Glossary

ABSOLUTE ZERO Lowest temperature attainable. As a body is cooled, its atoms move more and more sluggishly. At absolute zero, equivalent to –273.15 degrees on the Celsius scale, they cease to move altogether. (Actually, this is not entirely true since the Heisenberg uncertainty principle produces a residual jitter even at absolute zero.)

ACCRETION Key astrophysical process in which the gravity of a body sucks in more and more matter from its surroundings. As the matter swirls inwards, like water going down a plug hole, it can create an ‘accretion disc’. Friction within the disk heats it up and this is thought to be the source of the prodigious luminosity of powerful galaxies such as ‘quasars’. Here the accretion is onto a ‘supermassive’ black hole, as much as 10 billion times the mass of the Sun.

ALPHA DECAY The spitting out of a high-speed alpha particle by a large, unstable nucleus in an attempt to turn itself into a lighter, stable nucleus.

ALPHA PARTICLE A bound state of two protons and two neutrons – essentially a helium nucleus – which rockets out of an unstable nucleus during radioactive alpha decay.

ALPHA PROCESS The build-up of heavy atomic nuclei inside stars by the addition of alpha particles. It requires a temperature of about a billion degrees.

ANDROMEDA The nearest big galaxy to our own Milky Way, about 2.5 million light years away. Andromeda and the Milky Way are the dominant, big galaxies in a cluster of at least 40 galaxies known as the Local Group.

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 Term for a large accumulation of antiparticles. Antiprotons, anti-neutrons and positrons can in fact come together to make anti-atoms. And there is nothing in principle 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 pretty much 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.

    

BERSERKER Malevolent extraterrestrial machine bent on the destruction of life. Such machines appeared in a number of novels by the science-fiction writer Fred Saberhagen.

BETA DECAY The ejection of a high-speed electron by an unstable atomic nucleus. The nucleus left behind is of an element with one more proton.

BETA RAY An electron ejected during beta decay. The electron does not exist in the nucleus beforehand but is ‘created’ when a neutron changes into a proton.

BIG BANG The titanic explosion 13.7 billion years ago in which the Universe is thought to have been born. ‘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.

BINARY Way of expressing numbers using only 0s and 1s. In a binary number, the digit at the end represents 1s, the next digit 2s, the next 4s, the next 8s, and so on. So, for instance, 1101 = 1 + (0 × 2) + (1 × 4) + (1 × 8) = 13.

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 supermassive black hole at 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.

BLACK HOLE INFORMATION PARADOX A difficulty which arises because the laws of physics do not allow the destruction of information, yet, when a black hole disappears, or ‘evaporates’, the information which described the precursor star appears to vanish for ever. However, some physicists believe that the missing information is encoded in the membrane, or ‘event horizon’, surrounding the hole. When the black hole evaporates in a hail of Hawking radiation, this radiation then returns the information to the Universe.

BOLTZMANN BRAIN An intelligent observer created spontaneously out in the depths of space by an improbable convulsion of the quantum vacuum.

BOSON A microscopic particle with integer spin – 0 units, 1 unit, 2 units, and so on. By virtue of their spin, such particles are hugely gregarious, participating in collective behaviour that leads to lasers, superfluids and superconductors.

BOYLE’S LAW The observation that the pressure of a gas is inversely proportional to its volume; that is, halving its volume doubles its pressure.

BROWNIAN MOTION The random, jittery motion of a large body under machine-gun bombardment from smaller bodies. The most famous instance is of pollen grains zig-zagging through water as they are repeatedly hit by water molecules. The phenomenon, discovered by botanist Robert Brown in 1827 and triumphantly explained by Einstein in 1905, was powerful proof of the existence of atoms.

    

CAUSALITY The idea that a cause always precedes an effect. Causality is a much-cherished principle in physics. However, quantum events such as the decay of atoms appear to be effects with no prior cause.

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, for instance, say: ‘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.’

CEPHEID VARIABLE A very luminous yellow star that swells and shrinks periodically. The pulsation period is related to the intrinsic luminosity of the star. This means whenever a Cepheid is observed, its period reveals its true luminosity. A comparison with its apparent luminosity yields its distance. Cepheids have played a key role in measuring the distance to nearby galaxies such as Andromeda.

CHAIN REACTION See Nuclear Chain Reaction.

CHARON Pluto’s largest moon.

CHEMICAL BOND The ‘glue’ that sticks atoms together to make molecules. It involves the sharing, donating or borrowing of the atoms’ outer electrons.

CHEMICAL FUEL A fuel such as coal, oil or dynamite. A rearrangement of the electrons in the material’s atoms is associated with the liberation of heat energy.

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.

CNO CYCLE The series of nuclear reactions by which stars significantly more massive than the Sun turn hydrogen into helium. It is called a cycle because it comes back to its starting point, in the end recreating the carbon used by the nuclear reactions.

COMET Small icy body – usually mere 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 EFFECT The recoil of an electron when exposed to high-energy light, just as if the electron is a tiny billiard ball struck by another tiny billiard ball. The effect is a graphic demonstration that light is ultimately made of tiny, bullet-like particles, or photons.

COMPUTATIONAL UNIVERSE 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.

CONDUCTOR A material through which an electrical current can flow.

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.

CONTRACTION HYPOTHESIS The idea that the Sun stays hot because gravitational energy is constantly being converted into heat energy as it slowly shrinks. The hypothesis is wrong.

COPERNICAN PRINCIPLE The idea that 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 RADIATION The ‘afterglow’ of the Big Bang fireball. Incredibly, it still permeates all of space 13.7 billion years after the event, a tepid microwave radiation corresponding to a temperature of –270°C.

COSMOLOGY The ultimate science, whose subject matter is the origin, evolution and fate of the entire Universe.

COSMOS Another word for Universe.

CRITICAL MASS The threshold mass of a material such as uranium or plutonium necessary to trigger a runaway chain reaction of nuclear fission.

    

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 be speeding up the expansion of the Universe. Nobody has much of a clue what it is.

DARK MATTER Matter in the Universe which gives out no 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 one of the outstanding problems of astronomy.

DECOHERENCE The mechanism that destroys the weird quantum nature of a body, so that, for instance, it appears localised rather than in many different places simultaneously. Decoherence occurs if the outside world gets to ‘know’ about the body. The knowledge may be taken away by a single photon of light or an air molecule which bounces off the body. Since big bodies like tables are continually struck by photons and air molecules and cannot remain isolated from their surroundings for long, they lose their ability to be in many places at once in a fantastically short period of time – far too short for us to notice.

DEGENERACY PRESSURE The bee-in-a-box-like pressure exerted by electrons squeezed into a small volume of space. A consequence of the Heisenberg uncertainty principle, it arises because a microscopic particle whose location is known very well necessarily has a large uncertainty in its velocity. The degeneracy pressure of electrons prevents white dwarfs shrinking under their own gravity, whereas the degeneracy pressure of neutrons does the same thing for neutron stars.

DENSITY The mass of an object divided by its volume. Air has a low density and iron has a high density.

DETERMINISTIC LAW A law of physics which predicts some aspect of the future with 100 per cent certainty. All classical – that is, non-quantum – laws do this.

DEUTERIUM A rare isotope of hydrogen. Deuterium contains a neutron as well as a proton in its nucleus.

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.

DOUBLE-SLIT EXPERIMENT Experiment in which 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 Richard Feynman, highlighted the ‘central mystery’ of quantum theory.

    

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 charge attract.

ELECTRIC CURRENT A river of charged particles, usually electrons, which flows through a conductor.

ELECTRIC FIELD The field of force which surrounds an electric charge.

ELECTROMAGNETIC FORCE One of the four fundamental forces of nature. It is responsible for gluing together all ordinary matter, including the atoms in our bodies and those in the rocks beneath our feet.

ELECTROMAGNETIC WAVE A wave that consists of alternating electric and magnetic fields transverse to its direction of motion. An electromagnetic wave is generated by a vibrating electric charge and travels through space at the speed of 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.

ELEMENT A substance which cannot be reduced any further by chemical means. All atoms of a given element possess the same number of protons in their nucleus. For instance, all atoms of hydrogen have one proton, all atoms of chlorine 17, and so on.

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 Principle that energy can never be created or destroyed, only converted from one form to another.

ENTANGLEMENT The intermingling of two or more microscopic particles so that they lose their individuality and in many ways behave as a single entity.

ENTROPY The degree of disorder of a system. More specifically, it is the number of possible ways that the components of a system can be arranged and still yield the object.

EPOCH OF LAST SCATTERING The period, about 380,000 years after the beginning of the Universe, when the fireball of the Big Bang had cooled sufficiently for electrons and nuclei to combine to form the first atoms. Since free electrons are very good at redirecting, or ‘scattering’, photons, before this time light could not travel in a straight line and the Universe was opaque. Once the electrons were mopped up by atoms, it was possible for photons to travel unhindered in straight lines and the Universe became transparent. Today, we pick up photons from this epoch, greatly cooled by the expansion of the Universe over the past 13.7 billion years, as the cosmic background radiation.

EVENT HORIZON The one-way ‘membrane’ that surrounds a black hole. Anything that falls through, whether matter or light, can never get out again.

EXPANDING UNIVERSE The fleeing of the galaxies from each other in the aftermath of the Big Bang.

    

FERMI PARADOX The mystery, highlighted by the Italian-American physicist Enrico Fermi, of why, if intelligence has arisen elsewhere in our Galaxy, it hasn’t come here to Earth. ‘Where is everybody?’ asked Fermi.

FERMI PROBLEM A back-of-the-envelope calculation for which Enrico Fermi was well known.

FERMION A microscopic particle with half-integer spin – ½ unit,  units, units, and so on. By virtue of their spin, such particles shun each other. Their unsociability is the reason why atoms exist and the ground beneath our feet is solid.

FLY-BY ANOMALY The mystery of why the five space probes that have swung by the Earth since 1990 have gained or lost speed relative to the Earth. According to Newton’s law of gravity, there should be no change of speed.

FORCE-CARRYING PARTICLE A subatomic particle whose exchange, like a tennis ball being batted back and forth between tennis players, give rise to force. For instance, the exchange of photons gives rise to the electromagnetic force.

FUNDAMENTAL FORCE One of the four basic forces which are believed to underlie all phenomena. The four forces are the gravitational force, the electromagnetic force, the strong force and the weak force. The strong suspicion among physicists is that these forces are actually 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.

FUNDAMENTAL PARTICLE One of the basic building blocks of all matter. Currently, physicists believe there are six different quarks and six 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.

FUSION See Nuclear Fusion.

    

GALAXY One of the 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 billion stars.

GAMMA RAY The highest-energy form of light, generally produced when an atomic nucleus rearranges itself.

GAS Collection of atoms that fly about through space like a swarm of tiny bees.

GENERAL THEORY OF RELATIVITY Einstein’s theory of gravity which 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 predicted black holes, the expanding Universe and that gravity could bend the path of light.

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 by Einstein’s theory of gravity – the general theory of relativity. General relativity breaks down over 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 POTENTIAL ENERGY The energy a mass possesses by virtue of its position in a gravitational field. For instance, a loose slate on a roof is said to have more potential energy than one on the ground. If it falls to the ground, the potential energy is converted into other forms – in the first instance, energy of motion.

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.

    

HALF-LIFE The time it takes half the nuclei in a radioactive sample to disintegrate. After one half-life, half the atoms will be left; after two half-lives, a quarter; after three, an eighth; and so on. Half-lives can range from the merest split second to many billions of years.

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 partner to annihilate with, is boosted from a virtual particle to a real particle. Such particles stream away from a black hole – although admittedly the effect is small for a stellar black hole – as radiation with a characteristic temperature.

HEAT DEATH The hypothetical state in which all temperature differences in the Universe have been ironed out so that all activity dwindles to a standstill.

HEISENBERG UNCERTAINTY PRINCIPLE A principle of quantum theory stating 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 momentum 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 momentum 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 dots.

HELIUM Second-lightest element in nature and the only one to have been discovered on the Sun before it was discovered on the Earth. Helium is the second most common element in the Universe after hydrogen, accounting for about 10 per cent of all atoms.

HORIZON See Light Horizon, Cosmic.

HORIZON PROBLEM The problem arising from the fact 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 each other’s horizon. 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.

HYDROGEN The lightest element in nature. A hydrogen atom consists of a single proton orbited by a single electron. Close to 90 per cent of all atoms in the Universe are hydrogen atoms.

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.

HYDROSTATIC EQUILIBRIUM The state in which the gravitational force trying to crush a star is perfectly balanced by the force of its hot gas pushing outwards.

    

INFLATION, THEORY OF The idea that in the first split second of 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 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.

INFLATON The hypothetical force field that drove inflation.

INFRARED Type of invisible light given out by warm bodies.

INSTANTANEOUS INFLUENCE See Non-Locality.

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 others.

INTERSTELLAR MEDIUM The tenuous gas and dust floating between the stars. In the vicinity of the Sun this gas comprises about one hydrogen atom in every three cubic centimetres, making it a vacuum far better than anything achievable on the Earth.

INTERSTELLAR PLANET Hypothetical planet that wanders alone in the deep freeze of interstellar space. Such planets could have been ejected from the vicinity of stars during the process of planet formation.

INTERSTELLAR SCINTILLATION The interstellar equivalent of twinkling. Just as starlight is affected by turbulence in the atmosphere, radio waves from distant celestial objects are affected by turbulence in the interstellar medium. This can cause the radio signal to fluctuate in brightness.

INTERSTELLAR SPACE The space between the stars.

ION An atom or molecule which has been stripped of one or more of its orbiting electrons and so has a net positive electrical charge.

ISOTOPE One possible form of an element. Isotopes are distinguishable by their differing masses. For instance, chlorine comes in two stable isotopes, with masses of 35 and 37. The mass difference is due to the differing number of neutrons in their nuclei. For instance, chlorine-35 contains 18 neutrons, and chlorine-37 contains 20. (Both contain the same number of protons – 17 – since this determines the identity of an element.)

    

KUIPER BELT A belt of icy bodies orbiting in the outer Solar System. There may be tens of thousands of them. Pluto is now recognised to be one of the larger – though not the largest – Kuiper Belt object.

    

LASER Light source in which the gregarious nature of photons, which are bosons, comes to the fore. Specifically, the more photons there are passing through a material, the greater the probability that other atoms will emit others with the same properties. The result is an avalanche of photons all travelling lockstep.

LIGHT, CONSTANCY 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, SPEED OF The cosmic speed limit – 300,000 kilometres per second.

LIGHT FILL-UP TIME The time required for stars to fill empty space with light – pretty much like water filling a bath – so that the night sky would appear bright rather than dark. In fact, this time is far greater than the average lifetime of stars.

LIGHT HORIZON, COSMIC 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 Universe gets older (by one day), so every day the horizon expands outwards and new things become visible, just like ships coming over the horizon at sea.

LIGHT YEAR Convenient unit for expressing the distances in the Universe. It is simply the distance light travels in one year, which turns out to be 9.46 trillion kilometres.

LUMINOSITY The total amount of light pumped into space each second by a celestial body such as a star.

    

MAGNETIC FIELD The field of force which surrounds a magnet.

MANY WORLDS The idea that quantum theory describes everything, not simply the microscopic world of atoms and their constituents. Since quantum theory permits an atom to be in two places at once, this must mean that a table can be in two places at once. According to the Many Worlds scenario, however, the mind of the person observing the table splits into two, one which perceives the table in one place and another which perceives it in another. The two minds exist in separate realities, or universes.

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 100,000 tonnes of dynamite.

MASS SPECTROGRAPH Device for comparing the masses of atoms – or, to be more precise, ions. It does this by measuring how much a magnetic field bends the trajectory of an atom flying through space. The more massive the atom, the less its path is bent.

MAXWELL’S EQUATIONS OF ELECTROMAGNETISM The handful of elegant equations, written down by James Clerk Maxwell in 1868, which neatly summarise all electrical and magnetic phenomena. The equations reveal that light is an electromagnetic wave.

METEORITIC HYPOTHESIS The idea that the Sun is kept hot by a constant raining down of meteorites onto its surface. Unfortunately, it is wrong.

MILKY WAY Our Galaxy.

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 The momentum of a body is a measure of how much effort is required to stop it. For instance, an oil tanker, even though it may be going at only a few kilometres an hour, is far harder to stop than a Formula 1 racing car going at 200 kilometres per hour. We say the oil tanker has more momentum.

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.

    

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.

NEUTRON STAR A star that has shrunk under its own gravity to such an extent that most of its material has been compressed into neutrons. Typically, such a star is only 20 to 30 kilometres across. A sugar cube of neutron-star stuff would weigh as much as the entire human race.

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 and 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.

NON-LOCALITY The spooky ability of objects subject to quantum theory to continue to ‘know’ about each other’s state even when separated by a large distance.

NOVA Close binary star system in which one star is a super-dense white dwarf. Matter sucked from the other star spirals down to the white dwarf and, when enough has accumulated, can trigger an orgy of heat-generating nuclear reactions and an explosion.

NUCLEAR CHAIN REACTION An event triggered when an unstable nucleus such as uranium-235 splits, or ‘fissions’, releasing energy and neutrons, which split further uranium nuclei, releasing more energy and more neutrons, and so on. If such a chain reaction is controlled, the result is a nuclear reactor. If it is an uncontrolled runaway chain reaction, the result is an atomic bomb.

NUCLEAR ENERGY The excess 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 byproduct is sunlight.

NUCLEAR REACTION Any process which converts one type of atomic nucleus into another type of atomic nucleus.

NUCLEAR STATISTICAL EQUILIBRIUM Situation in which nuclear reactions are so fast and furious that a steady state is reached in which nuclear reactions building up each nucleus are perfectly balanced by reactions destroying it. Under such conditions, the mix of element ‘freezes out’ – that is, it does not change with time.

NUCLEON Umbrella term used for protons and neutrons, the two building blocks of the atomic nucleus.

NUCLEOSYNTHESIS The gradual build-up of heavy elements from light elements, either in the Big Bang – Big Bang nucleosynthesis – or inside stars – stellar nucleosynthesis.

NUCLEUS See Atomic Nucleus.

    

OORT CLOUD A cloud of comets thought to orbit the Sun beyond the orbit of the outermost planet. Estimates put the total number of comets at up to 100 billion.

    

PANSPERMIA The idea that the seeds of life spread across space from planetary system to planetary system and that simple life on Earth was therefore ‘seeded’ from the stars.

PAULI EXCLUSION PRINCIPLE The prohibition on two microscopic particles (fermions) sharing the same quantum state. The Pauli exclusion principle stops electrons, which are fermions, piling on top of each other and, consequently, explains the existence of different atoms and the variety of the world around us.

PHOTOCELL A practical device which exploits the photoelectric effect. The interruption of an electric current when a body breaks the light beam falling on a metal can be used to control something, such as an automatic door at the entrance to a supermarket.

PHOTOELECTRIC EFFECT The ejection of electrons from the surface of a metal by photons striking the metal.

PHOTON Particle of light.

PHYSICS, LAWS OF The fundamental laws which orchestrate the behaviour of the Universe.

PLANCK ENERGY The super-high energy at which gravity becomes comparable in strength to the other fundamental forces of nature.

PLANCK LENGTH The fantastically tiny length scale at which gravity becomes comparable in strength to the other fundamental forces of nature. The Planck length is a trillion trillion times smaller than an atom. It corresponds to the Planck energy. Small distances are synonymous with high energies because of the wave nature of matter.

PLANCK TEMPERATURE The super-hot temperature which corresponds to the Planck energy at which gravity becomes comparable in strength to the other fundamental forces of nature.

PLASMA An electrically charged gas of ions and electrons.

PLUM PUDDING MODEL Early model of the atom in which negatively charged electrons were imagined to be spread throughout a diffuse cloud of positive charge like raisins in a plum pudding.

PLUTO Until recently, the outermost planet. Now it has been demoted to ‘dwarf planet’ status and is considered to be just one of many tens of thousands of icy bodies in the outer Solar System known as Kuiper Belt objects.

PLUTONIUM Element 94. The nucleus of this man-made element can undergo nuclear fission and sustain a runaway chain reaction, liberating a vast amount of nuclear energy. It can therefore make an atomic bomb.

POPULATION I STAR Hot, blue stars found in the Milky Way’s ‘spiral arms’. Population I stars are young and relatively rich in heavy elements.

POPULATION II STAR Cool, red stars found in the central region of the Milky Way. Population II stars are old and relatively poor in heavy elements.

POSITRON Antiparticle of the electron.

PRISM Wedge of glass or some other dense, transparent medium. Since light of different colour travels at different speed in such a medium, on passing through a prism the colours of white light fan out to form a rainbow-like spectrum.

PROTON One of the two main building blocks of the nucleus. Protons carry a positive electrical charge, equal and opposite to that of electrons.

PROTON–PROTON CHAIN The chain of nuclear reactions by which stars up to about one and a half times the mass of the Sun turn hydrogen into helium.

PULSAR A rapidly rotating neutron star which sweeps an intense beam of radio waves around the sky, much like a lighthouse.

    

QED See Quantum Electrodynamics.

QUANTUM The smallest chunk into which something can be divided. Photons, for instance, are quanta of the electromagnetic field.

QUANTUM COSMOLOGY Quantum theory applied to the whole Universe. Since the Universe was once smaller than an atom, such a theory is necessary to try and understand the birth of the Universe in the Big Bang.

QUANTUM ELECTRODYNAMICS The theory of how light interacts with matter. It explains almost everything about the everyday world, from why the ground beneath our feet is solid to how a laser works, from the chemistry of metabolism to the operation of computers.

QUANTUM FLUCTUATION 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 INDISTINGUISHABILITY The inability to distinguish between two quantum events. They may be indistinguishable because they involve identical particles, for instance, or simply because the events are not observed. The crucial thing, however, is that the probability waves associated with indistinguishable events interfere. This leads to all manner of quantum phenomena.

QUANTUM NUMBER A number which specifies a microscopic property which comes in chunks, such as spin or the orbital energy of an electron.

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 Essentially, the theory of the microscopic world of atoms and their constituents. Those who favour the Many Worlds interpretation believe it also describes the large-scale world.

QUANTUM TUNNELLING The apparently miraculous ability of microscopic particles to escape their prisons. For instance, an alpha particle can tunnel through the barrier penning it in the nucleus, the equivalent of a high-jumper jumping a four-metre-high wall. Tunnelling is yet another consequence of the wave-like character of microscopic particles.

QUANTUM UNPREDICTABILITY The unpredictability of microscopic particles. Their behaviour is unpredictable even in principle. Contrast this with the unpredictability of a coin toss. It is unpredictable only in practice. In principle, if we knew the shape of the coin, the force exerted on it, the air currents around it, and so on, we could predict the outcome.

QUANTUM VACUUM The quantum picture of empty space. Far from empty, it seethes with ultra-short-lived microscopic particles which are permitted by the Heisenberg uncertainty principle to blink into existence and blink out again.

    

RADIOACTIVE DATING The use of the radioactive disintegration of an element such as uranium to date a rock. In practice, unstable uranium ultimately decays into stable lead, so as time passes the proportion of lead to uranium increases, and this can be used to date the material.

RADIOACTIVE DECAY The disintegration of unstable heavy atoms into lighter, stabler atoms. The process is accompanied by the emission of either alpha particles, beta particles or gamma rays.

RADIOACTIVITY Property of atoms which undergo radioactive decay.

RADIUM Highly unstable, or radioactive, element, discovered by Marie Curie in 1898.

RED DWARF Star less massive than the Sun which glows like a dying ember. About 70 per cent of the stars in the solar neighbourhood are red dwarfs, exploding the myth that the Sun is a typical star. In fact, it is more massive, and therefore more luminous, than the average star.

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. It can also be caused when light loses energy climbing out of the strong gravity of a compact star such a white dwarf, something which is known as a gravitational red shift.)

RELATIVITY, GENERAL THEORY OF Einstein’s generalisation of his special theory of relativity. General relativity 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.

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.

    

SCANNING TUNNELLING MICROSCOPE (STM) A device which drags an ultra-fine needle across the surface of a material and converts the up-and-down motion of the needle into an image of the atomic landscape of the surface.

SCHRöDINGER EQUATION Equation which governs the way in which the probability wave, or wave function, describing, say, a particle changes with time.

SELF-REPRODUCING SPACE PROBE See Von Neumann Probe.

SETI The Search for Extraterrestrial Intelligence. Most searches involve looking for radio signals from other stars, though optical searches are being undertaken as well.

SILICON BURNING The fast and furious chain of element-building reactions that ensues once a massive star makes silicon. In only a day they turn silicon into iron and nickel, the endpoint of normal stellar nuclear reactions. The star is then on the brink of catastrophe and primed to explode as a supernova.

SOLAR SYSTEM The Sun and its family of planets, moons, comets and other assorted rubble.

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.

SPECTRAL LINE Atoms and molecules absorb and give out light at characteristic wavelengths. If they swallow more light than they emit, the result is a dark line in the spectrum of a celestial object. Conversely, if they emit more than they swallow, the result is a bright line.

SPECTRUM The separation of light into its constituent ‘rainbow’ colours.

SPIN Quantity with no everyday analogue. Loosely speaking, subatomic particles with spin behave as if they are tiny spinning tops (only they are not spinning at all).

STAR A giant ball of gas which replenishes the heat it loses to space by means of nuclear energy generated in its core.

STRING THEORY See Superstring Theory.

STRONG NUCLEAR FORCE The powerful short-range force which holds protons and neutrons together in an atomic nucleus.

SUBATOMIC PARTICLE A particle smaller than an atom, such as an electron or a neutron.

SUN The nearest star.

SUPERFORCE Hypothetical force from which each of the four fundamental forces of nature ‘froze out’ as the Universe cooled in the aftermath of the Big Bang.

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.

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.

    

TELEPORTATION The clever use of entanglement to pin down the exact state of a microscopic particle, in apparent violation of what is permitted by the Heisenberg uncertainty principle. This enables the information necessary to reconstruct the state of the particle to be sent to a remote site.

TEMPERATURE The degree of hotness of a body. Related to the energy of motion of the particles that compose it.

THERMODYNAMICS, SECOND LAW OF The decree that entropy can never decrease. This is equivalent to saying that heat can never flow from a cold body to a hot body.

TOTAL ECLIPSE OF THE SUN The coverage of the Sun by the disc of the Moon when the Moon moves between the Sun and the Earth.

TRIPLE-ALPHA PROCESS The unlikely process by which stars weld three helium nuclei into a nucleus of carbon, opening the way to the build-up of all heavy elements.

TUNNELLING, QUANTUM See Quantum Tunnelling.

    

ULTRAVIOLET Type of invisible light which is given out by very hot bodies. Responsible for sunburn.

UNCERTAINTY PRINCIPLE See Heisenberg Uncertainty Principle

UNIFICATION The idea that at extremely high energy the four fundamental forces of nature are one, united in a single theoretical framework.

UNIVERSE All there is. This is a flexible term which was once 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 100 billion within the observable universe.

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.

URANIUM The heaviest naturally occurring element.

URANIUM-235 Isotope of uranium that undergoes nuclear fission and which can support a runaway chain reaction, liberating a vast amount of energy. This makes an atomic bomb.

    

VACUUM, QUANTUM See Quantum Vacuum.

VON NEUMANN PROBE 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.

    

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.

WAVELENGTH The distance it takes for a wave to go through a complete oscillation cycle.

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.

WHITE DWARF A star which has run out of fuel and which gravity has compressed until it is about the size of the Earth. A white dwarf is supported against further shrinkage by electron degeneracy pressure. A sugar cube of white dwarf material weighs about as much as a family car.

WORMHOLE A tunnel through space–time which connects widely spaced regions and provides a short cut. Though permitted to exist by Einstein’s theory of gravity, a wormhole is unstable and would snap shut in a split second unless held open by a type of matter with repulsive gravity. Nobody knows whether such ‘exotic matter’ exists in sufficient quantities to make a traversable wormhole.

WOW! SIGNAL A 37-second-long signal picked up by the Ohio State ‘Big Ear’ radio telescope on 15 August 1977. When astronomer Jerry Ehman saw the off-the-scale pen-recorder trace, he scrawled across it ‘Wow!’ It never repeated and, to this day, nobody knows whether the Big Ear picked up a real extraterrestrial broadcast.

    

X-RAY A high-energy form of light.

    

YOUNG’S DOUBLE-SLIT EXPERIMENT Experiment in which light of a single colour encounters an opaque screen with two closely spaced vertical slits cut into it. On the far side of the screen ripples of light emerge from each slit and overlap with each other. Where the crests coincide, the waves reinforce, enhancing the brightness, and where the crests of one set of ripples coincide with the troughs of the other, the waves cancel, creating dark regions. This ‘interference’ pattern – a series of alternating dark and bright bands – can be seen on a white screen interposed in the path of the light. The pattern is a defining characteristic of a wave and was seen for the first time by Thomas Young in 1801.

    

ZOO HYPOTHESIS The idea that the human race, as an emerging intelligence, is off limits to intelligent space-faring races, thus explaining why the Earth has not been visited by extraterrestrials.