Massive, Slow

COVARIANCE

Massless, Fast

Energy-Momentum

$E=p^2/2m$

Einstein’s

$E=\sqrt{{\left(cp\right)}^2+{\left(m{c}^2\right)}^2}$

$E=cp$

Internal

Space-time

Symmetry

$S_3$

$S_1\mathrm{,}{S}_2$

Wigner’s

Little Groups

$S_3$

Gauge

Transformation