# Light box

If it were possible to build a box of out of two way mirrors where all the reflective sides were inside and bright lights were shown in, whould it be possible to trap the light in the box and would there be a maximum amount of light it could hold?

We discussed this once in a freshman physics class. IIRC, which is highly doubtful, we decided that the refective surfaces would have to be perfectly reflective, as in an ideal material that doesn’t exist. Otherwise, all the energy would evenutally be absorbed and radiated away as heat. Also, you wouldn’t want to put the light source inside, as it would be nonreflective, ruining everything. You’d have to have a door which would provide entry for a photon, and which could be shut before it could escape again. Once shut, it would also have to prefectly merge with the rest of the perfect reflecting surface.

As to the maximum amount it could hold, I have this gut feeling that Bad Things would happen if you had too much light energy in there, but my head is hurting too much to try and figure out why.

" As to the maximum amount it could hold, I have this gut feeling that Bad Things would happen if you had too much light energy in there, but my head is hurting too much to try and figure out why."

Well, it would get really really hot, for starters.

If it got really hot, that would mean that you are loosing energy. If you are loosing energy, that means that the surfaces of the box are either not perfect, or they are not perfectly reflective.

Well, it would be incredibly hot inside the box, but you can’t actually detect the heat, since anything that could detect it would make it lose energy. Basically, I think you could theoretically keep adding light until the energy of the photons hitting a side would either melt or break it.

Well, eventually, if you cram enough light in there, you’ll get a black hole. It’s a bit silly to worry about it, though, since there are no perfectly reflective substances, etc., and any approximation to perfection would break down long before that point.

I don’t know what you mean by a “two-way mirror.” If you are thinking of a contraption where light can enter but not leave the box, I don’t think that is possible. There is no material through which light can propagate in only one direction. If nothing else, it would violate the laws of thermodynamics - such a box left in a room will suck all radiation and get hotter and hotter.

If you’re talking about just opening the lid, pouring some light in and closing the lid again so that light is trapped inside - it should be possible, theoretically. As for the theoretical limit of the energy that can be stored - maybe if there is enough energy in there, the box will reach critical mass and become a black hole. I can’t think of any other theoretical limitations. (Plenty of practical limitations though, as others have posted)

Not to change the subject, but I’m going to anyway…

What if you had a loop of superconducting wire. Much more practical than a perfectly reflective surface. You could pass a current into the loop (or induce one magnetically) and, theoretically, it would loop forever.

As for the maximum amount of energy it could contain?.. With zero resistance there would be no heat buildup, but there must be a maximum to the amount of electrons available for current flow.

Of course you couldn’t measure it without bleeding off the energy, but you could let it go for a few years then measure it just to make sure it was still there.

Two way mirrors are actually glass with a highly reflective coating on it. Light shone from the non-reflective side towards the room the reflective side is facing would be visible in the room, and if the light was shined into the room towards another mirror opposite from the two way mirror, once the light source is shut off, why exactly would the light stop bouncing back and forth between these two reflective surfaces?

Aren’t photons “without mass”? You’d need to start with extremely dense mass to create a black hole wouldn’t you? We’ve dubbed photons “wave-acles”, because they exhibit characteristics of a wave, and also exhibit characteristics of a particle (although they have no mass).

I’m thinking that “if” it were possible to build said box, and you kept forcing photons into it, you’d eventually end up with a box full of light that did nothing at all until you opened it. Once you opened the box, you’d be blasted by an enormous amount of photons all at once. That can’t be good. Hmm, the guys in the defense department might want to look into this. Give us something else to drop from the Stealth Bomber.

It wouldn’t stop, it would escape through the “openings” that allowed the light to enter in the first place. In essence, it would continue to travel back and forth between the reflective surfaces untill it found a crack. Check out the theory behind the design of a ruby laser. This is the technique applied to create the beam of collimated, coherent light that goes forth from the source. I won’t go into population inversion and changes in state here, but you can find it readily online.

Just because photons don’t have mass doesn’t mean you can’t make a black hole out of them. Remember, mass and energy are two forms of the same thing, so if you need m grams of mass in your box before a black hole formed, you’d just need mc^2 ergs of light energy.

You can measure the amount of current in a superconducting wire- you just reduce the current some depending on how much energy you took to do the measurement. You wouldn’t need to check continuously to see if current were still flowing- just once in a while. Of course, eventually you’d use it up, but just don’t check that often

Arjuna34

Quoth Ozone:

A common misconception. Photons are without rest mass, which means that if you could somehow stop them in place (impossible for various other reasons), then they would have no mass. They do, however, have energy, and you can refer to the mass equivalent of that energy as the photon’s (relativistic) mass. This “mass” does gravitate just like any other mass, although it’s very small. You would need to cram a heckuva lot of photons into that box in order to make a black hole, (exactly how many depends on the size of the box) but it does at least put some sort of theoretical upper limit on how many you could squeeze in.

As for the superconducting wire, I think that self-inductance still causes some slight impedance. You can minimize that effect by making the loop very large, though.

Okay, who’s gonna be the one to bring up Heisenberg’s Uncertainty Principle, now that we’re discussing measurement?

Devil in disguise said:"
Two way mirrors are actually glass with a highly reflective coating on it. Light shone from the non-reflective side towards the room the reflective side is facing would be visible in the room, and if the light was shined into the room towards another mirror opposite from the two way mirror, once the light source is shut off, why exactly would the light stop bouncing back and forth between these two reflective surfaces?"

sorry devil,
scr4 was correct; the one-way mirrors as you seem to envision them do not exist. there aint no such animal.
the one-way mirrors used to watch customers in some stores are simply half silvered mirrors that pass a relatively small fraction of the light (say 20%) and reflect the rest. it has the exact same reflectance from either direction; but that’s why they keep that back room dark: if your in the back then the transmitted light from the store (bright lights) is stronger than the reflected light from the room your in so you can see out. but from the store, the reflected light from the store is brighter than the tranmitted light from the back room, so you see your reflection. if you dimmed the light in the store and turned on a bright light in the back room, the mirror would seem one way in the other direction.
The is no mirror that only allows light to pass on way (or even makes any distinction at all between time reversed directions of travel). As scr4 pointed out, such a beast would allow cheating on thermo, And as a wise cartoon character once said: “In this house we obey the laws of thermodynamics!”
-Luckie

ps: hmmm, Devil in disguise might really be Maxwell’s demon. hmmm…