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 diff --git a/physics/units b/physics/unitsnew file mode 100644index 0000000..b1968f4--- /dev/null+++ b/physics/units@@ -0,0 +1,53 @@+======================+Units+======================++.. contents::++SI Units+--------------------+The SI system uses meters-kilograms-seconds. It also defines the Coulomb as +a unit for measuring electric charge, which introduces redundant conversions+between mass-length-time units and the electric charge.++cgs Units+--------------------+The cgs system uses centimeters-grams-seconds, and also defines electric charge+in terms of the fundamental quantities of mass, length, and time. The unit of +charge is "esu" or electrostatic unit.++Natural Units+--------------------+Natural units are a system of units which replace (or re-scale) the usual mass,+length, and time bases with quantities which have "natural" (physical)+constants associated with them. The two constants usually chosen are the speed+of light (c) and Plank's constant (:m:$\hbar$); the gravitational constant+(G) is a possibility for the third constant/unit, but energy (in+electron-volts: eV) is often used instead because it gives more useful+relations and because there is no accepted theory of quantum gravity to unite+these three constants. See _Plank Units for more on using G as a unit.++Working with natural units simplifies physical relations and equations because+many conversion factors drop out.++Given the relations between cgs units (gm, cm, sec) and natural units (c,+:m:$\hbar$ , eV), we can find the natural units of an arbitrary quantity+:m:$[Q]=[gm]^{a}[cm]^{b}[sec]^{c}=[c]^{\alpha}[\hbar]^{\beta}[eV]^{\gamma}$:++:m:$$(\alpha,\beta,\gamma)=\left(\begin{array}{ccc} -2 & 1 & 0\\ 0 & 1 & 1\\ 1 & -1 & -1\end{array}\right)\left(\begin{array}{c} a\\ b\\ c\end{array}\right)=(-2a+b,b+c,a-b-c)$$++or in reverse:++:m:$$(a,b,c)=\left(\begin{array}{ccc} 0 & 1 & 1\\ 1 & 2 & 2\\ -1 & -1 & -2\end{array}\right)\left(\begin{array}{c} \alpha\\ \beta\\ \gamma\end{array}\right)=(\beta+\gamma,\alpha+2\beta+\gamma,-\alpha-\beta-2\gamma)$$++Plank Units+----------------+Plank units (defined by Plank soon after defining his constant :m:$\hbar$) are a version of _Natural Units using the gravitational constant G as the the +third unit (instead of the common measure of energy). When converted back into+mass-length-time units we get three quantities which define the "Plank Scale",+which may provide estimation of the domain where quantum gravity effects become+important (similar to how the speed of light and Plank's constant provide +estimation of when special relativistic and quantum mechanical effects become+important).++