How will life on Earth end? International nuclear bombardment? Another asteroid? Enslavement by aliens? The devouring of all of our resources by a Skynet-esque batch of self-replicating microscopic robots? You laugh, but a lot of people (including a batch of anti-science terrorists) believe our demise will come in the form of self-replicating nanobots. So let's take a look at whether such a thing could happen, and what we could do to stop it.

The Grey Goo Scenario

"Grey goo" is a term coined by Eric Drexler in his 1986 book about nanotechnology, Engines of Creation. In one section of the book, Drexler speculates about what could happen if we created self-reproducing "nanobots." If they got out of control, they could conceivably use the resources of the entire planet to replicate in an exponential manner akin to bacterial replication. Every living creature, every useful mineral, would be converted into more of the "gray goo."

This point of view was no doubt informed by the concept of Von Neumann probes, a method of interstellar exploration wherein a single, self-sufficient craft is sent out programmed with the ability to reproduce itself once it finds enough resources. One craft becomes two, which then becomes four, each going in different directions and communicating back with Earth, until the universe is mapped by a phenomenal number of self-reproducing spacecrafts. Regardless of the origin of the concept, the idea of grey goo quickly became fodder for science fiction writers and end-of-the-world scenarios in pop culture.

The question is, could a grey goo scenario really come to pass?

How would the nanobots replicate?

Theoretically, it is believed that these nanobots would replicate through the manipulation of individual atoms in a discrete manner to create a desired product. This is a technology we are far from accomplishing, but the goal of many researchers. The rate of reproduction would be rather quick if the technology was available, akin to the exponential replication seen with bacteria.

There are some problems with self-assembly on the atomic level, however, with Drexler himself pointing out some of them in his paper Safe Exponential Manufacturing:

The chemistry proposed for machine-phase systems requires that every atom of every molecule be in a known position. A high-entropy mix of polysaccharides, proteins, lipids, and water, such as a biological material, does not meet this constraint, and hence would not be a suitable input. An MNT mechanochemical system would instead require as inputs simple chemical feedstocks, such as acetylene or acetone, in which every molecule is identical and impurities are easy to recognize by their shape and size.

Problems with self-replicating nanobot assemblers

So if they do go on a self-replicating frenzy, the nanobots simply won't have access to enough viable energy sources. Organic life forms lack the degree of organization and orientation necessary for a situation by which a nanobot could simply "pluck" a necessary atom off of a wiggly lipid in a bilayer. If our oceans were made of an organic solvent like acetone, with little or no impurities, we would be in trouble, but instead, our land and sea environments have enough diversity on an atomic level to prevent their assimilation.

Also, it's currently unknown as to what these nanobots would be made of – carbon would play some role, but if they were to made of a metal alloy like steel, there would be problems. Steel, though prevalent in industrial manufacturing, is a complex mixture of iron, chromium, tungsten, vanadium, manganese and a small amount of carbon. Any impurities in steel, as would theoretically occur in the process of nanobot assembly – say a small amount of copper is added – would greatly decrease the strength and durability of the steel. Steel in just one example, as there would be a variety of materials theoretically necessary for nanobots to reproduce.

What could we do if the grey goo started to take over?

So, let's say this nice little bit of debunkery is all wrong, and nanobots are ceaselessly replicating around us. Even if we couldn't see the demon robots, theoreticians suggest that amount of heat generated by the replication process would be quickly noticeable. What are some of ways we could defeat them?

Electromagnetic Pulse
Let's give the nanobots the benefit of the doubt, and say they are able to reproduce. A strong enough electromagnetic pulse could more than likely sway their domination of the earth, assuming we are willing to lose a vast amount of our information and telecommunication infrastructure in the process. This could be accomplished by detonating a sufficient yield nuclear device in the atmosphere, disrupting the magnetic field of the earth temporarily, or through a manufactured EMP device.

Halt their Programming Directives
A nanobot would more than likely not have enough internal memory to carry out each step necessary for reproduction of its constituent parts, but instead rely upon communication with other nanobots for instructions. These instructions in the programming thread could possibly be altered or stopped, leaving a portion of the nanobots un-assembled and halting replication. This would be akin to growing bacteria for protein replication in the presence of an antibiotic in order to select for which bacteria would replicate.

We're Safe!?!
While there are still many unknowns surrounding nanotechnology, it's important to separate the science fact from the science fiction. In doing so, it looks like we're safe from being devoured by creatures of our own creation. Drexler, who coined the term "grey goo", has gone on the record stating he wish he would have never used the terminology due to the scientific fear mongering that followed. So, grey goo won't get you, at least in your lifetime. What happens to your corpse a couple thousand years from now? That might be another matter.

You can read Drexler's new edition of Engines of Creation for free here.
Images from the AP, Eric Drexler, and the The Royal Society of Chemistry.