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## Friday, September 25, 2015

### Today’s Speed of Light Is 1.5180 Dollars per Pound

An article on pop physics, reportedly written by a retired Emory physics professor, is not entirely without merit or value, but it hangs itself of, and by, its subtitle question:

Light travels at around 300,000 km per second. Why not faster? Why not slower? A new theory inches us closer to an answer

To an interested layman this may seem a valid or even intriguing question. It is not.

For to the best knowledge of accepted physics, theoretical and experimental, the speed of light in vacuum $$c$$ everywhere, at any time, and to infinite precision, has only ever had one value:

$${\Huge 1}$$

The article’s kilometer-second figure of $$c=299792.458\frac{km}{s}$$ escapes falsehood merely by the happy coincidence that a second just happens to be exactly 299792.458 kilometers long.By happy coincidence, the author here means deliberate choice of definition made for that very reason. Plug in that definition and the equation really only says $$c=1$$. So the kilometer-second figure is merely needlessly verbose, not wrong. But then the dollar-pound figure for the speed of light in the title of this post is just as true and for just the same reason.

One could certainly change the definitions of kilometer and second. If one did, one would have also have to change the kilometer-second figure for the speed of light. But that would not imply that the speed of light actually changed; any more than the necessity of adjusting the dollar-pound figure for the speed of light when the markets reopen would imply that the speed of light actually changed.

The fact that a second has a length measured in kilometers, and vice versa, may be surprising to many. Yet, it undisputedly is so for physics draws (almost) no distinction between time and space. What to call the one and what the other, is purely a question of convenience and habit and will differ from observer to observer, in much the same way that what is left,, right, up, or down will be different for different people at different times and places.