# Storing light

## Main Question or Discussion Point

Why are we not able to capture and store light between two mirrors? I'm going to take a guess at the answer but I'd like to hear from someone who knows for sure...
I'm wondering if you actually can for a small amount of time, but the inefficiencies of an imperfect mirror cause some light to be absorbed and converted to heat. Since the speed of light is so fast, a tiny inefficiency will be multiplied millions of times in a short fraction of a second and whittle away at the light until it's gone, giving the appearance that it goes away instantly when the source is removed. Is this anywhere close to correct?

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Danger
Gold Member
Welcome to PF, Fluoronator.
That is correct.
There's also the consideration that it's almost impossible to get two mirrors perfectly parallel, so there'll always be 'leakage'.

f95toli
Gold Member
Yes, that is correct.
However, using the right equipment (high-finesse cavities) it is actually possible to "store" light between two mirrors for many milliseconds which is actually a very long time if you think about it.

Yes thats pretty much the reason. When light is reflected by a mirror energy is converted to other forms of energy, mostly heat. This is described by the reflection coefficient.
Although I suppose you could store photons by using bose-einstein condensate, it would slow the speed of the photons and thus increase the time they can be stored for, but the bose-einstein condensate would then start to absorb the photons.

tot
if you had a hollow sphere with mirrors on the inside and somehow got light on the inside could it retain the light for a few seconds?

if you had a hollow sphere with mirrors on the inside and somehow got light on the inside could it retain the light for a few seconds?
Like Ed Aboud said, it will depend on the reflection coefficient, in other words how good can you mirror reflect the light.

Cheers

A.T.
How about a fibre-optic cable loop? Assuming you made the join in an illuminated room, wouldn't light be contained in the cable for all time?

russ_watters
Mentor
Fiber optics involve losses too.

How efficient is the best mirror we can make (or have observed?) (This is an actual question, not food for thought).

f95toli
Gold Member
How efficient is the best mirror we can make (or have observed?) (This is an actual question, not food for thought).
Depends on the frequency, if you mean at ANY frequency I would assume that the Nb cavities used in cavity-QED have the record, although they are of course operated at MW frequencies.
I know Serge Haroche's group have access to some really good cavities that can "store" a photon for several ms, somewhere on the web there is a picture of the cavity they are using; you should be able to find it if you search.

How about photons orbiting a miniature black hole?
http://en.wikipedia.org/wiki/Photon_sphere
this is by my understanding the only way in wich one could store light, any reflection no matter how close to perfect would lose energy on contact with the reflecting surface, in a black hole, the ring of photons will always be there no matter what unless one deviates them from their paths. its like the earth orbiting the sun, once its going its not going to stop or change for a long long time haha

this is by my understanding the only way in wich one could store light, any reflection no matter how close to perfect would lose energy on contact with the reflecting surface, in a black hole, the ring of photons will always be there no matter what unless one deviates them from their paths. its like the earth orbiting the sun, once its going its not going to stop or change for a long long time haha
One question. How do you get them back out again ?
A bigger black hole ?

DaveC426913
Gold Member
this is by my understanding the only way in wich one could store light, any reflection no matter how close to perfect would lose energy on contact with the reflecting surface, in a black hole, the ring of photons will always be there no matter what unless one deviates them from their paths. its like the earth orbiting the sun, once its going its not going to stop or change for a long long time haha
The issue, I believe, is that photons do not really "orbit". An orbit involves a tradeoff between gravity and velocity. Photons do not lose velocity.

While you can theoretically get photons to travel around a black hole, it is not stable like an orbit is.

One question. How do you get them back out again ?
A bigger black hole ?
well i dont know what you mean by get them out again, as in get them out of their orbit? and i believe that since they travel at their speed limit, the speed of light, then they will stay there for ever unless you disrupt them and making them bounce of out of this ring. ie maybe a mirror? or fiber optics?

The issue, I believe, is that photons do not really "orbit". An orbit involves a tradeoff between gravity and velocity. Photons do not lose velocity.

While you can theoretically get photons to travel around a black hole, it is not stable like an orbit is.
well that is the idea, since photons always travel at the speed of light then there must be at a certain point close to the event horizon that when a photon reaches that point at a tangencial vector, the gravitational pull of the black hole will then create the centripedal force exactly equivilant to the centerfugial force caused by the photon going around in a circle at the speed of light, at that radius from the center of the black hole. and i belive that since billions upon billions of photons pass by the black hole, one is bound to hit the "sweet spot"

tot
why do fiber optics have losses? I suppose there is actually some equation that is larger and more accurate than snell's law?

Well snell's law is correct, and i dont think that there is something more accurate because snell's law only deals with the actual difraction of light not the energy, the only problem is that when light goes through a medium, energy is lost in the transition because some of it is absorbed and then turned into heat or other forms of energy. It happens in mirrors too.

Why are we not able to capture and store light between two mirrors? I'm going to take a guess at the answer but I'd like to hear from someone who knows for sure...
I'm wondering if you actually can for a small amount of time, but the inefficiencies of an imperfect mirror cause some light to be absorbed and converted to heat. Since the speed of light is so fast, a tiny inefficiency will be multiplied millions of times in a short fraction of a second and whittle away at the light until it's gone, giving the appearance that it goes away instantly when the source is removed. Is this anywhere close to correct?
actually, light is special. it is a particle but not totally coz it's not like other stuff that we can store. it is also a wave which makes it like a ray.

however, when it comes to mind whether we can store the light, and in anytime, we may take it out of the box and use it for some purpose?

is it feasible? the answer is YES.

there is one thing that uniquely exists in the universe: light

and there is also another thing that uniquely exists in the universe which can catch the light.

that is the Black hole!

if you look up some documentation about the universe, you will surprisingly find that the black hole can store the light amazingly.