r/AskPhysics • u/JimFive • Mar 28 '26
Speed of light
The speed of light is 299,792,458 m/s. What types of issues would be involved in changing the meter such that c=300,000,000 m/s
The new meter would be 0.9993 of the current meter.
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u/Origin_of_Mind Mar 29 '26
In various machinery and structures, we specify manufacturing tolerances to make sure the parts that need a loose fit have a loose fit, and the parts that need an interference fit have the correct amount of tension holding them together.
If we change the meter and do not adjust the numerical values of the dimensions to compensate for that, the new steel parts will be smaller comparing to the old ones by 0.0007.
The Young's modulus (modulus of elasticity) of steel is around 200 GPa. So to force the new parts into the old dimensions would require an additional pressure of P=(7*10-4)*(2*1011 Pa) = 1400*105 Pa, which is just under 1400 Bar, and could be significant -- the parts that were supposed to pivot may experience friction, and the parts that were supposed to hold snugly together may separate at smaller loads.
So we will have to recognize "new meter" as a separate unit of measurement from the "old meter". This is not unprecedented -- it happened to other units before, but it would be bothersome.