Arcadian Functor

occasional meanderings in physics' brave new world

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Marni D. Sheppeard

Tuesday, September 19, 2006

Measuring mass

The only basic standard unit that still requires an artefact is the kilogram. Masses are measured in comparison to the standard using an ordinary balance. Today the Hall effect can give us accurate values of Planck's 'constant' h and electric charge e, and an independent definition of the Volt and Ohm. Since the metre and second are defined without an artefact, this would allow a definition of mass unit independent of an artefact using a Watt balance.

A mass spectrometer measures the mass of a charged particle travelling at known speed by noting the distance it moves in a circular arc, given by r = mv/qB for magnetic field B.

The mass of the sun is measured by applying Kepler's Law M = (4 pi^2 R^3)/(GT^2) in terms of the period T of the Earth's orbit and the mean distance R of the Earth to the sun. Observe that Kepler's law is essentially the same as Riofrio's universal law rescaled by 4 pi^2. The value of R itself is now known fairly accurately, but the mass of the sun also depends on an accurate value for the gravitational constant G.

Observe that in all of the above settings the measurement of mass actually depends on the measurement of other quantities, most noticeably distances. In gravity a mass has a characteristic Schwarzschild radius for which r = 2Gm/c^2. Similarly, the quantum mechanical Compton wavelength is given by l = h/mc, but this indicates an inverse relation between mass and length. As is well known, these lengths agree at the Planck mass. This suggests that any theory attempting to explain the meaning of quantised mass ought to incorporate some form of T-duality, a correspondence between lengths and their inverses, up to a scale factor.


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