A molecular assembler is a molecule capable of assembling other molecules given a supply of smaller "building block" atoms or molecules to work from. It requires a specific environment, with temperature, viscosity, and purity constraints. Biological enzymes are examples of such molecules - they work within a cell's environment, and by reading strands of DNA and RNA they assemble specific large protein molecules out of more fundamental parts.
It is a matter of some debate among scientists whether more general, more like "mechanical", self-replicating molecules can exist, or whether naturally-evolved enzymes, cells, and DNA and RNA strands that instruct them on how to build proteins, are the only possible example. Richard Smalley in an essay in Scientific American in 2001 took the position that any non-natural assembler is simply impossible.
This has been an influential position in the US National Nanotechnology Initiative and the Texas Nanotechnology Initiative which have close ties to the US military-industrial complex, and seek to do molecular engineering of advanced materials for military use, with little or no scrutiny or regulation. As K. Eric Drexler notes in a letter to Smalley, "your words have been remarkably effective in changing how this issue is perceived."
Drexler and Ralph Merkle, like the late Richard Feynman, assert that assemblers are inevitable. Drexler and his colleagues at the Foresight Institute also assert that they represent a potential threat to all natural life in various ways:
- If there is an industrial accident on a scale like that of Chernobyl or Bhopal
- If weapons of mass destruction are made of them.
Accordingly, Drexler advocates quite strict Foresight Guidelines on Molecular Nanotechnology - including guidelines on self-replicating assemblers.
According to Smalley, of course, the regulations are unnecessary, since the replicators are impossible. However, he and the TNI advocate a regulations-free environment where presumably they can continue research in secret, including for military use. This parallels how the US biotechnology industry prevailed on George Walker Bush to abandon treaties on biotechnology supervision, and on "dual-use technology", to prevent the use of either in weaponry.
The technical point appears to have been clearly won by Drexler. In an open letter to Smalley he writes that
- "I follow Richard Feynman  in arguing the feasibility of building with atom-by-atom control. You [Smalley] endorsed this goal in 1999, stating that we will "learn to build things at the ultimate level of control, one atom at a time" , then rejected it in 2001, stating that "To put every atom in its place -- the vision articulated by some nanotechnologists -- would require magic fingers" , but apparently retract this rejection in 2003, stating that "The ultimate nanotechnology builds at the ultimate level of finesse one atom at a time, and does it with molecular perfection" .
- "Your 2001 essay  created the impression that you had shown building with atom-by-atom control to be impossible, but my open letter  pointed out that your argument misrepresents the basic idea (shared by myself and Feynman) that the goal is to control where each atom ends up in the product structure -- as happens in chemical and biological synthesis -- not to grab and manipulate impossibly many neighboring atoms separately and simultaneously. Your recent silence and 2003 statement (above) now suggest that you have abandoned your 2001 position and rejoined Feynman in endorsing the feasibility of atom-by-atom control."
There is no record of a response by Smalley.
The UK Royal Society and UK Royal Academy of Engineering has commissioned a study to deal with the assembler question, other related issues and their larger social and ecological implications, led by mechanical engineering professor Ann Dowling.
- Center for Responsible Nanotechnology
- House Science Committee Hearing on the Societal Implications of Nanotechnology
- Environmental Defense Dances With DuPont On Nanotechnology
- ETC Group
- National Nanotechnology Initiative
- Nanotechnology PR
- Project on Emerging Nanotechnologies
- "rage against the green machine" - Wired
- Biosafety discussion on assembler risk
- [UK "Government launches nano study" - EducationGuardian, June 11, 2003
- "Toward Closure" - Drexler - quoted above.
-  Drexler, K. E., 2003. "Open Letter to Richard Smalley." Reprinted in Small Times, May/June, p.10. http://www.foresight.org/NanoRev/Letter.html
-  Feynman, R., 1959. "There's Plenty of Room at the Bottom: An invitation to Enter a New Field of Physics," Talk at the Annual Meeting of the American Physical Society. http://www.its.caltech.edu/~feynman/plenty.html
-  Smalley, R. E, 1999. Written statement, U.S. Senate Committee on Commerce, Science, and Transportation, May 12. http://www.senate.gov/~commerce/hearings/hearin99.htm
-  Smalley, R. E,, 2001. "Of chemistry, love and nanobots - How soon will we see the nanometer-scale robots envisaged by K. Eric Drexler and other molecular nanotechologists? The simple answer is never." Scientific American, September, 68-69. http://www.ruf.rice.edu/~smalleyg/rick%27s%20publications/SA285-76.pdf
-  Smalley, 2003. Presentation to the President's Council of Advisors on Science and Technology, March 3. http://www.ostp.gov/PCAST/march3meetingagenda.html