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Claim analyzed
Science“The metre is defined as the distance light travels in a vacuum in 1/299,792,458 of a second.”
Submitted by Silent Bear e22a
The conclusion
Open in workbench →The claim accurately describes the metre in operational terms. Current SI sources define the metre by fixing the speed of light in vacuum at exactly 299,792,458 m/s, which is exactly equivalent to saying one metre is the distance light travels in vacuum in 1/299,792,458 of a second. The only caveat is that modern formal wording states this through c and the SI second rather than the older direct sentence.
Caveats
- The current formal SI wording does not literally use this sentence; it defines the metre by fixing the numerical value of the speed of light in vacuum.
- The statement depends on the SI second, which is itself defined separately from the caesium-133 transition frequency.
- User-generated or social-media sources are unnecessary here and should not be relied on over BIPM, NIST, or other national metrology institutes.
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Sources
Sources used in the analysis
The Conférence Générale des Poids et Mesures (CGPM) decides: "1. The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second." It also notes that this definition was abrogated in 2018 by the 26th CGPM (Resolution 1), which became effective in 2019.
The metre, symbol m, is the SI unit of length. It is defined by taking the fixed numerical value of the speed of light in vacuum, c, to be 299 792 458 when expressed in the unit m s−1, where the second is defined in terms of the caesium frequency ΔνCs. Inverting this relation gives an exact expression for the metre in terms of the defining constants c and ΔνCs: 1 m = (c / 299 792 458) s. The effect of this definition is that one metre is the length of the path travelled by light in vacuum during a time interval with duration of 1/299 792 458 of a second.
2. Definition of the metre The metre, symbol m, is the SI unit of length. It is defined by taking the fixed numerical value of the speed of light in vacuum c to be 299 792 458 when expressed in the unit m s−1, where the second is defined in terms of the caesium frequency ΔνCs. This definition implies that ‘The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second’, as it was stated in the previous definition of the metre.
The metre, symbol m, is the SI unit of length. It is defined by taking the fixed numerical value of the speed of light in vacuum c to be 299 792 458 when expressed in the unit m s−1, where the second is defined in terms of the caesium frequency ΔνCs. This definition implies the exact relation c = 299 792 458 m s−1. Inverting this relation gives an exact expression for the metre in terms of the defining constants c and ΔνCs: the effect of this definition is that one metre is the length of the path travelled by light in vacuum during a time interval with duration of 1/299 792 458 of a second.
The metre, symbol m, is the SI unit of length. It is defined by taking the fixed numerical value of the speed of light in vacuum c to be 299 792 458 when expressed in the unit m s−1, where the second is defined in terms of the caesium frequency ΔνCs. Inverting this relation gives an exact expression for the meter in terms of the defining constants c and ΔνCs: 1 m = (c / 299 792 458) s. The effect of this definition is that one meter is the length of the path traveled by light in vacuum during a time interval with duration of 1/299 792 458 of a second.
The SI is the system of units in which … the speed of light in vacuum c is 299 792 458 m/s. The SI is defined by the SI Brochure, published by the BIPM. These defining constants imply exact relations between units; for the metre, the fixed numerical value of c in m/s combined with the definition of the second establishes the length unit via the distance light travels in vacuum in a given fraction of a second.
The meter (m) is defined by taking the fixed numerical value of the speed of light in vacuum c to be 299,792,458 when expressed in the unit m s−1, where the second is defined in terms of ∆νCs. The speed of light in vacuum is 299 792 458 meters per second. A meter is the distance light travels in a tiny fraction of a second.
Since 1983, the metre has been internationally defined as the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second. The metre is defined by taking the fixed numerical value of the speed of light in vacuum, c, to be 299 792 458 when expressed in the unit m s−1, where the second is defined in terms of the caesium frequency. The wording of the definition was updated in 2019, but the numerical value and physical basis remained the same.
The SI metre is defined by fixing the numerical value of the speed of light in vacuum c to be 299 792 458 when expressed in the unit m s−1, where the second is defined via the caesium frequency ΔνCs. This implies that the metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.
The meter is the SI unit of length. It is defined by taking the fixed numerical value of the speed of light in vacuum c to be 299 792 458 when expressed in the unit m s−1, where the second is defined in terms of the cesium frequency ΔνCs. Thus, the meter is the length of the path traveled by light in vacuum during a time interval of 1/299 792 458 of a second.
The document states: "The current definition in its most recent phrasing at the 26th CGPM Meeting in 2018, resolution 1 is: 'The metre, symbol m, is the SI unit of length. It is defined by taking the fixed numerical value of the speed of light in vacuum to be 299792458 when expressed in the unit m·s−1, where the second is defined in terms of the caesium frequency.'" It adds: "This is a rephrasing of a definition at the 17th CGPM Meeting (1983), resolution 1: 'The metre is the length of the path travelled by light in vacuum during a time interval of 1/299792458 of a second.'"
NIST explains that in 1983, "the meter was redefined by international agreement in 1983 as the length of the path traveled by light in a vacuum in 1/299,792,458 of a second." The same paragraph notes: "This definition also locked the speed of light at 299,792,458 meters per second in a vacuum."
Under "Official definition (1983, 17th CGPM)", LNE writes: "Consequently, the meter became directly contingent on the second at the 17th CGPM in 1983, where the following definition of the meter was adopted: Definition of the meter: The metre is the length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second." It adds that this definition "established the numerical value for the speed c of light in a vacuum as 299,792,458 meters per second."
This earlier resolution is cited in later discussions of the metre: "1. The metre is the length equal to 1 650 763,73 wavelengths in vacuum of the radiation corresponding to the transition between the levels 2p10 and 5d5 of the krypton 86 atom." The page notes: "This definition was abrogated in 1983 by the 17th CGPM (Resolution 1)." This shows that the 1983 light‑travel definition replaced the 1960 krypton‑86 wavelength definition.
At the 1983 Conference Generale des Poids et Mesures, the following SI definition of the metre was adopted: "The metre is the length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second." This defines the speed of light in vacuum to be exactly 299,792,458 m/s.
In the revised SI, the meter is realized by fixing the speed of light in vacuum to exactly 299,792,458 meters per second. Operationally, this means the meter is defined as the distance light travels in vacuum in 1/299,792,458 of a second, with the second itself defined by the caesium-133 hyperfine transition.
Metre, also spelled meter, in measurement, is the fundamental unit of length in the metric system. Since 1983, it has been defined in terms of the speed of light: the metre is the distance traveled by light in a vacuum in 1/299,792,458 of a second. The definition ties the unit of length to a fundamental physical constant.
The SI base unit of length, the metre, is defined by specifying the value of the speed of light in vacuum c to be exactly 299 792 458 when expressed in the unit m s−1. This definition is equivalent to stating that the metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.
The metre (or meter in US spelling; symbol: m) is the base unit of length in the International System of Units (SI). Since 2019, the metre has been defined as the length of the path travelled by light in vacuum during a time interval of 1/299792458 of a second, where the second is defined by a hyperfine transition frequency of caesium. From 1983 until 2019, the metre was formally defined as the length of the path travelled by light in vacuum in 1/299792458 of a second. After the 2019 revision of the SI, this definition was rephrased to include the definition of a second in terms of the caesium frequency ΔνCs.
Today, the meter is no longer defined in terms of an inevitably imperfect physical object. Since 1983, the meter has been based on the distance that light travels in 1/299,792,458 of a second through empty space. But there’s a tiny issue: The 1983 definition doesn’t specify exactly what a “second” is. So in 2019, the official meter was refined to include the official definition of the second, which is equal to about 9 billion “ticks” of a cesium atom.
The SI Brochure is the definitive publication of the International System of Units (SI), including the formal definitions of the base units such as the metre. The current edition presents the definition of the metre via the fixed numerical value of the speed of light in vacuum, c = 299 792 458 m s−1, and the definition of the second. Appendices to the Brochure, such as Appendix 2 on the metre, explain that this definition implies that the metre corresponds to the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.
The metre (m) is the SI unit of length. It is defined by fixing the numerical value of the speed of light in vacuum c to 299 792 458 when expressed in m s−1. Consequently, the metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.
The article states that in 1983, "the 17th CGPM adopted a definition of the metre, in terms of the 1975 conventional value for the speed of light: 'The metre is the length of the path travelled by light in vacuum during a time interval of 1⁄299,792,458 of a second.'" It notes that "This definition was reworded in 2019" to the form: "It is defined by taking the fixed numerical value of the speed of light in vacuum c to be 299792458 when expressed in the unit m⋅s−1, where the second is defined in terms of the caesium frequency ΔνCs."
The paper describes: "In 1983, the CGPM-17 adopted a new definition of the basic unit of length (meter): 'The meter is the length of the travel of light in vacuum in (299792458)-1 seconds'." It explains that "This is to take the speed of light in vacuum as an accepted convention, that is =299792458 m/s" and that by specifying this value, "length units can be derived from time (frequency) units."
In its historical description, APS notes that precision measurements of the speed of light at NBS/NIST "were critical in enabling the 17th General Conference on Weights and Measures in 1983 to redefine the meter as the distance light travels in vacuum in 1/299,792,458 of a second." The text highlights that this redefinition "effectively fixes the speed of light at exactly 299,792,458 m/s."
The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second. The speed of light is a fundamental constant of the universe. It has a fixed value of 299792458 m/s. That means that in exactly 1 second, light will travel that many meters.
In 1983, the 17th General Conference on Weights and Measures (CGPM) adopted the definition: "The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second." This choice fixed the speed of light in vacuum at exactly 299 792 458 m/s and replaced the earlier definition based on krypton-86 radiation.
A meter is defined as the distance light would go in a vacuum at 1/299792458 of a second. The number 299,792,458 was chosen so that the 'new' meter would not be very much different from the old one based on a physical prototype. The second is equal to the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the hyperfine levels of the unperturbed ground state of the 133Cs atom.
A precise way of thinking about what a meter is is that we'd say that if you wait for 1 second, whatever distance light travels, by definition, it is 299,792,458 m. Which means you take that distance, divide it into so many parts, and then each part is exactly 1 m. That's the current definition of the meter: it is defined using the speed of light, so the speed of light is exact in the SI.
The metre is currently defined as the length of the path travelled by light in a vacuum in 1/299 792 458 of a second. This modern definition replaces older ones based on physical bars or wavelengths of light and relies on the constancy of the speed of light.
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The arguments
Two AI advocates debated this claim using the research gathered.
Argument for
The official definition of the metre established by the BIPM directly equates the unit of length to the exact distance light travels in a vacuum during a time interval of 1/299,792,458 of a second (Source 2, Source 3). Both the BIPM and NIST explicitly state that the current SI definition, which fixes the speed of light at 299,792,458 m/s, mathematically and operationally means the metre is this precise light-travel distance (Source 4, Source 5, Source 10).
The Proponent conflates an implied consequence of the current definition with the definition itself — Source 3 explicitly states that the light-travel phrasing merely 'implies' the 1983 wording, and Source 2 is unambiguous that the metre is formally defined by fixing the numerical value of c in conjunction with the caesium-frequency definition of the second, not by a simple light-travel distance statement. The claim omits the essential role of the caesium frequency ΔνCs, which Source 2 and Source 4 identify as a constitutive element of the current definition, rendering the claim's formulation a legally abrogated simplification rather than an accurate statement of the operative SI definition.
Argument against
The claim uses the phrasing of the outdated 1983 definition, which Source 1 (BIPM) explicitly states 'was abrogated in 2018 by the 26th CGPM (Resolution 1), which became effective in 2019.' The current, authoritative definition per Source 2 (BIPM SI Brochure, 9th edition, 2024) is that the metre is defined by fixing the numerical value of the speed of light in vacuum c to 299 792 458 m s⁻¹, with the second defined via the caesium frequency ΔνCs — making the claim's simple 'distance light travels' framing a legally superseded and technically incomplete description of the metre's definition.
The Opponent's argument relies on a false dichotomy between the 2019 rephrasing and the 1983 wording, ignoring that the BIPM explicitly states the current definition directly "implies" and has the exact "effect" of defining the metre as the distance light travels in vacuum in 1/299,792,458 of a second (Source 2, Source 3, Source 4). Furthermore, authoritative bodies like NIST and the National Physical Laboratory confirm that while the wording was updated to incorporate the caesium frequency, the physical basis and operational definition of the metre remain entirely unchanged (Source 5, Source 8, Source 10).
Expert review
3 specialized AI experts evaluated the evidence and arguments.
Expert 1 — The Logic Examiner
The logical chain from the evidence to the claim is sound, as multiple authoritative sources (Sources 2, 3, 4, and 5) explicitly state that the current SI definition of the metre directly implies and has the exact operational effect of defining it as the distance light travels in a vacuum in 1/299,792,458 of a second. While the formal wording was updated in 2019 to explicitly reference the caesium frequency, the physical basis and mathematical equivalence of the definition remain entirely unchanged, making the claim logically and factually true.
Expert 2 — The Source Auditor
High-authority primary metrology sources (BIPM: Sources 2–4 and NIST: Source 5 and 10) state that the metre is defined by fixing the numerical value of the speed of light in vacuum and explicitly note that the effect/implication of this definition is that one metre equals the length of the path travelled by light in vacuum during 1/299,792,458 of a second. Although the exact 1983 wording in Source 1 was formally abrogated in the 2019 SI rephrasing, the most reliable current sources still affirm the claim's statement as an exact equivalent consequence of the operative definition, so the claim is substantively true.
Expert 3 — The Precision Analyst
The claim states 'The metre is defined as the distance light travels in a vacuum in 1/299,792,458 of a second.' The number 1/299,792,458 is exactly correct per all authoritative sources. The substantive precision question is whether the claim's phrasing accurately captures the current definition. Sources 2, 3, 4, 5, 8, 10, and many others confirm that the current (post-2019) formal definition fixes the numerical value of c at 299,792,458 m/s with the second defined via caesium frequency, but all these same sources explicitly state this definition 'implies' or has the 'effect' that the metre is the distance light travels in vacuum in 1/299,792,458 of a second. Source 3 (BIPM Mise en pratique, 2024) states: 'This definition implies that The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second, as it was stated in the previous definition.' Source 8 (NPL) confirms 'the numerical value and physical basis remained the same.' The claim's phrasing is the operationally equivalent consequence of the current definition, universally acknowledged by BIPM, NIST, NPL, and others as an accurate description of what the metre means in practice. The only precision issue is that the claim omits the dependency on the caesium-frequency definition of the second, which is a constitutive element of the formal 2019 definition — but this omission does not make the claim false, as the light-travel distance formulation remains an accurate and BIPM-endorsed description of the metre's physical meaning. The claim is essentially true as worded, with only a minor incompleteness regarding the caesium-frequency anchor for the second.