# Speed of Light in a Vacuum?

How do we know the speed of light in a vacuum? How can we observe light in a vacuum?

We have been able to measure the speed of light using some fairly simple math for a couple hundred years now. Here is a good page which summarizes the history of measuring the speed of light:

Thanks, but that page is describing light’s speed through space (not a complete vacuum).

My educated guess would be that scientists measured c in as close to a vacuum as they could get, then extrapolated to find the “real” c. It was found to be approximately 299,792,458 m/s. Since then, c has been defined to be exactly 299,792,458 m/s, and the meter redefined to be the distance traveled by light in 1/299,792,458th of a second. So we don’t have to worry about whether or not our measurement of the speed of light in a vacuum is correct - it is, by definition.
Jeff

http://www.nidsci.org/articles/seti.html
K. Scharnhorst, Phys. Lett. B 236, 354 (1990)

The Scharnhorst effect is interesting in a technically esoteric way, but it’s a bit less exciting than it may appear. For a start it’s an extremely small effect at best. Even Scharnhorst has concluded of his calculations:

(And, IMHO, Hal Puthoff is nuts …)

Well, it’s impossible to create a true vacuum in the laboratory or for a vacuum to exist anywhere in the universe. But you can calculate the speed of light based on the theory of electromagnetism formulated by James Clerk-Maxwell.