Half-life is the period of time it takes for a substance undergoing decay to decrease by half. The name was originally used to describe a characteristic of unstable atoms (radioactive decay), but it may apply to any quantity which follows a set-rate decay.

The original term, dating to 1907, was "half-life period", which was later shortened to "half-life" in the early 1950s.

Half-lives are very often used to describe quantities undergoing exponential decay—for example, radioactive decay—where the half-life is constant over the whole life of the decay, and is a characteristic unit (a natural unit of scale) for the exponential decay equation. However, a half-life can also be defined for non-exponential decay processes, although in these cases the half-life varies throughout the decay process. For a general introduction and description of exponential decay, see the article exponential decay. For a general introduction and description of non-exponential decay, see the article rate law. Corresponding to sediments in environmental processes, if the half-life is greater than the residence time, then the radioactive nuclide will have enough time to significantly alter the concentration. The converse of half-life is doubling time.

The table on the right shows the reduction of a quantity in terms of the number of half-lives elapsed.

Etymology

half- +‎ life

Noun Half-life

Wikipedia half-life (plural half-lives)

1. (physics) The time required for half of the nuclei in a sample of a specific isotope to undergo radioactive decay.
2. (chemistry) In a chemical reaction, the time required for the concentration of a reactant to fall from a chosen value to half that value.
3. (medicine) The time it takes for a substance (drug, radioactive nuclide, or other) to lose half of its pharmacological, physiologic, or radiological activity.
4. The time it takes for an idea or a fashion to lose half of its influential power.
   • 1991, Robert Ackerman, Introduction to Jane Ellen Harrison's Prolegomena to the Study of Greek Religion (1903)

     Most books of scholarship have surprisingly short intellectual 'half-lives during which they make a difference"

Half-Life is a science fiction first-person shooter video game series which consists of two full games, two half-length "episodes", one side-story and three expansion packs; the first game, Half-Life, was released in 1998. The protagonist of the series is Gordon Freeman, a theoretical physicist whose actions at the Black Mesa Research Facility inadvertently create a post-apocalyptic world. Expansions of the game Half-Life feature other controllable characters.

Contents

• 1 Half-Life (1998)
• 2 Half-Life: Opposing Force (1999)
• 3 Half-Life: Blue Shift (2001)
• 4 Half-Life 2 (2004)
• 5 Half-Life 2: Episode One (2006)
• 6 Half-Life 2: Episode Two (2007)
• 7 See also

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