Death Rays 101 – The Laser

 

laser-guide

 

This is the first in a series of articles discussing the technology of science fiction, both from the perspective of existing usage in fiction and in the real world. It’s aimed at writers of genre fiction, and is intended to act as a resource for creating fiction over a wide range of tones, “hardness”, and even genres.

Rather than exhaustive technical details, I am intending to provide information about what technology will look like, and sound like, how much it might weigh and what might distinguish it from more mundane technology. I’m going to prioritize information that might be of narrative significance and I’m going to discuss existing genre conventions and how well they fit in with the Science.

Or at least, that’s the plan.

I welcome any feedback on how things work out. I’m intending that this blog turn into a substantial resource for writers, producers and anyone else involved in bringing the future to life, so if you enjoy this article please don’t hesitate to tell as many people as possible.

I’m going to start by looking at that perennial mainstay of futuristic killing – the laser.

 

So what is it –

LASER is an acronym, it stands for Light Amplification by Stimulated Emission of Radiation. This is one of those acronyms that is probably trying harder to sound cool than actually provide useful information, but it’s worth noting that “Radiation” is referring to electromagnetic radiation in general, not (usually) the ionizing radiation that many people associate with the term.

The laser fires a coherent beam of light, that is, the light is released in such a way as to all be travelling in the same direction and all at the same wave length. From a practical point of view, this means that a laser beam is simply very consistent and intense light.

The beam will travel in a perfectly straight line towards the target at the speed of light. The beam will eventually spread out over a wider area, but, depending on the laser, this will often occur over a very large distance. The minimum rate of spread can be very precisely calculated, as there are hard physical limits at work, but spread will also be influenced by the quality of construction of the laser.

 

How does it work –

There are a lot of different types of lasers, but the basics are as follows.

An existing light source, as well as a lot of additional energy, is released into a material called a Gain Medium. This material has the property of amplifying light that passes through it, by emitting new photons as other photons are absorbed, once this reaches a critical point is reached (the lasing threshold) more light is released than absorbed.

This is not is not free energy, as other forms of energy must be absorbed by the Gain Medium for lasing to occur, but the result is that light within the device is amplified. The laser amplifies only the light that meets specific criteria, which results in an extremely consistent beam.

Once the beam has been created, lenses can be used to focus it into a tighter or wider beam, or onto a small area. Different laser designs can produce continuous beams or shorter duration pulses.

 

Is any of that technical stuff actually important to me? –

The gain medium is the main thing that differentiates the different types of laser. The common association of lasers with “crystals” in fiction likely stems from early lasers using ruby rods in the gain medium, and not from any requirement to use crystals as lenses.

Lenses are not required for the production of a beam, although focusing the beam is important to a lot of applications.

Therefore the “laser requires specific rare crystal maguffin” plot line does not really work in hard science.

The ability of some systems to produce discrete pulses of energy allows for higher power, but shorter duration, energy release.

Many modern lasers user specially designed diodes as the medium, these are effectively LED’s that are also capable of producing the lasing effect. These laser diodes are easy to manufacture and integrate with other electronics, they also have properties that allow the pulsing, described above, to be achieved easily.

 

Important stuff that you should know

The thing that never seems to be talked about with regards to lasers in genre fiction is blindness. Eyes are extremely sensitive to light, so any laser that is capable of doing significant damage to a person, let alone any type of armored vehicle, would have the capacity to blind not just on a direct hit to the eyes but via reflection off of any nearby vaguely reflective surfaces. This is a major problem when considering the use of lasers as often depicted within science fiction.

This is also likely to become a real world problem as increasingly powerful lasers become readily available to the general public. Eye protection is possible to some degree, but might rely on some knowledge of the specific types of device that your opponent is using. The range of lasers and their capacity to produce continuous beams, also serve to increase the potential for accidents.

Lasers can be reflected, but a proportion of the energy will still be absorbed, depending on the quality of the mirror, if this absorption is large enough the mirror will be destroyed. Don’t have a character reflect a super-laser with their makeup compact. Lasers are also extremely prone to being blocked by suspended particles such as fog, as these particles absorb the laser’s energy without being able to transfer it anywhere else, or reflect the beam unpredictably, diffusing it.

 

Genre conventions

Lasers are generally depicted as being extremely accurate, and firing at long range, and are now typically depicted as beams rather than projectiles. They are often shown firing as continuous beams and depicted as having tremendous potential to penetrate and cut, but are sometime instead shown as rapidly firing discrete pulses. They are often depicted as red, especially in older media, with blue and green lasers becoming increasingly common reflecting changes in real world technology.

 

Is this correct?

The main issue is probably the depiction of laser beams within the atmosphere. A laser beam will be invisible until it hits something, Very high power lasers will produce enough fluorescence to be weakly visible and they are much easier to see in smoke or fog, although this will significantly reduce the range of the beam.

Lasers used in military applications are likely to release their energy as short pulses instead of a continuous beam, as this allows for more energy in the individual pulses.

The depiction of lasers as a hyper powerful cutting tool is also somewhat problematic. This may stem from the usage of lasers in industry to produce very accurate cutting, as well as an expectation that a continuous beam will continue cutting through a material. However, lasers transfer the majority of energy to the target in the form of heat which tends to be transmitted poorly.

Essentially, lasers are intrinsically quite poor at penetrating through most materials, but a very high power laser can be an effective cutting implement because of its ability to focus a lot of energy onto a small area.

 

Size and weight

This really depends on the laser, or more probably, on the power supply. The lasing component itself is likely to be insignificant in size compared to the power supply used to support it, at least with modern laser technology. The power supply could take almost any form, but in the near future it is most likely to be comprised of batteries or fuel cells. Capacitors are also likely to be involved and these also tend to be heavy.

It would also be possible for a handheld device to be made lighter by tethering it to a power supply by cable. This obviously reduces its mobility and flexibility.

There is unlikely to be any physical requirement for a handheld weapon to have a long “barrel”, but practically speaking it probably will, as this makes it easier for a person to aim and use the weapon, especially if they are accustomed to firearms. If the weapon has the capacity to adjust or refocus the light that it emits then this might significantly increase the size and unwieldiness of the device.

A larger laser is likely to be directly connected to the power grid or have its own dedicated generator and/or capacitors. Large lasers may incorporate complicated lens and mirror assemblies that may require some degree of mechanical insulation from movement or explosions.

 

Recoil

The question of whether lasers can produce recoil is a rather complicated one. The obvious knee-jerk response is that photons do not have mass (although this itself is an oversimplification), and therefore according to the classical physics equation they have no momentum.

However, Photons do have momentum, and thus also produce recoil, but this recoil is exceedingly negligible compare to a firearm delivering an equivalent amount of energy, although obviously more significant if the laser is releasing that energy in very short duration pulses.

I’d suggest that writers research this topic carefully if they somehow need to make a definitive statement on the issue in a story, but otherwise treat lasers as if they do not produce recoil, as even a very well educated audience’s expectation is likely to be that they don’t.

It’s worth noting that the same issues mean that a laser beam striking a target will not knock them over, although it might cause an explosion of evaporated material that could.

 

Sound

The laser beam itself makes no sound. Given that they are often associated with a large power supply, this may produce a humming noise and associated capacitors can make a distinctive whining noise as they charge. A very high power laser could make produce noise as the beam ionizes the air and there may be hissing, sizzling, or popping, as the water in a target evaporates.

 

Colour

A laser can be any colour of visible light as well as light in the invisible portions of the spectrum. This is obviously a function of wavelength of the light and is usually determined by the type of laser. The colour of laser can be important for specific applications and the colour of the target object can be relevant for a low power laser, for example a red object reflects red light, although obviously if a high power laser is used, the target colour is likely to change as the object burns.

You can’t change a lasers colour with a filter, as all of the light is the same wavelength, meaning that the filter will either stop all of it or none of it.

Invisible laser beams will still blind, and are actually more dangerous, both because the invisible beam is easier to misdirect, and because they will not trigger the eye’s blink reflex.

 

How destructive?

By convention lasers are rated according the energy they transmit to the target. Here are some approximate power values for modern lasers.

1-5mw                  – Laser Pointer

5-10mw               -DVD reader

250mw                 – DVD burner

30 -100 W           – Surgical Laser

100 W  – 3 KW   – Industrial cutting laser

100 KW                – Projected output for military laser in near future

Some lasers designed for use in high energy physics are listed as producing even higher energies, but they are pulsed lasers and produce those energies only for a very short period of time.

 

A worked example

The following are very approximate figures for energy delivered by a single round from representative modern weaponry. Starting at 30 Joules for an air rifle, up to around 500 joules for a handgun, 1000 – 2000 Joules for an modern rifle, and 10-20 thousand joules for a high powered sniper rifle.

Let’s go with the handgun.  Wattage indicates Joules per second. You can see that a laser that puts out as much energy as the impact from a handgun each second (500 watts), is well within the range of modern technology. However we cant expect this to make a terribly effective weapon.

The kinetic energy from a handgun is delivered over a very short time period and comes with a lot of momentum that will transfer it throughout the target. The energy from the laser is delivered over an entire second and will tend to stay in one place as heat.

500 Joules is roughly enough energy to raise 2 grams of water in the target from body temperature to boiling point, which will certainly cause a nasty surface injury, but is unlikely to cause serious harm in itself, especially if the laser has been unable to remain fixed on a single spot, or the target is wearing lightly coloured or thick clothing.

However any laser over 5mw is considered a very significant risk to sight. Any laser over half a watt (500mw) is capable of causing eye damage due to reflection from light coloured matte surfaces. Therefore our laser gun is 100 Thousand times more intense than the level that is considered to pose a major risk to sight, and a thousand times over the level at which the operator can accidentally blind himself by firing at a nearby unreflective object such as the target’s white t-shirt.

Assume our laser is 30% efficient, which is a solid upper range for modern lasers. So the laser gun requires a pretty hefty 1.7kw power supply to run continuously.

You can see that the military laser is predicted to deliver around ten times that of a very high powered rifle every second, which probably would be enough energy to destroy or badly damage aircraft or other light vehicles, but it’s likely that such a laser would require almost all the interior space of a very large aircraft to operate.

 

Lasers as weapons

The use of lasers as handheld weapons is therefore problematic.

1)      The heat energy supplied by lasers is rather inefficient at transferring itself throughout a target compared to the kinetic energy applied by projectile weapons. High power lasers are likely to rely on extremely limited or bulky power sources for the foreseeable futures.

2)      The tremendous potential of lasers to maim by blinding is likely to make their use against human targets unpalatable, although this could obviously increase their attraction as terror weapons. They put both their operator, and any bystanders at significant risk of accidental injury.

3)      Lasers can be defeated or attenuated by reflective surfaces or smoke. The latter of which is readily found on a battlefield. Deliberately formulated reflective particle aerosols would be even more effective. Insulating material can also provide effective protection from the transfer of heat.

Essentially a laser transfers energy in a form that is not particularly effective at damaging physical objects, and is relatively easy to protect against, but predisposed to maim unprotected targets such as civilians.

However, Lasers do excel in other respects

1)      Speed – There is no possibility to evade an unexpected laser beam, as it is moves at the speed of light. You can not detect it before it hits you and you are unlikely to be able to move fast enough to get out of the way.

2)      Accuracy – The lasers path to target is predictable, and the laser beam may also provide feedback about the current impact point allowing rapid adjustments to be made.

3)      Range – Especially in a low atmosphere or vacuum, lasers can travel incredible distances, but perhaps more importantly, retain accuracy over that entire distance.

4)      Scalability – Lasers can concentrate an extremely large amount of energy on a very small target over a very short period of time.

5)      Stealth (sometimes)– A laser can fire light that invisible to the naked eye, probably very quietly. The nature of the attack might not be obvious to the victim immediately and it will defeat modern systems designed to identify the firing point of firearms. However, if the beam is visible, especially to an automated system that can detect a wider range of wavelengths than our eyes, the situation could be dramatically reversed.

This means that lasers will tend to work well against relatively fragile, fast moving targets at long ranges, especially if they can be deployed from a fixed emplacement with a large power supply. They are ideal for air defence, especially as this is unlikely to result in bystanders being blinded.

The ability to reflect lasers off of mirrors could make them useful in situation where establishing a direct line of sight to the target is difficult and can also allow a single laser to fire from multiple emplacements.

They are particularly viable in situations where obtaining sufficient power is not an issue.

Lasers may offer some potential as long range sniping weapons, because of their accuracy, potential for stealth, and because, properly implemented, usage at range would reduce the danger to the operator. The possibility of injuries to bystanders would obviously remain a concern. Some mechanism would need to be in place to protect an operator viewing the target through a scope, but this could be done by occluding the scope at the instant that the weapon fires, or just not having the operator use traditional optical sights.

 

Other Applications

Lasers are important components in a large number of different devices. They are used for cutting, for reading information from or recording on optical media. Their consistency makes them very important for surveying and rangefinding applications.

On the battlefield, Lasers can be directed onto an object and used to guide munitions that will home in on the reflected laser dot.

They can concentrate very high energies onto a small space for a short time and are critical for high energy physics research, or for processes that must be triggered by a large initial energy input such as nuclear fusion.

I’m going to focus, however, on the two incidental applications of lasers that fiction tends to stuff up the most.

 

Laser sights

Laser sights make for great cinema, they just don’t make any sense as typically depicted. The first issue is that laser sights are intended for use with short range weapons, and allow the operator to see where the gun is aiming without using sights. If the operator is using properly calibrated optical sights he doesn’t need a laser dot to see where he is aiming. It certainly doesn’t offer any advantage that would override the massive disadvantage of allowing the target to see the dot.

If the sights are electronic, there is no good reason for a light dot to use visible wavelengths of light. A real sniper might use laser range finding equipment to calibrate his optical sights, but this will use an invisible laser and only needs a single discrete pulse to measure the distance to the target.

I understand that this trope has narrative value, although it’s probably way over-used if nothing else. There is potential to score some credibility points here by incorporating invisible lasers into the scenario, especially with sci-fi. Have the cyborg character able to rescue the ambassador specifically because he is the only one present that can see the dot, or recognize the flash of a range finder.

I’d also note that modern armies tend to use light within the invisible range of the spectrum for a lot of different applications such as target marking, pulsating friendly troop beacons, and landing site marking. There is potential here for them to have unexpected problems when fighting an enemy that can see these things. You might have soldiers suddenly realize that the aliens can see the pulsating beacons that they had all forgotten that they were wearing.

 

Laser tripwire

Once again, problem A is that the real tripwire is likely to use invisible wavelengths of light. This would mean that you need special equipment to see the beam, rather than just squirting aerosols about.

The second problem is that writers sometimes fail to understand that the alarm is triggered by the beam being blocked from reaching a sensor, this means that reflecting the beam with a mirror will trigger the alarm. It is theoretically possible for a complex arrangement of mirrors to divert the beam back to the sensor but in probably impossible to maneuver this into place without interrupting the signal. It’s also worth noting that using an aerosol to make a beam visible might also occlude the beam enough to trigger the alarm, especially if the sensors are carefully calibrated.

There are a lot of obvious ways of playing with this scenario. A clever thief might somehow set up mirrors to reflect the lasers while the sensors are not activated, such as when the museum is open during the day. He might disable the emitters and train his own lasers on the detectors.

A clever villain might user visible laser traps to guide an intruder into invisible sensors. He could have aerosol detectors. The system could even be designed so that the obviously visible beams have to be broken to disable the security.

 

Key concepts –

A laser strikes its target instantly.

A laser can be fired very accurately.

Lasers require large amounts of energy.

Lasers can be diverted using mirrors and are easily blocked by airborne particles

They can easily cause blindness at any intensity that could cause significant physical damage to a target.

Lasers are therefore unlikely to be used as hand held weapons.

They may be well suited to use as anti-aircraft weapons.

Lasers are inefficient weapons, but scale well in transferring very high energies to very precise targets.

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