Imagine I have a very, very large box (say, 3E8 meters long) with a
perfectly reflective surface on all interior walls.  Lets say I opened
one end of the box to allow light in.  If I then closed that end of
the box, what would happen to the light inside?  Would it keep getting
reflected around forever?  With perfectly reflective (zero absorption)
surfaces on the inside, what could possibly stop it from being
reflected?

---

Clarification of Question by azdoug-ga on 21 May 2006 10:31 PDT

OK, lets say there is a perfect vacuum inside the box - no air, no
particulate matter, etc. and the interior walls are still 100%
reflective.

How would the scenario play out if light is photons?

How would the scenario play out if light is waves?

How would things change if the box was 0 Kelvin?

How "ideal" is this situation? Essentially the moment there's anything
else in the box (such as air) it will absorb the light over time until
none is left. How "deep" is this meant to be ... there's several
levels of answers, including photons quantum tunneling through the
walls until none are left.

For a hypothetically oversimplified model system in which the walls
and the light are 100% elastic, it will indeed just reflect endlessly
within the chamber. For more realistic scenarios, it just won't
happen.

I just tried the experiment:  used a foil-lined bag for carrying frozen food.
It works.  Every time I open the bag to check if the light is still
there, it is, just as bright as the previous time I looked.

The inside of my refrigerator also turns out to be a very efficient
internal reflector. Every single time I open the door, there's still
light inside.

OK. Perfect vacuum, 100% reflective surfaces.

Light as photons are treated as particles, so they would never slow
down, never stop. They would just bounce forever.

Light as waves would eventually (and I mean over a LONG period of
time) quantum tunnel thier way out of the box, 100% reflectivity or
not. Eventually (note I've used eventually twice now) the light waves
would find themselves in a situation where the probability of them
being outside the box would be greater then 0. Greater then 0% chance
of showing up outside the box is the same thing as all the light
eventually leaking out.

0 degree kelvin is easy. Not even light has enough energy to move at
absolute zero. In a type of catch 22 if you have a box that is 0K,
there can be no light inside, because as soon as any light is in the
box, it will raise the temperature above 0K.

Hope this helps.

This is essentially the definition of the perfect black box.

Take any box with any level of less than 100% perfect internal
reflection (i.e. any box that can exist) and open an aperture to look
into it.

The results is perfectly black. Any light that falls into the box from
outside is utlimately absorbed by the walls. It can realistically
never escape. The only light that does escape is the light emitted by
the walls because they are not at abolute zero. The amount of energy
emitted as radiation by the black body is determined by the
Stefan-Blotzmann constant multiplied by the fourth power of the
thermodynamic temperature of the box (i.e. the emperature above
absolute zero).

Schrodinger's cat lived/died in a box just like your one.

http://en.wikipedia.org/wiki/Schrodingers_cat

how about if it was made completely out of one way mirrors what sort
of effect could this have

One way mirrors cannot be perfectly reflective since some of the light
has to escape through the mirror for the other side to see. So, if
side A appears to be reflective and side B appears to be more of a
window, then some of the light from side A has to travel through the
mirror to side B for side B to see side A. Hence, a one-way mirror
cannot be a true mirror and the light would escape over time.

I once got into a box like that! While at Cal-Tech a Brazilian artist
set up a similar box that people were led into wearing sox on their
feet. The box was about 2.5 meters high, 4 meters in length and a
couple wide. All built entirely out of mirrors. To dramatize the
effect, strips of translucent tape were applied to the sides that
allowed colored lights from the outside to enter the box...What a
trip! you had to be in it to believe the feeling.

just a thought,

__peter