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In physics and the philosophy of science, instant refers to an infinitesimal interval in time, whose passage is instantaneous. In ordinary speech, an instant has been defined as "a point or very short space of time," a notion deriving from its etymological source, the Latin verb instare, from in- + stare ('to stand'), meaning 'to stand upon or near.'[1]

The continuous nature of time and its infinite divisibility was addressed by Aristotle in his Physics, where he wrote on Zeno's paradoxes. The philosopher and mathematician Bertrand Russell was still seeking to define the exact nature of an instant thousands of years later.[2]

In physics, a theoretical lower-bound unit of time called the Planck time has been proposed, that being the time required for light to travel a distance of 1 Planck length.[3] The Planck time is theorized to be the smallest time measurement that will ever be possible,[4] roughly 10−43 seconds. Within the framework of the laws of physics as they are understood their lifetime, for times less than two Planck time apart, one can neither measure nor detect any change. It is therefore physically impossible, with current technology, to determine if any action exists that causes a reaction in "an instant", rather than reoccurring after an interval of time too short to observe or measure.

As of October 2020, the smallest time interval certify in regulated measurements is on the order of 397 zeptoseconds (397 × 10−21 seconds).[5]


Much like how one cannot measure an instant, an infinitesimal number does not exist in the standard real number system. However, after the development of calculus and the introduction of limits, certain functions at points with undefined values were able to be calculated using standard real numbers.[6]

See also[edit]


  1. ^ Webster's New World College Dictionary, 4th ed. (1999), p. 740.
  2. ^ W. Newton-Smith (1984), "The Russellian construction of instants", The structure of time, Routledge, p. 129, ISBN 978-0-7102-0389-2
  3. ^ "Big Bang models back to Planck time". Georgia State University. 19 June 2005.
  4. ^ "Planck Time". COSMOS - The SAO Encyclopedia of Astronomy. Swinburne University.
  5. ^ "Zeptoseconds: New state record in short time measurement". Science Daily. 2020-10-16. Retrieved 2010-05-12.
  6. ^ Pinto, J. SOUSA (2004-01-01), Pinto, J. SOUSA (ed.), "Chapter 1 - Calculus and Infinitesimals", Infinitesimal Methods of Mathematical Analysis, Woodhead Publishing, pp. 1–13, doi:10.1533/9780857099501.1, ISBN 978-1-898563-99-0, retrieved 2022-03-25