual jitter even at absolute zero.)
Acceleration The rate of change of velocity. Since velocity is defined as having both a magnitude (speed) and a direction, a body accelerates whenever it changes its speed, its direction or both. For instance, a car pulling away from traffic lights accelerates as does the Moon circling the Earth at pretty much constant speed.
Accretion disc CD-shaped disc of in-swirling matter that forms around a strong source of gravity such as a black hole. Since gravity weakens with distance from its source, matter in the outer portion of the disc orbits more slowly than in the inner portion. Friction between regions where matter is travelling at different speeds heats the disc to millions of degrees. Quasars are thought to owe their prodigious brightness to ferociously hot accretion discs surrounding ‘supermassive’ black holes.
Algorithmic Information Theory A field of mathematics in which the complexity of a number is defined as the length of the shortest computer program capable of generating the number. Since it is actually never possible to know for certain that you have found the shortest program – determining whether you have is an uncomputable problem – the whole field is founded on paradox and uncertainty.
Alpha Centauri The nearest star system to the Sun. It consists of three stars and is 4.3 light years distant.
Angular momentum A measure of how difficult it is to stop a rotating body.
Angular momentum, conservation of The edict that, in the absence of any turning force – torque – angular momentum can never be created or destroyed.
Annihilation The mutual destruction of a subatomic particle and its antiparticle when they run into each other. In the process, their mass-energy is converted into radiant energy – gamma rays.
Anthropic principle The idea that the Universe is the way it is because, if it was not, we would not be here to notice it! In other words, the fact of our existence is an important scientific observation.
Antimatter A large accumulation of antiparticles. Anti-protons, antineutrons and positrons can in fact come together to make anti-atoms. And in principle there is nothing to rule out the possibility of anti-stars, anti-planets and anti-life. One of the greatest mysteries of physics is why we appear to live in a Universe made solely of matter when the laws of physics seem to predict a 50/50 mix of matter and antimatter.
Antiparticle Every subatomic particle has an associated antiparticle with opposite properties such as electrical charge. For instance, the negatively charged electron is twinned with a positively charged antiparticle known as the positron. When a particle and its antiparticle meet, they self-destruct, or ‘annihilate’, in a flash of high-energy light, or gamma rays.
Atom The building block of all normal matter. An atom consists of a nucleus orbited by a cloud of electrons. The positive charge of the nucleus is exactly balanced by the negative charge of the electrons. An atom is about a ten-millionth of a millimetre across.
Atomic energy See Nuclear energy.
Atomic nucleus The tight cluster of protons and neutrons (a single proton in the case of hydrogen) at the centre of an atom. The nucleus contains more than 99.9 per cent of the mass of an atom.
Axiom A self-evident truth. Each field of mathematics is built on a set of axioms from which theorems are deduced by the application of logic.
Big Bang The titanic explosion in which the Universe is thought to have been born 13.7 billion years ago. ‘Explosion’ is actually a misnomer since the Big Bang happened everywhere at once and there was no pre-existing void into which the Universe erupted. Space and time and energy all came into being in the Big Bang.
Big Bang theory The idea that the Universe began in a super-dense, super-hot state 13.7 billion years ago and has been expanding and cooling ever since.
Big Crunch If there is enough matter in the Universe, its gravity will one day halt and reverse the Universe’s expansion so that it shrinks down to a Big Crunch. This is a sort of mirror-image of the Big Bang.
Binary A way of representing numbers. In everyday life we use decimal, or base 10. The right-hand digit represents the 1s, the next digit the 10s, the next the 10 × 10s, and so on. For instance, 9217 means 7 + 1 × 10 + 2 × (10 × 10) + 9 × (10 × 10 × 10). In binary, or base 2, the right-hand digit represents the 1s, the next digit the 2s, the next the 2 × 2s, and so on. So, for instance, 1101 means 1 + 0 × 2 + 1 × (2 × 2) + 1 × (2 × 2 × 2), which in decimal is 13. Binary is particularly useful for computers which are composed of ‘transistors’ – devices with two states.
Black hole The grossly warped space-time left behind when a massive body’s gravity causes it to shrink down to a point. Nothing, not even light, can escape, hence a black hole’s blackness. The Universe appears to contain at least two distinct types of black hole – stellar-sized black holes, formed when very massive stars can no longer generate internal heat to counterbalance the gravity trying to crush them, and ‘supermassive’ black holes. Most galaxies appear to have a super-massive black hole in their heart. They range from millions of times the mass of the Sun in our Milky Way to billions of solar masses in the powerful quasars.
Brane A lower-dimensional ‘island’ in a higher-dimensional space. In superstring theory, for instance, space has ten dimensions, which has led to the suggestion that our Universe is a four-dimensional brane, or four-brane.
Butterfly effect The fact that in a chaotic system – which is infinitely sensitive to initial conditions – an arbitrarily small change in the initial state of the system can in time produce a dramatic effect. For instance, in the case of the global weather system, the beat of a butterfly’s wings in China can eventually trigger a hurricane in the Caribbean.
Carrier wave A periodic wave on which an information-carrying signal is superimposed. In the case of a radio wave, for instance, the frequency or the amplitude is continuously varied, or ‘modulated’.
Cellular automaton A simple computer program that takes an input – a pattern of coloured cells – and applies a simple rule to the input to produce an output – another pattern of coloured cells. The key thing is that the output is fed back in as the next input rather like a snake swallowing its own tail. In the case of a two-colour, adjacent cell, one-dimensional cellular automaton, the input is the pattern of black-and-white cells on one line and the output is the pattern of cells on the next line. Whether a cell in the second line is black or white depends on a rule applied to its two nearest neighbours in the first line. The rule might say, for instance: ‘If a particular cell in the first line has a black square on either side of it, it should turn black in the second line.’
Cellular automaton rule 110 One of the 256 possible rules for a two-colour, adjacent cell, one-dimensional cellular automaton. The rule is special because it generates unending complexity – a pattern that never repeats itself.
CERN The Organisation Européenne pour la Recherche Nucléaire (European Organization for Nuclear Research) – the laboratory for particle physics near Geneva, Switzerland. The acronym originates from the council that set it up, the Conseil Européen pour la Recherche Nucléaire (European Council for Nuclear Research).
Chaos A system which is infinitely sensitive to initial conditions – for instance, the global weather system. This makes it unpredictable not in principle but in practice – because it is never possible to know the starting conditions precisely enough to make a long-term prediction.
Classical physics Non-quantum physics. In effect, all physics before 1900 when the German physicist Max Planck first proposed that energy might come in discrete chunks, or ‘quanta’. Einstein was the first to realise that this idea was totally incompatible with all physics that had gone before.
Cloud chamber A sealed box with a window in which ultra-pure water vapour is cooled well below the temperature at which it normally condenses to form water droplets. The physicist Charles Wilson invented it to investigate how clouds form but soon realised it could be used to reveal the tracks of subatomic particles such as electrons. Water droplets formed like tiny beads around the ions created by the subatomic particles.
Comet Snowy body – usually only a few kilometres across – that orbits a star. Most comets orbit the Sun beyond the outermost planets in an enormous cloud known as the Oort Cloud. Like asteroids, comets are builders’ rubble left over from the formation of the planets.
Compton wavelength The size an electron appears to have in experiments when photons rebound, or ‘scatter’, off it.
Computational irreducibility The property of a computer program whose output can be deduced from its input only by running the program. There is no shortcut.
Computational reducibility The property of a computer program whose output can be deduced from its input in less time than it takes to run the program. There is a shortcut.
Computer program A series of instructions which is applied to an input to create an output.
Computer program, recursive A computer program whose output is fed back in as its next input like a snake swallowing its tail.
Conservation law Law of physics that expresses the fact that a quantity can never change. For instance, the conservation of energy states that energy can never be created or destroyed, only converted from one form to another. For example, the chemical energy of petrol can be converted into the energy of motion of a car.
Copenhagen Interpretation of quantum theory An attempt to explain why denizens of the microscopic world such as an electron exhibit bizarre ‘quantum’ behaviour – for instance, being in two places at once – while those of the everyday world do not. According to the Copenhagen, the act of ‘observing’ an electron somehow forces it to behave and plump for being in one place at once. Since the act of observation is not defined, the whole interpretation is open to interpretation. Most commonly, physicists talk of a big, or ‘macroscopic’, object observing a microscopic object and killing its quantumness. Consequently, the Copenhagen Interpretation splits the world into two – small things that obey quantum theory – and big things which do not.
Copernican principle The idea there is nothing special about our position in the Universe, either in space or in time. This is a generalised version of Copernicus’s recognition that the Earth is not in a special position at the centre of the Solar System but is just another planet circling the Sun.
Cosmic Background Explorer Satellite (COBE) Satellite launched in 1989 to ‘map’ the temperature of the cosmic background radiation – the ‘afterglow’ of the Big Bang fireball – across the sky. COBE found slight variations in the average temperature of the radiation, which were created by matter beginning to clump 450,000 years after the birth of the Universe. The clumps were the ‘seeds’ of giant superclusters of galaxies in today’s Universe.
Cosmic Background Radiation The ‘afterglow’ of the Big Bang fireball. Incredibly, it still permeates all of space 13.7 billion years after the event, a tepid radiation corresponding to a temperature of -270 °C.
Cosmic rays High-speed atomic nuclei, mostly protons, from space. Low-energy ones come from the Sun, high-energy ones probably come from supernovae. The origin of ultra-high-energy cosmic rays, particles millions of times more energetic than anything we can currently produce on Earth, is one of the great unsolved puzzles of astronomy.
Cosmology The ultimate science. The science whose subject matter is the origin, evolution and fate of the entire Universe.
Cosmos Another word for Universe.
Coupling constant A number which is a measure of the strength of one of nature’s fundamental forces. For instance, the fine-structure constant governs the interaction between light (photons) and matter. Since this interaction is the basis of the electromagnetic force, it governs the strength of this force.
Dark energy Mysterious ‘material’ with repulsive gravity. Discovered unexpectedly in 1998, it is invisible, fills all of space and appears to be pushing apart the galaxies and so speeding up the expansion of the Universe. It accounts for about 73 per cent of the mass-energy of the Universe and nobody has much of a clue what it is! In fact, quantum theory predicts that it should have an energy density 10123 times bigger than it actually does.
Dark matter Matter in the Universe which gives out no detectable light. Astronomers know it exists because the gravity of the invisible stuff bends the paths of visible stars and galaxies as they fly through space. There is at least six times as much dark matter in the Universe as visible matter. The identity of the dark matter is the outstanding problem of astronomy.
Davies-Unruh effect The heat radiation a person accelerating through the quantum vacuum sees coming from in front of them. Its origin is in the virtual particles which are constantly popping in and out of existence in the quantum vacuum. From the point of view of an accelerated observer, they appear much like the heat from a furnace and have a temperature dependent on the person’s acceleration.
Decoherence The loss of the weird quantum nature of a microscopic particle when it makes a record on some kind of detector – for instance, the eye. All weird quantum behaviour ultimately stems from interference between the individual components of a wave representing a particle. If those waves do not overlap – if they are not coherent – there can be no interference between the waves and no weird quantum behaviour. This is the case when a particle leaves a record on the large number of atoms of a detector like the eye. Wave overlap, or coherence, is irretrievably lost. Hence the term decoherence.
Dimension An independent direction in space-time. The familiar world around us has three space dimensions (left–right, forward–backward, up–down) and one of time (past–future). Superstring theory requires the Universe to have six extra space dimensions. These differ radically from the other dimensions because they are rolled up very small.
Density The mass of an object divided by its volume. Air has a low density and iron has a high density.
Double-slit experiment Experiment in which subatomic particles are shot at a screen with two closely spaced, parallel slits cut in it. On the far side of the screen, the particles mingle, or ‘interfere’, with each other to produce a characteristic ‘interference pattern’ on a second screen. The bizarre thing is that the pattern forms even if the particles are shot at the slits one at a time, with long gaps between – in other words, when there is no possibility of them mingling with each other. This result, claimed the American physicist Richard Feynman, highlighted the ‘central mystery’ of quantum theory.
DNA Deoxyribonucleic acid, the ultimate store of genetic information for all cells.
Einstein’s theory of gravity See Relativity, general theory of.
Electric charge A property of microscopic particles which comes in two types – positive and negative. Electrons, for instance, carry a negative charge and protons a positive charge. Particles with the same charge repel each other while particles with opposite charges attract.
Electric charge, conservation of The edict that electric charge can never be created or destroyed.
Electric field The field of force which surrounds an electric charge.
Electromagnetic field The field which underpins the electromagnetic force.
Electromagnetic force One of the four fundamental forces of nature and the one responsible for gluing together ordinary matter such as the atoms in our bodies.
Electromagnetic wave A wave that consists of an electric field which periodically grows and dies alternating with a magnetic field which periodically dies and grows. An electromagnetic wave is generated by a vibrating electric charge and travels through space at the speed of light. In fact, it is light.
Electron Negatively charged subatomic particle typically found orbiting the nucleus of an atom. As far as anyone can tell, it is a truly elementary particle, incapable of being subdivided.
Electron-positron pair Exactly what it says. When an electron is created, it is always created alongside its antiparticle, the positron.
Energy A quantity which is almost impossible to define! Energy can never be created or destroyed, only converted from one form to another. Among the many familiar forms are heat energy, energy of motion, electrical energy, sound energy, and so on.
Energy, conservation of The principle that energy can never be created or destroyed, only converted from one form to another.
Energy, heat One of the myriad forms of energy. Since heat is actually microscopic disorder – for instance, the chaotic motion of gas atoms, flying hither and thither, in a gas – it is the lowliest form of energy, inasmuch as, eventually, all forms of energy get degraded into heatenergy, the ultimate slag of the Universe.
Energy of motion The energy a body possesses by virtue of the fact it is moving.
Expanding Universe The fleeing of the galaxies from each other in the aftermath of the Big Bang.
Event horizon The imaginary one-way ‘membrane’ that surrounds a black hole. Anything that falls through – whether matter or light – can never get out of the hole again.
Field An entity which fills all of space, assigning to each point a unique property. In the case of the gravitational field, for instance, the property is the direction and strength of the force of gravity. In modern physics, fields are considered more fundamental than forces.
Field, colour The field which gives rise to the ‘strong nuclear’, or colour, force between quarks.
Field, Higgs Hypothetical field which fills the vacuum like cosmic treacle. By sticking to particles, it bestows mass on them.
Fine-structure constant The number – or coupling constant – which determines the strength of the interaction between light and matter. In everyday language, it determines the strength of the electromagnetic force which glues together the atom in our bodies.
Force, centrifugal A fictitious force which we invent to explain why we appear to be flung outwards when we, for instance, round a bend in a car. In fact, there is no such force and we are not flung outwards. We are merely continuing to travel in a straight line under our own inertia and it is the body of the car that is changing its motion, not us.
Force, colour One of the four fundamental forces of nature and the one responsible for gluing together the quarks inside the nuclei of atoms.
Force, electroweak A force which exists at high energies and therefore existed in the early moments of the Universe’s existence. The electromagnetic force and the weak nuclear force are both facets of this unified force.
Force, fictitious A force we invent to explain our motion when we refuse to recognise the reality – that we are in fact moving solely under our own inertia and that it is our surroundings that are changing their motion, not us.
Force, fundamental One of the four basic forces which are believed to underlie all phenomena. The four forces are gravitational force, electromagnetic force, strong force and weak force. The strong suspicion among physicists is that these forces are in fact merely facets of a single superforce. In fact, experiments have already shown the electromagnetic and weak forces to be different sides of the same coin.
Force, GUT A hypothetical force predicted by ‘Grand Unified Theories’ to exist at high energy, and therefore to have existed in the very early Universe. Today’s electromagnetic, weak nuclear and strong nuclear forces are mere facets of this unified force.
Force-carrying particle Microscopic conveyor of a force. Forces arise through the continual exchange of such particles in much the same way that the continual exchange of a tennis ball between tennis players results in a force between them.
Fourier analysis A mathematical technique for picking out patterns which repeat in a waveform or signal.
Frequency How fast a wave oscillates up and down. Frequency is measured in Hertz (Hz), where 1 Hz is 1 oscillation per second.
Frequency band A range of frequencies.
Fundamental particle One of the basic building blocks of all matter. Currently, physicists believe there are 6 different quarks and 6 different leptons, making a total of 12 truly fundamental particles. The hope is that the quarks will turn out to be merely different faces of the leptons.
Galaxy One of the basic building blocks of the Universe. Galaxies are great islands of stars. Our own island, the Milky Way, is spiral in shape and contains about 200 hundred thousand million stars.
Gamma ray The highest-energy light, or electromagnetic wave.
General theory of relativity See Relativity, general theory.
Geodesic The shortest path between two points in warped, or curved, space.
Global positioning system (GPS) System of satellites, each carrying a clock and broadcasting a precise timing signal. A receiver on the ground determines its position by comparing the different amounts of time the various signals take to reach it.
Gluon Force-carrying particle of the strong nuclear force.
Gödel’s incompleteness theorem The proof that, for any set of axioms, there exist theorems which cannot be deduced from the axioms – undecidable theorems, which are true but can never be proved to be true.
Grand unified theories (GUTs) Theories which attempt to show that, at high energies, and consequently early on in the Big Bang, the three electromagnetic, weak nuclear and strong nuclear forces were combined into a singly unified force.
Gravitational force The weakest of the four fundamental forces of nature. Gravity is approximately described by Newton’s universal law of gravity but more accurately described by Einstein’s theory of gravity – the general theory of relativity. General relativity breaks down at the singularity at the heart of a black hole and the singularity at the birth of the Universe. Physicists are currently looking for a better description of gravity. The theory, already dubbed quantum gravity, will explain gravity in terms of the exchange of particles called gravitons.
Gravitational wave A ripple spreading out through space-time. Gravitational waves are generated by violent motions of mass such as the merger of black holes. Because they are weak, they have not been detected directly yet.
Gravity See Gravitational force.
Gravity, repulsive The gravity of a material with a very large negative pressure – strictly speaking, a pressure which is less than -1/3 the material’s energy density. The vacuum is thought to have possessed this property at the beginning of the Universe and to have therefore ‘inflated’. Today’s Universe also seems to be dominated by such material, dubbed ‘dark energy’, which is remorselessly driving the galaxies apart.
Halting problem The remarkably simple problem that the Mathematician Alan Turing discovered was uncomputable by any conceivable computer. Given a computer program, is it possible to tell ahead of actually running the program whether it will eventually halt? The answer is no.
Hardware The unchangeable components of a computer – for instance, its transistors and disc drives.
Hawking radiation The heat radiation which is generated near the event horizon of a black hole. A consequence of quantum theory, it arises because pairs of virtual particles and their antiparticles are continually popping in and out of existence in the vacuum, as permitted by the Heisenberg uncertainty principle. Near the horizon of a black hole, however, it is possible for one particle of a pair to fall into the hole. The left-behind particle, with no antiparticle to annihilate with, is boosted from a virtual particle to a real particle. Such particles stream away from a black hole – though admittedly the effect is small for a stellar black hole – as radiation with a characteristic temperature.
Heat death The sorry fate of a universe that expands for ever. More and more of the energy of the Universe ends up as heat – microscopic disorder – characterised by a single temperature. Since all cosmic activity is ultimately driven by temperature differences, the Universe dies a death.
Heisenberg uncertainty principle A principle of quantum theory that there are pairs of quantities such as a particle’s location and speed that cannot simultaneously be known with absolute precision. The uncertainty principle puts a limit on how well the product of such a pair of quantities can be known. In practice, this means that if the speed of a particle is known precisely, it is impossible to have any idea where the particle is. Conversely, if the location is known with certainty, the particle’s speed is unknown. By limiting what we can know, the Heisenberg uncertainty principle imposes a ‘fuzziness’ on nature. If we look too closely, everything blurs like a newspaper picture dissolving into meaningless dots.
Higgs field See Field, Higgs.
Higgs particle See Particle, Higgs.
Horizon problem The problem that far-flung parts of the Universe which could never have been in contact with each other, even in the Big Bang, have almost identical properties such as density and temperature. Technically, they were always beyond one another’s horizons. The theory of inflation provides a way for such regions to have been in contact in the Big Bang and so can potentially solve the horizon problem.
Horizon of Universe The Universe has a horizon much like the horizon that surrounds a ship at sea. The reason for the Universe’s horizon is that light has a finite speed and the Universe has been in existence for only a finite time. This means that we only see objects whose light has had time to reach us since the Big Bang. The observable Universe is therefore like a bubble centred on the Earth, with the horizon being the surface of the bubble. Every day the horizon expands outwards and new things become visible, just like ships coming over the horizon at sea.
Hydrogen burning The fusion of hydrogen into helium accompanied by the liberation of large quantities of nuclear binding energy. This is the power source of the Sun and most stars.
Inertia The tendency for a massive body, once set in motion, to keep on moving – at constant speed in a straight line in unwarped space and along a geodesic in warped space. Nobody knows the origin of inertia.
Inflation, eternal A generic property of inflation. Although the inflationary, or false, vacuum continually decays into bubbles of normal vacuum – creating Big Bang universes – the false vacuum grows in volume at a faster rate than it is lost. Consequently, inflation, once begun, is unstoppable.
Inflation, theory of Idea that in the first split-second of the creation, the Universe underwent a fantastically fast expansion. In a sense inflation preceded the conventional Big Bang explosion. If the Big Bang is likened to the explosion of a hand grenade, inflation was like the explosion of an H-bomb. Inflation can solve some problems with the Big Bang theory such as the horizon problem. Inflation was driven by the repulsive gravity of the vacuum, which was in an unusual state, dubbed the false vacuum.
Information, irreducible Information that cannot be compressed, or reduced, into a more compact form. The ultimate example is the number Omega, which cannot be summarised by any number shorter in length than itself.
Information, reducible Information that can be compressed, or reduced, into a more compact form.
Information Gathering and Utilising System (IGUS) An abstract entity invented by the Nobel Prize-winner Murray Gell Mann. It consists of an input register, memory registers, a computer, and so on. The way a human being experiences reality can be modelled by an IGUS. But the entity is general enough that it can even model something like a Galaxy-spanning civilisation.
Interference The ability of two waves passing through each other to mingle, reinforcing where their peaks coincide and cancelling where the peaks of one coincide with the troughs of another.
Interference pattern Pattern of light and dark stripes which appears on a screen illuminated by light from two sources. The pattern is due to the light from the two sources reinforcing at some places on the screen and cancelling at other places.
Interstellar space The space between the stars.
Ion An atom or molecule either bereft of its full complement of electrons or with more than its full complement of electrons. Unlike an atom or a molecule, it has a net electric charge.
Ionisation The process by which one or more electrons is knocked from an atom to leave an ion.
Large Hadron Collider (LHC) A giant particle accelerator being built at CERN and due for completion in 2007.
Laws of physics The edicts which orchestrate the behaviour of our Universe.
Laws of physics, fine-tuning of The observation that the laws of physics are ‘just right’ to permit the existence of stars, planets and life. If the force of gravity, for instance, were even a few per cent weaker or stronger than we find it, human beings would never have arisen.
Lepton Umbrella term for a group of subatomic particles including the electron and neutrino. Leptons, along with quarks, are currently thought to be the ultimate building blocks of nature. There are 6 different quarks and 6 different leptons.
Liar’s Paradox The assertion by someone that: ‘I am a liar.’ If the statement is true, it is false; and if it is false, it is true.
Light, constancy of speed of The peculiarity that in our Universe the speed of light in empty space is always the same, irrespective of the speed of the source of light or of anyone observing the light. This is one of two cornerstones of Einstein’s special theory of relativity, the other being the principle of relativity.
Light curve The variation with time of the light coming from a celestial object such as a star or supernova.
Light, speed of The cosmic speed limit – 300,000 kilometres per second.
Light year Convenient unit for expressing the distances in the Universe. It is simply the distance light travels in one year, which is 9.46 trillion kilometres.
Local group The small cluster of galaxies of which our Milky Way and the Andromeda Galaxy are the two biggest members.
Lorentz Contraction The contraction of a body moving relative to an ‘observer’. The observer sees the body shrink in the direction of its motion. The effect is noticeable only when the body is moving close to the speed of light with respect to the observer.
Mach’s principle The idea of the nineteenth-century philosopher Ernst Mach that bodies have inertia – that is, a resistance to any change in their motion – because of the combined gravitational pull of all the stars and galaxies in the Universe. In other words, your fridge is difficult to budge because, when you budge it, the whole Universe pulls against you.
Magnetic Field The field of force which surrounds a magnet.
Many Worlds interpretation of quantum theory See Quantum theory, Many Worlds interpretation of.
Mass A measure of the amount of matter in a body. Mass is the most concentrated form of energy. A single gram contains the same amount of energy as about 100 tonnes of dynamite.
Mass, gravitational The mass that quantifies the response of a body to the force of gravity.
Mass, inertial The mass which quantifies the resistance of a body to changes in its motion.
Mass, rest The mass which quantifies the amount of energy a nonmoving body contains.
Mass-energy The energy a body possesses by virtue of its mass. This is given by the most famous equation in all of physics – E = mc 2, where E is energy, m is mass and c is the speed of light.
Matter The most concentrated form of energy.
Matter, dark Matter which gives out no discernible light and whose existence is inferred from the gravitational pull it exerts on visible matter such as stars and galaxies. The Universe’s dark matter outweighs its normal matter by a factor of between 6 and 7. It may consist of hitherto unknown subatomic particles.
Maxwell’s equations of electromagnetism The handful of elegant equations, written down by James Clerk Maxwell in 1873, which neatly summarise all electrical and magnetic phenomena. The equations reveal that light is an electromagnetic wave.
Microwave A type of electromagnetic wave with a wavelength in the range of centimetres to tens of centimetres.
Milky Way Our Galaxy.
Modulation The continuous variation of the amplitude or frequency of a carrier wave. It is by means of such modulation that information – for instance, a radio programme – is impressed on an electromagnetic wave.
Molecule Collection of atoms glued together by electromagnetic forces. One atom, carbon, can link with itself and other atoms to make a huge number of molecules. For this reason, chemists divide molecules into ‘organic’ – those based on carbon – and ‘inorganic’ – the rest.
Momentum A measure of how much effort is required to stop a body. For instance, an oil tanker, even though it may be going at only a few kilometres an hour, has far more momentum than a Formula 1 racing car going at 200 kilometres per hour.
Momentum, conservation of Principle that momentum can never be created or destroyed.
Multiverse Hypothetical enlargement of the cosmos in which our Universe turns out to be one among an enormous number of separate and distinct universes. Most universes are dead and uninteresting. Only in a tiny subset do the laws of physics promote the emergence of stars and planets and life.
Natural Selection The idea that the traits of creatures who compete for scarce resources and survive to produce offspring are the traits that end up in a population.
Neutrino Neutral subatomic particle with a very small mass that travels very close to the speed of light. Neutrinos hardly ever interact with matter. However, when created in huge numbers they can blow a star apart as in a supernova.
Neutron One of the two main building blocks of the atomic nucleus at the centre of atoms. Neutrons have essentially the same mass as protons but carry no electrical charge. They are unstable outside of a nucleus and disintegrate in about ten minutes.
Newton’s first law A body remains in a state of rest or uniform motion in a straight line unless acted upon by an external force.
Newton’s second law The force on a body is the rate of change of its momentum. Conventionally, the law is written as F = ma, where F is the force experienced by a body of mass, m, and a is the acceleration that results. In fact, the law is no more than a definition of inertial mass, which is defined as the ratio of the force applied to a body to the acceleration produced.
Newton’s universal law of gravity The idea that all bodies pull on each other across space with a force which depends on the product of their individual masses the inverse square of their distance apart. In other words, if the distance between the bodies is doubled, the force becomes four times weaker; if it is tripled, nine times weaker; and so on. Newton’s theory of gravity is perfectly good for everyday applications but turns out to be an approximation. Einstein provided an improvement in the general theory of relativity.
Nuclear energy The excess energy – binding energy – released when one atomic nucleus changes into another atomic nucleus.
Nuclear fusion The welding together of two light nuclei to make a heavier nucleus, a process which results in the liberation of nuclear binding energy. The most important fusion process for human beings is the gluing together of hydrogen nuclei to make helium in the core of the Sun since its by-product is sunlight.
Nuclear reaction Any process which converts one type of atomic nucleus into another type of atomic nucleus.
Nucleon Umbrella term used for protons and neutrons, the two building blocks of the atomic nucleus.
Nucleus See Atomic nucleus.
Ockham’s Razor The rule of thumb, promulgated by the fourteenth-century Franciscan monk, William of Ockham, that, if there are two competing theories both of which explain some phenomenon, the one that makes the least assumptions is invariably the true one.
Omega The jewel in the crown of Algorithmic Information Theory. Omega is a number that cannot be generated by a computer program shorter than itself. It is irreducible, incompressible information. Omega is also related to the halting problem.
Omega Point The endpoint of the Omega Point Universe. Here all light rays from the past Universe converge and an infinite amount of information processing may be carried out.
Omega Point Universe A universe which contracts faster in one direction than all other directions. In such a universe, the temperature differences grow without limit, enabling an infinite amount of information processing before the universe ends in the Omega Point.
Particle, fundamental One of the basic building blocks of all matter. Currently, physicists believe there are 6 different quarks and 6 different leptons, making a total of 12 truly fundamental particles. The hope is that the quarks will turn out to be merely different faces of the leptons.
Particle, Higgs A localised knot in the hypothetical Higgs field. There may be more than one Higgs particle. It all depends on how nature implements the Higgs mechanism for bestowing mass on matter.
Particle, virtual A subatomic particle which is permitted by the Heisenberg uncertainty principle to pop into existence as long as it pops back out again in a very short time.
Particle accelerator Giant machine, often in the shape of a circular race track, in which subatomic particles are accelerated to high speed and smashed into each other. In such collisions, the energy of motion of the particles becomes available to create new particles.
Particle physics The quest to discover the fundamental building block and the fundamental forces of nature.
Photon Particle of light and force-carrier of the electromagnetic force.
Physics, classical Non-quantum physics. Classical physics is a recipe for predicting the future – for instance, the location of Mars the day after tomorrow – with 100 per cent certainty.
Physics, laws of The fundamental laws which orchestrate the behaviour of the Universe.
Physics, quantum Essentially, the physics of the atomic and subatomic realm (although it can have manifestations in the large-scale world). Whereas classical physics predicts the future with 100 per cent certainty, quantum physics predicts only the probabilities of events – for instance, the chance that a particular atom will decay some time in the next hour. Fortunately, the unpredictability is predictable – or we would live in a world of utter chaos!
Pi (π) The ratio of the circumference to the diameter of a circle.
Planck energy The super-high energy at which gravity becomes comparable in strength to the other fundamental forces of nature.
Planet A small sphere-shaped body orbiting a star. A planet does not produce its own light but shines by reflecting the light of its star. There is currently some dispute over what constitutes a planet and whether Pluto, for instance, deserves the name.
Positron Antiparticle of the electron.
Pressure The force per unit area exerted on a container – for instance, by air molecules drumming on the inside of a balloon.
Pressure, negative The opposite of normal, positive pressure. Whereas stuff with positive pressure wants to expand, like the air in a balloon, stuff with negative pressure wants to shrink. If it were possible to fill a balloon with it, the fabric of the balloon would be sucked inwards rather than blown outwards.
Principle of computational equivalence The idea, proposed by Stephen Wolfram, that systems of equivalent complexity are truly equivalent. In other words, a system such as the Earth’s atmospheric circulation, which is as complex as a living thing, has just as much right to be classed as a living thing. The principle implies that life can be implemented in a multitude of systems not just the system of biochemicals of terrestrial life.
Principle of equivalence The idea that gravity and acceleration are indistinguishable, at least in a small enough region of space. This observation was the cornerstone of Einstein’s general theory of relativity.
Probability A number between 0 and 1, with 0 corresponding to a 0 per cent chance and 1 corresponding to a 100 per cent chance.
Protein A long-chain molecule composed of amino acids and used by living things for a multitude of tasks, ranging from providing cellular scaffolding to speeding up chemical reactions.
Proton One of the two main building blocks of the nucleus. Protons carry a positive electrical charge, equal and opposite to that of electrons.
Quantum The smallest chunk into which something can be divided. Photons, for instance, are quanta of the electromagnetic field.
Quantum chromodynamics The quantum theory of the strong nuclear force. The force arises from the exchange of gluons.
Quantum computer A machine that exploits the fact that quantum systems such as atoms can be in many different states at once to carry out many calculations at once. The best quantum computers can manipulate only a handful of binary digits, or bits, but in principle such computers could massively outperform conventional computers.
Quantum field theory Theory in which the fundamental entities of nature are deemed to be fields and the particles are mere ‘excitations’ of the fields.
Quantum fluctuations The appearance of energy out of the vacuum as permitted by the Heisenberg uncertainty principle. Usually, the energy is in the form of virtual particles.
Quantum interference The interference between the components of the quantum wave representing a particle. This is essentially the source of all weird quantum behaviour.
Quantum probability The chance, or probability, of a microscopic event. Although nature prohibits us from knowing things with certainty, it nevertheless permits us to know the probabilities with certainty!
Quantum superposition A situation in which a quantum object such as an atom is in more than one state at a time. It might, for instance, be in many places simultaneously. It is the interaction, or ‘interference’, between the individual states in the superposition which is the basis of all quantum weirdness. Decoherence prevents such interaction and therefore destroys quantum behaviour.
Quantum theory See Physics, quantum.
Quantum theory, Copenhagen interpretation of The idea that a particle exists as a weird superposition of possibilities until the moment it is ‘observed’. It is the act of observation – ill-defined in the interpretation – that suppresses weird quantum behaviour and makes a particle behave itself.
Quantum theory, Many Worlds interpretation of The idea that all possibilities encapsulated in the ‘wave function’ that describes a microscopic particle are real. In other words, if a particle can be in two places at once and we make an observation that pins it down to one place, there is another reality, or Universe, containing another version of us, who observes the particle to be in the other place. In this way, events at the quantum level cause the universe to continually split into a multitude of parallel realities.
Quantum vacuum The quantum picture of empty space. Far from empty, it seethes with ultra-short-lived microscopic particles which are permitted to blink into existence and blink out again by the Heisenberg uncertainty principle.
Quark Particle out of which the protons and neutrons of atoms are made. Quarks, along with leptons, are currently thought to be the ultimate building blocks of nature. There are 6 different quarks and 6 different leptons.
Quasar A galaxy which derives most of its energy from matter heated to millions of degrees as it swirls into a central giant black hole. Quasars can generate as much light as a hundred normal galaxies from a volume smaller than the Solar System, making them the most powerful objects in the Universe.
Qubit A quantum bit, or binary digit. Whereas a normal bit can only represent a ‘0’ or a ‘1’, a qubit can exist in a superposition of the two states, representing a ‘0’ and a ‘1’ simultaneously. Because strings of qubits can represent a large number of numbers simultaneously, they can be used to do a large number of calculations simultaneously.
Radiation Energy which is radiated through space in the form of photons.
Radio wave A type of electromagnetic wave with a long wavelength, longer than about a centimetre.
Red giant A star which has exhausted the energy-generating hydrogen fuel in its core. Paradoxically, the shrinkage of the core – which is deficient in heat to hold it up against gravity – heats up the interior of the star. Furious burning of hydrogen in a ring of fire around the core causes the outer envelope of the star to balloon up and cool to a dull red colour. A red giant – the future of the Sun – often pumps out about 10,000 times as much heat as the Sun, principally because of its enormous surface area.
Red shift The loss of energy of light caused by the expansion of the Universe. The effect can be visualised by drawing a wiggly light wave on a balloon and inflating it. The wave becomes stretched out. Since red light has a longer wavelength than blue light, astronomers talk of the cosmological red shift. (A red shift can also be caused by the Doppler effect, when a body emitting light is flying away from us. And it can be caused when light loses energy climbing out of the strong gravity of a compact star such a white dwarf. This is known as a gravitational red shift.)
Refractive index The ratio of the speed of light in empty space to the speed of light in a medium. In practice, it causes light to be bent as it traverses a medium such as a piece of glass.
Relativity, general theory of Einstein’s generalisation of his special theory of relativity. The theory relates what one person sees when they look at another person accelerating relative to them. Because acceleration and gravity are indistinguishable – the principle of equivalence – general relativity is also a theory of gravity. General relativity shows gravity to be nothing more than the warpage of space-time. The theory incorporates several ideas that were not incorporated in Newton’s theory of gravity. One is that nothing, not even gravity, can travel faster than light. Another is that all forms of energy have mass and so are sources of gravity. Among other things, the theory predicts black holes, the expanding Universe and that gravity should bend the path of light.
Relativity, principle of The observation that all the laws of physics are the same for observers moving at constant speed with respect to each other.
Relativity, special theory of Einstein’s theory which relates what one person sees when they look at another person moving at constant speed relative to them. It reveals, among other things, that the moving person appears to shrink in the direction of their motion while their time slows down, effects which become ever more marked as they approach the speed of light.
Schrödinger equation Equation which governs the way in which the quantum wave describing, say, a particle, changes with time.
Science, equation-based Conventional science.
Science, program-based The ‘new kind of science’ proposed by Stephen Wolfram.
Scientific notation Shorthand for writing big numbers. For instance, 1099 equals 1 followed by 99 zeroes.
Search for extraterrestrial intelligence (SETI) The use of telescopes to scan the sky for either a radio or optical signal from an extraterrestrial civilisation.
SETI See Search for extraterrestrial intelligence.
Set theory A branch of mathematics regarded as simpler and more fundamental than most others. A set is merely a group of entities. For instance, there is the set of all mammals, and the set of all countries beginning with ‘B’.
Singularity Location where the fabric of space-time ruptures and therefore cannot be understood by Einstein’s theory of gravity, the general theory of relativity. There was a singularity at the beginning of the Universe. There is also one in the centre of every black hole.
Software The infinitely rewritable instructions of a computer which enable it to simulate any other machine.
Solar System The Sun and its family of planets, moons, comets and other assorted rubble.
Space network A network of nodes connected together that may underpin space itself.
Space-time In the general theory of relativity, space and time are seen to be essentially the same thing. They are therefore treated as a single entity – space-time. It is the warpage of space-time that turns out to be gravity.
Spectrum The separation of light into its constituent ‘rainbow’ colours.
Square kilometre array (SKA) A giant array of telescopes with 100 times the collecting area of any existing telescope, which is being planned for operation in 2020. It will consist of a cluster of dishes within five kilometres of one another, with outriders up to 3,000 kilometres away. The central dishes could be built either in Argentina, Australia, South Africa or China.
Standard model of particle physics The picture in which the fundamental forces are explained as the result of the exchange of force-carrying particles – photons, in the case of the electromagnetic force; vector bosons in the case of the weak nuclear force; and gluons in the case of the strong nuclear force.
Star A giant ball of gas which replenishes the heat it loses to space by means of nuclear energy generated in its core.
Stochastic electrodynamics Theory in which the quantum vacuum is absolutely central to the creation of the world. Ultimately, all bizarre quantum behaviour of microscopic particles can be traced back to the relentless buffeting they receive from the ceaselessly churning quantum vacuum.
String theory See Superstring theory.
Strong nuclear force The powerful short-range force between quarks. It holds protons and neutrons together in an atomic nucleus.
Subatomic particle A particle smaller than an atom such as an electron or a neutron.
Supernova A cataclysmic explosion of a star. A supernova may, for a short time, outshine an entire galaxy of 100 billion ordinary stars. It is thought to leave behind a highly compressed neutron star.
Supernova, Type Ia The explosion of a white dwarf star triggered by the dumping of matter on it from a companion star. Since all such supernovae arise from essentially the same type of star, they are considered to be of equivalent intrinsic brightness. This makes them useful as cosmological distance indicators since we can be sure that a Type Ia that is fainter than another is also farther away.
Superstring theory Theory which postulates that the fundamental ingredients of the Universe are tiny strings of matter. The strings vibrate in a space-time of ten dimensions. The great pay-off of this idea is that it may be able to unite, or ‘unify’, quantum theory and the general theory of relativity.
Symmetry Symmetry concerns the things that do not change when an object is transformed in some way. For instance, a face that looks the same when it is reflected in a mirror is said to show ‘mirror symmetry’.
Symmetry, broken A symmetry which is no longer apparent. Often the laws of physics are symmetric whereas their consequences are not. Take a pencil perfectly balanced on its point. The law of gravity is perfectly symmetric. There is a vertical force – down towards the centre of the Earth – but there is no sideways force. The sideways force is therefore the same in all directions – that is, symmetric with respect to orientation in space. But, when the pencil falls, it will fall in one direction in particular. The symmetry will be broken.
Temperature The degree of hotness of a body. It is related to the energy of motion of the particles that compose it.
Theorem A mathematical statement deduced from a set of axioms by the application of logic.
Theory of everything A theory that physicists dream of which captures all the fundamental features of reality in one simple set of equations.
Thermal radiation Radiation given out by a body that absorbs all the heat that falls on it. The heat is shared among the atoms in such a way that the heat radiation given out takes no account of what the body is made of but depends solely on its temperature and has a characteristic and easily recognisable form. Often such a body is known as a ‘black body’. An approximate example is the Sun.
Thermodynamics The theory of heat.
Time, flow of A popular idea which cannot be correct since anything that flows changes with time. How can time change with time? It cannot.
Time dilation The slowing down of time for someone moving close to the speed of light or experiencing strong gravity.
Transit The passage of one celestial body in front of another as seen from our vantage point on Earth. In the case of a planet orbiting a nearby star this causes a noticeable dimming of the star’s light as part of its luminous disc is obscured.
Turing machine, universal A theoretical machine devised by the mathematician Alan Turing in the 1930s and capable of simulating any other conceivable machine. In today’s jargon, it is simply a computer.
Uncertainty principle See Heisenberg uncertainty principle.
Uncomputability The idea that some things in mathematics cannot be computed by any conceivable computer.
Undecidability The idea that some theorems in mathematics are true but cannot be proved to be true. This is also called incompleteness.
Unification The idea that at extremely high energy, the four fundamental forces of nature were one, united in a single theoretical framework.
Universal constructor A machine capable of constructing any other machine.
Universe All there is. This is a flexible term, which once was used for what we now call the Solar System. Later, it was used for what we call the Milky Way. Now it is used for the sum total of all the galaxies, of which there appear to be about ten billion within the observable Universe.
Universe, bouncing The idea that our Big Bang was triggered after a previous contracting phase in which the Universe shrunk down to a Big Crunch and bounced into a new expansion phase. Big Bangs and Big Crunches therefore alternate throughout eternity. The idea has several fatal flaws and so is no longer considered a viable possibility.
Universe, closed A universe that contains sufficient matter that the combined gravity of all that matter eventually brakes and reverses its expansion, shrinking Creation down to a Big Crunch.
Universe, computational The abstract universe of all conceivable computer programs. Since we can be considered computer programs, we – or at least, cyber versions of us – exist in the computational universe.
Universe, cyclic An idea in which our Universe is a four-brane – a four-dimensional island in the ten-dimensional space of string theory. Periodic collisions between our brane and another brane drive periodic bursts of expansion within our Universe. Instead of Big Bangs alternating with Big Crunches, as in the bouncing universe, Big Bangs are followed by Big Bangs.
Universe, ekpyrotic Universe in which the Universe is a four-brane – a four-dimensional island in the ten-dimensional space of string theory – and the Big Bang was triggered by a collision between our brane and another brane.
Universe, expansion of The fleeing of the galaxies from each other in the aftermath of the Big Bang.
Universe, observable All we can see out to the Universe’s horizon. The Universe has a horizon because it was born only 13.7 billion years ago. This means that we can see only the stars and galaxies whose light has taken less than 13.7 billion years to reach us. All other objects are currently beyond the horizon of the observable Universe.
Universe, open A universe that contains insufficient matter for the combined gravity of all that matter ever to reverse its expansion. Such a universe will expand for ever.
Universe, oscillating Another name for bouncing universe.
Vacuum, false An unusual state of the vacuum with sufficient negative pressure that it generates repulsive gravity. Such a state is believed to have existed at the beginning of the Universe and to have driven the super-fast expansion of ‘inflation’.
Vacuum, polarised The distortion of the quantum vacuum by the presence of the electric charges in matter. If the charges in matter are positive, for instance, they will attract the vacuum’s negative virtual particles and repel its positive virtual particles.
Vacuum, quantum See Quantum vacuum.
Vector boson Particles whose exchange by other particles gives rise to the weak nuclear force.
Virtual particle Particle which has a fleeting existence, popping into being and popping out again according to the constraint imposed by the Heisenberg uncertainty principle.
Von Neumann probe, self-reproducing A cross between a starship and an intelligent factory. Such a probe would reach a target planetary system and use the resources there to make two copies of itself. In this way such probes could visit every star in the Galaxy in a relatively short time.
Wavelength The distance for a wave to go through a complete oscillation cycle.
Wave function A mathematical entity that contains all that is knowable about a quantum object such as an atom. The wave function changes in time according to the Schrödinger equation.
Wave-particle duality The ability of a subatomic particle to behave as a localised billiard-ball-like particle or a spread-out wave.
Weak nuclear force The second force experienced by protons and neutrons in an atomic nucleus, the other being the strong nuclear force. The weak nuclear force can convert a neutron into a proton and so is involved in beta decay.
Weight The force of gravity acting on a body.
Zero-point fluctuations See Quantum fluctuations.
Zitterbewegung Trembling motion. The idea, proposed by Louis de Broglie and Erwin Schrödinger, that an electron is a point-like charge which jitters about randomly within a sphere of diameter the Compton wavelength.