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**Physics, Astronomy & Cosmology / Re: How does light move in a vacuum?**

« **on:**02/12/2020 20:39:48 »

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.

**Historical Background**

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/√(ε

_{0}μ

_{0})

- This was before Einstein's relativity

**An Analogy**

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 = ε

_{0}ε

_{r})

- 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.

See: https://en.wikipedia.org/wiki/International_System_of_Units#Evolution_of_the_SI

**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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