Thread: Mirrored box and Eternal Light?

this is a purely theoretical thing, please don't roast me over the illogicalities of it



Supposing we have a light source in a box where all the walls are total mirrors and will reflect ALL electromagnetic radiation (heat, light etc etc)

if we shut off the light and assume the bulb or whatever will absorb nothing what will happen? I was thinking that theoretically the light would continue to bounce around


now assuming the light did continue to bounce around (because theres no way out after all) what would you see if you could somehow look into the box without loosing the light?

assuming the light continued, if you compressed the box would the light intensify?, I know light has no mass but it would have less space to move essentially meaning it increases and Finally, if you could compress the box containing the light to diameters of 200nm X 200nm (the wavelength of light in this case) what would happen? would the light stop? could we transform light into matter by stopping it from moving?

It's really hard to draw meaningful conclusions from faulty premises. Basically, to ask "what would we see" implies certain things that you've already ruled out, so it's difficult to make much sense of the question.

Anyway, inside the box would basically be a uniform glow, and compressing the box would make the glow brighter. This would also increase the energy density of the space inside the box, but not the energy of the light itself. I don't know what happens to light bouncing between two mirrors that are spaced one wavelength apart, but it sounds like a good radio/microwave experiment (though someone else has probably already done it).

Originally Posted by Booms

Supposing we have a light source in a box where all the walls are total mirrors and will reflect ALL electromagnetic radiation (heat, light etc etc)

If there were such a thing as a 100% reflecting surface (in a perfect vacuum) then the photons would continue bouncing around forever. In reality they would lose energy each time they hit a surface. Should they last long enough, they would finally redshift to infra-red radiation and be soaked up as heat, warming the mirrors.

Originally Posted by Booms

...and Finally, if you could compress the box containing the light to diameters of 200nm X 200nm (the wavelength of light in this case) what would happen? would the light stop? could we transform light into matter by stopping it from moving?

This is an awfully good question, but no one seems to have understood it yet.

Here's a version of the problem.

We have an ideal microwave cavity containing radiation, that is one wavelength long. Its length is adjustable with a piston. What will happen if we try to move the piston?

Should we be using a box, or a sphere? In either case, some portions of the light wave will be traveling different distances between reflections, because different angles put the walls different distances apart.

Originally Posted by kojax

Should we be using a box, or a sphere? In either case, some portions of the light wave will be traveling different distances between reflections, because different angles put the walls different distances apart.

In the simplest case, we'd want a box only large enough to contain one wavelength.

Originally Posted by Booms

I know light has no mass but it would have less space to move essentially meaning it increases and Finally, if you could compress the box containing the light to diameters of 200nm X 200nm (the wavelength of light in this case) what would happen? would the light stop? could we transform light into matter by stopping it from moving?

This is basically a configuration you find in gas lasers, and it is a typical exercise in quantum mechanics when studying physics. First of all, white light consists of an infinite number of different wavelengths. So, such an apparatus would only be significant for those waves that have that wavelength or integer fractions of it. You would produce "standing waves" inside such a cube that could lead to an increased excitation of light with such a wavelength, while other parts of the spectrum would fade out. In essence, I suppose, such a box would be a spectral filter that prefers some wavelengths to others.