y goo" (runaway biosphere-eating self-replicators), leading to excessive fear and attempts to counteract that fear. It has also been associated with with extreme science-fictional projections-though it can be hard to tell fantasy from sober calculation, because the calculations predict some pretty amazing capabilities. In reaction to these factors, some mainstream scientists have attempted to shut down discussion entirely by declaring that molecular manufacturing is impossible or that its major proponents are not credible. Discussion has been further distorted by a variety of widespread conceptual confusions.
The position that molecular manufacturing is impossible is not supportable. Living organisms are not an example of molecular manufacturing, because they are not based on engineering but rather on interlocking complex systems. However, the biochemistry of life could be adapted almost unchanged to a molecular manufacturing system.
A few prominent scientists have nonetheless claimed that molecular manufacturing is impossible, and others have echoed them. However, study of their objections shows that the arguments are weak, based on intuition rather than calculation. Some of the arguments elevate engineering difficulties to the status of fundamental limitations. Others are built on basic misunderstandings of the proposals. See "Molecular Manufacturing Myths" for examples of arguments that have been advanced against the theory, and why they are incorrect.
Although there are some practical questions remaining to be answered, there is no scientific study demonstrating a limitation or problem with molecular manufacturing theory. Indeed, the chemical mechanisms of life demonstrate that machines of a sort can be constructed out of molecules and can build more machines. Thus the scientists who dismiss the entire field out of hand have not bothered to study it in any depth. An intermediate position is that some form of molecular manufacturing will be possible, but will be limited. This is usually based on the assumption that engineering cannot improve on biology. This may be true for complex problems, but not for the relatively simple tasks that are tackled by engineering.
Perhaps because of the opposition of these scientists, not many people are working on molecular manufacturing in the lab; most of the work has been theoretical study, which is easier to do without funding. For now, it remains a controversial theory. But after more than a decade, no one has managed to find a real problem with the theory, and evidence is accumulating in favor of it. Some predicted phenomena, such as exceptionally low friction in certain cases, have now been observed. A single atom has been mechanically removed from a crystal and put back in the same place. Although these were not intended as demonstrations of molecular manufacturing theory, they indicate that at least some of the predictions are valid.
Separating science from science fiction, and alarmism from serious policy concerns, will not be an easy task. Most nanotechnology researchers have not paid enough attention to molecular manufacturing to be able to give informed commentary. The section on "Molecular Manufacturing Myths" lists some of the common misconceptions about molecular manufacturing.
According to its proponents, molecular manufacturing could be revolutionary. Because chemistry is very precise and repeatable, the manufacturing operations should be reliable enough to allow complete automation. Because the chemical steps are simple, a single fabricator could build a wide variety of products by changing the control program. Furthermore, machines built this way could have very precise features. This is a good thing, because building with molecules would be quite slow, and small machines can be built more quickly. In fact, if the calculations are right, a complete nanoscale manufacturing system could build a complete copy of itself in a few hours-and this remains true if many small manufacturing systems are combined into one large one, so that even human-scale factories and produc