Self Replication and Nanotechnology
Source: Nanotechnology Industries
A crucial objective of nanotechnology is the ability to make
products inexpensively. While the ability to make a few very small, very precise
molecular machines very expensively would clearly be a major scientific
achievement, it would not fundamentally change how we make most products.
Fortunately, we are surrounded and inspired by products that are marvelously
complex and yet very inexpensive. Potatoes, for example, are made by intricate
molecular machines involving tens of thousands of genes, proteins, and other
molecular components; yet the result costs so little that we think nothing of
mashing this biological wonder and eating it.
It's easy to see why potatoes and other agricultural products are
so cheap: put a potato in a little moist dirt, provide it with some air and
sunlight, and we get more potatoes. In short, potatoes are self replicating.
Just as the early pioneers of flight took inspiration by watching
birds soar effortlessly through the air, so we can take inspiration from nature
as we develop molecular manufacturing systems. Of course, "inspired by" does not
mean "copied from." Airplanes are very different from birds: a 747 bears only
the smallest resemblance to a duck even though both fly. The artificial self
replicating systems that have been envisioned for molecular manufacturing bear
about the same degree of similarity to their biological counterparts as a car
might bear to a horse.
Horses and cars both provide transportation. Horses, however, can
get their energy from potatoes, corn, sugar, hay, straw, grass, and countless
other types of "fuel." A car uses only a single artifical and carefully refined
source of energy: gasoline. Putting sugar or straw into its gas tank is not
recommended!
The machines that people make tend to be inflexible and brittle in
response to changes in their environments. By contrast, living biological
systems are wonderfully flexible and adaptable. Horses can pick their way along
a narrow trail or jump over shrubs; they get "parts" (from their food) in the
same flexible way they get energy; and they have a remarkable self repair
ability.
Cars, on the other hand, need roads on which to travel; have to be
provided with odd and very unnatural parts; are often difficult to repair (let
alone self repairing!); and in general are simply unable to cope with a complex
environment. They work because we want them to work, and because we can fairly
inexpensively provide carefully controlled conditions under which they can
perform as we desire.
In the same way, the artifical self replicating systems that are
being proposed for molecular manufacturing are inflexible and brittle. It's
difficult enough to design a system able to self replicate in a controlled
environment, let alone designing one that can approach the marvelous
adaptibility that hundreds of millions of years of evolution have given to
living systems. Designing a system that uses a single source of energy is both
much easier to do and produces a much more efficient system: the horse pays for
its ability to eat potatoes when grass isn't available by being less efficient
at both. For artificial systems where we wish to decrease design complexity and
increase efficiency, we'll design the system so that it can handle one source of
energy, and handle that one source very well.
Horses can manufacture the many complex proteins and molecules
they need from whatever food happens to be around. Again, they pay for this
flexibility by having an intricate digestive system able to break down food into
its constituent molecules, and a complex intermediary metabolism able to
synthesize whatever they need from whatever they've got. Artificial self
replicating systems will be both simpler and more efficient if most of this
burden is off-loaded: we can give them the odd compounds and unnatural molecular
structures that they require in an artifical "feedstock" rather than forcing the
device to make everything itself -- a process that is both less efficient and
more complex to design.
The mechanical designs proposed for nanotechnology are more
reminiscent of a factory than of a living system. Molecular scale robotic arms
able to move and position molecular parts would assemble rather rigid molecular
products using methods more familiar to a machine shop than the complex brew of
chemicals found in a cell. Although we are inspired by living systems, the
actual designs are likely to owe more to design constraints and our human
objectives than to living systems. Self replication is but one of many abilities
that living systems exhibit. Copying that one ability in an artificial system
will be challenge enough without attempting to emulate their many other
remarkable abilities.
Complexity of self replicating systems
If our designs are going to be very different from the living
systems that inspired us, what approach are we going to follow? The study of
artificial self replicating systems was first pursued by von Neumann in the
1940's. Subsequent work, including a study by NASA in 1980, confirmed and
extended the basic insights of von Neumann. More recent work by Drexler
continued this trend and applied the concepts to molecular scale systems. The
author has also contributed a few articles, including: Self Replicating Systems
and Low Cost Manufacturing, Self Replicating Systems and Molecular Manufacturing
and Design Considerations for an Assembler. (A web page on artificial self
replication maintained by Moshe Sipper has links to and information on other
references). One conclusion from this body of work is that the design complexity
of artificial self replicating systems need not be excessive. One of the
simplest "self replicating systems" (when executed, it prints itself out on the
standard output) is the following one line C program:
main(){char q=34,n=10,*a="main(){char
q=34,n=10,*a=%c%s%c;printf(a,q,a,q,n);}%c";printf(a,q,a,q,n);} (From
Self-reproducing programs, Byte magazine, August 1980, page 74. Those interested
in a deeper understanding of the recursion theorem and its applications are
referred to Introduction to the Theory of Computation by Michael Sipser, 1996,
PWS Publishing Company, chapter 6.)
The following table illustrates the design complexity of several
other systems:
The estimate of the complexity of the internet worm is simply an
approximation to the number of bits in the C source code. For the biological
systems, the complexity is derived by multiplying the number of base pairs in
the DNA times 2. For humans, the number of base pairs is for the haploid, rather
than diploid, system. The complexity for the the NASA proposal was taken from
Advanced Automation for Space Missions. Mycoplasma genitalium is the simplest
natural living system that can survive on a well defined chemical medium. Its
genomic complexity of 1,160,140 bits (twice the 580,070 base pairs sequenced by
TIGR) is less than 150 kilobytes -- about one tenth of a typical floppy disk.
TIGR is pursuing the Minimal Genome Project to reduce to a minimum the number of
genes required for a simple living system. (While viruses are simpler they
require a living system to infect: they need additional molecular machinery
provided in their environment. For this reason, we exclude them from the table).
The primary observation to be drawn from this data is that simpler
designs and proposals for self replicating systems both exist and are well
within current design capabilities. The engineering effort required to design
systems of such complexity will be significant, but should not be greater than
the complexity involved in the design of such existing systems as computers,
airplanes, etc. A recent proposal is "Exponential growth of large
self-reproducing machine systems," by Klaus S. Lackner and C. H. Wendt, Mathl.
Comput. Modelling Vol. 21, No. 10, pages 55-81, 1995.
One last point: self replication is used here as a means to an
end, not as an end in itself. A system able to make copies of itself but unable
to make much of anything else would not be very useful and would not satisfy our
objectives. The purpose of self replication in the context of manufacturing is
to permit the low cost replication of a flexible and programmable manufacturing
system -- a system which can be reprogrammed to make a very wide range of
molecularly precise structures. This lets us economically build a very wide
range of products.
Systems that function in a complex environment
If artificial self replicating systems will only function in
carefully controlled artificial environments, how can we develop applications of
nanotechnology that function in complex environments, such as the inside of the
human body or a (rather messy) factory floor? While self replicating systems are
the key to low cost, there is no need (and little desire) to have such systems
function in the outside world. Instead, in an artificial and controlled
environment they can manufacture simpler and more rugged systems that can then
be transferred to their final destination. Medical devices designed to operate
in the human body don't have to self replicate: we can manufacture them in a
controlled environment and then inject them into the patient as needed. The
resulting medical device will be simpler, smaller, more efficient and more
precisely designed for the task at hand than a device designed to perform the
same function and self replicate. This conclusion should hold generally:
optimize device design for the desired function, manufacture the device in an
environment optimized for manufacturing, then transport the device from the
manufacturing environment to the environment for which it was designed. A single
device able to do everything would be harder to design and less efficient.
Conclusions
Self replication is an effective route to truly low cost
manufacturing. Our intuitions about self replicating systems, learned from the
biological systems that surround us, are likely to seriously mislead us about
the properties and characteristics of artificial self replicating systems
designed for manufacturing purposes. Artificial systems able to make a wide
range of non-biological products (like diamond) under programmatic control are
likely to be more brittle and less adaptable in their response to changes in
their environment than biological systems. At the same time, they should be
simpler and easier to design. The complexity of such systems need not be
excessive by present engineering standards.
http://onward.to/inventions/
Complexity of self replicating systems (bits)
Von Neumann's universal constructor ~500,000
Internet worm (Robert Morris, Jr., 1988) ~500,000
Mycoplasma genitalium 1,160,140
E. Coli
9,278,442
Drexler's assembler ~100,000,000
Human ~6,400,000,000
NASA Lunar Manufacturing Facility over 100,000,000,000