Re: SI and nothing else

[David Bowman <dbowman@tiger.gtc.georgetown.ky.us>]

Ludwik K. wrote:

> David: can you briefly explain a system of units in which the number of
> fundamental units is zero (plank's system, as you wrote).

James Wheeler already mentioned this but Planck units are the set of units
one gets by setting c = h_bar = G = k (Boltzmann's constant) = 1.  This 
system has no fundamental base units whatsoever.  Crudely speaking (i.e.
up to factors of 2*[pi], etc.) one Planck unit of time corresponds to the
period of a photon whose energy corresponds to the mass of a black hole whose
Schwarzschild radius is equal to the wavelength of the photon.  One Planck
unit of energy is the photon's energy; one Planck unit of mass is the
corresponding mass to this energy if it was a rest energy; one Planck unit of
length is this wavelength/Schwarzschild radius; one Planck unit of momentum
is the photon's momentum; one Planck unit of temperature it the Hawking/
Bekenstein temperature of the black hole's event horizon; etc.

David Bowman
dbowman@gtc.georgetown.ky.us

p.s. Sometimes Planck units are chosen so that 8*[pi]*G = 1 rather than G = 1
to make the connection between spacetime (Ricci) curvature and the
stress-energy-momentum tensor of matter most simple.  This is a General
relativistic version of rationalized units--kind of like using rationalized
rather than unrationalized units in electromagnetism.