Capturing Light
 

Ok.. this is purely conceptual, but here goes - and follow along with me and try to realize what I'm describing...

We know light moves really fast and usually has a direct source that we can pin-point. Would it be possible to capture light in a box? The box would have to be perfectly reflective on the inside so that no part of the surface could absorb any of the light.

First, with the box slightly open, light shines in. Then, with light constantly bouncing off the insides of the box, it's closed up (theoretically, it's closed up faster than the speed of light).

Would the light be 'trapped' inside the box and continue to bounce off the perfectly reflective surfaces? Would the light then escape from the box in and instant once it was opened?

My brain is weird.

I think the light would lose heat energy (a little with each bounce) because your box could never be 100% efficient. The light would just dim and eventually go out... unless this concept also involved a box that also reflected heat at 100% efficiency. How much reality are we trying to maintain here?

Cow has it right, its just the same as the idea of trapping sound or creating perpetual motion. Light is created by a reaction that has a certain force. Once the force is diffused it is no longer light but rather the remnants of it.

If you want something interesting, read up on quantum theory. at any time there is an infintesimal chance that a carrot riding a bicycle will pop into your room, or you could teleport to another dimension. Also quantum theory states that light even if you could keep it at full power within the box would eventually escape through the same ways. *we are talking 10 to the nth power chance though*

i've always thought about something similar. Placing small mirrors in a position such that pointing a laser at one will send the laser beam into a repeated circuit of mirrors. Then the beam would just keep going in circles and never be able to leave.

Without the Partition Theory, nothing can generate it's own power.
Only "pure" light can be reflected with a "generic source" ie, flashlight (for all intents and purposes) Pure light can only be generated thru a perfect refractor. No such element has been made or discovered thus far. Once this refractor is made possible, the theory of light travel (quatum physics) will be possible.
Just an FYI: NASA has been trying to build such a refractor for over 30 years. They have spent over 13 billion dollars on the research and have not had any success. It's a good question and a fascinating subject.

a theoretical box is sometimes used to help explain special relativity to students. If you imagine a beam of light reflecting between two mirrors in a box, and you carry that box with you when you go on a really fast airplane, observing the way the light behaves is a good way of getting understanding of the time dilation and contraction effects.

It's been several years since my quantum course so pardon me if I'm totally wrong but wouldn't the photons lose momentum with each collision within the box? Eventually, the wavelenths would shift to lower energy wavelengths and you wouldn't be able to see the light anymore?

photons can't lose momentum. the photon momentum is determined by it's energy/frequency. And, since it's called quantum physics, a phonton can only be fully absorbed or not at all. You can't absorb only part of a photon (of course, a material can absorb a photon and then spit out another photon of some fraction of the energy of the original, thus effectively absorbing part of the photon). Anyway, there was a thread a couple months ago about this very subject. See here:
http://www.tfproject.org/tfp/showthr...threadid=54544

What about optical hard drives? How do those work, or is what I heard total BS?

From what I recall, an optical hard drive is kind of like a multi-layer dvd. Data is stored in small images that are stacked on top of each other.

They are amazingly fast at retrieving data and the amount of data stored per unit volume is huge. The downside is that as of now the writing process is painstakingly slow.

fckm:

http://online.cctt.org/physicslab/co...re/Compton.asp

When the photon collides with the crystal in this example, it re-emerges from the crystal with a longer wavelength and therefore less energy and less momentum. Are you saying that the photon with the longer wavelength is not the same one that collided with the object?

Halx did say PERFECTLY reflective. Sort of like the theoretical fritionless surface.

If the inside of the box was perfectly reflective and the beam of light lost no energy with each "collision" then in theory we might be able to create a sort of "light battery." But I doubt it.

kutulu:
well, yeah, more or less. Think about it this way. I have a blue laser. It only produces one wavelength, lets say... 400nm.
Now, i have a crystal that only absorbes red light. This means that the separation of the energy levels of the crystal correstpond to red light, of say... 700nm.
So, if I shine the laser on the crystal, the light passes through with NO absorption. This happens even though the blue light is at a higher energy (lower wavelength) than the energy separation of the crystal. Classically, this makes no sense. The blue light has plenty of energy, it should be absorbed by the crystal. But, according to quantum mechanics, (well, QED, quantum electrodynamics) this doesn't happen. And indeed, experiments confir m this.
EDIT:
as for Compton, the electron is no long in a bound state, so it's energy levels are no longer discrete. I'm not too sure how this effects photon absorption/reimission.
And remember, as much as a photon is a particle, it is also a quantization of the field. In some cases, especially when there is light-matter interaction, it doesn't really make sense to talk about "the same photon". Matter takes a certain amount of energy from the field, and gives a certain amount back. The energy is quantized in multiples of plank's constant. Beyond that, whether the photon is the same or not doesn't really matter.
A photon pretty much has three properties, wavelength/momentum, spin, and polarization. If these are the same before and after an interaction, then you can think of the photon as being the "same", it doesn't really matter. (unless you're talking about entropy and information destruction)

As for the perfectly reflecting box. I seem to remember thinking about that in E&M class. For now, I'll say that you could, in this impossibel case, make a 'light battery'. I do seem to recall there being problems with beam injection and interference patterns and something doesn't work out quite right. i don't remember what it was but if I do, i'll post it here.

Hmm, a perfectly reflective mirror is going to be difficult to come by; might it be more possible to make a sort of gravity bottle? In that case it might be possible to inject light into what amounts to space bent in on itself, so the light is traveling in an infinite loop. With this system no energy would be lost at all, and the capacity for storage could perhaps be limitless. Of course, constructing one of these containers would be really hard, and it would probably be difficult to do anything nearby. It would also have to be adjustable, because as you added a lot of energy it would get a gravity field of its own...

i remember an article in pop sci about trapping light.

the used a laser to melt a substance that turns clear when its melted and shined a second laser through it. when the melting laser was shut off a smnall bit of light was trapped insid of the substance.

if i find the article ill scan it up.

fckm->

That's not entirely true, as the absorbtion spectrum looks like a lorentzian centered on 700nm. A Lorentzian is like a gaussian with larger "wings", and doesn't ever quite go to zero. So given enough passes through the crystal it will still absorb a blue photon. Oh, and scattering processes don't have the same narrow spectrum, so you can still get things like rayleigh scattering. You can also excite vibrational modes in the crystal lattice.

Oh, btw, I have a 493nm blue laser that I use to cool and trap single atoms.

mpgreen: having not taken solid state or lasers yet, i'll defer to your expertise. I was ignoring vibrational modes. I figured since we're talking about "perfect mirrors" ...
One question tho, is the absorbtion spectrum empirical, or theoretical? Is the orentzian the solution to the two state system? (i just took this last semester, i can't believe i'm forgetting already)

My atoms hurt.

Was that the Harvard cesium vapor light trap?

This is a bit off topic but a scientist at MIT managed to slow light down by passing it through a super-cooled tube. Dunno if that adds anything to your thought processes.

Y'know, this just goes to show how little we know about light, and likely the universe in general.

Many of our beliefs are based on the speed of light being a constant, and the fact that it can be slowed by conventional means makes me believe that we are grossly incorrect.

I suppose, though, that it isn't too surprising as gravity is able to affect light as well.

no the speed of light being constant means in a vaccum. it's different in the air/water esc. remember, light travesl through no medium. what they mean by "slowing" probably means that the light hits so many obstacles and bounces so much, it takes a long time for it to travel through. the speed of light IS constant but for practicality sake, the number "C" is the measurement of hte speed of light in vaccum as the light travels without hitting anything, hope this explains something ----- [/off topic]

As mentioned, there was a post about this some time ago.

The "theoretical box" would have to reflect 100% of the EM energy that is light. The photons will expecience a "lamda shift" and eventually lose all energy to adsorption. Blue light would eventually fade to red, and down to radio, and subsequently further down the spectrum ...

Energy always flows down hill - this is an important concept to remember in these conceptual situations.

"Sort of like the theoretical fritionless surface."

I know this is off topic, but I think I remember supercritical helium-2 is frictionless. If not, it's pretty damn near it.