# Applications of Trapped Photons

by student34
Tags: applications, photons, trapped
 P: 261 First off, I must ask if the reflection of a photon is truly an elastic collision; is it? If it is, then wouldn't that mean that there is perpetual momentum? And if that is true, then can't this momentum be converted into perpetual energy? Apparently, they have successfully trapped a photon, http://www.cosmosmagazine.com/news/e...on-trap-built/ . A second thought that I have is how this could help the solar energy business. The obvious problem with solar energy is the unpredictability of sun hours. If we can trap photons at will and for a relatively low cost, then maybe we can store access photons for unusually cloudy periods of time. Does anyone have any other ideas or suggestions?
 Homework Sci Advisor HW Helper Thanks P: 12,447 A photon bouncing between two perfect mirrors, imparting a recoil each time, is a common conceptual problem for students. It is not a perpetual motion machine - the trick is to figure out why not. On the scale of photons, mirrors are a bit odd anyway- i.e. the reflection path is only followed on average. See the Feynman QED lectures for more on this at an accessible level. IRL: there are no perfect mirrors. It does not make all that much sense to trap photons for later use - why not plate the area the trap takes up with photocells and just store the resulting energy?
P: 261
 Quote by Simon Bridge A photon bouncing between two perfect mirrors, imparting a recoil each time, is a common conceptual problem for students. It is not a perpetual motion machine - the trick is to figure out why not. On the scale of photons, mirrors are a bit odd anyway- i.e. the reflection path is only followed on average. See the Feynman QED lectures for more on this at an accessible level. IRL: there are no perfect mirrors.
Ah, thanks!

 It does not make all that much sense to trap photons for later use - why not plate the area the trap takes up with photocells and just store the resulting energy?
I guess it would come down to what is cheaper and more effective. Apparently, a photon trap has never been successful before, so who knows how we can further develop this technology.

 Homework Sci Advisor HW Helper Thanks P: 12,447 Applications of Trapped Photons It's always possible to say "who knows" and imply that anything can happen - but it's not a helpful approach to science. For reflection see also: http://www.physicsforums.com/showthread.php?t=497487
HW Helper
P: 4,300
 Quote by student34 If it is, then wouldn't that mean that there is perpetual momentum? And if that is true, then can't this momentum be converted into perpetual energy?
That is also not true. In theory it is not hard to imagine something with "perpetual momentum": take any lossless system such as an ideal pendulum or a ball bouncing without friction.
The thing is that as soon as you start extracting energy, that energy has to come from somewhere. Although you can mask it pretty well by making the conversion process increasingly convoluted (every time someone says "but aren't you losing energy here?" you replace that component by a more complicated subsystem), in the end any extracted will be taken out of the objects "energy budget". This will be momentum or temperature or rest mass... in any case some finite quantity.

The second problem, as pointed out, is that in practice there ARE no ideal situations like that.
P: 261
 Quote by Simon Bridge It's always possible to say "who knows" and imply that anything can happen - but it's not a helpful approach to science.
That is most certainly not what I meant.
 Homework Sci Advisor HW Helper Thanks P: 12,447 No worries - you'll notice as you learn that there are ways of talking about ideas that are generally considered "unhelpful" and it is good discipline to avoid those ways. Avoiding "who knows" type phrases means you have to look more closely to see what sort of development paths are likely ... you'll see that photon storage is not a likely direction. If you look through the related discussions below, you'll get more of an idea. Note: the link in post #2 is pretty much what compuchip is talking about along with a description of why one might come to think that the momentum changes at each reflection could be used to generate energy. Pretty much the entire site is good for developing the kind of thinking you need to figure out various "perpetual motion" ideas. It's valid to think them up as a way to tell if you are missing some physics somewhere.
 Sci Advisor P: 1,474 Qubit storage. The quantum state of photons are easy to manipulate but difficult to store. A scheme for storing photons and preserving their quantum state would be a massive boon to the field of quantum optics! Claude.

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