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AnthonyC comments on Feynman Paths - Less Wrong
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"How fast does the little arrow rotate? As fast as the photon's wavelength - that's what a photon's wavelength is. The wavelength of yellow light is ~570 nanometers: If yellow light travels an extra 570 nanometers, its little arrow will turn all the way around and end up back where it started."
Which would seem to make it a ruler as well as a clock. But then, since general relativity made time an axis like space, I have sometimes wondered why we don't measure time in meters or distance in seconds.
We do.
To expand on that point, we also measure energy in hertz, and temperatures in Joules, and ultimately everything in pure numbers. :)
The speed of light is used to define not only the lightyear, but also the common metre.