Quote from: RobC
The complexity of the mathematics overwhelmed me especially when I found permittivity being defined in terms of 'c'.You are correct - today it has become a circular argument.
- In in the past 100 years, all our basic measurements have been redefined based on the theory and observation that the speed of light is constant.
When Maxwell originally developed his equations, he combined several laws that had been developed by Gauss, Faraday and Ampere into a set of differential equations.
- One of the predictions of these equations is that if you disturb the electric or magnetic field, part of that energy will set off through space as a self-propagating wave.
- And the constants in his equations define the speed of this wave, which is c= 1/√(ε0μ0)
- This was before Einstein's relativity
This is not so different from the equations for vibration of a string, which you may have studied in senior high school.
- In this case, the velocity of the wave is v= √(T/μ)
- where T is the tension of the string, and μ is the mass per unit length of the string
- And we are familiar with musicians tuning a guitar by changing the tension, and also using thicker strings for lower notes
- Of course, a string is a medium, and a vacuum isn't - but if you substitute a medium....
- For the internet, we all make use of pulses of light traveling through optical fiber, which has a different value of ε than a vacuum, here light travels at about 2/3 of c. (ε = ε0εr)
- see: https://en.wikipedia.org/wiki/String_vibration
50 years after Einstein's Relativity
Einstein's assertion that c is the ultimate unchanging speed limit has been thoroughly tested in many different ways.
- And so, in 1960, the definition of length was changed from a certain platinum bar to a certain number of wavelengths of light.
100 years after Einstein's Relativity
Einstein's assertion that c is the ultimate unchanging speed limit has been tested in far more precision in far more scenarios.
- So now, the definition of length has been changed to the distance light travels in a certain amount of time.
- Time is now measured by the frequency of electromagnetic radiation
- Since speed = distance/time, and both distance and time are now defined in terms of light, c is a fixed and unchangeable value, by definition. (Hence the circular argument mentioned at the start of this post.)
- And finally, the kilogram has recently been changed from a certain lump of platinum to a relationship based on light and Plank's constant.
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