Percentage of the speed of light | Wikipedia audio article
Channel: wikipedia tts
Date: 06/06/2019

This is an audio version of the Wikipedia Article:
https://en.wikipedia.org/wiki/Speed_of_light


00:01:23 1 Numerical value, notation, and units
00:04:57 2 Fundamental role in physics
00:07:19 2.1 Upper limit on speeds
00:07:30 3 Faster-than-light observations and experiments
00:09:09 4 Propagation of light
00:10:02 4.1 In a medium
00:10:23 5 Practical effects of finiteness
00:12:36 5.1 Small scales
00:14:30 5.2 Large distances on Earth
00:18:17 5.3 Spaceflights and astronomy
00:21:03 5.4 Distance measurement
00:21:16 5.5 High-frequency trading
00:26:20 6 Measurement
00:26:49 6.1 Astronomical measurements
00:27:34 6.1.1 Astronomical unit
00:28:28 6.2 Time of flight techniques
00:31:08 6.3 Electromagnetic constants
00:32:13 6.4 Cavity resonance
00:32:53 6.5 Interferometry
00:34:16 7 History
00:37:06 7.1 Early history
00:38:49 7.2 First measurement attempts
00:39:44 7.3 Connections with electromagnetism
00:41:32 7.4 "Luminiferous aether"
00:41:59 7.5 Special relativity
00:42:32 7.6 Increased accuracy of ic/i and redefinition of the metre and second
00:42:41 7.7 Defining the speed of light as an explicit constant
00:44:49 8 See also
00:46:45 9 Notes
00:47:32 10 References
00:50:30 11 Further reading
00:53:06 11.1 Historical references
00:54:34 11.2 Modern references
00:56:32 12 External links
00:57:18 Increased accuracy of c and redefinition of the metre and second
00:58:41 299792456.2±1.1 m/s. This was 100 times less uncertain than the previously accepted value. The remaining uncertainty was mainly related to the definition of the metre. As similar experiments found comparable results for c, the 15th General Conference on Weights and Measures in 1975 recommended using the value 299792458 m/s for the speed of light.
00:59:23 Defining the speed of light as an explicit constant
00:59:34 In 1983 the 17th CGPM found that wavelengths from frequency measurements and a given value for the speed of light are more reproducible than the previous standard. They kept the 1967 definition of second, so the caesium hyperfine frequency would now determine both the second and the metre. To do this, they redefined the metre as: "The metre is the length of the path travelled by light in vacuum during a time interval of 1/299792458 of a second." As a result of this definition, the value of the speed of light in vacuum is exactly 299792458 m/s and has become a defined constant in the SI system of units. Improved experimental techniques that prior to 1983 would have measured the speed of light, no longer affect the known value of the speed of light in SI units, but instead allow a more precise realization of the metre by more accurately measuring the wavelength of Krypton-86 and other light sources.In 2011, the CGPM stated its intention to redefine all seven SI base units using what it calls "the explicit-constant formulation", where each "unit is defined indirectly by specifying explicitly an exact value for a well-recognized fundamental constant", as was done for the speed of light. It proposed a new, but completely equivalent, wording of the metre's definition: "The metre, symbol m, is the unit of length; its magnitude is set by fixing the numerical value of the speed of light in vacuum to be equal to exactly 299792458 when it is expressed in the SI unit m s−1." This is one of the proposed changes to be incorporated in the next revision of the SI also termed the New SI.
01:01:48 See also



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SUMMARY
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The speed of light in vacuum, commonly denoted c, is a universal physical constant important in many areas of physics. Its exact value is 299,792,458 metres per second (approximately 300,000 km/s (186,000 mi/s)). It is exact because by international agreement a metre is defined as the length of the path travelled by light in vacuum during a time interval of 1/299792458 second. According to special relat ...