  Two different luminosity indicators have been recently proposed for Gamma Ray
Bursts that use the lag between light curve peaks with hard and soft photons as
well as the variability ('spikiness') of the burst light curve. Schaefer, Deng,
& Band have proven that both of these luminosity indicators are valid by
finding a lag/variability relation for 112 BATSE bursts. Here, I provide simple
and general explanations for both indicators. The lag/luminosity relation is a
consequence of conservation of energy when radiative cooling dominates. High
luminosity bursts cool rapidly so the lag between when hard and soft photons
peak is short, while low luminosity bursts cool slowly so the lag is long. The
variability/luminosity relation arises within internal shock models because the
luminosity and variability both scale as strong functions of the bulk
relativistic motion of the jet, Gamma. With luminosity roughly proportional to
Gamma^5 and variability scaling as Gamma^2, we get the luminosity being
correlated to variability just as the observed power law.
