# Light battery

This is more of a theoretical question.
If one could constrct a spherical chamber one meter in diameter with a virtually perfect interior relective surface how much energy in the form of light could one cram into it?

Is it theoretically possible that at some point in the future we will have the materials and technology to come close enough to such optical perfection for such a device to be a pratical energy storage medium?

Hi Opal!

Read just a few days ago of a beam of light entering a chamber being switched off before it could emerge from far side of such a chamber. This was in connection with new theory that the speed of light is decreasing, i.e. it is not a constant as previously thought.

`` &lt;http://wnd.com/news/article.asp?ARTICLE_ID=39733&gt;``

I don’t think it will ever be a practical energy storage medium. Let’s say you use a 20-meter chamber, roughtly the size of a subway car. If you let light into it and seal both ends, light will bounce back and forth 15 million times a second. Even if the reflectivity of each mirror is 99.99999%, almost all the light will be absorbed by the mirrors within a second or two.

Even if you could create perfect mirrors, it’d be difficult to create a high enough energy density. You can’t accumulate energy in this container; all you can do is shine a bright light into it and close the lid. The photons that happened to be inside will be trapped. Let’s say you use a light source as strong as the sun (which is pretty darn difficult to achieve!). That’s 1300W/m[sup]2[/sup]. For a subway-car size container (10x3x3m), you can trap about 1x10[sup]-3[/sup] J. That’s just enough to light a tiny LED for 1 second (assuming 1 volt and 1 mA).

That’s just the energy flux of the sun’s radiation at the Earth’s surface. It’s only that low because the Earth is so far away. If you contained the energy from a square meter of the Sun’s surface, it would be much, much higher.

Ahem, I could be wrong, but I don’t believe that the energy density of the sun at the earth’s surface is 1300 degrees.

Yes, but I used a 10x3x3 meter box in my example - that means an opening of 3x3 meters. If you use a football-field sized solar reflector or an array of mercury lamps, maybe you can get 100-solar brightness over the entire opening. That still won’t allow you to store a useful amount of energy in that box. It’s something like 0.01% of the energy stored in a AA battery.

No one said anyting about degrees. The reference was to energy flux, which is not a measure of temperature. Watts per square meter does not equal degrees.

I was mistaken, BTW. That’s the energy flux above the atmosphere. Also, the number is closer to 1400 W/m^2, anyway. (Cite, page 2)

But the point remains. That’s how much power the Sun delivers per unit area at the Earth’s orbital distance…not how hot the Sun makes the Earth’s surface.

I did say I was speaking theoretically. So, theoretically what if you had some way of making the material strictly one-way wioth regard to transparency?

Then how do you get the energy out? Even if you open it as quickly as possible and close it immediately, most of the light will probably escape, right? And now that the light is shining out, how do you harness its energy efficiently? Sounds pretty impractical to me.

If youo had some way to make the inside an absolutely perfect reflector as well as being one-way (which is a logical contradiction, I think), it would accumulate photons wherever you put it that wasn’t complete darkness - the only trouble is that, being a perfect reflector on the inside, there’s no way to know for sure how much energy you’ve caught without opening it - an action that might be catastrophic if a lot of energy had been collected.

I’m pretyy sure that the second law of thermodynamics comes into play here - such a device would passively decrease local entropy (or am I wrong?).

Could you not weigh the chamber? Wouldn’t its mass slowly increase as more light entered?

I wondered that myself.

On a sort of tangent, assuming that such a perfect device was possible (and assuming it was made from magical materials that wouldn’t simply catch fire or disintegrate under the conditions inside), is there a limit to how many photons you can cram into a given region of space? Would it become ‘full up’?

I believe that photons are massless.

Our SDSAB members Q.E.D. and Chronos addressed this topic in an earlier thread

IANA physicist, but I thought I would run my musings on this OP past the teeming masses.

We have a spherical chamber. This chamber is constructed from a material that has two special physical properties:

A) the inside of the chamber reflects light perfectly
B) the outside of the chamber is transparent to light

So it is built out of the stuff they use to make mirror sunglasses. The *really * expensive stuff.

We can postulate that the concentration of light photons inside the chamber will increase with time. From this we can make two inferences:

1. Individual photons have momentum. Photons reflecting off the inside of the chamber will exert pressure. That pressure will increase with time.
2. A system of photons has mass. The mass of the chamber will increase with time.

So, if you wait long enough, this is what I think will happen to your light battery:

i) The pressure acting on the inside of the sphere will increase until such time it explodes. At this point be careful to look away.

Now if we add a third attribute to the physical properties of our light battery, we can overcome this problem:

C) the chamber is very, very strong

In this case, I think a different destiny might await our battery:

ii) The mass of the sphere will increase until such time it becomes a black hole. At this point be careful not to stand too close.

A bit of a WAG really, we need a physicist’s view!

There is no such material. As Mangetout said, if it existed, it would violate the laws of thermodynamics. It would continue to accumulate energy and long before it explodes due to photon pressure, it would exceed the temperature of the light source. What you have is a device that pumps heat against a temperature gradient and decreases entropy.

Try reversing your mirrored sunglasses. You’ll see that it transmits the same amount of light. One side has higher reflectivity than the other, but the transmission is the same both ways.

Actually, something like this was suggested in an old science fiction story. I’m drawing a blank on who just now (might have been Microcosmic God by Theodore Sturgeon). It bothered me at the time.

The problem is, in any real system, if the absorption of the walls is not absolutely zero then, no matter how small it is, it will eventually absorb all of your light, even if none is lost to transmission through the sides, or to absorption of any residual stuff inside the box. Light travel being as fast as it is, for any box that’s not ponderously huge, the light energy will thus be absorbed by the walls and transformed into heat energy in no time at all.

I am sure that you are right that there is no such material. It may also be correct to say that such a material could violate one or more of the laws of thermodynamics, but I am less certain about this. While the energy of the system under question will increase, it does not necessarily follow that its temperature will increase. Hence, no contradiction of the 2nd law of thermodyanamics.

We’re talking about Maxwell’s Daemon here, assigned to light duties. It’s a nice thought, but it doesn’t work.

(Note that this page has a pretty good explanation, but spells it wrong.)