User:Brews ohare/Speed of light

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Speed of light set by definition[edit]

In 1983 the 17th Conférence Générale des Poids et Mesures defined the metre to be the length of the path travelled by light in vacuum during a time interval of 1 ⁄ 299 792 458 of a second. The reasons for using this definition are stated in Resolution 1.[1]

The term 'vacuum' is used in this definition and in this article to refer to the reference state of free space. Like absolute zero, it is an idealized state that only can be approximated in the physical world. Measurements in any real-world medium, such as air[2][3] or a medium perturbed by gravity must be corrected so as to relate to vacuum.

The effect of this definition is to fix the speed of light in vacuum at exactly c = 299 792 458 m/s;[4] thus the speed of light is now "a defined constant,[5] not to be measured again..." although "...length measurement can be continuously improved without resorting to a new definition."[6]

It is important to separate the definition of a physical quantity itself from the definition of the unit in which it is measured.[7] Measurement is basically comparison between the quantity being measured and the standard unit; it is a matter of ratios.[8] Setting the speed of light to a definite numerical value when measured in the SI units of m/s simply means comparisons of length become comparisons of transit times of light.[9][10] In particular, comparison of a length with the metre is a comparison of its transit time with 1/299,792,458 s.[11]

Because the second is defined in terms of atomic transitions which can be measured with a fair degree of accuracy, the new definition allows for a more precise definition of the metre, assuming an adequate realization of "vacuum" is available.[9] Suggested atomic transitions for determining the second, with their associated uncertainties, are tabulated.[12]

Rather than measure a time-of-flight, one implementation of this definition is to use a recommended source with established frequency (f ), and delineate the metre in terms of the wavelength (λ) of this light as determined using the defined numerical value of c and the relationship λ = c / f.[13] Practical realizations of the metre use recommended wavelengths of visible light in a laboratory vacuum with corrections being applied to take account of actual conditions such as diffraction, gravitation or imperfection in the vacuum.


In-line references and notes[edit]

  1. ^ Resolution 1
  2. ^ BG Zagar (1999). "Laser Interferometer Displacement Sensors; §6.5". In JG Webster (ed.). The Measurement, Instrumentation, and Sensors Handbook. CRC Press. p. 6-69. ISBN 0849383471.
  3. ^ J Flügge; et al. (2004). "Fundamental length metrology: Practical issues; §D.2.1.3". In CE Webb & JDC Jones (ed.). Handbook of Laser Technology and Applications. Taylor & Francis. pp. 1737 ff. ISBN 0750309660. {{cite book}}: Explicit use of et al. in: |author= (help)
  4. ^ "Definition of the meter".
  5. ^ James Jespersen, Jane Fitz-Randolph and John Robb (1999). From Sundials to Atomic Clocks: Understanding time and frequency (Reprint of National Bureau of Standards 1977, 2nd ed.). Courier Dover. p. 280. ISBN 0486409139.
  6. ^ "NIST document" (PDF). p. 191.
  7. ^ David A Cardwell, David S Ginley (2003). Characterization, Applications and Cryogenics. CRC Press. p. 1251. ISBN 0750308974.
  8. ^ Lee Smolin (2007). The Trouble with Physics: The Rise of String Theory, the Fall of a Science, and What Comes Next. Houghton Mifflin Harcourt. p. 215. ISBN 061891868X.
  9. ^ a b PH Sydenham (2003). "Measurement of length". In Walt Boyes (ed.). Instrumentation reference book (3rd ed.). Butterworth-Heinemann. p. 56. ISBN 0750671238. … if the speed of light is defined as a fixed number then, in principle, the time standard will serve as the length standard …
  10. ^ "Fundamental Physical Constants: Speed of light in vacuum; c, c0". physics.nist.gov.
  11. ^ Peter Woit (2006). Not even wrong: the failure of string theory and the search for unity in physical law. Basic Books. p. 6. ISBN 0465092756. Setting the speed of light equal to one determines the choice of units used to measure time in terms of the units to measure space, and vice versa.
  12. ^ The table is p. 249; CI-2006, Recommendation 1 of CIPM of 2006.
  13. ^ A list of the resulting wavelengths based upon these frequencies and λ = c/f is found at BIPM mise-en-pratique, method b.

Sources[edit]

NIST timeline fundamental paper setting up standard historical resolutions of CIPM relativity experiments

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