# Why can't we store light in the form of light?

We can store cold (ice), heat (i.e. hot water bag) and electrical charge (batteries). We can even "store" a magnetic field in a magnet. We can convert light into energy and then, if we want, back to light. But we can't store light in form of light in significant amounts. What is the explanation of that in physics terms?

• We can store light wavelengths Mar 3, 2013 at 23:18
• There's a specifier missing from the question. If we're interested in generating photons and then using them for practical purposes later, the current answer applies. If, however, you want to store light from the sun for later use, a very different limitation exists, which is that transmission and reflectivity are tradeoffs. By opening the "door" to let photons in, you lose an unacceptable number of the ones already in. Mar 3, 2013 at 23:49
• You don't store those things, you only delay the return to equilibrium such that you can exploit that process more to your advantage Mar 4, 2013 at 11:07
• Sorry Tobias, I'm missing something here (and it is for sure my fault). Isn't what you are saying true for all the things that begin to release their energy in the very moment in which we store them? According to what you say an airplane which is air-to-air refueling in not storing the fuel in the tank of the other plane, because it is just delaying the moment in which it will fall. And storing food in our stomaches is just delaying the moment in which we will be hungry again. Where is my misconception? Mar 4, 2013 at 11:55
• @AldCer Nice analogy with the stomach ;-) What I mean is you do not store the specific form of energy (light, heat of a fire or solar heat, electrical potential of a generator, ...) but convert it into another form of energy (photovoltaic cell, heat in water, chemical potential in a battery) which has a longer half-life time so you have more time to e.g. physically distribute it somewhere else and/or release it at a later time. And light (or any electromagnetic radiation) is one of the - if not the - shortest-living forms of energy at all due its restricted existence at the speed of light... Mar 4, 2013 at 12:57

For the photons that make up light to exist they have to be travelling at the speed of light. This means that to store them you have to put them in a container where they can move around at the speed of light until you want to let them out.

You could build the container out of mirrors, but no mirror we can currently build is 100% reflective, or indeed can be ever 100% reflective. Usually when a photon "hits" the mirror it is absorbed by one of the atoms in the mirror and then re-emitted back out into the container. However, occasionally the photon either won't get re-emitted (leaving the atom in an excited state) or it doesn't hit one of the atoms and makes it way through the mirror and out of the container.

While the chances of this happening for an individual photon are low, there are lots of photons travelling very fast so it happens many times thus causing the light to "leak" or decay.

Building a near perfect mirror is hard, so it's easier to convert the light into something that can be stored and then convert that back into light when you need it.

• +1. One remark: maybe you should make it more clear that the perfect mirror is not just a technical issue, rather that quantum tunneling is inherent to nature. So not even a mirror we can imagine building would be able to reflect light 100%. Mar 3, 2013 at 23:50
• @Wouter - I wasn't sure about that, but yes. Mar 4, 2013 at 8:47
• Thanks Chris...I'm going to store your enlightening answer:-) Mar 4, 2013 at 12:36
• Maybe one should add that there exist real energy storage methods while the examples given by the questioner are not really storage, because storage means subsequent recovery and use. A thermos stores heat or lack of it for a limited number of hours, as the container you are proposing. While gas oil natural gas wood, and dams and even batteries are real storage methods with unlimited time and can be tapped as needed. Mar 4, 2013 at 13:46
• @annav - see John Rennie's answer for that argument. Mar 4, 2013 at 13:50

We can store cold (ice),heat (i.e. hot water bag)

But we can only store heat temporarily, just as we can only store light temporarily. Your ice pack will eventually heat up and your hot water bottle will eventually cool down, just as light stored between two mirrors will eventually escape.

and electrical charge (batteries)

Charge isn't stored as charge in a battery. A chemical reaction generates the charge. This would be the same as converting the light to something else, storing that something else then regenerating the light when needed.

We can even "store" a magnetic field in a magnet.

Not the same thing, as we are not storing magnetic charge in a magnet.

A method of storage that might just fit your criteria is storing light in a Bose Einstein condensate. Light pulses can be brought to a halt in a BEC, and in principle stored indefinitely.

• Many thanks John, I definitely got your points related to batteries and magnetes. Coming to storing light between two mirrors vs storing heat or cold is clear again your point from a physicist's perspective, even though my question was actually related to storing significant amounts of light (i.e. useful for many and different purposes). In this case, my next question would be how light "stored" between two (or millions of) mirrors could prove itself useful. p.s. no way to access your link Mar 4, 2013 at 9:18
• Hmm, I don't know why the link isn't working for you as it works here. Anyhow the link is deas.harvard.edu/haulab/publications/pdf/Stopped_Light_2001.pdf Mar 4, 2013 at 10:03
• Re your next question storing light as light seems a pointless exercise. We don't store electricity as charge, we store it as chemical energy in a battery because that's easier, cheaper and more useful. If you want to store light put the energy in a battery then use the energy to power an LED. Mar 4, 2013 at 10:06
• Your last two points seem to be technicalities, as we can store charge in capacitors and magnetic field in inductors Feb 25, 2015 at 4:51
• @raptortech97: we can store charge temporarily in a capacitor and we can store a magnetic field temporarily in an inductor. But then we can store light temporarily between two mirrors - a point I make in my post. Admittedly I concede a well designed supercapacitor can store charge for hours or days, but not for weeks or years. Feb 25, 2015 at 6:31

I'll try to answer your question in the spirit of how you asked it. Basically you can't really store anything you've mentioned. The ice will eventually heat up, the heat will cool down and the battery will lose charge. A box of mirrors with light shined in it will "store" light like your other examples but it will lose the energy much faster then any of them. Imagine throwing a superball into a box and closing the lid, does it bounce forever and "store" the energy?

We can store light - just for very small amounts of time. I'm no physicist though, so perhaps this link isn't what you intended?

http://news.bbc.co.uk/1/hi/sci/tech/3308109.stm

It's hard to store light as light because the most common way light interacts with matter is through absorption and emission, which is how mirrors work. However light rays can be bent by gravity, so it would be possible to arrange several massive stars in a way such that a light ray would move in a loop around the stars without energy loss.

The answer by John Rennie and subsequent comments reminded me of this TEDtalk about energy storage from light.

I don't know the details, but this is what I understand they did: they've studied the electronic and absorption properties of foils made of nanotubes, in particular when combined with the result of some impressive research on infrared imagery. The combined product gains the extraordinary property that it can absorb light and store the energy for longer periods of time and in a cleaner way than batteries (our main and perhaps only real method for energy storage). This energy could be free (because you could just attach these flexible foils to your window for example) and it could even be shared through the coherent re-emission of light (from your window to your neighbours window for example).

It's not storing light in the form of light - what the question asked for - but I think it's as close as we're able to get to storing sunlight for a semi-long period of time efficiently and conveniently, something photovoltaic cells are still struggling with. I imagine Justin Hall-Tipping is making it sound more advanced than it is at this time, but nonetheless it has some great potential and I think it's definitely useful to mention it here. I reiterate that I'm unaware of the details and am not an expert in this field though.

Isn't that what we call "heat" is just a form of electromagnetic waves in the infrared spectrum? So we may say that we can store an electromagnetic wave in a certain media, like water in a thermos. But the first thing, we do not use a thermos filled with hot water as an energy storing container, we just use it for a very simple purpose, to store hot water! But if we were using it as, let's say, a kind of a battery, with a some kind of devise inside that would convert heat into electricity or a mechanical power, to move a vehicle, that would be a different story. So, may be, the very first thing that we need is to find such a media to store the sun light, as that hot gas containing atoms of rubidium or may be that should be some sort of a solid matter, and a second step is to create a sort of a convertor to transform that collected energy into a mechanical or electrical power. By the way, filling a car tank with hot boiling water instead of gas sounds like not a bad idea!))