The Fizeau experiment measured this quantity in moving water. Though it's not the age of the universe, this quantity's finitude can resolve Olbers's paradox. It's equal to the reciprocal of the square root of the product of permittivity and permeability of free space. Cerenkov radiation occurs when particles exceed the local value of this in a transparent medium, emitting a largely ultraviolet bluish glow. The interferometer used by Michelson and Morley failed to find any "ether" that would cause this quantity to vary. For 10 points, name this 
The Fizeau experiment measured this quantity in moving water. Though it's not the age of the universe, this quantity's finitude can resolve Olbers's paradox. It's equal to the reciprocal of the square root of the product of permittivity and permeability of free space. Cerenkov radiation occurs when particles exceed the local value of this in a transparent medium, emitting a largely ultraviolet bluish glow. The interferometer used by Michelson and Morley failed to find any "ether" that would cause this quantity to vary. For 10 points, name this constant symbolized in E equals m c squared by c, which is the fastest possible speed.
The Fizeau experiment measured this quantity in moving water.
speed of light [accept c]