is speed of light unmeasurable?

Question

 A recent Veritasium video suggested that the speed of light is unmeasurable 

all known experiments that measure light speed done by shining a light beam at a mirror with a known distance and having the beam bounce back to its origin to measure the time it took to complete the round-trip

These types of experiments introduce the possibility that light could travel at a different speed on its away trip than it would on its return trip, which confines the accuracy of those experiments to only measuring the average of the away and return speeds, and not the absolute speed of light in any given direction.

Answer 1

You have to bear in mind that despite his qualifications and stance as a serious YouTuber, he is prone to clickbait titles and severe errors. The actual issue with measuring may well be a real thing (although I'm not convinced) but it completely ignores the fact that the speed of light is classically determined by the wave solution to Maxwell's Equations, where the velocity is dependent only on the permeability and permittivity of free space. And no one is suggesting the permeability and permittivity of free space are directional. In which case measuring over two directions is perfectly adequate to confirm the velocity.

c = 1/√μ₀ε₀

https://en.wikipedia.org/wiki/Electromagnetic_wave...

Answer 2

NO

There is no indication whatsoever that light slows down within any medium

Answer 3

He might want to run this idea past Maxwell...Aw crap. Max died.

Oh well I guess we can just muddle along...

Answer 4

186,000 miles per second. Thanks to Bill Nye.

Answer 5

there are many ways the speed of light has been measured, and they all agree within the measurement accuracy.  Reference has a list...

excerpt:

Historically, the most accurate results have been obtained by separately determining the frequency and wavelength of a light beam, with their product equaling c.

Source(s): https://en.wikipedia.org/wiki/Speed_of_light#Measu...