In 1983 the General Conference on Weights and Measures officially redefined the meter as the distance that light travels in vacuum in 1/299,792,458 of a second. The second is defined in terms of a certain atomic frequency in a way that does not concern us here. The nine-digit integer was chosen to make the assigned value of c agree with the most accurate measured value to well within the uncertainty in the latter. Henceforth the velocity of light is, by definition, 299,792,458 meters/sec. An experiment in which the passage of a light pulse from point A to point B is timed is to be regarded as a measurement of the distance from A to B, not a measurement of the speed of light.

While this step has no immediate practical consequences, it does bring a welcome simplification of the exact relations connecting various electromagnetic units. As we learn in Chapter 9, Maxwell's equations for the vacuum fields, formulated in SI units, have a solution in the form of a traveling wave with velocity c = (μ0ε0)-1/2;. The SI constant μ0 has always been defined exactly as 4π × 10-7, whereas the value of ε0 has depended on the experimentally determined value of the speed of light, any refinement of which called for adjustment of the value of ε0.