Energy: 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. Entropy: A measure of the disorder of a system and of its constituent molecules. 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.
quantum field theory
Any matter or light that falls through the event horizon of a black hole can never leave, and any event inside the event horizon cannot affect an outside observer. Exogenesis: 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 with electromagnetism or the strong nuclear force , axions particles with no electric charge , very small mass 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 matter almost undisturbed.
It also suggests that, in the distant past, the universe was much smaller and ultimately had its beginning in a Big 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 energy conditions 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.
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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. Galaxies may be anywhere from 1 to , 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 rays have the highest frequency and energy 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.
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The initial burst is usually followed by a longer-lived 'afterglow' emitted at longer wavelengths X-ray, ultraviolet, optical, infrared and radio. Gas: A state of matter consisting of a collection of particles molecules , atoms , ions , electrons , etc without a definite shape or volume, and that are in more or less random motion. 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 universe , time dilation , length 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. 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 electromagnetic , weak nuclear , and strong nuclear forces are all fused into a single unified field.
It is by far the weakest of the four fundamental 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 with mass , it is instrumental in the formation of galaxies , stars , planets and black holes. Half-Life: A measure of the speed of radioactive decay of unstable, radioactive atoms.
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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. Over long periods of time, as a black hole loses more matter through radiation than it gains through other means, it is therefore expected to dissipate, shrink and ultimately vanish. Horizon: Click for a picture The horizon is the edge of the observable universe from a particular position Source: U.
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 centered on the Earth. 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.
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. 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. Any frame of reference that moves with constant velocity relative to an inertial system is itself an inertial system. This results, for example, in an interference pattern of light and dark stripes on a screen illuminated by light from two sources.
Ion: 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 negative electric charge is also a type of ion, known as an anion. Isotope: A possible form of an element , distinguishable from other isotopes of the same element by its differing mass , 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.
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. The total amount of energy in an isolated system or in the universe as a whole therefore remains constant. Newton observed that the force of gravity between two objects is proportional to the product of the two masses , and inversely proportional to the square of the distance between them.
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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. Life: 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. 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. Light: 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.
Light travels at a constant speed of about , kilometers 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 , kilometers per second, works out to about 9,,,, kilometers 9. 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.
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. Mass: A measure of the amount of matter in a body.
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, conversely, any energy has an associated mass.
Given that c is a very large number, it becomes apparent that mass is in fact a very concentrated form of energy. Matter: Anything that has both mass and volume i.
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