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As mentioned before, it's exactly that value as it is defined as such in SI. In reality it's not exactly that value.
"After centuries of increasingly precise measurements, in 1975 the speed of light was known to be 299,792,458 m/s (186,282 mi /s) with a measurement uncertainty of 4 parts per billion. In 1983, the metre was redefined in the International System of Units (SI) as the distance travelled by light in vacuum in 1/299792458 of a second. As a result, the numerical value of c in metres per second is now fixed exactly by the definition of the metre.[6]"
In terms of order of magnitude, we knew the value to 1 part in one hundred million (since the uncertainty was on the order of 1 part in per billion) and truncated the rest for convenience of the SI definition.
This is not unlike pretty much any other physical constant, in that we only know them to a certain accepted level of uncertainty (e.g., Avogadro's Number, Universal Gravitational Constant, and Planck's constant).
The way this is handled in practice would be to use the accepted values as a best estimate and then perform the appropriate error propagation calculations to then place an uncertainty / error value on the quantity calculated.
"After centuries of increasingly precise measurements, in 1975 the speed of light was known to be 299,792,458 m/s (186,282 mi /s) with a measurement uncertainty of 4 parts per billion. In 1983, the metre was redefined in the International System of Units (SI) as the distance travelled by light in vacuum in 1/299792458 of a second. As a result, the numerical value of c in metres per second is now fixed exactly by the definition of the metre.[6]"
In terms of order of magnitude, we knew the value to 1 part in one hundred million (since the uncertainty was on the order of 1 part in per billion) and truncated the rest for convenience of the SI definition.
This is not unlike pretty much any other physical constant, in that we only know them to a certain accepted level of uncertainty (e.g., Avogadro's Number, Universal Gravitational Constant, and Planck's constant).
The way this is handled in practice would be to use the accepted values as a best estimate and then perform the appropriate error propagation calculations to then place an uncertainty / error value on the quantity calculated.
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