Thought Expirement: Light In a Mirrored box

[SilverFlashII]

A theoretically perfect mirror with no gaps cracks, (no absorbing surfaces), so small that only a single photon could tightly fit inside

Is it stopped? If so let's get our scales ;)

 

[mfb]

so small that only a single photon could tightly fit inside

Photons can "stack" without any problems. Your box would have a discrete spectrum of allowed energies (corresponding to standing waves in the box), similar to a laser. For each allowed frequency, you can have an arbitrary amount of photons inside.