[coinshen]

In a sealed box (it should be regarded as an ideal state of vacuum), if you want to fill the entire box with light, according to the linear propagation of light in the vacuum, it can be concluded that the box must be full of fine particles, forming diffuse reflection. Fill the whole box! In the process of diffuse reflection, energy is consumed, that is, light particles (particle nature in the wave-particle duality of light) collide with fine particles, resulting in energy loss (light energy-kinetic energy-internal energy), when the flashlight is turned off At the moment, the light particles are still running at high speed, and the energy is lost after the collision until the energy is exhausted. However, because the human eye has a certain time interval for capturing the light, it will cause the illusion that the light disappears in an instant! On the other hand, light has volatility. We all know that the propagation of waves actually propagates energy, and light waves are no exception. When the flashlight is turned off, the light source stops emitting light, and the wave source stops emitting energy. We also see No light! Energy cannot be generated or disappeared out of thin air. As long as you find the essence of energy transformation, you can grasp the idea of ​​solving the problem!

[alancalverd]

Never mind the geometry and nitpicking stuff like reflectivity. There is an answer, which lies at the root of nuclear weapons and fusion experiments.

Light is electromagnetic energy, so the amount you can contain in a box depends on the strength of the box. HYdrogen bombs use an atomic explosion to generate x-rays which rupture a box outwards, some fusion laboratories use lasers to collapse a box.

Radiation pressure on a reflective surface P = 2I/c where I is the intensity of radiation and c is the speed of light.

[Bored chemist]

There is an answer, which...

...ignores the conditions set by the OP.

if the box was unbreakable and the pressure element thus eliminated,

[alancalverd]

Ah, yes. You could in principle stuff an infinite amount of anything into an unbreakable box. All you need is an irresistible force (an infinite laser) and an immovable object (the sides of an unbreakable box).

Fortunately physicists are less stupid than philosophers and we realise that the photons would eventually reach such a high density that they would coalesce into matter, then into a black hole. So the answer depends on the diameter of a black hole that doesn't quite touch the sides of the box.

[Bored chemist]

Light is electromagnetic energy, so the amount you can contain in a box depends on the strength of the box

Only if you are doing some despicable philosophy.
In the real world, we know that you can't make a true1 way mirror, nor a perfect mirror.

Fortunately physicists are less stupid than philosophers and we realise that the photons would eventually reach such a high density that they would coalesce into matter, then into a black hole

I'm sure I heard someone say that before.
https://www.thenakedscientists.com/forum/index.php?topic=82373.msg642468#msg642468