This question already has an answer here:
From Wikipedia - "Its exact value is 299792458 metres per second"
I'm wondering how we can be so precise as to the speed of light given that we cannot create a true vacuum on Earth and as I understand it the interstellar vacuum is a lot more active than we used to think with particles and anti-particles appearing and annihilating each other constantly. Is there a caveat here that should say "Exact value as measured at vacuum density x"?
Apologies if this is a duplicate, I did find several questions regarding the speed of light, the type of vacuum and whether it's a constant but couldn't find one that dealt with the precision of the measurement.