8 February 2010
Last month I spent a week out at Los Alamos National Laboratory vaporizing things with a high powered laser. Now, as I drown in data that I collected out there, I thought I’d take a moment to talk about lasers.
When I tell people that I zap things with lasers, I can almost see the mental images flickering behind their eyes. They tend to look something like this:
Man, I wish.
I hate to burst your bubble, but working with lasers, although very cool, is not as showy as most sci-fi depictions. To help understand why, let’s first talk about how lasers work.
The word laser is actually an acronym for Light Amplification by Stimulated Emission of Radiation, and that actually sums up how they work quite well. There are lots of different types of lasers these days but they all share a few common characteristics. First, you need the “lasing medium” – that is, the stuff that will give off the light. The first lasers used artificial ruby crystals, but now there are lasers that are based on everything from CO2 gas to organic dyes to various semiconductors. The laser I use for my research is a Nd: YAG which stands for Neodymium-doped Yttrium Aluminum Garnet crystal.
Ok, so we have a “lasing medium”, now we need to make it shine. Things give off light when they have electrons in high energy levels jumping back down to lower energies and getting rid of the excess energy as photons. In a laser, the goal is to get something called “population inversion”, meaning that there are more electrons in excited energy levels than there are in the ground state. This is typically done with a flash lamp in a process called “pumping“. By shining very intense light on the lasing medium, the electrons all get excited and the laser is ready to, well, lase.
Of course, the goal of a laser is to have a nice narrow beam, but if you just have a lump of stuff with excited electrons, the light will be given off in all directions. A fluorescent bulb is a good example of this. A lasing medium acts in much the same way, shining a diffuse light in all directions, unless we do something to it. The secret is to place it between two mirrors, one which reflects all light, and one which reflects only some of the light that hits it.
Initially, the atoms in the lasing medium give off light in all directions, but some of those photons will end up traveling along the laser, bouncing back and forth between the two mirrors. Here is where the laser really starts working. It turns out that when you have photons of a certain energy traveling along through a bunch of atoms with excited electrons that have the same energy, you get “stimulated emission“. The first photons cause the electrons to jump down and emit identical photons. And I do mean identical. Yes they have the same energy (and therefore the same frequency/wavelength/color), but the new photons also have the same phase, polarization and direction as the initial ones. They are completely indistinguishable at the quantum level.
As you might expect, this stimulated emission leads to a chain reaction. Each photon of laser light can stimulate new photons to join it. Since one end of the laser is partially transparent, the result is a narrow beam of light made up of identical photons: a frickin’ laser beam!
Wonderful. Now that we understand how they work, I want to address a few misconceptions about lasers in science fiction and popular culture in general.
1. Laser beams are visible.
With a laser, the idea is to have all of the light going in the same direction, right? That means that if you can see the laser beam from the side, as shown in this picture from Star Trek, and in pretty much every depiction of lasers ever, then something isn’t right! The light is being scattered out of the beam. If you’ve ever used a laser pointer you know that even though it gives off visible (usually red or green) light, you just see a dot where it is pointing. Now, if you shine it at someone who is smoking, or if you use it outside in the fog, or in a dusty room, you can see the beam because the light is reflecting off of particles in the air (smoke or water droplets or dust).
So, yes sometimes visible lasers in air are plausible because there could be stuff in the way, but visible lasers in space? No way!
There are some other caveats to this also. Not all lasers use visible light! The Nd:YAG that I use for my research and the similar laser used by ChemCam emit infrared light. It is completely invisible, no matter what. This makes it incredibly dangerous to work with lasers like this, especially when first lining up the optics, because you can’t tell if the laser is being reflected around the room! Just because these lasers are not visible doesn’t mean they can’t destroy your retina in a millisecond, so we wear special protective goggles designed for the specific wavelength that the laser emits at all times when the laser is on.
Also: you can’t see the laser beam traveling from the source to the target. It’s going at the speed of light. So all those sci-fi depictions of laser blasts whizzing by the hero’s head like tracer bullets: wrong.*
*Yes, I know, some sci-fi explains this by invoking pulses of plasma and not actual lasers. That’s a whole different can of worms with its own issues. Suffice it to say that most people *think* those blasters, phasers, etc. are supposed to be lasers, so I’m debunking that misconception.
2. Pew pew pew!
That’s not what they sound like. I know. I’m sorry.
Low powered lasers don’t really sound like anything. And can you imagine how annoying it would be if they did? At the grocery store checkout: pew pew pew! Using a CD or DVD player: pew pew pew! Laser pointer: pew pew!
Yes, but the “pew pew” sound really comes from things like Star Wars, depicting lasers used as weapons. So what about big lasers, capable of vaporizing things? Nope. With higher powered lasers, at least the kind I work with, the main sound comes from the flash lamp. It’s sort of a ticking noise, one tick per flash, one flash per laser pulse. Now, when we crank up the power or use something called a “q-switch” to make each pulse shorter and more intense, you get another noise that comes from the laser actually vaporizing things. That noise is more of a “crack” or “pop” noise. In fact, I once popped some bubble wrap in the laser lab while my collaborators were aligning the laser and totally freaked them out because they thought it was the laser. Oops…
The popping noise is essentially the same thing as thunder: a rapidly expanding ball of plasma causes the air to be compressed in a shockwave. Our laser plasmas are tiny, so they just make a little noise. Lightning bolts (plasma formed by electrical discharge) are rather larger, and so is their noise. Many of my experiments are done zapping rocks inside a vacuum chamber, and it’s always fun to hear the noise fade away as we decrease the air pressure in the chamber.
3. Lasers as weapons.
They’re really not that great.
There are a lot of issues with using lasers as weapons. First of all: the optics. For a laser to be useful as a weapon, you would have to focus the light as tightly as possible on the target. De-focus at all, and you might still blind them, but there won’t be much vaporization going on. The precision required for the optics to do this makes a hand-held laser really impractical. The slightest bump or wiggle and all of a sudden your gun is a high-powered flashlight. There’s also the issue of air. Anyone who has looked through a telescope or out over a parking lot on a hot day has seen the shimmering mess that the air can make of an otherwise clear image. Now imagine trying to shine a tightly focused beam of light through that mess and hitting a target. Not an easy task. The military has worked on this to some extent with adaptive optics used for giant plane-mounted anti-missile laser, but it is a significant problem.
The air poses another problem: it absorbs light. In fact, a high enough powered laser can cause the air itself to break down into a ragged line of plasma. I’ve seen this in the lab and it is awesome. The problem is that plasma is full of free-flying electrons, so it absorbs light. A laser strong enough to use as a weapon would also be strong enough to turn the air to a plasma, which would then block the laser from hitting its target. One way around the plasma problem is to use a pulsed laser. As long as the pulses are timed so that the plasma has dissipated before the next pulse is fired, the plasma is not as much of a problem.
I mentioned lightning earlier and that’s relevant here. There is a way to make use of the “plasma issue”, because plasmas conduct electricity. So in theory it would be possible to use a laser as a long-distance taser! The laser would first create a conduit of plasma out of the air, and then with a high enough voltage, an electric shock could be send down the plasma to the target. This would not be a subtle weapon: at this point the lightning analogy is not really an analogy anymore. It would basically be a lightning gun, and would make a noise to match. I thought I was being really clever when I thought of this, but it turns out I’m not the first: the US military has experimented with them.
Another problem with lasers as weapons is the power source. It takes quite a lot of power to make a laser capable of doing damage, and it would probably not be practical for a person to carry such a power source around. I’m playing the video game “Fallout 3” right now, and the energy weapons use things called “microfusion cells” for ammunition. Right now, we don’t even have power-positive macro-fusion cells, so bullet-sized fusion powerplants are not available yet.
Finally, there is the issue of collateral damage. The thing with light is that it tends to reflect off of things. This means that anyone using a laser weapon better be wearing the appropriate protective eyewear or else their own target is going to blind them. Aside from the practical issues with blindness, the Geneva conventions also specifically forbid laser weapons that cause blindness (in other words, all of them).
In my opinion, I highly doubt that lasers will ever be practical as pistols or rifles. Maybe as large mounted guns on tanks or something. But really, the most likely place for lasers as a viable weapon is space. Without air, the difficulties with plasma creation and turbulence are removed. The issue of power and optics remain, but I could plausibly see a satellite or space station with the stability and power to use a laser as a weapon. It might still be difficult to focus on a distant target, just due to the physical limits on the optics, but the advantage of near-instant travel-time might be of benefit when you’re aiming at a target thousands of km away, traveling at thousands of km per hour.