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All sites give this value as "exact" value. I mean, what's after the comma? 299792458,000 m/s?

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    $\begingroup$ The speed of light is used to define the metre so it's not measured to be 299792458 m/s, it's that speed by definition. In other systems of units the speed of light is 1, exactly. $\endgroup$ – M. Enns Jun 20 '16 at 14:52
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    $\begingroup$ Possible duplicate: physics.stackexchange.com/q/92969/2451 $\endgroup$ – Qmechanic Jun 20 '16 at 15:03
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This is an exact value. Meter is defined by the speed of light.

Meter is a distance that the light travels during $\frac{1}{299792458}$ of a second.

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  • $\begingroup$ You've got 9 significant figures here. What more do you want? $\endgroup$ – user16622 Jun 20 '16 at 14:55
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The BIPM (Bureau International de Poids et Mesures) defines the meter as the distance traveled by light in $\frac{1}{299792458}$ seconds.

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 as $299792458$ m/s is therefore exact and not a measured value.

Similarily, the vaccuum permeability $\mu_0$ also has a defined value of $$\mu_0 = 4\pi \times 10^{-7} N/A^2$$

This is also used to calculate the vaccuum permittivity $$\varepsilon_0 = \frac{1}{\mu_0 c^2} = 8.854\ 187\ 817\ \cdots\times 10^{−12} F/m$$

Sources:

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