Glossary
anthropic principle The idea that the answer to the question ‘Why is the universe, as we find it, so suitable for our existence?’ is that if it were different, we could not be here to ask the question.
antimatter Matter consisting of antiparticles.
antiparticle For every type of particle there exists an antiparticle with opposite properties, such as the sign of its electrical charge (for example, the electron has negative electrical charge; the antielectron, or positron, has positive electrical charge), and other qualities that we haven’t dealt with in this book. However, the antiparticles of photons and gravitons are the same as the particles.
arbitrary element Something which isn’t predicted in a theory but must be learned from observation. For example, an alien who had never seen our universe couldn’t take any theory we have so far and use it to work out what the masses and charges are of the elementary particles. These are arbitrary elements in the theories.
Big Bang singularity A singularity at the beginning of the universe.
Big Bang theory The theory which says that the universe began in a state of enormous density and pressure and exploded outwards and expanded until it is as we see it today.
black hole A region of spacetime shaped like a sphere (or a slightly bulged-out sphere in the case of a rotating black hole) which cannot be seen by distant observers because gravity there is so strong that no light (or anything else) can escape from it. Black holes may form from the collapse of massive stars. This was the ‘classical’ definition of a black hole. Hawking showed that a black hole does radiate energy and may not be entirely ‘black’. (See also primordial black hole.)
boson Particle with spin expressed in whole numbers. The messenger particles of the forces (gluons, W+, W–, Z°, photons and gravitons) are bosons.
boundary conditions What the universe was like at the instant of beginning, before any time whatsoever had passed. Also what it is like at any other ‘edge’ of the universe – the end of the universe, for example, or the centre of a black hole.
classical physics Physics that doesn’t take quantum mechanics into account.
conservation of energy The law of science that says that energy (or its equivalent in mass) cannot be either created or destroyed.
cosmological arrow of time The direction of time in which the universe is expanding.
cosmological constant Albert Einstein introduced a ‘cosmological constant’, to counteract gravity, into his theory of general relativity. Without it, the theory predicted that the universe ought to be either expanding or collapsing, neither of which Einstein believed to be true. He later called it ‘the greatest blunder of my life’. We now use the term to mean the energy density of the vacuum.
cosmology The study of the very large and of the universe as a whole.
dark energy The mysterious energy that makes up about 73 per cent of the universe and is thought to be responsible for the current acceleration of the universe’s expansion.
determinism The idea that the future is completely predictable from the present, completely determined by the present.
Einstein’s general theory of relativity (1915) The theory of gravity in which gravity is explained as a curvature in four-dimensional spacetime caused by the presence of mass or energy. It provides a set of equations that determine how much curvature is generated by any given distribution of mass or energy. It is a theory that we use to describe gravity at the level of the very large.
Einstein’s special theory of relativity (1905) Einstein’s new view of space and time. The theory is based on the idea that the laws of science should be the same for all freely moving observers, no matter what their speed. The speed of light remains unchanged, no matter what the velocity of the observer measuring it is.
electromagnetic force One of the four fundamental forces of nature. It causes electrons to orbit the nuclei of atoms. At our level it shows up as light and as all other electromagnetic radiation, such as radio waves, microwaves, X-rays and gamma rays. The messenger particle (boson) of the electromagnetic force is the photon.
electromagnetic interaction The interaction in which an electron emits a photon and another electron absorbs it.
electromagnetic radiation All forms of radiation that make up the electromagnetic spectrum, such as radio waves, microwaves, visible light, X-rays and gamma rays. All electromagnetic radiation is made up of photons.
electroweak theory A theory developed in the 1960s by Abdus Salam at Imperial College, London, and Steven Weinberg and Sheldon Glashow at Harvard which unified the electromagnetic force and the weak force.
elementary particle A particle that we believe is not made up of anything smaller and that cannot be divided.
entropy The measurement of the amount of disorder in a system. The second law of thermodynamics states that entropy always increases, never decreases. The universe as a whole, or any isolated system, can never become more orderly.
escape velocity The speed necessary to escape the gravity of a massive body such as the Earth and escape to elsewhere in space. Escape velocity for the Earth is about 7 miles (11 kilometres) per second. Escape velocity for a black hole is slightly greater than the speed of light.
event A point in spacetime, specified by its position in time and space, as on a spacetime diagram.
event horizon The boundary of a black hole; the radius where escape velocity becomes greater than the speed of light. It is marked by hovering photons, which (moving at the speed of light) cannot escape and also cannot be drawn into the black hole. Light emitted inside it is drawn down into the black hole. To calculate the radius at which the event horizon forms, multiply the solar mass of the black hole (the same as for the star that collapsed to form it, unless that star lost mass earlier in the collapse) by 2 for miles or 3 for kilometres. Thus, a 10-solar-mass black hole has its event horizon at a radius of 20 miles or 30 kilometres. You can see that if the mass changes, the radius where the event horizon is will also change, and the black hole will change in size.
fermion For the purposes of this book you need to know that particles of ordinary matter (the particles in an atom, such as electrons, neutrons and protons) belong to a class of particles called fermions, and, like all fermions, they exchange messenger particles. A more technical definition of a fermion is a particle with half-integer spin which obeys the Pauli exclusion principle. We have not dealt with the exclusion principle in this book.
forces of nature The four basic ways that particles can interact with one another. They are, in order from strongest to weakest, the strong force, the weak force, the electromagnetic force and the gravitational force.
fractal A geometric pattern in which parts of the pattern repeat when viewed at any scale.
frequency For a photon, the rate at which the electromagnetic field associated with the photon changes. For the purposes of this book, all you need to know is that the higher the frequency, the greater the energy of the photon.
gamma rays Electromagnetic radiation of very short wavelengths.
gluon The messenger particle which carries the strong force from one quark to another and causes the quarks to hold together in protons and neutrons in the nucleus of the atom. Gluons also interact with one another.
grandfather paradox The idea that someone could travel back in time and prevent their grandparents from giving birth to their parents, thus preventing their own birth.
gravitational force One of the four fundamental forces of nature, and also the weakest. Gravity usually attracts (but not during inflation), and can work over extremely long distances.
gravitational radius Photons cannot escape from a black hole to the outside universe from within this radius. You can think of it in the same way as the event horizon, though the two terms are used differently. To calculate roughly what this radius will be, multiply the solar mass of the black hole by 2 for miles and 3 for kilometres. Thus a 10-solar-mass black hole will have a radius of 20 miles or 30 kilometres.
> graviton The messenger particle which carries the gravitational force among all particles in the universe, including gravitons themselves. None has ever been directly observed.
gravity See gravitational force
Hawking radiation Radiation produced by a black hole when quantum effects are taken into account. You can think of it as a type of virtual particle pair production near the event horizon of a black hole in which one of the two falls into the hole, allowing the other to escape into space.
Heisenberg uncertainty principle In quantum mechanics, it is impossible to measure precisely, at the same time, the position and the momentum of a particle. Likewise it is impossible to measure precisely, at the same time, the value of a field and the way the field is changing over time.
helium The second lightest of the chemical elements. The nucleus of a helium atom contains two protons and either one or two neutrons. There are two electrons orbiting the nucleus.
homogeneous Having the same quality and appearance at all places.
hydrogen The lightest of the chemical elements. The nucleus of ordinary hydrogen consists of just one proton. There is a single electron orbiting the nucleus. Hydrogen is fused into helium in the cores of stars.
imaginary numbers Numbers that when squared give a negative result. Thus the square of imaginary two is minus four. The square root of minus nine is imaginary three.
imaginary time Time measured using imaginary numbers.
inflationary-universe model Model in which the early universe went through a short period of extremely rapid expansion.
initial conditions The boundary conditions at the beginning of the universe, before any time whatsoever had passed.
isotropic Looking the same in all directions.
microwave radiation Electromagnetic radiation that has wavelengths longer than those of visible light and shorter than radio waves. The particles of microwave radiation, as of all radiation in the electromagnetic spectrum, are photons. A background of microwave radiation that we detect in the universe is evidence used to support the idea of the Big Bang model.
N=8 supergravity A theory that attempts to unify all the particles, both bosons and fermions, in a supersymmetric family, and to unify the forces. This was the theory Hawking spoke of in his 1980 Lucasian Lecture and which he thought might turn out to be the Theory of Everything.
naked singularity A singularity that is not hidden inside an event horizon.
neutron One of the particles that make up the nucleus of an atom. Neutrons have no electrical charge. Every neutron is made up of three smaller particles called quarks.
neutron star The final stage of a star that is too massive to form a white dwarf star but not massive enough to collapse to a black hole.
Newton’s theory of gravity Each body in the universe is attracted towards every other by a force that is stronger the more massive the bodies are and the closer they are to one another. Stated more precisely: bodies attract each other with a force that is proportional to the product of their masses and inversely proportional to the square of the distance between them.
no-boundary proposal The idea that the universe is finite but has no boundary (in imaginary time).
nucleus The central part of an atom, made up of protons and neutrons (which in turn are made up of quarks). The nucleus is held together by the strong force.
optical telescope A telescope that produces images of stars and galaxies in the part of the electromagnetic spectrum that is visible to human eyes.
particle pairs Pairs of particles that are being created everywhere in the vacuum and all the time. They are usually thought to be virtual particles, are extremely short-lived, and cannot be detected except indirectly by observing their effect on other particles. In a fraction of a second the two particles in a pair must find each other again and annihilate one another.
photon The messenger particle of the electromagnetic force. At our level, photons show up as visible light and as all the other radiation in the electromagnetic spectrum, such as radio waves, microwaves, X-rays and gamma rays. Photons have zero mass and move at the speed of light.
Planck length Thought to be the smallest meaningful length. It is 10–33 centimetres.
positron Antiparticle of the electron. It has positive electric charge.
primordial black hole Tiny black hole created not by the collapse of a star but the pressing together of matter in the very early universe. According to Hawking the most interesting ones are about the size of the nucleus of an atom, with a mass of about a billion tons.
proton One of the particles that make up the nucleus of the atom. Protons have a positive electric charge. Every proton is made up of three smaller particles called quarks.
psychological arrow of time Our everyday experience of the way time passes, from past to future.
pulsar A neutron star that rotates very rapidly and sends out regular pulses of radio waves.
quantum fluctuations The constant appearance and disappearance of virtual particles that occur in what we think of as empty space (the vacuum).
quantum gravity The scientific theory that successfully unites general relativity and quantum mechanics. At present we do not have such a theory.
quantum mechanics or quantum theory The theory developed in the 1920s that we use to describe the very small, generally things the size of the atom and smaller. According to the theory, light, X-rays and any other waves can only be emitted or absorbed in certain ‘packages’ called ‘quanta’. For instance, light occurs in quanta known as photons, and it can’t be divided up into smaller ‘packages’ than one photon. You can’t have half a photon, for example, or one and three-quarter photons. In quantum theory, energy is said to be ‘quantized’. The theory includes the uncertainty principle.
quantum wormhole A wormhole of an unimaginably small size. (See also wormhole.)
quarks The fundamental particles (meaning they can’t be divided into anything smaller) which, banded together in groups of three, make up protons and neutrons. Quarks also band together in groups of two (one quark and one antiquark) to form particles called mesons.
radio waves Electromagnetic waves with longer wavelengths than those of visible light. The particles of radio waves, as of all radiation in the electromagnetic spectrum, are photons.
radioactivity The spontaneous breakdown of one type of atomic nucleus into another.
radius The shortest distance from the centre of a circle or sphere to the circumference or surface.
renormalization A process that is used to remove infinities from a theory. It involves putting in other infinities and allowing the infinities to cancel one another out.
second law of thermodynamics Entropy, the amount of disorder, in an isolated system can only increase, never decrease. If two systems join, the entropy of the combined system is as great as or greater than the entropy of the two systems added together.
singularity A point in spacetime at which spacetime curvature becomes infinite, a point of infinite density. Some theories predict that we will find a singularity at the centre of a black hole or at the beginning or end of the universe.
solar mass Mass equalling the mass of our sun.
spacetime The combination of the three dimensions of space and one dimension of time.
spacetime curvature Einstein’s theory of general relativity explains the force of gravity as the way the distribution of mass or energy in spacetime causes something that resembles the warping, denting and dimpling in an elastic surface by heavy pellets of different weights and sizes lying on it.
strong nuclear force The strongest of the four fundamental forces of nature. It holds the quarks together, in neutrons and protons for instance, and is responsible for the way protons and neutrons hold together in the nucleus. The messenger particle (boson) of the strong force is the gluon.
supernova An enormous explosion of a star in which all but the inner core is blown off into space. The material blown off in a supernova forms the raw mat
erial for new stars and for planets.
superstring theory The theory that explains the fundamental objects in the universe not as pointlike particles but as tiny strings or loops of string. It is a leading candidate for unifying all the particles and forces.
Theory of Everything Sometimes called the TOE, this is the nickname for the theory that explains the universe and everything that happens in it.
thermodynamic arrow of time Entropy (disorder) increases over time.
uncertainty principle A particle cannot have both a definite position and a definite momentum at the same time. The more precisely you measure the one, the less accurate your measurement of the other will be. Similarly you cannot measure precisely the value of a field and its rate of change over time. There are other pairs of quantities that present the same problem. The uncertainty principle was discovered by the German physicist Werner Heisenberg and is more properly called the Heisenberg uncertainty principle.
unified theory A theory that explains all four forces as one ‘superforce’ showing up in different ways and that also unites both fermions and bosons in a single family.
vacuum energy The energy that is there in what we think of as empty space.
velocity The speed at which something is moving away from some fixed place, and the direction in which it is moving.
virtual particle In quantum mechanics a particle that can never be directly detected, but whose existence we know about because we can measure its effect on other particles.
W+, W-, Z° The messenger particles (bosons) of the weak force.
wave function In quantum theory, a wave function describes all the possible paths a particle could follow between two points. If the value of the wave function is high for a particular path, then the particle is more likely to be found on that path.
wave function of the universe Hartle and Hawking treat the universe like a quantum particle. The Hartle–Hawking wave function of the universe represents all the physically possible histories our universe might have. If the value of the wave function is high for a history, then that is a more likely history.
Stephen Hawking, His Life and Work Page 37
