Data Analysis
Our experimental data was inconsistent, which illustrates some of the problems with using lasers as a weapon. We ran two different tests with our laser in order to determine the amount of power that would be needed to realistically weaponize a laser. In the first test, we held a piece of paper in front of the laser and varied the power to record how long it would take the laser to set the paper on fire. The laser we is a green Verdi laser (wavelength = 532 nm) with a maximum power of 5 W.
The first test we ran was the paper test. The results are presented graphically below:
As we can see, the amount of time it took the laser to burn the paper generally hovered around five seconds, with a high of 11.8 seconds. The top group represents the tests that failed to burn the paper after 30 seconds. In this experiment, we found that the minimum power needed to burn a piece of paper was 2.75 W.
The balloon data was less consistent than the paper data. We did discover that the color of the balloons had an impact on the amount of time it would take the laser to pop a balloon – pink balloons (which are much further from green on the electromagnetic spectrum then green or yellow) popped much faster and at significantly less power then either green or yellow balloons. For the pink balloons, we were able to pop a balloon instantly at only 2 W and puncture (but not pop) a balloon at 1 W. We were able to pop one green balloon at 2.5 W, but could not replicate this result when we attempted it a second time. We were able to pop both green and pink balloons at 3 W. This data tells us that the color of the target object does play a significant role in determining how much power and how long is necessary to burn or puncture a target.
The reason behind this difference is color absorption. Colors with shorter wavelengths (such as blue) reflect more light at shorter wavelengths and absorb more light at longer wavelengths. On the other hand, colors with longer wavelengths (such as red) do the opposite – reflecting more light at longer wavelengths and absorbing more light at shorter wavelengths. Since our laser was green (a shorter wavelength color), balloons that were also colors that have shorter wavelengths, such as the green and yellow balloons, should take more power and more time to pop than balloons with colors that have longer wavelengths, such as pink.
Military Applications
We saw in the lab that even at a low power of 2.75W, a laser can burn a hole through a piece of paper, or at 2W, pop a pink balloon. But how close are we to actually utilizing lasers as combat weapons? Not too far. Our research is still years away from achieving the ray guns, blasters, and light sabers that we see so much of in science fiction. Current laser weapons work by using light to excite an atom to shift its outermost electrons from one state to the next. The electrons then get ionized and torn from the atom, which leaves behind a host of positive charges. These positive charges repel and explode, and this explosion produces a wave of radiation. There are three types of lasers: gas, liquid, and solid-state. While all work in generally the same way, only gas and solid-state lasers have been utilized as weapons.
Chemical lasers, a type of gas laser, use chemical reactions to create energy. In February 2009, installed on a Boeing jet, the megawatt-class Airborne Laser (ABL) was able to shoot down long-range ballistic missiles, which could eventually progress to shooting down enemy attacks and aircrafts. The ABL is a 1.3 pm chemical oxygen-iodine laser (COIL). COILs produce a single wavelength radiation, which allows for a very well focused beam. When fired, it produces enough energy in a five-second burst to power a typical household for more than an hour. But such power comes at a price. Not just the ABL, but all chemical lasers built for the military have been bulky and logistically complex, and the cooling required to keep them running also makes them heavy. As seen in the picture, only the largest of aircrafts would be able to carry such a weapon.
Photo by Edwards Air Force Base
The need for a more efficient device prompted the government to invest money in solid-state lasers; they typically use crystal or glass as lasing media. Solid-state lasers are electrically powered, which makes them smaller and more attractive for combat, but they have only just been able to reach enough power to make them weapon-grade lasers. In the case of firearms, lasers have only been used as a tool to guide the targeting of other weapons. A laser sight for example, beams a small, visible-light laser onto a target to help aim a gun, but does no direct damage. They are now commercially available as an attachment to firearms. Other laser firearms include non-lethal laser weapons. These are typically used to disorient an opponent. The Personnel Halting and Simulation Response (or PHaSR) is a non-lethal laser weapon developed by the Air Force. It temporarily impairs aggressors by illuminating, or dazzling them, removing their ability to see the source of the laser, and is the first man-portable, non-lethal deterrent. Future uses include protecting troops and crowd control. Additional research has gone into making laser weapons that cause blindness, but the ethical considerations raised by permanently handicapping someone caused the United Nations to issue the Protocol on Blinding Laser Weapons in 1995 that forbids laser weapons whose sole purpose is to cause permanent blindness. As such, taken as a whole, the technology on building handheld laser weapons has lagged; we will not be seeing any ray guns or blasters anytime soon.
Photo by U.S. Air Force
We are making headway in solid-state lasers on a larger scale however. In 2009, engineers at Northrop Grumman Corporation were able to develop and test an electric laser that emitted 100 kW continuously for five minutes, and in April of this year, a 15 kW laser mounted on a cruiser ship was able to blast a weaving boat from the Pacific Ocean. The boat first caught fire, and then burst into flames. It is the first test at sea of such a gun and a major milestone in outfitting future Naval fleets with laser weapons. The Navy hopes to get the energy up to 100 kW and eventually take down incoming missiles.
In conclusion, existing laser weapons look promising, but they are still a long way off from reaching the ones portrayed in science fiction. Gas lasers have the potential to produce a great amount of power, but their bulk makes them inefficient to actually use in the battlefield. Solid-state lasers also have potential, but still lack the necessary power needed to create a true weapon-grade laser. But perhaps in a few years, we will finally be closer to making a little piece of fiction real.
Portrayals in Popular Culture
The Star Wars films provide us with many classic examples of laser use in science fiction. One of the most famous laser weapons in Star Wars is the Death Star – a massive space station that also functions as a high-powered laser. As shown in the clip below, the Death Star’s capabilities are quite devastating as it can destroy an entire planet. But could scientists ever develop lasers to be this powerful?
It is possible to scale our data in order to predict just how much power would be needed to achieve such a feat. We can estimate that the wavelength of this laser pulse was around 550 nm, due to the fact that the Death Star’s laser was green. Thus we can calculate the energy per photon in the Death Star’s pulse:
Energy per Photon = h*f
F = c/lambda
Lambda = 550 * 10^-9 m
C = 3*10^8
F = 5.45 * 10^14
h = 6.63*10^-34
Energy per photon = 6.63*10^-34 * 5.45*10^14 = 3.6 * 10^-19 joules per photon
In order to isolate the rate at which the Death Star emits photons, we can divide the estimated power of the Death Star (watts) by the above value. But in order to do this, we should first predict the energy that a laser would need to produce in order to explode a planet of this size/material.
The density of the earth is 5.5 grams per cubic centimeter.
The density of the paper we used for laser testing, on the other hand, has a density of .72 grams per cubic centimeter.
The ratio of the densities between these two materials is 5.5/.72 = 7.64
This ratio can be used to scale the wattage we used for paper to the wattage necessary for a planet like Alderaan.
Obviously, the destruction of Alderaan was nearly instantaneous, so we can assume a higher power is necessary for the instantaneous burning of paper. A regression analysis of our data will help us predict this new value for the instantaneous burning of paper.
Regression equation à Y = 97.251e^-.608x
Y = 97.251e^-.608(0) = 97.25 watts.
The thickness of standard paper is .103 mm. The thickness of Alderaan, of course, is much larger than this. Let us say that Alderaan has a radius similar to Earth’s: 6378.1 kilometers. We can use this radius value as the measurement of Alderaan’s “thickness.” We use it as a thickness value because we predict that a laser would only need to bore halfway through a planet in order to destroy it. There are two reasons for this. First of all, as we mentioned in the earlier in our post, the ionized particles that are shot from a laser repel each other, resulting in an “explosion” effect. The second reason has to do with a process called spallation. Spallation occurs when a high-powered laser cuts through a rock, heating up the moisture contained within that rock. Vapor within the subsurface of the rock begins to explode outwards, which results in a high degree of stress on the overall system. Alderaan is one big rock, most likely trapping a large amount of moisture within its subsurface. Therefore the laser would only need to travel far enough to result in a heating of the inner layer of Alderaan, resulting in an explosion.
Thus for our purposes, we will use the planet’s radius as its “thickness” value. The ratio of the planet’s thickness to the paper’s thickness is…
(6378.1*10^3)/(*10^-3) = 6.2*10^10
Using these ratios, we can use the effective power of our lab laser in order to predict the effective power of the Death Star.
Death Star Power = Lab Laser Power (for instantaneous burning of paper) * Thickness Ratio * Density Ratio
Death Star Power = 97.25 * (6.2 * 10^10) * 7.64 = 4.6*10^13 watts
This is the minimum number of watts the Death Star would need in order to generate a laser that could destroy Alderaan. Given the Power, and the Energy per photon of the Death Star’s laser, we can also find the rate at which the Death Star emits photons.
Power = Rate of photon emission * energy per photon
4.6*10^13/3.6*10^-19 = 1.28 * 10^32 photons per second
Given today’s standards of technology, these are incredibly large numbers for power output and photon emission rate. However, new developments in laser technology suggest that it could be possible for a laser to be built with these power specifications. For example, scientists are currently building an unprecedented laser that can produce 200 * 10^15 watts of power. This laser is unlikely to be militarized, however; researchers will use it to investigate the nature of matter and antimatter particles in space. Still, it seems that the writers of Star Wars created the Death Star with relatively good scientific underpinnings. Interestingly, if you closely watch the Death Star clip, several of its lasers appear to “join” together before creating a unified, larger laser. This appears to violate very basic properties of light – light beams would not “join together” upon intersection. Rather, they would simply pass right by one another and continue heading in a straight direction. However, the aforementioned “super laser” under construction does combine multiple lasers into one “focal point” that results in a single, incredibly powerful beam.
Thus, Star Wars appears to have a decent amount of science behind its fictional technology.
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U.S. Air Force. Accessed 29 November 2011. http://www.af.mil/news/story.asp?storyID=123012699