On 8/11/2011 12:03 AM, Stephen P. King wrote:
On 8/11/2011 2:54 AM, meekerdb wrote:
On 8/10/2011 11:24 PM, Stephen P. King wrote:
And interesting choice of examples since that exactly what man has done. The speed of light is nothing but a conversion constant between units. In 1983 the speed of light in SI units was *defined* to be 299,792,458 m/s.

Umm, not so fast. The permitivity and permiability of the vacuum are not set by prior theory! It is still only a known fact due to measurement.

No. The length of the meter was set as the distance traveled by light in 1/299792458 of a second. So permitivity and permiabilty are not fundamental but are derived values (for the vacuum).


Hi Brent,

Seriously? The speed of light in a vacuum is proportional to the ration of permeability to permittivity of the vacuum (see Maxwell's equations). Those quantities are not derived from any theory. So we measure the time it takes a beam of light to travel some distance in a vacuum.... how is the vacuum achieved?

You're still assuming it's measured. It's not measured; it's used to define the meter so it's an exact number.

What determines the accuracy of the clocks used?

Accuracy compared to what? The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom.


We can go around and around on this, for what? In your accounting what defined that the speed of light in SI units is not 299,792,458.000000000000000000000000001212423452345 m/s or 299,792,458.00000000000000000000000000000000000000000004567 m/s or ... ?



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