Researchers for the Navy announce a significant achievement on the way to producing a Free Electron Laser or FEL. rdbrewer at Ace of Spades passes on the news, and Ace, umm, gets excited. The press release isn’t very informative — in particular, it isn’t clear what “producing [electrons] from thin air” means; I don’t believe in perpetual motion machines, so the energy has to come from somewhere — and I haven’t tried to track down more information.
FELs are interesting to me because they really aren’t “lasers” at all. The inventor, one John Madey at Stanford University, called them that because they produce light at a fixed wavelength, and that’s what lasers do, but the principle of operation of the FEL is quite different.
Light is a form of electromagnetic radiation, and electromagnetic radiation is peculiar. There exists such a thing as an electric field, which is really a stress or modification of space within a certain area; there also exists such a thing as a magnetic field, which is a different modification of spacetime. If you move an electric field you get a magnetic field, and if you move a magnetic field you get an electric field; scientists long ago decided that this means that the two types of space-stress, which seem different, are really the same thing looked at from different directions.
It’s possible to inject energy into space in such a way as to create a moving magnetic field. That field produces a moving electric field, which produces a moving magnetic field, which produces… that’s electromagnetic radiation. The little zone of space containing energy alternating between electric and magnetic is called a photon, which can be thought of as a thing, a particle, that moves through space at the speed of light. The rate at which the magnetic and electric fields alternate — the frequency — can be changed, and since the photon always moves at the speed of light, you can pick one of the two fields and find its strength varies with distance. At one point, the magnetic field (for instance) is strongest; move a bit, and first it’s less, then it comes back to its original value farther along. The distance between the two maximum strength points is the wavelength.
There exist electrons, zones of space that contain a permanent electric field surrounding a point, and there are ways to make electrons move. Moving electrons moves the electric field, which creates a magnetic field, which creates, etc. etc. The least-effort way to do that is a transmitting antenna. There are materials in which electrons can move fairly freely. If you take a piece of such material and move electrons in it, that creates photons, electromagnetic radiation, and there’s an effect rather like a spring. If the antenna is the right size the electrons get to the end and bounce back, so you don’t have to expend energy forcing them to move through the material. Electrons oscillate, bounce back and forth, losing only the energy that goes into space; on each bounce you can touch them up with a bit more energy to compensate for the energy radiated, lost as photons, so they keep bouncing back and forth.
As with any spring, the size of the antenna controls the bounce rate. If the antenna is big, it takes a (relatively) long time for the electrons to get to the end and bounce back to where their energy can be replenished. The electric-magnetic fields swap (relatively) slowly, so the photon created has a large wavelength. As it turns out, this relationship is directly proportional. By tuning — making the antenna a specific size — it’s possible to produce photons of any wavelength. However, it’s at this point that you hit the first major snag.
Remember that the photon isn’t really an object, a thing, it’s a zone in space containing energy expressed as electric and magnetic fields. It turns out that the relationship between how much energy is stored in the photon and what you might think of as its size is backwards — the more energy the photon has, the smaller it is. Light has a high frequency, a short wavelength, and therefore more energy per photon than radio, which has low frequency and a long wavelength. If, as with a lot of explanations, you think of the photon as a particle, an object or thing, this is confusing. That’s why people prefer to talk about “waves” at low frequency (or energy) and “particles” at high energy (or frequency).
The inverse relationship means that if you want to produce photons of light you need a really, really small transmitting antenna, so small that it stops being a piece of material and starts being individual atoms. Fortunately for us, it turns out that atoms have electrons around them, and those electrons are confined to specific energy states. Every electron has the same energy, but if they’re “squeezed” into small zones more energy can be stored, just like identical springs can have different amounts of energy stored in them by compressing them differently. Chemistry is what happens when atoms swap electrons with one another, and moving electrons cause moving electric fields which cause moving magnetic fields which cause… in some cases the electrons have enough stored energy that when they move a photon with enough energy to be “light” results, and that’s the earliest and easiest way to make light. Set up the right chemical reaction, and the moving electrons emit useful photons: fire.
That works backwards, too. If a photon with the right amount of energy arrives, its energy can be stored as electron-squeeze. When the electron “springs back” its energy is released as a photon identical to the one whose energy was stored. Pump energy into a bunch of atoms, and some of them store energy in their electrons. It turns out that a passing photon of the same energy can “trigger” the electron to release its photon, so if you trap your atoms between two mirrors to insure that there are lots of passing photons, you build up a syndrome where the stored energy is released as photons which release more photons. That’s how a laser works.
If you simply pump a lot of energy into a collection of atoms, they start moving faster and faster until they start shedding electrons, and the moving electrons produce photons. That’s incandescence, as in light bulbs.
The trouble with dealing with atoms is that it isn’t efficient. Atoms have many electrons, all with different amounts of energy, and pumping energy in results in photons of many different energies, most of them too low to be useful as light. In an incandescent light bulb we simply live with it, pumping in enough energy that at least some of the photons have enough energy to be visible; in a laser we carefully select atoms that have electrons of the right energy level and painstakingly insure that the right electrons get swapped around, but it’s still only a minority that do what we want. It would be really handy to have an antenna the size of a light-wavelength, so that we could use the spring effect to create light efficiently. Nothing is small enough to use for an antenna, though.
But remember that, at root, it’s all just electrons moving back and forth to create magnetic fields, which produce electric fields, and so on. It turns out to be possible, in fact relatively easy as such things go, to produce a “beam” of electrons — a bunch of electrons moving together, more or less in the same direction — and the more energy you put into the beam the faster the electrons move. If you then wiggle the beam back and forth, like waving the end of a hose to make the water stream wave, the electrons are moving sideways to the direction of the beam. Moving electrons produce photons, and if you trap the photons with mirrors they’ll come back and make the electron beam wiggle harder — a “Free Electron Laser”, so called because the electrons are “free” in space rather than being bound to atoms. It isn’t really a laser as such, because the photons aren’t triggering the release of other photons that are bound up with the electrons in atoms, but the effect is analogous.
It takes excrutiatingly exact arrangement of the apparatus. Remember that the size of the antenna controls how fast the electrons bounce back and forth to produce photons. The equivalent in the FEL is to make sure the wiggle happens at the right rate, and that the photons returned by the mirrors are synchronized with the wiggle to make it bigger, instead of just randomly pushing or even canceling the motion. If it can be made to work, it would be a method of efficiently producing light of any desired frequency (color) in amounts dependent only on how much energy can be pumped through the system, and that’s a sort of Holy Grail in the technology, especially if you want to build a weapon.
A weapon is a device for pumping enough energy into its target to disrupt its functioning. The problem with making a weapon out of light is that it’s absorbed by the atmosphere, and the absorption depends on the exact composition of the atmosphere, which changes according to weather and the amount of energy already absorbed. Your death ray isn’t of much use if it’s absorbed by the air before it hits the bad guys. Theoretically, an FEL can be tuned instant-by-instant to change the wavelength it emits, in order to get past the atmosphere molecules that were affected an instant ago — which would mean the energy gets past and on to the target. The “breakthrough” described in the press release appears to be a way to get the electrons and the photons better synchronized, which is a big deal. It’s too early to tell whether or not it’s a big enough deal.
 or maybe not, but if you pretend such a thing exists you can make useful devices. Physics is weird, down deep.
or not, but once again, if you pretend (“assume”) that, you can build something that works.
why? Nobody really knows. It just happens.
there’s no corresponding “thing” for magnetic fields, which is inconvenient.
or photons — the process creates many of them.
”energy” is an abstraction with no independent existence — things move, and we call whatever made them move “energy”, but all we have to deal with is the motion.
not really, of course — they can’t go faster than light — but the faster they go the more energy they have, which is all same-same as “speed” as far as what we want to do goes.