Limitations of Nanoweapons
Nanoweapons have a number of inherent limitations which mean that, in practice, countermeasures can be devised to slow down or stop many or most forms of nanoattack.
Theorists who lived during the Information Age made a number of predictions about the military capabilities of nanotechnology. The general consensus was that nanoweapons would be unstoppable short of deploying nanotech defenses of at least equal sophistication. The events of the nanoswarms era seemed to bear this out. Since that time theory has given way to practice. Various powers have employed nanotechnology as a weapon of war and found their creations stalemated or even defeated by supposedly inferior tech. The current state of the art now acknowledges that nanoweapons have some inherent limitations. These limits mean that countermeasures based on pre-nano technologies are often effective at slowing the advance of nanoweapons and sometimes capable of stopping nanotechnology altogether.
Limit #1: Physics Nanotechnology can do some truly wondrous things, but it is not exempt from the laws of physics. Nanomachines are subject to the laws of conservation, motion, gravity and so on. Whatever material a nanite or foglet is made of reacts to external energies and to other chemicals in the normal manner. Nanotechnology has no miraculous powers or immunities. This limit is the basis for all of the other limitations listed below.
Limit #2: Size Nano-agents, by their very nature, are incredibly tiny. An individual nanomachine is too small to carry much of an onboard fuel supply (typically no more than a few hours worth). Defenders can sometimes defeat nanoweapons simply by outlasting them. Sufficiently thick fortifications can delay the units long enough for their fuel to run out, provided that the nano can't get in through weak points such as doors and air vents. Designers of nanoweapons counter this disadvantage by incorporating feedstock into delivery systems — artillery shells containing nanites suspended in a fuel solution are the earliest known form of this — or by deploying refueling units alongside their nanoweapons. The latter approach adds to both the expense and the strategic risk of using nanoweapons because it is necessary that the refueling units be well defended to avoid attacks on them, crippling the nanoweapons (synsects excel at this function). The most advanced nanotechnology can refuel itself by scavenging materials from the environment; bionano that fuels itself by consuming organic matter is one of the more reliable ways of implementing this strategy.
Because of their dimensions (in the 10-9 meter range) nanoscale machines are limited in their ability to detect and utilize EM radiation. A nano-agent cannot effectively absorb long wavelengths even by using its entire body surface as a receiver. Radio-frequency communication and detection using radar or infrared are quite impossible. Even generating photoelectric power using visible light is problematic. Wavelengths in the ultraviolet range and beyond are usable, but X-rays and gamma rays are so energetic that individual units can be damaged by constant exposure. Nanotech devices are limited to using ultraviolet light or visible light in the blue to violet range as a medium of sensing or as a photovoltaic power source. This well-known limitation means that purely military installations that require protection from nanoweapons are often built in places that are poor in those frequencies of light. On the one hand this denies the nanites an abundant power source. On the other hand, any artificial source of those wavelengths is easily detected and constitutes a warning of possible nano attack. An alternate strategy is to place an installation under a bright X-ray or gamma ray source so that invading nanotechnology is "sterilized" before it can do any harm. Joining many nanomachines into larger arrays can overcome the EM limitations inherent at the nanoscale — many nanites working in tandem can detect or utilize longer wavelengths — but each nanite remains vulnerable to damage from the more energetic frequencies. Arrays are also easier to detect and attack than individual nanites would be.
The same resolution problems that apply to EM radiation also limit a unit's ability to use or detect sound. Nanites cannot detect audible frequency sound, much less infrasound, without forming receiver chains. The necessity of joining in large groups in order to hear certain frequencies robs the nanomachines of the advantage of stealth that they gain from their small size. Nano-agents can and do make use of ultrasound for both detection and communication, and some types can derive power from it as well (like all broadcast power, this is vulnerable to jamming).
A basic engineering limit of being small is that it's hard to move fast. Even the most advanced utility fogs can barely muster speeds faster than a running baseline can achieve, and nanites that are limited to surface travel can't move faster than a typical hu can walk. But small linear dimensions aren't the only movement problem. With low mass comes low inertia and correspondingly reduced ability to overcome resistance. The faster a nanomachine travels the more it must contend with atmospheric resistance. Nano can usually travel much more easily in vacuum than it can within a gaseous medium. Nanotechnology that's designed to operate in liquid is often unable to overcome the resistance of the medium; the nano is typically designed to reach its target by drifting with the prevailing current. Nanites are often positively buoyant because of their low weight, though agents made of especially dense materials may be neutrally buoyant. Most nanomachines can only submerge by forming clumps that are large enough to sink (and therefore large enough to detect via sonar) or by attaching themselves to more massive objects. Nano-agents that ride to their targets on the surfaces of vehicles or animals are common. Installation defense is often achieved by building bases underwater or by installing positive-pressure systems to make the atmospheric pressure higher inside the building than it is outside. This creates an artificial wind that offers further resistance to the movement of attacking nano (the usual increase over ambient pressure is 20% or more). Positive-pressure systems work very well against airborne units but are less effective against surface-bound nanites. Finally, low inertia makes nanoweapons highly vulnerable to adverse weather conditions. Nanomachines find it difficult to operate in a driving rain or in gale force winds. No known nanoweapon, not even those created using transapientech, can be effectively deployed in the face of a hurricane or tornado. Several of the polities that have pursued weather control technology are known to have done so because of the strategic advantage that they hoped to gain against potential aggressors who rely heavily on military nanotechnology.
Size also severely limits the amount of processing power that a nanite can muster and the number of devices — sensors, manipulators, etc. — that it can carry. Designers of nanotechnology can easily overcome this limit is by designing nanoweapons to operate in networks. This, however, makes the entire conglomerate vulnerable to communications disruption. The network collapses if the component nanites can't pass signals to each other. Units that communicate via signal lasers lose cohesion if immersed in smoke or reflective chaff that is tuned to the proper frequency. Ultrasound communication is vulnerable to phase cancellation (projecting a wave of the same frequency but of opposite phase to cancel out the signal). Both ultrasound and light-based communications are easily jammed by stronger signals.
The greatest consequence of small size is a low damage capacity; there is very little structure to harm, and armoring individual units is simply impossible. Any macro-scale destructive force that manages to reach a nano-agent will probably destroy it.
The reader should note that the size limits above apply mainly to nanoscale machines. Microscale machines, which are comparable in size to naturally occurring microbes (around 10-6 meters), suffer from these limits to a far lesser degree. Micromachines are quite capable of using visible light, for example, and have greater capacity for fuel supplies or onboard processors. They can also move faster and have more inertia. Being larger, of course, makes micromachines easier to detect and attack than nanomachines are. Some theorists don't consider micro-agents that are designed to operate on the nano-level to be true nanotechnology, but the distinction is largely academic.
Limit #3: Physical Properties Nanomachines are made of matter, and that matter behaves normally according to its nature. Diamondoid has the same qualities of reactivity and conductivity as natural diamond, corundumoid has the same properties and vulnerabilities as corundum, and so on. A defender who knows what attacking nano-agents are made of can sometimes neutralize them by using the right corrosive. The process is usually quick because destroying a single unit requires very little reactant. Insulating nano-agents against corrosive attack is completely ineffective; a layer of protective material even a single molecule thick would compromise a unit's function. Agents that are made of naturally corrosion-resistant materials are vulnerable to adhesives that gum up the works or simply impede movement. Some disassemblers can simply "eat" their way through such an obstruction, but this slows the advance toward the primary target (which is often the entire point of using such obstructives). The best way for nanomachines to defend themselves from chemical attack is to disperse widely enough that some of them will be outside of the area of effect. This defense, while highly effective, also increases system lag as the units have to communicate with each other over greater distances.
Temperature is an intractable problem for nanomachines. Individual units are too small to incorporate either cooling systems or thermal insulation; the only thermoregulation possible for nano-agents is that derived from their physical architecture (either cooling fins or heat-conserving shapes) or their composition (i.e. reflectivity or thermal conductivity). For this reason most nano-agents are restricted to operating within fairly narrow temperature ranges — hylonano is superior to bionano in this regard. Most nanoweapons that are designed to operate in a shirtsleeves environment can't survive the extremes of heat or cold that bionts can. Nano-agents that are optimized for extreme heat will freeze in temperatures that hu would find unbearably hot, while nano that's meant to work in the cold of deep space burns out at temperatures as low as the boiling point of nitrogen. Of even greater concern is thermal shock. A sudden temperature change can shatter every nano-agent within the area of effect. Flamethrowers and liquified gas grenades are common anti-nano countermeasures for this very reason. As with chemical attack, a nanoweapon's only real defense against temperature-based attacks is dispersal, and even that is ineffective against ambient temperatures. Designing nano-agents to work in the inner regions of a star system is very difficult; the machines must often deal with rapid changes from extreme cold to searing heat. Nano that's designed for orbital conditions is typically optimized for cold and made of material that reacts to sudden exposure to sunlight by becoming more reflective. Macroscale delivery systems are another common solution.
No material, no matter how strong, is immune to a sudden application of kinetic energy. Stronger materials simply require more input energy before failing. Nanomachines are too small to have much structural strength even when they are made of exceptionally strong materials. A sharp impact destroys every nano-agent that it hits. Small size makes nanoweapons difficult to target with most kinetic energy weapons, however, so projectiles and close combat weapons are ineffective. Sophonts who have had occasion to develop anti-nano countermeasures have found that ultrasound is the best way to apply kinetic energy to nanomachines. Sound can deliver large force loads, especially when tuned to the resonant frequency of the target material, and sonic waves naturally disperse over wide areas (thereby neutralizing a nanoweapon's best defense). Insulating nanomachines against ultrasound is impossible for the same reason that they can't be insulated against temperature extremes or corrosive chemicals.
Limit #4: Fire with Fire Effectively defending against nanoweapons doesn't require nanotechnology — but having nanotechnology of equal or greater sophistication certainly works. History contains few examples of nanoweapons being successfully deployed against an enemy that had more advanced nanoweapons, and most of those examples involve transapient intervention. In all military conflicts between combatants using similar weapons, the party who has superior technology has a significant advantage.
Limit #5: Strategy All of the above problems limit the ways in which nanoweapons can be effectively deployed. An enemy who has the technological capacity to use chemicals or ultrasound, let alone countering nano-agents, isn't particularly vulnerable to nano-attack unless caught by surprise. This is why the nanoswarms caused so much damage initially but were eventually defeated. Military strategists have learned that the most effective use of nanoweapons is as ambush weapons that incapacitate their targets before there is time to prepare countermeasures. This strategy is most effective if the attacker employs multiple types of nano-agents. In this scheme the first agents deployed are specialized units designed to attack the countermeasures before they can be activated. The actual combat units may then operate unimpeded. Nanoweapons are also effective as rear-guard devices. Deploying nano-agents to cover a retreat can buy an army valuable escape time, especially if the pursued can achieve surprise in that deployment.
The other main use of nanoweapons is for sieges. Self-replicating agents that can refuel from the environment come into their own in this strategy; wide dispersal prevents even the most effective countermeasures from destroying all of the units (particularly if some of them deliberately wait outside the combat area). The surviving units then need only avoid detection long enough to build their numbers back up, at which point the attack begins again.
The problem with both of the above strategies is that an opponent who is familiar with nanotechnology probably knows of them. Countermeasures that can be activated quickly enough to minimize the element of surprise were the first anti-nano devices created. Ultrasound projectors, flamethrowers and cryonic fluid dispensers are common in military units throughout the known galaxy. Quick-deploy Anti-nano Countermeasures, or QDANCs, can't stop nanoweapons from doing any damage at all but they can keep a surprise attack from being a complete success. QDANCs have an advantage over blue goo in that, when used properly, a QDANC can destroy attacking nano-agents before they reach their targets. As for nanotech siege weapons, defeating them is often as simple as building secure installations in places where nano-agents can't operate effectively. Denying the machines fuel or replication resources eliminates the long-term danger of nanotech siege. A given type of nanotech disassembler is generally effective against a narrow range of materials (universal disassemblers, if they exist, are beyond the technological capabilities of everyone but high-level transapients).
Building an installation's outer walls out of a given material and placing the base in an environment that won't support units that are effective against that substance is a common technique. Building in an area that is both poor in energy resources and rich in environmental hazards is even more effective. Underwater locations are among the best; they aren't 100% proof against nano-attack, but the pressure, salinity, currents and fluctuating temperatures of a marine environment are powerful defenses. Nanomachines — and even micromachines — can't cross haloclines or thermoclines without piggybacking on larger objects. The acidity of anoxic water keeps most nano-agents at bay; bionano is particularly ineffective under such conditions. Finally, water is an effective barrier to light. Even the energetic wavelengths that nanomachines can use for photovoltaic or broadcast power can't penetrate very deep even into clear water. This limits underwater operation to nano-agents that rely on other sources of energy.
An often-overlooked strategic problem of using nanotechnology is expense. Even post-scarcity economies still have to allocate resources. The resources — in terms of materials, infrastructure and brainpower — required for designing and building nanotech weapons are much greater than what is needed for developing effective anti-nano countermeasures. Attackers who are too reliant on nanoweapons are in real danger of meeting effective resistance from defenders who are less technologically advanced. Mass production of QDANCs is usually faster and cheaper than manufacturing nano-agents in sufficient quantity to overwhelm those defenses. Complexity is also a major issue. It can take years to educate a sophont to the point of being a competent nano-engineer, but a technician can learn how to build a flamethrower or a cryo-grenade after just a few hours of instruction.
Nanoweapons remain among the most feared threats in existence, but this is largely because most sophonts are ignorant of the limits of the technology. As weapons of mass destruction, nuclear and antimatter bombs work faster and are harder to counter. As personal combat weapons, handheld directed energy weapons are cheaper. The greatest advantage of nanoweapons is their selectivity. A nano-based attack can destroy a rebellious population (or just a specific segment thereof) without doing any property damage at all. Nanoweapons can even be tuned to attack particular types of equipment or specific structures. It is also easy to deploy nano-agents in secret. Even with these advantages nanoweapons are most effective when used suddenly against defenders who lack the technology or the time to deploy countermeasures. This is why the nanoswarms failed to drive the Terragens to extinction; while the short-term damage was great, even populations that consisted mostly of baselines were clever enough to deal with the long-term threat in one way or another. Those who truly understand nanotechnology don't regard it as an ultimate weapon. It is simply one tool among many, and its effective use is only possible for those who understand both the strengths and the weaknesses of the technology.