The Laser Weapon Myth
by J. L. Doty
copyright (c) 2009 by J. L. Doty
released: April 9, 2009
“He climbed into his 1967 Chevy Impala, brought it up to light-speed and headed for the center of the galaxy.”
Now wait a minute! 1967 Chevy Impala? Light speed? Center of the Galaxy? Something’s wrong here. It doesn’t add up. I guess my suspension of disbelief has just been disrupted.
There are any number of definitions for the term suspension of disbelief, but here I refer to the reader’s ability to suspend their natural inclination to disbelieve the fantastic or unusual and enjoy the story. It certainly is necessary in science fiction and fantasy, but I believe it’s necessary in any form of fiction, and in some forms of non-fiction, especially that generated by the government, Wall Street and Madison Avenue. We’ve all experienced that moment where you’re deeply immersed in a good story, only to have something disrupt that immersion. Perhaps the hero entered the room wearing a green shirt and exited wearing a red one. And suddenly, where a moment before you were almost part of the story, now you’ve been thrown out of it and are a person looking at the pages of a book in which something doesn’t add up. It’s the author’s responsibility to avoid such disruption.
The ‘67 Chevy that travels at light speed is an intentionally blatant example of this. Almost everyone knows what a Chevy is. It’s a nice car. It goes fairly fast, but not light-speed; it goes down the road, but not to the center of the galaxy; and it certainly is not going to protect its passengers from the ravages of interstellar space. So if an author of a work of science fiction wrote such a line, and he didn’t explain why this 1967 Chevy Impala is different from those we know, he risks disrupting the reader’s suspension of disbelief.
I recently read an excellent novel, well plotted, well written, with nicely orchestrated battles between warships in space using laser weapons. But the laser weapons constantly violated the laws of physics, repeatedly disrupting my suspension of disbelief. Now the author might have gotten away with it if all he had done was violate the laws of physics, because students of physics know the laws of physics are always changing. A few hundred years ago Isaac Newton taught us all that the Aristotelian physics of air, earth, fire, water and aether was all bunk. Then Einstein taught us that Newtonian physics, while not bunk, was still incorrect, though the error is inconsequential until objects of finite mass approach the speed of light.
Like the Chevy, lasers are a common part of ordinary, everyday life. There’s a small solid-state laser in every CD and DVD player to read and write information on the disk. Ultraviolet (UV) lasers are used extensively for laser vision correction in LASIK procedures. Cable television signals travel most of the way to your home in an optical fiber, and optical fiber carries most of the information in the internet’s backbone, all with signals generated by tiny solid-state lasers. Laser weapons, on the other hand, are much more formidable devices, but they are still just a laser.
For several decades now the most common laser weapon has been a gas dynamic laser (GDL), which combusts some rather nasty materials and passes the byproducts through a supersonic expansion nozzle, essentially a two-dimensional rocket nozzle. GDLs have been explosively pumped with materials like hexanitrobenzene and/or tetranitromethane; chemically pumped with substances like deuterium-fluoride, hydrazine, gaseous chlorine and/or molecular iodine, chemicals which are nasty, toxic and frequently explosive. GDL’s emit electromagnetic radiation (light) in the 1 to 10 µm (µm = micron or micrometer: one-millionth of a meter, 40 millionths of an inch) range, and researchers really try hard to be far away when a GDL decides to malfunction and turn into a big bomb.
Note: I’m going to frequently use the term radiation here because light is electromagnetic radiation. But don’t confuse the term with x-ray or gamma-ray radiation, which we all know can be quite harmful in sufficient dosages. In this context, radiation is a much more general term.
If you’d like to know a little more about the properties of light, check out What is Light? Or, if you’d like to know a bit more about lasers, check out What is a Laser? Like this article, both of these keep the equations to a minimum since they’re not really needed to explain these concepts.
The laser as weapon
Science fiction can reduce a laser weapon like a GDL from the size of a small building to a sidearm that the hero can carry on his hip. But as long as it’s a laser, its output (electromagnetic radiation, light) has a well-known set of operational characteristics and limitations. Once the light leaves the weapon, it must obey the laws of physics and the limitations that those laws impose. Perhaps the limitations of laser radiation are not as widely known as those of a Chevy, but they are nevertheless there, and much too well understood to be ignored, even if for no other reason than good form. But for those who know and understand them, and we are not that rare, and we are frequently avid readers of science fiction, when an author improperly employs a laser as a weapon in his work, it can ruin a perfectly good piece.
In any case, the author of the book that bothered me employed lasers that violated both the laws of physics and the operational parameters of known lasers. And at no time did he explain why his lasers were different. With liberal paraphrasing, he wrote something like:
“The defender’s warships kept their infrared lasers focused to a tight spot on the enemy warships at a distance of 50,000 km, eventually burning their way through enemy armor.”
Like most weapons, this weapon’s function is to concentrate a large amount of energy into a small area where that energy is absorbed by the target, but there are any number of phenomena, both simple and complex, that can disrupt the effectiveness of any weapon. For instance, fire an artillery shell at a cotton bed sheet, and as long as the sheet is not too close to the muzzle flash, the shell punches a nice hole through the sheet without exploding. Here, the weapon is not matched to the target so little of its destructive force is delivered to the target: the sheet. For a laser weapon, the spot might be blurred into a larger area, the energy might not be absorbed efficiently, or the energy of the beam might be reduced in transit. More often than not, all of the foregoing play a role in reducing the energy density at the target. In this case, the author of the novel apparently did not understand a number of the aspects of laser beam propagation. The first we’ll discuss, and probably the most relevant, is the physics of diffraction, the effects of which were first observed by Francesco Maria Grimaldi in the mid 17th century.
Diffraction of light
Consider a lens that is uniformly filled with light of a single wavelength, as in the figure below. Note that the distribution of energy in the aperture of a laser is not uniform as in this example, but that doesn’t change these numbers much, and in fact it actually exacerbates the situation. The lens redirects the rays of the beam, and in simple ray theory all of the rays exit the lens and converge to the same spot known as the focal point, concentrating the energy of the beam. Many of us will remember as children spending a lazy, sunny afternoon burning up ants with a magnifying glass—please, no unpleasant e-mails from the SPCA—an example of concentrating the energy at the target. In fact, the lens does redirect the rays to that single, infinitesimal point, but as the light approaches the spot, diffraction causes it to bend outward, producing a finite spot diameter.
The spot is actually a central disk with concentric rings around it. The central disk is named after George Biddell Airy who first described the phenomena. The Airy disk contains 83.8% of the energy in the spot, while the outer rings contain the rest. The diameter of the central disk (Ds) is related to the diameter of the aperture of the lens (Da), the wavelength of the light (λ), and the focal length of the lens (f) in the following way:
This relationship holds even when f is far greater than Da.
So, back to the space opera: what constitutes a “tight spot,” a few centimeters, one meter, ten meters? Assuming an infrared wavelength of 1 µm and a focal distance of 50,000 km, the figure below is a plot of the relationship between the aperture and spot diameters on a logarithmic scale.
It’s not the lens that’s important here, because science fiction can come up with any number of futuristic ways of redirecting the rays to the focal spot. Rather, the issue is the propagation characteristics of the energy once it leaves the lens or its equivalent, and what’s important is the required beam diameter at the source (Da) to achieve a given spot diameter (Ds) at the target. The information in the figure tells us the smallest that the spot can be. It can be larger through defocusing, but never smaller.
Notice that for a 10 cm spot (0.1 m or 4”) the aperture must be larger than 1 km. Even for a 10 m spot, the aperture is still larger than 12 m; and yet, can one really call a 10 m spot a “tight spot?” Furthermore, in this example I used essentially the shortest of the infrared wavelengths. Most laser weapons produce light with a wavelength closer to 10 µm, so multiply all the aperture sizes above by ten—a 10 km diameter lens? The physics of light implies that the warships in that space opera employed weapons apertures hundreds of meters in diameter, which is so unlikely, or perhaps merely inelegant, that it reduced the enjoyment in what was an otherwise excellent book.
Next, consider the small laser sidearm carried by our hero. Assume the sidearm has an aperture of 1 cm and a wavelength of 1 µm. By the time the 1 cm beam reaches a target at 100 m, it has grown to 2.44 cm in diameter, reducing the energy density at the target by a factor of six (the square of 2.44). At 200 m the spot diameter is 4.88 cm and the energy density is down by a factor of 24.
What about the classic pencil-thin beam? A pencil is about 5 mm in diameter, so for such an aperture the beam is nearly 5 cm (2”) in diameter at 100 m, 10 cm at 200 m. Or we can turn it around and ask what aperture is required in the laser rifle to produce a pencil thin spot at a target 100 m away: the answer is 5 cm. The classic image of a laser rifle firing a pencil-thin beam of light just doesn’t wash, and the situation gets even worse when considering the atmospheric effects discussed later in this article.
Target reflectivity is critically important. A ruby laser that emits intense pulses of bright red light will punch a hole through a piece of black paper, but have no effect on red or white paper because most of the energy is reflected. I recall once having had a shirt and tie destroyed by an infrared laser that emitted about ten pulses per second. A mirror in our experimental setup had been accidentally misaligned, and since the beam was invisible I walked through it, leaving a line of burn spots across my clothing at chest height—I was wearing my laser eye protection and the laser wasn’t powerful enough to penetrate my clothing. Interestingly enough, the damage to the tie was minimal with burn spots that were almost undetectable, while the cotton shirt had several, clearly defined burns. The tie apparently reflected most of the energy because of the characteristics of its material—as I recall it was some sort of polyester—whereas the cotton in the shirt clearly absorbed most of the energy. I think the lesson to be learned here is that in a good science fiction story, the best personal body armor against infrared lasers may be a good polyester-knit-leisure-suit. It was the 70s, after all.
This phenomenon is used to positive effect in some cosmetic surgical applications where it’s termed selective absorption. For example, that tattoo you got after that party when you were 18—the one that says “Live Hard Die Young”—just doesn’t work now that you’re trying to get that promotion in corporate America. Simply put, a red laser is strongly absorbed by blue tattoo ink, but not by the pink skin of many races, and can be used to selectively damage the tattoo ink with minimal harm to the surrounding un-tattooed skin. Unfortunately, the tattoo ink is itself embedded in skin, so such treatment is not without side effects and can result in blistering and discomfort.
Since lasers emit light at very specific wavelengths, and only certain lasers emit enough energy to be considered as weapons, everyone knows the few wavelengths that might be employed, and everyone knows what wavelengths to reflect to defeat the absorption of the energy. It’s a sad weapon that can be defeated by holding up a piece of paper of the right color.
Thermal blooming and dielectric breakdown
Thermal blooming and dielectric breakdown occur only in atmosphere, not in the vacuum of space. When an intense laser beam propagates through atmosphere, if the air absorbs even the tiniest bit of energy from the beam—and it always does—then the beam slightly heats the path through which it propagates, reducing the density of the air at the center of the beam. This creates a negative lens in the beam path; the heating may only be a fraction of a degree and the lens very weak, but the effect is cumulative over long paths. Hence, while the initial wave front of the beam is undistorted, the energy which follows diverges rapidly (it blooms), broadening the beam into uselessness.
One solution to this is to concentrate the energy into pulses in which the back of the pulse is so close to the front of the pulse that the air hasn’t had time to heat up yet. But if we compress the energy into too short a pulse, the electric fields in the pulse become so intense that the air experiences dielectric breakdown—it sparks, the same effect as lightning—sucking considerable energy out of the pulse to produce the spark, and further distorting the propagation path. Additionally, a single pulse is rarely sufficient for a kill, so even if the pulses are long enough to avoid dielectric breakdown, if they are too close together, the first few pulses may not experience thermal blooming, but subsequent pulses will. So, even when science fiction produces a small, hand-held, laser weapon that can emit enormous amounts of energy, atmospheric effects place fundamental limitations on the amount of energy that can be delivered to a target once it leaves the aperture of the weapon.
Scattering occurs when small structures deflect minute amounts of energy from the beam. Particulate matter like fog—tiny, airborne water droplets—can completely obliterate the most intense beam in a few dozen meters. This is known as Mie scattering, named after G. Mie who, in 1908, published a theoretical treatment for scattering from particles of a size greater than or equal to the wavelength of the light. Dust, or large amounts of debris and vapor created in the vacuum of space during an interstellar battle, while probably not as dense as fog, will have a cumulative effect over large distances. Let’s go back to that space opera and consider an enemy warship that has just been vaporized by a thermonuclear warhead, or some such weapon. I’ll hypothesize—purely conjecture here since I myself haven’t recently vaporized any enemy warships in interstellar space—well, actually I have, but only in my fiction—that for a considerable time afterward, and throughout a large volume surrounding such a detonation, the cloud of metal vapor and carbonized plastic and tissue will be all but impenetrable by a laser, producing a sizeable blind spot for any laser weapons. Large battle, lots of such detonations, lots of such blind spots, and the laser becomes a useless doorstop.
Furthermore, such vapor can act in much the same way as atmosphere. So the atmospheric effects of thermal blooming and dielectric breakdown previously ignored because this battle is taking place in the vacuum of interstellar space, now begin to play a role.
Incidentally, in the late 19th century Lord Rayleigh developed the theory for scattering from particles much smaller than the wavelength. Recognizing that all matter is not a truly homogenous structure but consists of small, discrete particles—atoms and molecules—even if we could make perfectly clear, non-absorbing glass or air, the molecules in the medium will themselves scatter light. It’s interesting to note that Rayleigh’s theory proved why the sky is blue. Certainly he didn’t anticipate this, but Raleigh scattering from the glass molecules in optical fibers is one of the primary loss mechanisms and limiting factors in long distance fiber-optic communication. Again, a minute effect, but the cumulative result of kilometers of fiber eventually attenuates the signal drastically. The very structure of matter itself attempts to defeat us.
Another scattering mechanism with considerable effect over long distances is atmospheric turbulence: small fluctuations in the density of air due to localized motion or other factors. Turbulence cells are referred to as turbules, range is size from a few millimeters to a few centimeters across, and have the effect of a potato-chip like prism of glass that varies from a thickness of zero on one edge, to a few tenths of a wavelength on the other. Again, a minute effect, but over a distance of 100 m such a potato-chip prism deflects a small portion of the energy by a meter or more. Over long distances the accumulated effect of many turbules can easily cut the beam energy in half, or worse.
So, this is all very negative and one might ask: Then how do I use a laser as a weapon in my fiction? My opinion is, don’t.
Consider this, in the last thirty years billions of dollars have been dumped into laser weapons research—as part of the Space Defense Initiative and otherwise. Now I am not privy to any classified information, so I can’t say that laser weapons haven’t been widely deployed by our military. But when classified weapons research is successful—think of the stealth fighter here—and the devices are deployed significantly, while we, the public, may not know the classified details behind them, we know what they are, we know they are out there, and we know they’re being used regularly; again think of the stealth fighter. Lasers just don’t make good weapons.
So the next question might be: But I want to use a laser as a weapon in my fiction, so how do I make it work without violating the laws of physics? The answer is that you probably have to change it in such a way, and to such an extent, that it’s no longer a laser.
Let’s go back to that 1967 Chevy Impala. If you’re not writing humor or satire, why use a Chevy to travel at light speed, with all its known limitations that must be explained away? Why not use a spaceship, a starship, or something like that? If it really doesn’t exist, you can make it anything you want it to be, and you don’t have to explain away all the aggravating details of physics. Do the same with weapons, make up something like phasers or photon torpedoes, and in the story line don’t go too deeply into the physics of your weapons. You might get it wrong. And if you do, you’ll lessen the impact and entertainment value of an otherwise good work of fiction, at least for some of us.