| 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.
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.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.
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. 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).
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