Let's say you have 2 plane mirrors, placed perfectly parallel to each other. There is a beam of light bouncing back and forth between these 2 mirrors. Assume that there is no energy lost from the beam of light, and both mirrors are perfectly smooth. What would happen if you move the 2 mirrors closer and closer together, until they finally touch? Since light is a form of EM energy, and EM energy has to be moving constantly at the speed of light, what would happen to the beam of light then? Can you consider it to have been 'stopped'?
In practice such devices are called optical cavities and they are used to trap light inside the cavity. This makes a number of interesting things possible including strong coupling between light and atoms and enhancement/suppression of spontaneous emission. The frequency of the lowest available mode is determined by the cavity length. As the length decreases, the lowest cavity mode increases in frequency and thus energy. If you compress the cavity adiabatically, then you expect the field to be in the instantaneous ground state of the cavity which corresponds to ever increasing energy. The source for this energy increase is, of course, the work you do to compress the cavity. Does this mean you could really stop light by compressing the cavity? No and here is why: First, it would require an infinite amount of energy to compress the cavity to zero separation. Second, as the frequency of the light is increased, the light will find it easier and easier to escape from the cavity. Third, zero separation doesn't mean much anyway since quantum effects would take over long before you reached zero separation. In practice you can't even get close to stopping the light in this way, cavity lifetimes are just too short and you can't manipulate the cavity very well beyond a certain point (piezos are usually used, but they can only get you so far).
This is related to the how heavy is light thread (http://www.sciforums.com/showthread.php?t=49333). The pressure from the light between the two mirrors is equal to E = PV. This is different from the result in the other thread because you are constraining the light to go in only one direction. So, as you move the mirrors together keeping E constant V goes to 0 and P becomes infinite. -Dale
An additional factor leading to energy loss is diffraction, since the morrors have finite area so energy gets loss at the edges.
Whether this is relevant or not, I read something a few days ago that some scientists slowed light down to a few hundred (feet or meters, can't remember) per second. I don't remember the details, but they passed it through some kind of material, I think it was a rare earth metal. I'll try to find the article.
Found some articles on it. I read around a bit and looks like they're able to increase and slow down the speed at which light travels. I read one article that said they got a laser to travel at 300 times the speed of light through caesium atoms. Some of the slowing down light articles: http://www.google.com/search?client...hl=en&q=slowing light down&btnG=Google Search
So this means in practice you can never be able to get the 2 mirrors to touch because P becomes infinite? But since P=0 shouldn't E=0 as well?
dzerzhinsky, See my post above, the frequency of the light is always increasing as you compress the cavity, so the energy of the light is increasing. Combined with the fact that the volume is going to zero, the pressure goes to infinity which is why it takes an infinite amount of energy to compress the plates.
Nope, write P as a function of V: E = P V ---> P = E/V By conservation of energy E is constant so you simply plot this as V for your x axis and P for your y axis. Then as V goes to 0 (from the right) P increases without bound. Physics Monkey's point is that, in addition to this effect from classical mechanics you also have a quantum mechanics effect. As the cavity gets smaller the longest wavelength gets smaller. A smaller wavelength means a higher frequency which means more E. This means that the pressure actually increases faster than predicted with just my classical treatment. Either effect alone causes an infinite pressure as V goes to zero, both together mean that you better push really hard! Please Register or Log in to view the hidden image! -Dale
