The Laser Weapons Myth
J. L. Doty
copyright © 2009–2016 by J. L. Doty
released: June 19, 2016
Some years ago I read an excellent science fiction novel, well plotted, well written, with nicely orchestrated battles between warships. Unfortunately the author chose to use lasers as weapons, and he constantly violated the laws of physics in doing so, repeatedly disrupting my suspension of disbelief. At one point he wrote something to the effect of:
The good guys spaceships focused their infrared lasers to a tight spot on the enemy spaceships at a distance of 50,000 km, and slowly melted through their steel armor.
This is heavily paraphrased because we’re not mentioning names here, but what he wrote made me cringe. 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 modern automobile, lasers are a technology that is 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. All modern telecommunications signals—including cable television, the bits and bytes on the internet, and telephony—are carried on optical fibers to within a mile or two of our homes, and those fiber signals are all generated by small, solid-state injection laser diodes no larger than the head of a pin. Laser weapons, on the other hand, are much more formidable devices, but they are still just a laser.
We all know that lasers are used extensively in modern warfare. But they’re used exclusively for guidance, control and targeting, not as the primary means of delivering destructive energy to the target. The image below is that of a Reaper drone firing a Hellfire missile.
The small turret beneath the “chin” of the drone sends out a laser signal that the missile can detect and use to guide itself to the target. That’s how our military is able to send a missile or smart bomb right through the door of a bunker with pinpoint accuracy. For more details on how that’s accomplished, see What is a Laser?
I began my career in the early 70’s at Bell Laboratories working in a laser lab rather like the one pictured below.
I remember one day several of us were in a lab working with an IR laser, and suddenly the traces on the oscilloscopes went flat. At first we were perplexed, but as I stepped to one side, we all heard, and I felt, little pops as the laser burned holes in my clothing; it wasn’t powerful enough to burn all the way through the cloth so I didn’t suffer any skin burns.
We turned off the laser, turned on the lights, and discovered that one of us had accidentally elbowed a mirror, deflecting the laser beam off the table and across the room. It was a pulsed laser operating at 10–20 pulses per second, and we also noticed a straight line of blackened spots running across my chest spaced about a centimeter apart. The shirt was pale blue, and the tie brown. We also noticed that while there were burn marks on the shirt on either side of the tie, there were none on the tie itself. The width of the tie and the spacing of the burns on my shirt indicated that there should have been four or more burns on the tie—it was the 70’s after all, and wide ties were fashionable. At that point Bell Laboratories paid a bunch of PhDs their exorbitantly high salaries to research why the laser burned Jim’s shirt, but not his tie.
It turned out that the shirt was cotton—or perhaps a cotton-polyester blend—and natural fibers like cotton tend to absorb the infrared, converting it into heat. On the other hand, the tie was polyester knit, and synthetic fibers tend to reflect the infrared, so very little energy was absorbed and converted to heat. The moral of this story is: If you’re writing a science fiction novel, and you want to properly attire your hero or heroine in laser body armor, put them in a polyester knit leisure suit. It was the 70’s after all.
Returning to that phrase that bothered me so much:
The good guys spaceships focused their infrared lasers to a tight spot on the enemy spaceships at a distance of 50,000 km, and slowly melted through their steel armor.
Visible light is composed of electromagnetic radiation with wavelengths in the range of 0.45 to 0.75 µm. The infrared extends from 0.75 to about 20 µm. Keeping that in mind, it’s interesting to note that steel exhibits the following reflectivity’s:
1.0 µm (near infrared)
10 µm (far infrared)
At 10 µm, close to where most of the big weapons lasers operate, steel reflects most of the energy that hits it. In fact, polished steel is frequently used to make mirrors for those wavelengths. An infrared laser is not going to melt steel armor, because the energy of the light isn’t absorbed and converted to heat.
Another problem with this statement is the velocity of light, which we all know is infinitely fast—but it really isn’t, is it? In fact, light propagates at 299,792,458 m/s, which is 186,755 miles/s. That’s really fast, but it still takes 0.1667 sec., or one sixth of a second, to cross 50,000 Km. So the round-trip time from observing a target, and hitting it with our destructive laser energy, is one third of a second. During that time, if the pilot of a modern fighter jet—not a futuristic fighter—pulls back on the stick, he or she can pull 10 Gs, which will deflect the aircraft to the side by 18 feet, so to make sure we hit the target, we have to smear the spot around by 36 feet.
When I first considered writing this article, I wondered who came up with the idea of using lasers as weapons: Was it we scientists pitching it to the government, or the government asking us to make a new weapon?
Charles H. Townes was an American physicist and inventor known for his work on the theory and application of the MASER (microwave amplification by stimulated emission of radiation). When he built a MASER in his laboratory, he was one of the first to actually demonstrate stimulated emission of radiation. In 1964, he shared a Nobel Prize with two other scientists for “. . . contributions to fundamental work in quantum electronics leading to the development of the maser and laser.” That’s a fancy way of saying “. . . he built a maser.” Born in 1915, he was an active member of the scientific community through much of the 20th century.
Some years ago I came across an old interview with him that had been posted to the internet. In it he pointed out that it was the science fiction writers who first came up with the fundamental concept of a laser weapon. He specifically noted that in 1898, in H. G. Wells’s War of the Worlds, when the Martians invaded earth, they used Death Rays to good effect against the human race. According to Townes, that triggered the following sequence of events:
- The government thought Death Rays were a great idea, so in the early part of the 20th century they pushed scientists to come up with something like that—what a great way to kill lots of people.
- The scientists, striving at all times to maintain scientific integrity, really didn’t know what the heck they’d build to make a Death Ray, but they took the money anyway.
- Then the success of the Manhattan Project—the atomic bomb program during WWII—validated the concept of throwing massive amounts of government money at scientists in the hope of getting a new weapon.
- 1954: Townes and Gordon build the first MASER
- 1957: Gordon Gould (different Gordon) coined the acronym LASER
- 1960: Maiman et. Al. build a ruby laser
It’s interesting to note that once Maiman built an actual laser, within a year’s time three or four other types of lasers were demonstrated in other laboratories. Such cascade effects are quite common in scientific endeavor.
Einstein theorized the existence of stimulated emission in a couple of papers in 1916 and 1917. But by the mid-20th century no one had been able to demonstrate it, and a large part of the scientific community had concluded that, while Einstein was a smart guy, in this case he was probably wrong. I’ve always felt that makes Townes’s persistence and tenacity even more laudable.
Before discussing weapons lasers, we need some way of comparing the output of a laser to the destructive energy of conventional weapons. It turns out that a 1 MJ (megajoule) pulse of optical energy, if completely absorbed by the target in a short period of time, delivers the destructive energy of about 200 grams (0.44 lbs) of high explosive.
If you’re not familiar with the joule as a unit of energy, think of a 100 Watt light bulb. A Watt is the rate of delivery of energy, so a 100 Watt bulb is dissipating 100 J/sec of energy.
For several decades now the most power laser weapon has been the 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. The configuration of a GDL is conceptually depicted in the figure below:
The fuels are combusted in a chamber then passed through two-dimensional nozzles, compressing the hot gasses. On the exhaust side of the nozzles, as the air expands, a very high percentage of the molecules in the flow are in an elevated energy state—see What is a Laser?—providing quite a bit of gain. Put mirrors on either side and we have a laser, though we have to come up with mirrors that will survive in hot rocket exhaust.
A few examples of GDLs are:
- Chemical Oxygen Iodine Laser (COIL)
- Chlorine, iodine, hydrogen peroxide, potassium hydroxide
- Mach ~1 flow, 20 KW (kJ/s) at 1.315 µm (1 MJ in 50 sec)
- Explosively pumped GDL
- Combusts hexanitrobenzene and/or tetranitromethane with metal powder
- Deuterium fluoride laser
- Nitrogen trifluoride combusted with ethylene
- MW range at 2.7–2.9 µm
The chemicals in the COIL might seem rather innocuous, but that’s only because we’re used to chlorine and hydrogen peroxide in 5% and 3% aqueous solutions respectively. Hydrogen peroxide in a 30–60% solution is highly corrosive, and dangerously explosive. And chemicals like hexanitrobenzene and tetranitromethane are dangerous and nasty under any circumstances.
A friend of mine—who goes by the moniker of Elkhorn—recently gave me a copy of
Ignition! An Informal History of Liquid Rocket Propellants, by John D. Clark, Rutgers University Press, 1972.
Clark was a chemist and rocket scientist during the first half of the 20th century. During his career he developed and experimented with rocket fuels, and in the early 70’s he decided to write this book as a compendium of the work done with liquid rocket propellants up to that time. There are some interesting and humorous anecdotes, but by and large it’s rather dry reading. However, I noticed one trend: there is a class of rocket propellants that are far more effective at propelling rockets than those we actually use today. Unfortunately, they also tend to combust rather dramatically on the launch pad, destroying the launch vehicle, payload and passengers, as well as the launch pad and everything within a few hundred yards of it.
Isaac Asimov, who worked with Clark circa 1942, wrote a preface to the book in which he stated:
Now it is clear that anyone working with rocket fuels is outstandingly mad. I don’t mean garden-variety crazy or a merely raving lunatic. I mean a record-shattering exponent of far-out insanity.
There are, after all, some chemicals that explode shatteringly, some that flame ravenously, some that corrode hellishly, some that poison sneakily, and some that stink stenchily. As far as I know, though, only liquid rocket fuels have all these delightful properties combined into one delectable whole.
After reading Ignition!, I realized that the laser weapons scientists use the fuels the rocket scientists aren’t allowed to use because they’re just too fucking dangerous.
By the way, Clark was not merely a contemporary of Isaac Asimov; his college roommate at Caltech was L. Sprague de Camp. Clark dabbled a bit at writing science fiction and produced a couple of short stories, but apparently he never really pursued writing with any great fervor.
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.
We’ve already covered target reflectivity, and the problem with time delay due to the finite velocity of electromagnetic radiation. Let’s look at some other issues with light.
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.
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, while the lens does redirect the rays to that single, infinitesimal point, as the light approaches the spot, diffraction causes it to bend outward, producing a finite spot diameter, as depicted here.
The picture below is a photograph of such a spot looking down the axis of the lens:
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.
The fundamental meaning of this is that when given
f, λ, and Da,
Ds is the diameter of the smallest spot that can be produced without violating the laws of physics. This is referred to as the diffraction limit or diffraction spot. It also has an inverse relationship with the aperture: larger aperture, smaller spot, and vice versa.
As an aside, this limits the resolution of any lens, including a camera lens. If the diffraction spot is larger than the pixel size in the camera’s detector array, then adding more pixels won’t provide more resolution. In fact, expensive dSLR camera lenses are capable of resolving 20–30 Mp (mega-pixels), but one must pay several hundred dollars for such a lens. So the next time you’re buying a new cell phone, and the salesman tells you it’s got a 12 Mp camera, make sure you’re not paying for 12 Mp, because that cheap little lens is probably limiting your resolution to 3–4 Mp.
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, so we end up with the following situation:
The lens must be larger than our spacecraft.
What about the classic pencil-thin beam of a laser carbine? Our science fiction hero needs to kill a bad guy who is 100 m away. A pencil is about 5 mm in diameter, so to have a pencil-thin beam at 100 m, the output aperture of the carbine must be 5 cm (2”) in diameter.
If it’s a pencil-thin beam at the carbine, it’s not pencil thin at the target.
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.
Fog is perhaps the simplest and most obvious example of scattering. If you can’t see through it, light can’t get through it. A laser is not going to penetrate fog any better than the light you’re trying to see with.
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 in 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.
Pictured below is the Airborne Laser Laboratory:
Built in the 70’s as a “proof of concept,” it was a modified KC-135 carrying a kW class CO2 laser, and operated out of the Air Force Weapons Lab at Kirtland AFB in Albuquerque, NM. The turret on the top had an aperture approximately 1 m in diameter. They learned from experiments that half the laser energy was scattered away from the target by those little turbules.
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.
To put that into perspective consider the most powerful GDL lasers, which emit about 1 MW. Let’s compare that to earliest of all atom bombs, the weapons used to destroy Hiroshima and Nagasaki near the end of WWII. Their rated yield strength was approximately equivalent to 15,000 tons of high explosive. We’ve already discussed the following:
- 1 MJ pulse = 200 grams high explosive = 0.44 lbs
- 1 MW = 1 MJ/sec = 200 gm/sec = 0.44 lbs/sec
For our most powerful laser weapons to deliver the same destructive energy as the earliest nuclear weapons, the laser would have to deliver:
- Hiroshima & Nagasaki: 15,000 tons = 30,000,000 lbs
- 30,000,000 lbs @ 0.44 lbs/sec = 68,181,818 sec
The laser would have to operate continuously for over two years. If one considers modern nuclear weapons in the 1 Megaton range, it would have to operate for 144 years. Even for something as small as a 500 lb bomb, the laser would have to operate continuously for almost 20 minutes.
This is why we don’t see lasers deployed as weapons on modern battlefields.
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.