Light Balast

Curious today. {Sorry}

A photon has effectively no mass. But they do have mass.

Photos can travel billions of light years before they reach us, meaning they can effectively be eternal.

A photon can go from the visible spectrum and lose energy along the way, slowing down and drifting out of the visible spectrum.

But what if you could capture it in a perfect containment unit?

Lets say you have a box. This box does not allow light to escape at any level, be it visible, ultraviolet, infrared, etc. It is perfectly insulated. The insides of this box are also perfectly reflective. Photons will bounce forever, and even if they do slow down, they never leave.

You turn on an LED inside this box.

Can this box gain mass?

If a photon in this box does decay, does it just pop out of existence?