In a vacuum, light travels at a speed of exactly 299,792,458 metres per second, or about 300,000 kilometres per second, a speed which remains constant irrespective of the speed of the source of the light or of the observer (one of the cornerstones ofAlbert Einstein’s Special Theory of Relativity). It is the term c in Einstein’s famous equation E = mc2.
A fundamental property of sub-atomic elementary particles that means that behave as though they are spinning or rotating (although in reality they are not spinning at all). The concept has no direct analogue in the everyday world. Particles of spin Ѕ (e.g. electrons, positrons, neutrinos and quarks) make up all the matter in the universe, while particles with integer spin (0, 1 or 2) give rise to, or mediate, the forces operating between thematter particles (e.g. photons, gluons, W and Z bosons).
A massive, luminous ball of gas or plasma, held together by its own gravity, that replenishes the heat it loses to space by means of nuclear energy generated in its core. Almost all of theelements heavier than hydrogen and helium were created by thenuclear fusion processes in stars. There are many different types of stars including binary stars, proto-stars, dwarf stars (like our nearest star which we call the Sun), supergiants, supernovas, neutron stars, pulsars,quasars, etc. There are a roughly estimated 10,000 billion billion stars (1022) in the observableuniverse.
Steady State Universe:
A cosmological model developed by Fred Hoyle, Thomas Gold and Hermann Bondi in 1948 as the main alternative to the standard Big Bang theory of the universe. Steady state theory holds that the universe is expanding but that new matter and new galaxies are continuously created in order to maintain the perfect cosmological principle (the idea that, on the large scale, the universe is essentially homogenous and isotropic in both space and time), and therefore has no beginning and no end. The theory was quite popular in the 1950s and 1960s, but fell out of favour with the discovery of distant quasars and cosmic background radiation in the 1960s.
String: An object with a one-dimensional spatial extent, length (unlike an elementary particle which is zero-dimensional, or point-like). According to string theory, the different fundamental particles of the standard model can be considered to be just different manifestations of one basic object, a string, with different vibrational modes. The characteristic length scale of strings is thought to be on the order of the Planck Length (about 10-35 metres, still too small to be visible in current physical laboratories), the scale at which the effects of quantum gravity are believed to become significant.
Cosmic string is a similar but separate concept which refers to one-dimensional topological defects, extremely thin but immensely dense, which are hypothesized to have formed as a result of phase changes soon after the Big Bang (analogous to the imperfections that form between crystal grains in solidifying liquids or the cracks that form when water freezes into ice). According to some theories, such cosmic strings grew as the universe expanded and were instrumental in the accretion of matterand the formation of galaxy clusters and large-scale structures in the universe.
String Theory (Superstring Theory):
A theory which postulates that the fundamental ingredients of the universe are tiny strings of matter (on the tiny scale of thePlanck Length of around 10-35 metres) which vibrate in a space-time of ten dimensions. It is considered one of the most promising of the quantum gravity theories which hope to unite or unify quantum theory and the General Theory of Relativity, and apply to both large-scale structures and structures on the atomic scale.
Superstring theory (short for supersymmetric string theory) is a refinement of the more general theory of strings.
Strong Nuclear Force: Also known as the strong interaction, this is the powerful but short-range force that holds protons andneutrons together in the nucleus of an atom despite the electromagnetic repulsion of same-chargeparticles, as well as holding together the constituent quarks which comprise neutrons and protons. It is one of the four fundamental forces of physics (along with the gravitational force, the electromagnetic force and weak nuclear force), and the most powerful, being 100 times the strength of theelectromagnetic force, about 1013 times as great as that of the weak force and about 1038 times that of gravity.
The force is mediated by elementary particles called gluons which shuttle back and forth between the particles being operated on and "glue" the particles together. Unlike the other forces, the strength of the strong force between quarks becomes stronger with distance, acting like an unbreakable elastic thread. However, it only operates over a very small distance (less than the size of the nucleus), outside of which it fades away abruptly.
A cataclysmic explosion caused by the collapse of an old massive star which has used up all its fuel. For a short time, such an explosion may outshine an entire galaxy of a hundred billion ordinary stars. It leaves behind a cloud of brightly coloured gas called a nebula, and sometimes a highly compressed neutron star or even a black hole.
Superposition: The ability in quantum theory of an object, such as an atom or sub-atomic particle, to be in more than one quantum state at the same time. For example, an object could technically be in more than one place simultaneously as a consequence of the wave-like character of microscopic particles.
The phenomenon, predicted by Albert Einstein’s Special andGeneral Theories of Relativity, whereby, from the relative context of one observer's frame of reference, another’s time (for example, an identical clock) appear to run slower. Thus, moving clocks run more slowly compared to stationary clocks and, the closer the speed of movement approaches to the speed of light, the greater the effect. Gravitational time dilation is a related phenomenon, whereby time passes more slowly the higher the local distortion of space-time due togravity (such as near a black hole, for example).
The principle in quantum theory, formulated by Werner Heisenberg in 1926, which holds that the values of certain pairs of variables cannot BOTH be known exactly, so that the more precisely one variable is known, the less precisely the other can be known. For example, if the speed or momentum of a particle is known exactly, then its location must remain uncertain; if its location is known with certainty, then the particle’s speed or momentum cannot be known. Formulated another way, relating the unvertainties of energy and time, the uncertainty principle permits the existence of ultra-short-lived microscopic particles (virtual particles) in apparently empty space, which briefly blink into existence and blink out again.
Everything that physically exists, including the entirety of space and time, all forms of matter, energy and momentum, and the physical laws and constants that govern them. The universe (or cosmos) is usually considered to have begun about 13.7 billion years ago in a gravitational singulary commonly known as theBig Bang, and has been expanding ever since. Some have speculated that this universe is just one of many disconnected universes, which are collectively denoted as the multiverse.
Wave-Particle Duality: The idea that light (and indeed all matter and energy) is both a wave and a particle, and that sometimes it behaves like a wave and sometimes it behaves like a particle. It is a central concept ofquantum theory.
Weak Nuclear Force: Also known as the weak interaction, it is one of the forces experienced by protons and neutrons in thenucleus of an atom, the other being the strong nuclear force. It is one of the four fundamental forces of physics (along with the gravitational force, the electromagnetic force and the strong nuclear force). It is called the weak force because it is about 1013 times weaker than the strong nuclear force and 1011times weaker than the electromagnetic force, and it is also very short range in its effect.
The weak interaction is mediated by the exchange of heavy elementary particles known as W and Z bosons. It is responsible for radioactive beta decay (as it converts neutrons into protons) and for the production of neutrinos.
White Hole: The theoretical time reversal of a black hole, which arises as a valid solution in general relativity. While a black hole acts as a vacuum, drawing in any matter that crosses its event horizon, a white hole acts as a source that ejects matter from its event horizon.
A hypothetical “tunnel” through space-time that connects widely distant regions, thus providing a kind of short-cut throughspace-time. Although there is no observational evidence for wormholes, they are known to be valid solutions under theGeneral Theory of Relativity.
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