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A measure of how much effort is required to stop a body, defined as the body’s mass multiplied by its velocity. Thus, a large heavy body (e.g. a train) going relatively slowly may have more momentum than a smaller body going very fast (e.g. a racing car). The Law of Conservation of Momentum rules that the total momentum of an isolated system (one in which no net external force acts on the system) does not change.

Multiverse (Parallel Universes):

A hypothetical set of multiple possible universes (including our own) which exist in parallel with each other. Our universe would then be just one of an enormous number of separate and distinct parallel universes, the vast majority of which would be dead and uninteresting, not having a set of physical laws which would allow the emergence of stars, planets and life.



A sub-atomic elementary particle with no electrical charge and very small mass that travels very close to the speed of light. They are created as a result of certain types of radioactive decay or nuclear reaction, such as the decay of a free neutron(i.e. one outside of a nucleus) into a proton and electron. Being electrically neutral and unaffected by the strong nuclear force or the electromagnetic force, neutrinos are able to pass through ordinary matter almost undisturbed and are therefore extremely difficult to detect, although when created in huge numbers they are capable of blowing a star apart in a supernova.


One of the two main building blocks (along with the proton) of the nucleus at the centre of an atom. Neutrons have essentially the same mass as a proton (very slightly larger) but no electric charge, and are made up of one “up” quark and two “down”quarks. The number of neutrons in an atom determines theisotope of an element. Outside of a nucleus, they are unstable and disintegrate within about ten minutes.

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

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

The rather spooky ability of objects in quantum theory to apparently instantaneously know about each other’s quantum state, even when separated by large distances, in apparent contravention of the principle of locality (the idea that distant objects cannot have direct influence on one another, and that an object is influenced directly only by its immediate surroundings).

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

Nuclear Fusion:

The welding together of two light nuclei to make a heaviernucleus, resulting in the liberation of nuclear energy. An example of this kind of nuclear reaction is the binding together of hydrogen nuclei in the core of the Sun to make helium. In larger, hotter stars, helium itself may fuse to produce heavierelements, a process which continues up the periodic table ofelements as far as iron. The fusion of ultra-stable iron nucleiactually absorbs energy rather than releasing it, and so iron does not easily fuse to create heavierelements.


The process of creating new atomic nuclei from pre-existingprotons and neutrons by a process of nuclear fusion. The primordial nucleons (hydrogen and helium) themselves were formed from the quark-gluon plasma in the first few minutes after the Big Bang, as it cooled to below ten million degrees, but nucleosynthesis of the heavier elements (including all carbon, oxygen, etc) occurs primarily in the nuclear fusionprocess within stars and supernovas.

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


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


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The hypothesis that "seeds" of life exist already all over theuniverse, and that life on Earth may have originated through these "seeds", driven by a steady influx of cells or viruses arriving from space via comets. It is a more limited form of the related hypothesis of exogenesis, which also proposes that lifeon Earth was transferred from elsewhere in the universe, but makes no prediction about how widespread it may be.

Pauli Exclusion Principle:
The prohibition on two identical fermions from sharing the same quantum state simultaneously. Among other implications it stops electrons (which are a kind of fermion) from piling on top of each other, thereby explaining the existence of different types of atoms and the whole variety of the universearound us.

Photoelectric Effect:

The phenomenon in which, when a metallic surface is exposed to electromagnetic radiation above a certain threshold frequency (typically visible light and x-rays), the light is absorbed andelectrons are emitted. The discovery of the effect is usually attributed to Heinrich Hertz in 1887, and study of it (particularly by Albert Einstein) led to important steps in understanding thequantum nature of light and electrons and in formulating the concept of wave-particle duality.

A particle (or quantum) of light or other electromagnetic radiation, which has no intrinsic mass and can therefore travel at the speed of light. It is an elementary particle and the basic unit of light, and effectively carries the effects of the electromagnetic force. The modern concept of the photon as exhibiting both wave and particle properties was developed gradually by Albert Einstein and others.

Planck Constant:
The proportionality constant (h) which provides the relation between the energy (E) of a photon and the frequency (v) of its associated electromagnetic wave in the so-called Planck Relation E = hv. It is essentially used to describe the sizes of individual quanta in quantum mechanics. Its value depends on the units used for energy and frequency, but it is a very small number (with energy measured in Joules, it is of the order of 6.626 CH 10-34 J·s).

Planck Energy:
The super-high energy (approximately 1.22 CH 1019 GeV) at which gravity becomes comparable in strength to the other fundamental forces, and at which the quantum effects of gravity become important.

Planck Length:
The fantastically tiny length scale (approximately 1.6 CH 10-35 metres) at which gravity becomes comparable in strength to the other fundamental forces. It is the scale at which classical ideas aboutgravity and space-time cease to be valid, and quantum effects dominate.

Planck Temperature:
The temperature of the universe at 1 Planck Time after the Big Bang, approximately equal to 1.4 CH 1032°C.

Planck Time:
The time it would take a photon travelling at the speed of light to cross a distance equal to the Planck Length. This is the “quantum of time”, the smallest measurement of time that has any meaning, and is approximately equal to 10-43 seconds.

Planck Units:
“Natural units” of measurement (i.e. designed so that certain fundamental physical constants are normalized to 1), named after the German physicist Max Planck who first proposed them in 1899. They were an attempt to eliminate all arbitrariness from the system of units, and to help simplify many complex equations in modern physics. Among the most important are the Planck Energy, the Planck Length, the Planck Time and the Planck Temperature.


A partially ionized gas of ions and electrons, in which a certain proportion of the electrons are free rather than being bound to an atom or molecule. It has properties quite unlike those of solids, liquids or gases and is sometimes considered to be a distinct fourth state of matter. An example of plasma present at the Earth's surface is lightning.

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

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

Principle of Equivalence:
The idea that no experiment can distinguish the acceleration due to gravity from the inertial acceleration due to a change of velocity (or acceleration).

Principle of Relativity:

The idea, first expressed by Galileo Galilei in 1632 and also known as the principle of invariance, that the fundamental laws of physics are the same in all inertial frames and that, purely by observing the outcome of mechanical experiments, one cannot distinguish a state of rest from a state of constant velocity. Thus, all uniform motion is relative, and there is no absolute and well-defined state of rest.

Probability Wave (or Wave Function):
A description of the probability that a particle in a particular state will be measured to have a given position and momentum. Thus, a particle (an electronphoton or any other kind of particle), when not being measured or located, takes the form of a field or wave of probable locations, some being more probable or likely than others.

Prokaryotes and Eukaryotes:

Prokaryotes are primitive organisms that lack a cell nucleus or any other membrane-bound organelles. Most prokaryotes are single-celled (although some have multicellular stages in their life-cycles), and they are divided into two main domains, bacteria and archaea.

Eukaryotes, on the other hand, are organisms whose cells contain a nucleus and are organized into complex structures enclosed within membranes. Most living organisms (including all animals, plants, fungi and protists) are eukaryotes.


One of the two main building blocks (along with the neutron) of the nucleus at the centre of an atom. Protons carry a positiveelectrical charge, equal and opposite to that of electrons, and are made up of two “up” quarks and one “down” quark. The number of protons in an atom’s nucleus determines its atomic number and thus which chemical element it represents.


A highly-magnetized rapidly-rotating neutron star that sweeps regular pulses of intense electromagnetic radiation (radio waves) around space like a lighthouse. The intervals between pulses are very regular, ranging from 1.4 milliseconds to 8.5 seconds depending on the rotation period of the star. A pulsar generally has a mass similar to our own Sun, but a diameter of only around 10 kilometres.


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

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

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

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

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

Quantum Tunnelling:

The quantum mechanical effect in which particles have a finite probability of crossing an energybarrier, or transitioning through an energy state normally forbidden to them by classical physics, due to the wave-like aspect of particles. Theprobability wave of a particle represents the probability of finding the particle in a certain location, and there is a finite probability that the particle is located on the other side of the barrier.

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


Short for QUAsi-StellAr Radio source, a quasar is an extremely powerful and distant active galactic nucleus (a compact region at the centre of a galaxy which has a much higher than normal luminosity). It derives most of its energy from very hot matterswirling into a central supermassive black hole, and can generate as much light as a hundred normal galaxies from a much smaller volume. It is one of the most powerful objects in the universe, and among the most distant things ever seen in space.


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


The shifting of emitted electromagnetic radiation (such as visible light) towards the less energetic red end of the electromagnetic spectrum when a light source is moving away from the observer. This occurs as the wavelengths of lightstretch as an object moves away (as opposed to being squashed by an approaching object), similar to the familiar Doppler effect on sound waves. Among other things, it can be used as a measure of the speed with which galaxies throughout the universe are moving away from us.

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

RNA and DNA:

Ribonucleic acid (RNA) is a type of single-stranded moleculethat consists of a long chain of nucleotide units, each of which consists of a nitrogenous base, a ribose sugar and a phosphate. RNA transmits the genetic information from DNAinto the nucleus of cells, and controls certain chemical processes in the cell. Both DNA and RNA are considered essential building blocks of life.


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

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

Singularity (or Gravitational Singularity):

A region of space where the density of matter, or the curvature of space-time, becomes infinite and the concepts of space and time cease to have any meaning. At this point, the whole fabric of space-time ruptures and the precepts of Einstein’s General Theory of Relativity (and physics in general) break down and no longer apply, similar to the way in which a calculator returns an error when asked to divide by zero. According to general relativity, the Big Bang started with a singularity, and there is a singularity at the centre of a black hole.


Space-time (or spacetime or the spacetime continuum) is any mathematical model that combines space and time into a single construct. The fourth dimension of time is traditionally considered to be of a different sort than the three dimensions of space in that it can only go forwards and not back but, in Albert Einstein’s General Theory of Relativity, space and time are seen to be essentially the same thing and can therefore be treated as a single entity.

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

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