# C = 299 792 458 m/s and uncertainty

1. ### DonnieD

9
c is 299 792 458 m/s without uncertainty? doesn't work the Heisenberg uncertainty principle for fotons?

2. ### AlphaNumeric

290
The distances typically involved in measuring c are sufficently big that quantum fluctuations are ignored.

Also, the length of 1m is defined by the distance light travels in a certain amount of time. Because of this definition, we're effectively defining light's speed, not measuring it, so it is exact.

3. ### Born2Perform

80
we define 1m, but we must still measure in how much time light crosses 1m; we measured c but it can be less or more, right?