I was challenged today and I came short.I was taking a walk with a female friend when she suddenly asked me the following. She asked me what would happen to light under the following conditions:-

"A perfectly square room completely and flawlessly mirrored. A light is hung in the centre of the space. Does the light shine for all eternity?"

Well I was stumped. I made some guess that the photons would disipate in time as conditions were never perfect and unchanging but she would not be fobbed off so easily. I could not for the life of me think of the relevant law of physics. Could any mirrors relect the light perfectly in a self perpetuating cycle? I know that energy cannot be created or destroyed but.....?

Can anyone please help me out as my friend still wants an answer so that she can show off to the person who first challenged her with this teaser(!).

Many thanks Andy.

The question is, "is there anyone - or anything (!) there to observe this light?"

Energy cannot be created or destroyed, but it can be converted. When the original light source itself runs out, as the law of entropy tells us it must, the light in the mirrors will eventually fade, and will probably break down into heat.

Both your answers are interesting. WilburrUK I did in fact first mention the Schrodinger's Cat theory to my friend as I thought that the riddle's solution might lie in that direction.She cleverly countered by mentioning that the mirrors were one way!!!

I then turned to the fact that there could not possibly be an absolutely perfect vacuum in the room (as with everywhere else in the universe).I think Scott might well be correct in thinking that the light would be converted in time to heat.As to how long the light would be visible after the original source has gone is anyone's guess and perhaps depends upon unknowns such as the degree of perfect smoothness of the mirror surface and the nature of the objects in the room (ie the atoms).

Thanks to you both.Andy.

You can't have a perfectly reflective one-way mirror (actually you can't have a perfectly reflective mirror at all, but that's a different matter).

I'm not even really refering to Shrodinger's cat, just that by letting your eyes (or camera or whatever detecting device you use), you'd be converting some of the energy in the system. As I understand it, Shrodinger's cat is about the system not resolving to a particular state until it is actually observed or measured. I'm sure someone will have a more accurate interpretation.

Assuming you could remove the light source instantly, the light would travel forever.

(Also assuming the mirrors were impossibly 100% reflective)

Consider the light that reaches us from stars billions of light years away.

The light traveling in a vacuum pretty much reaches us unchanged.

Air in room? Scattering.

So (forgive me if I am still not understanding correctly) in the real physical world the 2nd law of thermodynamics relating to entropy would apply as Scott states. But given the theoretically perfect conditions Philemon states then the light photons (which are presumably massless and would not scatter in the air) would reflect for ever within the room without diminishing.

WilburrUK your post is very deep.

"just that by letting your eyes (or camera or whatever detecting device you use), you'd be converting some of the energy in the system."

As a Buddhist practitioner I think I know what you are saying.

Thanks everyone for your answers. I think that this one is now sorted.