Has the speed of light in pure vacuum ever been measured?
edit:
If so when and where?
It is difficult (actually impossible) to create a pure vacuum on Earth, so it has not been done directly. This is not the way in which the accepted value for the speed of light is calculated, though. We know from Maxwell’s equations in electromagnetics that the speed of light can be simply calculated by the permeability and permittivity constants in a vacuum.
The most common methods of measuring the speed of light are by using a Michelson interferometer or by astronomical calculations. The astronomical calculations are done by measuring the time radio signals (EM radiation = light) to reach distant spacecraft. The position of the space craft is calculated by the effects of gravity of the sun and other planets on it. These measurements are done over and over and over and we can then analyze the data to accurately calculate the speed of light in a vacuum with relative uncertainty of 0.02 parts per billion to be 299,792,458 m/s.
In fact, this is technically a perfectly exact value, not because we could ever measure any value to be “exact”, but because the current definition of a meter is defined by the distance light travels in a vacuum in 1/299,792,458 s.
Matt Redman – Pure Light
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