Quantum Theory Cannot Hurt You

by Marcus Chown

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

  HORIZON 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 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.

  HORIZON PROBLEM The problem that far-flung parts of the Universe that 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.

  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.

  INERTIAL FORCE A force we invent to explain a motion that is actually due to nothing more than inertia. A good example is centrifugal force. There is no such force flinging us outwards in a car rounding a sharp corner. We are simply continuing to move in a straight line because of our inertia, and the interior of the car, because it is moving along a curved path, intercepts us.

  INFLATION, THEORY OF Idea that in the first split-second of its 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.

  INFRARED Type of invisible light that is given out by warm bodies.

  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 that 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 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 3 cubic centimetres, making it a vacuum far better than anything achievable on Earth.

  INTERSTELLAR SPACE The space between the stars.

  ION An atom or molecule that 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 a mass of 35 and 37. The mass difference is due to a differing number of neutrons in their nuclei. For instance, chlorine-35 contains 18 neutrons and chlorine-37 contains 20 neutrons. (Both contain the same number of protons—17—since this determines the identity of an element.)

  JOULE The standard scientific unit of energy. The energy of motion of a flying cricket ball is about 10 joules; the chemical energy provided by a single slice of bread is about 100,000 joules; and the electrical energy of a lightning discharge is about 10 billion joules.

  LAMBDA POINT Temperature below which liquid helium begins to turn into a superfluid.

  LASER Light source in which the gregarious nature of photons—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 in 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 BENDING See Gravitational Light Bending.

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

  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.

  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 that 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 idea, however, the mind of the person observing the table splits into two—one that perceives the table to be in one place and another that perceives it to be 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.

  MAXWELL’S EQUATIONS OF ELECTROMAGNETISM The handful of elegant equations, written down by James Clerk Maxwell in1868, that neatly summarise all electrical and magnetic phenomena. The equations reveal that light is an electromagnetic wave.

  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 200 kilometres per hour. The oil tanker is said to have 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, planets, and life.

  MUON Short-lived subatomic particle that behaves like a heavy version of the electron.

  NEUTRINO Neutral subatomic particle with a very small mass that travels very close to the speed of light. Neutrinos, of which there are three kinds, hardly ever interact with matter. However, when created in huge numbers, they can blow a star apart 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 10 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 that 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.

  NONLOCALITY 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.

  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 that 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 that 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.

  PARTICLE ACCELERATOR Giant machine, often in the shape of a circular racetrack, 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 blocks and fundamental forces of nature.

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

  PHOTOCELL A practical device that exploits the photoelectric effect. The interruption of an electric current when a body breaks the light beam falling on a metal is used to control something—for instance, 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 that orchestrate the be-havior of the Universe.

  PLANCK ENERGY The superhigh 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.

  PLASMA An electrically charged gas of ions and electrons.

  POSITRON Antiparticle of the electron.

  PRECESSION OF THE PERIHELION OF MERCURY The fact that the orbit of Mercury, the planet closest to the Sun, does not follow a straightforward elliptical orbit but rather an elliptical orbit whose nearest point to the Sun gradually moves around the Sun, resulting in the planet tracing out a rosettelike pattern. The explanation is that the gravity of the Sun weakens with distance from the Sun more slowly than in the case of Newtonian gravity, which uniquely predicts elliptical orbits. It weakens more slowly because, in the Einsteinian picture, gravity itself is a source of more gravity.

  PRINCIPLE OF EQUIVALENCE The idea that gravity and acceleration are indistinguishable.

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

  PULSAR A rapidly rotating neutron star that 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 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 ELECTRODYNAMICS Theory of how light interacts with matter. The theory 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 INDISTINGUISHABILITY The inability to distinguish between two quantum events. These may be indistinguishable, for instance, because they involve identical particles 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 that specifies a microscopic property that comes in chunks such as the spin or 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 Situation in which a quantum object such as an atom is in more than one state at a time. It might, forinstance, be in many places simultaneously. It is the interaction, or “interference,” between the individual states in the superposition that is the basis of all quantum weirdness. Decoherence prevents such interaction and therefore destroys quantum behaviour.

  QUANTUM THEORY The theory of objects isolated from their surroundings. Because it is very hard to isolate a big object, the theory is essentially a theory of the microscopic world of atoms and their constituents.

  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 4-metre-high wall. Tunnelling is yet another consequence of the wavelike 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 ultra-shorth ultra-short-lived microscopic particles that are permitted by the Heisenberg uncertainty principle to blink into existence and blink out again.

  QUASAR A galaxy that 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 power-ful 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.

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

  RADIOACTIVITY Property of atoms tha
t undergo radioactive decay.

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

  RELATIVITY, GENERAL THEORY OF Einstein’s generalisation of his special theory of relativity. General relativity relates what one person sees when looking 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 that relates what one person sees when looking 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 that become ever more marked as they approach the speed of light.

  SCANNING TUNNELLING MICROSCOPE (STM) A device that drags an ultrafine 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 that governs the way in which the probability wave, or wave function, describing, say a subatomic particle, changes with time.

  SIMULTANEITY The idea that events that appear to happen at the same time for one person should appear to happen at the same time for everyone in the Universe. Special relativity shows that this idea is mistaken.

  SINGULARITY Location where the fabric of space-time ruptures and so cannot be understood by Einstein’s theory of gravity, the general theory of relativity. There was a singularity—a point where quantities such as temperature skyrocketed to infinity—at the beginning of the Universe. There is also one in the centre of every black hole.