# Speed of Light in a Vacuum?
**URL:** https://boards.straightdope.com/t/speed-of-light-in-a-vacuum/185399
**Category:** Factual Questions
**Created:** [June 30, 2003, 6:44pm UTC](https://boards.straightdope.com/t/speed-of-light-in-a-vacuum/185399 "2003-06-30T18:44:04Z")
**Posts on this page:** 7
**Page:** 1
### Author:  [@Major\_Kong](https://boards.straightdope.com/u/Major_Kong)
#### Post date: [June 30, 2003, 6:44pm UTC](https://boards.straightdope.com/t/speed-of-light-in-a-vacuum/185399/1 "2003-06-30T18:44:04Z")
How do we know the speed of light in a vacuum? How can we observe light in a vacuum?
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### Author:  [@friedo](https://boards.straightdope.com/u/friedo)
#### Post date: [June 30, 2003, 6:49pm UTC](https://boards.straightdope.com/t/speed-of-light-in-a-vacuum/185399/2 "2003-06-30T18:49:44Z")
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:
[http://www.colorado.edu/physics/2000/waves\_particles/lightspeed\_evidence.html](http://www.colorado.edu/physics/2000/waves_particles/lightspeed_evidence.html)
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### Author:  [@Major\_Kong](https://boards.straightdope.com/u/Major_Kong)
#### Post date: [June 30, 2003, 6:57pm UTC](https://boards.straightdope.com/t/speed-of-light-in-a-vacuum/185399/3 "2003-06-30T18:57:35Z")
Thanks, but that page is describing light’s speed through space (not a complete vacuum).
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### Author:  [@ElJeffe](https://boards.straightdope.com/u/ElJeffe)
#### Post date: [June 30, 2003, 7:39pm UTC](https://boards.straightdope.com/t/speed-of-light-in-a-vacuum/185399/4 "2003-06-30T19:39:13Z")
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
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### Author:  [@Squink](https://boards.straightdope.com/u/Squink)
#### Post date: [June 30, 2003, 7:58pm UTC](https://boards.straightdope.com/t/speed-of-light-in-a-vacuum/185399/5 "2003-06-30T19:58:34Z")
> [@](#):
>
> For the case of propagation between closely spaced conducting boundaries as in discussions of the Casimir effect, we have an increase in the velocity of light which is associated with the reduction of vacuum fluctuation energy between the plates.
[http://www.nidsci.org/articles/seti.html](http://www.nidsci.org/articles/seti.html)
K. Scharnhorst, Phys. Lett. B 236, 354 (1990)
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### Author:  [@bonzer](https://boards.straightdope.com/u/bonzer)
#### Post date: [June 30, 2003, 11:28pm UTC](https://boards.straightdope.com/t/speed-of-light-in-a-vacuum/185399/6 "2003-06-30T23:28:42Z")
The Scharnhorst effect is interesting in a technically esoteric way, but it’s [a bit less exciting than it may appear](http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/FTL.html#12). For a start it’s an _extremely_ small effect at best. Even [Scharnhorst](http://arxiv.org/PS_cache/hep-th/pdf/9810/9810221.pdf) has concluded of his calculations:
> [@](#):
>
> It should finally be emphasized that this conclusion does not involve any serious conceptual dangers and does not, in particular, contradict the special theory of relativity.
(And, IMHO, Hal Puthoff is nuts …)
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### Author:  [@refusal](https://boards.straightdope.com/u/refusal)
#### Post date: [June 30, 2003, 11:43pm UTC](https://boards.straightdope.com/t/speed-of-light-in-a-vacuum/185399/7 "2003-06-30T23:43:51Z")
> [@](#):
>
> \*Originally posted by Major Kong \*
> \*\*Thanks, but that page is describing light’s speed through space (not a complete vacuum). \*\*
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](http://www.mathphysics.com/pde/Maxwell.html) formulated by James Clerk-Maxwell.