# Can you trap light indefinitely?

1. Dec 7, 2011

### ArcanaNoir

Can you "trap" light indefinitely?

One of my fifth grade students wants to know if you can "trap" light. that is, can you let some light into a container and then seal it off, and let it out again some time later?

I'm not a physics major so I wasn't sure at all, but instinct tells me "no". That's supposing you could even get it in a container and seal the container in the first place. At first I'm thinking you can bounce light off a bazillion mirrors and so why couldn't you just keep doing that, but then I realized you basically have to have all the mirrors set up already, since you could never keep up with the speed of the light, leading me to suppose it doesn't matter how far the light goes, what matters is how long it is preserved. So in that case, it may dissipate almost instantly. But then, there's the light from far away planets from a long time ago. So, light does in fact have an appreciable "shelf-life". Of course, this light was extraordinarily bright at the origin and is now extraordinarily dim. So, maybe your "light in a jar" would become so dim that you couldn't see it anymore extremely quickly. The most problematic thing of all, I think, would be that you would only have an instant of light, since you could only capture that last instant before sealing the container. Any light that had entered the container previously would have worked its way out almost instantly. Even if you could "collect" more than an instant of light, the instant you released the light, it would ALL come out at practically the same time.

Well, it's all very impractical, but does the concept violate any laws? If not, what "practical constraints" would make the whole endeavor pointless? Please try to use basic language.

2. Dec 7, 2011

### Curl

Re: Can you "trap" light indefinitely?

3. Dec 7, 2011

### DaveC426913

Re: Can you "trap" light indefinitely?

It's not a problem of being really quick. You could easily make a box large enough to trap the light inside.

The problem is that no mirror is perfectly reflective. It is not simply a practical constraint. No mirror can be perfectly reflective - even in principle. To reflect, the light must interact with the mirror. Ultimately (quickly) the light's energy will be absorbed into the mirrored walls of the box, resulting in heating up the box a tiny bit.

4. Dec 7, 2011

### I like Serena

Re: Can you "trap" light indefinitely?

Hey Arcana!

What's this about you having students?
Did you make a career change?

Oh, and here's a nice article about trapping light:
http://www.rsc.org/chemistryworld/News/2007/December/10120701.asp

5. Dec 7, 2011

### e.bar.goum

Re: Can you "trap" light indefinitely?

You can't trap light in a mirrored box, as there are no perfect mirrors (complicated dielectric mirrors can get to 99.999% reflectivity, but only for a very small region of the spectrum).

On the other hand, it is possible to "trap" light by slowing it down. If you can stop the light, you've trapped it. Indeed, this is crucial for the development of quantum memory.

Last edited: Dec 7, 2011
6. Dec 8, 2011

### ehild

Re: Can you "trap" light indefinitely?

The answer is "almost". You can reach reflectivity with internal total reflection much better than that of mirrors. There are optical resonators which trap light for a while. The lasers consist of such a resonator with a small opening on one of the end mirrors. Closing the output, the light is trapped. Think of fibre optics or other waveguides. The light is confined between the walls all along the length of the fibre or waveguide by the total reflection, and escapes at the open end. Of course, no surfaces are perfect, and no materials are completely free of absorption, so there will be some loss and the energy of the trapped light is dissipated sooner or later. But it is worth to show the students an experiment with a prism and laser pointer to demonstrate total reflection. With a prism of shape shown, at certain position and angle of the incident ray one can make the prism glow without any visible outgoing ray of light.

ehild

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7. Dec 8, 2011

### ArcanaNoir

Re: Can you "trap" light indefinitely?

These answers are great! Thank you. I did search some other threads like this, but the discussion was above my head. What you have posted here is exactly what I was looking for.

I haven't made a career change, I'm a teacher's aide during the day, stealth mathematician by night! I'll finally get to go to school full-time next year.

8. Dec 8, 2011

### Andy Resnick

Re: Can you "trap" light indefinitely?

Echoing ehild, total internal reflection is used in optical fibers, which are nearly lossless at telecommunications wavelengths (0.2 dB/km). Cavity ring-down spectroscopy also uses high-finesse cavities (reflectivity = 0.9999 appears to be common).

These roughly translate to light 'trapped' for 1.6 ms (fiber, starting at a 100 dB SNR) and 12 us (CRDS, 1-meter long cavity).

9. Dec 8, 2011

### ehild

Re: Can you "trap" light indefinitely?

Thank you Andy Resnick! Although that 1.6 ms is almost infinitely long with respect to the period of light, it is very short with respect to our lifespan But everything is relative...

ehild

10. Dec 8, 2011

### A.T.

Re: Can you "trap" light indefinitely?

The star light being dim has nothing to do with "shelf-life". The light doesn't decay with time, it just gets distributed over a huge area, and red shifted due to expansion.

You can trap light "locally" without reflection in orbit around a massive object. But these orbits are not stable.

11. Dec 9, 2011

### Staff: Mentor

Re: Can you "trap" light indefinitely?

Yes, researchers have been achieving that in just the last few years. But it's not quite as simple as capturing the beam in a mirrored lunchbox. But they claim to be able to stop a pulse of light and release it later at a time of their choosing. (They have been choosing what we would think of as very brief times, though, so far.)

12. Dec 9, 2011

### ArcanaNoir

Re: Can you "trap" light indefinitely?

Good point! Thank you for correcting me.

13. Dec 9, 2011

### silentbob14

Re: Can you "trap" light indefinitely?

14. Aug 17, 2013

### lenita24

i guess light can be trapped on pictures, I assume that is how astronomers study the universe!. Also I would like to try to trap light on frozen water, may be very possible.

15. Aug 17, 2013

### Drakkith

Staff Emeritus
Pictures are either the result of a chemical reaction caused when light hits film, or the absorption of light by electrons in a digital sensor. Neither of these "trap light".

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

16. Aug 17, 2013

### Fieldwaveflow

I wouldn't call it a lunch box

Metal is put on the backs of mirrors. It's like looking at the surface of atomic shells right?
So if someone where to put light on the inside of the shell it would be a light trap.
Maybe you could call fusion a way of constructing a "lunch box".

17. Aug 18, 2013

### Drakkith

Staff Emeritus
I'm not quite sure what you're trying to get at, but I don't think it works this way.

18. Aug 18, 2013

### Bobbywhy

Until last week when I received my copy of Science News, August 10, 2013; Vol.184 #3 (p. 8) I would have answered the OP’s question just like everyone so far has.

But Wait! This article changes all that! Physicist Chia Wei Hsu and colleagues at MIT report in the July 11 Nature and summarized in Science News:

“A new type of mirror that reflects light perfectly has been constructed; a feat many scientists thought wasn’t possible. The mirror could find its way into powerful lasers and other devices.”

http://www.sciencenews.org/view/generic/id/351485/description/Perfect_mirror_debuts

19. Aug 18, 2013

### Drakkith

Staff Emeritus
Note that this new mirror required that the light be at a specific angle, 35 degrees. I'm not sure you could design a container that could trap light indefinitely out of this. Yet.

20. Aug 18, 2013

### Fieldwaveflow

Atoms are energy traps right?

If most atoms are perfect energy traps, as they appear to be, then a person would need to create an atom ("lunch box") to contain energy. One of the results of atomic fusion is an atom of increased mass. E=mc² shows that energy is contained in mass. With an increase in mass there is an increase in contained energy. So fusion is a way of creating a "lunch box".

If I were explain a perfect lunch box to a 5 year old I might not go into such details but possibly say "It would take a special lunch box that would need big, special machines to make."

Last edited: Aug 18, 2013