# Inertia of a box of photons

Suppose a box one light year long (along its x-axis) with some mass M also contained a bunch of high energy photons with energy E that were travelling along the x-axis. Would the inertia of the box include the energy of the photons while they are travelling freely along the axis (not bouncing off the walls)?

• How do you know where in your box the photons are located? Oct 6 '16 at 0:48

The problem with your question is that you're trying to describe inertia in terms of what are, inherently, transient phenomena. Never mind the photons, if you push on a box that large it is going to take a long time for the sound waves to propagate from where you pushed to the rest of the box. So, for the specific limits you've given, you'll find an inertia that is on the order of $$\frac{v_s}{c} L \mu,$$ where $v_s$ is the speed of sound through the walls of the box, $L$ is the length of the box, and $\mu$ is the mass per unit length of the box.