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### How do we know the precise speed of light in a vacuum? [duplicate]

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 ...
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### Do we have a better approximation of $c$ than 299792458 m/s? [duplicate]

All sites give this value as "exact" value. I mean, what's after the comma? 299792458,000 m/s?
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### What is so special about speed of light in vacuum?

I will try to be as explanatory as possible with my question. Please also note that I have done my share of googling and I am looking for simple language preferable with some example so that I can get ...
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### No uncertainty for standard gravitational acceleration?

The other day I asked about the uncertainty of light, and this issue triggered me to start looking into other physical constants and try to understand why other constants have no uncertainty. One of ...
1 vote
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### How confident can we be that the speed of light in a medium is constant?

I have recently found this article http://news.harvard.edu/gazette/1999/02.18/light.html it tells that physicists have been able to slow the speed of light. Is this hokum? If not how is it possible to ...
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### Can a measurement be 100% accurate? [closed]

I have never heard about any experiment performed which is 100% accurate (Sorry,if there is any).By the term accurate I mean measurements exactly equal to the theoretical value .Is it every time that ...
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### How is the Length of a Meter Physically Measured?

I have two parts to this question. First, I understand that the meter is defined as the distance light travels in 1/299,792,458 seconds. But how is this distance actually measured? The second is ...
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### Is the definition of the meter arbitrary? [duplicate]

From Wikipedia, the definition of the meter is The meter is defined as the distance traveled by light in a vacuum in 1/299792458 seconds. Why is this number of seconds chosen? Is there a ...
1 vote
### Is it experimentally proven that photons travel at speed $c$ in vacuum?
Are there experiments which show that single photons (not classical em waves) travel exactly at $c$ in vacuum? What is the error bar in that case? The question is posed due to the fact that loop ...