Relativistic Units
From the definition of 4-vectors shown earlier, we know that
[math]\mathbf{R} \equiv
\begin{bmatrix}
x^0 \\
x^1 \\
x^2 \\
x^3
\end{bmatrix}=
\begin{bmatrix}
ct \\
x \\
y \\
z
\end{bmatrix}
\qquad \qquad
\mathbf{P} \equiv
\begin{bmatrix}
p^0 \\
p^1 \\
p^2 \\
p^3
\end{bmatrix}=
\begin{bmatrix}
\frac{E}{c} \\
p_x \\
p_y \\
p_z
\end{bmatrix}[/math]
The 4-vectors and 4-momenta are defined to be in units of distance and momentum and as such must be multiplied or divided respectively by the speed of light to meet this requirement. For simplicity, the units of c can be chosen to be 1.
The Planck-Einstein relation can be used to rewrite the 4-momenta vectors
[math]E=\hbar \omega \qquad k=\frac{p}{\hbar} \rightarrow p=k \hbar[/math]
[math]
\mathbf{P} \equiv
\begin{bmatrix}
p^0 \\
p^1 \\
p^2 \\
p^3
\end{bmatrix}=
\begin{bmatrix}
\frac{E}{c} \\
p_x \\
p_y \\
p_z
\end{bmatrix}=
\begin{bmatrix}
\frac{\hbar \omega}{c} \\
\hbar k_x \\
\hbar k_y \\
\hbar k_z
\end{bmatrix}\rightarrow
\mathbf{\frac{P}{\hbar}}\equiv
\mathbf{K} \equiv
\begin{bmatrix}
k^0 \\
k^1 \\
k^2 \\
k^3
\end{bmatrix}=
\begin{bmatrix}
\frac{\omega}{c} \\
k_x \\
k_y \\
k_z
\end{bmatrix}[/math]
[math]\hbar[/math] in SI units is defined as:
[math]\hbar \approx 1.0545718 \times 10^{-34} m \cdot kg \cdot \frac{m}{s}[/math]
Since c is already to be defined as equal to zero, this implies unit of mass must also be equal to one. By convention, the mass of the proton is used
[math]M_{p} \equiv 1.6726219 \times 10^{-27} kg =1[/math]