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Electromagnetic Radiation (or Electromagnetic Waves)

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Electromagnetic Radiation (or Electromagnetic Waves):

A wave that travels though space at the speed of light, consisting of an electrical field that periodically grows and dies, alternating with a magnetic field that periodically dies and grows. Electromagnetic waves carry energy and momentum, which may be imparted when it interacts with matter.

In order of increasing frequency, the electromagnetic spectrum includes radio waves, microwaves, terahertz radiation, infrared radiation, visible light, ultraviolet radiation, x-rays and gamma rays.


A negatively-charged sub-atomic particle. It is an indivisible,elementary particle, and is usually to be found orbiting thenucleus of an atomElectrons in an atom (which exist in the same quantity as the number of protons in the nucleus of the particular atom, so that the overall electric charge is zero) are constrained to occupy certain discrete orbital positions or “shells” around the nucleus. Interactions between the electrons of different atoms play an essential role in chemical bonding and phenomena such as electricity, magnetism and thermal conductivity. The discovery of electrons is credited to the British physicist J. J. Thomson in 1897.


A substance that cannot be reduced any further by chemical means. It is a pure chemical substance composed of atomswith the same atomic number (i.e. the same number of protonsin its nucleus). There are 92 naturally occurring elements on Earth, and all chemical matter consists of these elements (although a further 25 have been discovered as products of artificial nuclear reactions). Elements with atomic numbers 83 or higher are inherently unstable, and undergo radioactive decay. The list of elements is usually shown in the form of a Periodic Table, in order of their atomic number (see box at right, or click ther source link for a more detailed interactive Periodic Table).

Elementary Particle:

A particle with no substructure (i.e. not made up of smaller particles) and which is therefore one of the basic building blocks of the universe from which all other particles are made.Quarkselectonsneutrinosphotons, muons and gluons (along with their respective antiparticles) are all elementary particles;protons and neutrons (which are made up of quarks) are not.

Sometimes defined as the ability to do work or to cause change, energy is notoriously difficult to define. In accordance with the Law of Conservation of Energy, energy can never be created or destroyed but it can be changed into different forms, including kinetic, potential, thermal, gravitational, sound, light, elastic and electromagnetic. The standard scientific unit of energy is the Joule.


The phenomenon in quantum theory whereby particles that interact with each other become permanently dependent on each other’s quantum states and properties, to the extent that they lose their individuality and in many ways behave as a single entity. At some level, entangled particles appear to “know” each other’s states and properties.

A measure of the disorder of a system and of its constituent molecules. More specifically, in thermodynamics it is a measure of the unavailability of a system’s energy to do work. The Second Law of Thermodynamics embodies the idea that entropy can never decrease, but rather will tend to increase over time, approaching a maximum value as it reaches thermal equilibrium. A classic example of increasing entropy is ice melting in water until both reach a common temperature.

Event Horizon:

A one-way boundary in space-time surrounding a black hole. Any matter or light that falls through the event horizon of ablack hole can never leave, and any event inside the event horizon cannot affect an outside observer.

The hypothesis that life on Earth was transferred from elsewhere in the universe. A related but more limited concept is that of panspermia, the idea that "seeds" of life exist already all over the universe, and that life on Earth may have originated through these "seeds".

Exotic Particle:
A kind of theoretical particle said to exist by some theories of modern physics, whose alleged properties are extremely unusual. Examples include tachyons (particles that always travels faster than the speed of light), WIMPs (weakly interacting massive particles which do not interact withelectromagnetism or the strong nuclear force), axions (particles with no electric charge, very smallmass and very low interaction with the strong and weak forces) and neutrinos (particles that travel close to the speed of light, lack an electric charge and are able to pass through ordinary matteralmost undisturbed).

Expanding Universe:

universe which is constantly growing in size and in which the constituent parts (galaxies, clusters, etc) are flying ever further away from each other. Although contrary to the static universewhich had been assumed throughout most of history, an expanding universe was confirmed by Edwin Hubble’s 1929 observations of the redshifts of distant Cepheid variable stars, and is consistent with most solutions to Albert Einstein’s general relativity field equations. It also suggests that, in the distant past, the universe was much smaller and ultimately had its beginning in aBig Bang type event.


Fundamental (or Elementary) Forces:
There are four basic forces of physics that are believed to underlie all phenomena in the universe. Listed in order of strength they are: the strong nuclear force, the electromagnetic force, the weak nuclear force and the gravitational force (or gravity). It is thought likely that, in extremely high energyconditions such as occurred near the beginning of the Big Bang, the four fundamental forces of nature are actually unified in a single theoretical framework (known as the Grand Unified Theory).
According to quantum field theory, the forces between particles are mediated by other particles, and the fundamental forces can be described by the exchange of virtual force-carrying particles: the strong nuclear force mediated by gluons; the electromagnetic force by photons; the weak nuclear force by W and Z bosons; and gravity by hypothetical gravitons.



One of the basic building block of the universe, a galaxy is a massive system of stars, stellar remnants, gas, dust, and possibly a hypothetical substance known as dark matter, bound together by gravity. Galaxies may be anywhere from 1 to 100,000 light years across and are typically separated by millions of light years of intergalactic space. They are grouped into clusters, which in turn can form larger groups called superclusters and sheets or filaments. There are many different kinds of galaxy including spiral (like our own Milky Way galaxy), elliptical, ring, dwarf, lenticular and irregular. There are estimated to be over a hundred billion galaxies in the observable universe.

Gamma Ray:

A form of electromagnetic radiation produced by some kinds ofradioactive decay. Gamma rays have the highest frequency andenergy and the shortest wavelength in the electromagnetic spectrum, and penetrate matter more easily that either alpha particles or beta particles.

Gamma Ray Burst:
A narrow beam of intense electromagnetic radiation released during a supernova event, as a rapidly rotating, high-mass star collapses to form a black hole. They are the brightest events known to occur in the universe, and can last from milliseconds to several minutes (typically a few seconds). The initial burst is usually followed by a longer-lived 'afterglow' emitted at longer wavelengths (X-ray, ultraviolet, optical, infrared and radio).

A state of matter consisting of a collection of particles (moleculesatomsionselectrons, etc) without a definite shape or volume, and that are in more or less random motion. A gas tends to have relatively low density and viscosity compared to the solid and liquid states of matter, expands and contracts greatly with changes in temperature or pressure (“compressible”), and diffuses readily, spreading and homogeneously distributing itself throughout any container.

General Theory of Relativity:

Sometimes known as the Theory of General Relativity, this wasAlbert Einstein’s refinement (published in 1916) of his earlierSpecial Theory of Relativity and Sir Isaac Newton’s much earlierLaw of Universal Gravitation. The theory holds that acceleration and gravity are indistinguishable - the Principle of Equivalence - and describes gravity as a property of the geometry (more specifically a warpage) of space-time. Among other things, the theory predicts the existence of black holes, an expanding universetime dilationlength contraction, gravitational light bending and the curvature of space-time. Although classical physics can be considered a good approximation for everyday purposes, the predictions of general relativity differ significantly from those of classical physics. They have become generally accepted in modern physics, however, and have been confirmed by all observations and experiments to date.


The shortest path between two points in curved space. It originally meant the shortest route between two points on the Earth's surface (namely a segment of a great circle) but, since its application in general relativity, it has come to mean the generalization of the notion of a straight line as applied to all curved spaces. In non-curved three-dimensional space, the geodesic is a straight line. In general relativity, a free falling body (on which only gravitational forces are acting) follows a geodesic in curved four-dimensional space-time.

Grand Unified Theory (or Unified Field Theory):
Also known as Grand Unification or GUT, this refers to any of several unified field theories that predict that at extremely high energies (such as occurred just after the Big Bang), the electromagneticweak nuclear, and strong nuclear forces are all fused into a single unified field. Thus far, physicists have only been able to merge electromagnetism and the weak nuclear force into the “electroweak force”. Beyond Grand Unification, there is also speculation that it may be possible to merge gravity with the other three gauge symmetries into a “theory of everything”.

Gravity (or Gravitational Force):

The force of attraction that exists between any two masses, whether they be stars, microscopic particles or any other bodies with mass. It is by far the weakest of the fourfundamental forces (the others being the electromagnetic force, the strong nuclear force and the weak nuclear force), and yet, because it is a consistent force operating on all bodies withmass, it is instrumental in the formation of galaxiesstars, planets and black holes. It was approximately described by Sir Isaac Newton’s Law of Universal Gravitation in 1687, and more accurately described by Albert Einstein’s General Theory of Relativity in 1916.


A measure of the speed of radioactive decay of unstable, radioactive atoms. It is the time taken for half of the nuclei in a radioactive sample to disintegrate or decay. Half-lives can vary from a split-second to billions of years depending on the substance.

Hawking Radiation:

Random and featureless sub-atomic particles and thermal radiation predicted to be emitted by black holes due to quantum effects. Over long periods of time, as a black hole loses morematter through radiation than it gains through other means, it is therefore expected to dissipate, shrink and ultimately vanish.


The horizon of the universe is much like the horizon on Earth: it is the furthest that can be seen from a particular position. Because light has a finite speed and the universe has a finite age, we can only see objects whose light has had time to reach us since the Big Bang, so that the observable universe can be thought of as a bubble centred on the Earth.

Hubble’s Law:
Formulated by Edwin Hubble in 1929, the law states that the redshift in light coming from distantgalaxies is proportional to their distance, so that every galaxy appears to be rushing away from us (or from any other point in the universe) with a speed that is directly proportionate to its distance from us. It is considered the first observational basis for an expanding universe (or the metric expansion of space), and the most often cited evidence in support of the Big Bang theory, and arguably one of the most important cosmological discoveries ever made.

Hydrostatic Equilibrium:

The state in which the force of gravitation working to crush astar is exactly balanced by the thermal pressure of its hot gaspushing outwards. It is the reason that stars in general do not implode or explode, and it also explains why the Earth's atmosphere does not collapse to a very thin layer on the ground.


The natural tendency (as defined in Sir Isaac Newton’s First Law of Motion of 1687) of objects to resist changes in their state of motion. Therefore, a body at rest tends to stay at rest and, once set in motion, a body tends to stay moving at a constant speed in a straight line (or along a geodesic in curved space) unless acted on by an outside force. An example of an inertial force is centrifugal force, which in reality is just due to a body trying to continue in a straight line while constrained to move along a curved path.

Inertial Frame (or Inertial System):
A reference frame in which the observers are not subject to any accelerating force. An inertial frame is a frame of reference in which a body remains at rest or moves with constant linear velocity unless acted upon by outside forces (as stipulated by Sir Isaac Newton’s First Law of Motion, Force = MassCH Acceleration). Any frame of reference that moves with constant velocity relative to an inertial system is itself an inertial system.


The ability of two waves passing through each other to mingle, reinforcing each other where crests coincide and cancelling each other out where crests and troughs coincide, similar to the way ripples in water interfere with each other. This results, for example, in an interference pattern of light and dark stripes on a screen illuminated by light from two sources.

An atom or molecule that has been stripped of one or more of its orbiting electrons, thus giving it a net positive electric charge. Technically, an atom which gains an electron (thus giving it a net negativeelectric charge) is also a type of ion, known as an anion.

A possible form of an element, distinguishable from other isotopes of the same element by its differingmass, which is caused by a different number of neutrons in the nucleus (the number of protons, which gives the atomic number of the element, must be the same). Around 75% of isotopes are stable, while some are unstable or radioactive, and will decay over time into other elements.


Law of Conservation of Energy:

Also known as the First Law of Thermodynamics, this is the principle that energy can never be created or destroyed, only converted from one form to another (e.g. the chemical energy of gasoline can be converted into the energy of motion of a car). The total amount of energy in an isolated system (or in the universe as a whole) therefore remains constant.

Law of Universal Gravitation:

Published by Sir Isaac Newton in 1687, and sometimes also known as the Universal Law of Gravity, this was the first formulation of the idea that all bodies with mass pull on each other across space. Newton observed that the force of gravitybetween two objects is proportional to the product of the twomasses, and inversely proportional to the square of the distance between them. Although the theory has since been superseded by Albert Einstein's General Theory of Relativity, it predicts the movements of the Sun, the Moon and the planets to a high degree of accuracy and it continues to be used as an excellent approximation of the effects of gravity for everyday applications (relativity is only required when there is a need for extreme precision, or when dealing with the gravitation of very massive objects).

Length Contraction:

The phenomenon, predicted by Albert Einstein’s Special andGeneral Theories of Relativity, whereby, from the relative context of one observer's frame of reference, space or length appears to decrease as the relative velocities increase.

A difficult and contentious phenomenon to define, life is usually considered to be a characteristic of organisms that exhibit certain biological processes (such as chemical reactions or other events that results in a transformation), and that are capable of growth through metabolism and are capable of reproduction. The ability to ingest food and excrete waste are also sometimes considered requirements of life (e.g. bacteria are usually considered to be alive, whereas simpler viruses, which do not feed or excrete, are not).

The two distinguishing features of living systems are sometimes considered to be complexity and organization (negative entropy). Some organisms can communicate, and many can adapt to their environment through internally generated changes, although these are not universally considered prerequisites for life.

Technically, this refers to electromagnetic radiation of a wavelength that is visible to the human eye, although in the broader field of physics, it is sometimes used to refer to electromagnetic radiation of all wavelengths, whether visible or not. It exhibits “wave-particle duality” in that it can behave as both waves and particles (photons). Light travels at a constant speed of about 300,000 kilometres per second in a vacuum.

Light Year:
A convenient unit for measuring the large distances in the universe. It is the distance that light travels in one year which, given that light travels at 300,000 kilometres per second, works out to about 9,460,000,000 kilometres (9.46 trillion kilometres).


Magnetic Field:
The field of force that surrounds a magnet (in the same way as an electric field is the field of force that surrounds an electric charge). Together, the magnetic and electric fields make up the electromagnetic field which underlies light and other electromagnetic waves, and changes in either field will induce changes in the other, as indicated by James Clerk Maxwell’s Equations of Electromagnetism.

Magnetic Monopole:
A hypothetical particle that is a magnet with only one pole, and which therefore has a net magnetic charge. Although the existence of monopoles is indicated by both classical theory and quantum theory (and predicted by recent string theories and grand unified theories), there is still no observational evidence for their physical existence.

A measure of the amount of matter in a body. It can also be seen as a measure of a body’s inertia or resistence to change in motion, or the degree of acceleration a body acquires when subject to a force (bodies with greater mass are accelerated less by the same force and have greater inertia). Mass is often confused with weight, which is the strength of the gravitational pull on the object (and therefore how heavy it is in a particular gravitational situation), although, in everyday situations, the weight of an object is proportional to its mass.

Mass-Energy Equivalence:
The concept that any mass has an associated energy, and that, conversly, any energy has an associated mass. In Einstein’s Special Theory of Relativity, this relationship is expressed in the famous mass-energy equivalence formula, E = mc2, where E = total energym = mass and c = thespeed of light in a vacuum. Given that c is a very large number, it becomes apparent that mass is in fact a very concentrated form of energy.

Anything that has both mass and volume (i.e. takes up space). Matter is what atoms and moleculesare made of, and it exists in four states or phases: solid, liquid, gas and plasma (although other phases, such as Bose-Einstein condensates, also exist).


A collection of atoms glued together by electromagnetic forces. A more formal definition might be: a sufficiently stable electrically neutral group of at least two atoms, in a definite arrangement, held together by very strong chemical bonds. A molecule may consist of atoms of the same chemical element(e.g. oxygen: O2) or of different elements (e.g. water: H2O). Organic molecules are those which include carbon, and the others are called inorganic.

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