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Is there is any uncertainty in the measurement of the speed of light?

Arnav Badan
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    The very first paragraph of the Wikipedia article answers your question. – ACuriousMind Jun 20 '21 at 13:48
  • My dad was a buyer for Apollo components.

    He told me back in the 1960's (or very early 1970's), that the speed of light was 186,282.3960−mile/second with a tolerance of ±3.6−feet/second.

    Even then, the tolerance seemed like it should have had the same units as the average stated value.

    However, if the tolerance was mi/s, then the excessive digits in the main value were extraneous.The stated tolerance works out to about ±0.0007−mi/s.

    With the SI value of 299,792,458−m/s, the tolleranced value shown above corresponds closely as (299,792,456.31 +/- 1.13)−m/s (an error of 0.56-m/s).

     – Jim Clark Jul 04 '21 at 15:36
  • There are three different things. One, the defined value of the constant c. Second, the real speed of light , as close at is can measured. Three, the limit speed that arises from special relativity and is assumed, for several reasons, to be equal to the speed of traveling photons. – Gyro Gearloose Nov 07 '23 at 19:02

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See page 131 of the SI brochure:

The metre, symbol $\mathrm m$, 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 $\mathrm m\mathrm s^{−1}$, where the second is defined in terms of the caesium frequency $\Delta\nu_{Cs}$.

This definition implies the exact relation $c = 299\, 792\, 458\mathrm m\mathrm s^{−1}$.

(Emphasis mine)

jng224
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Dale
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