These are the main arguments I've heard for why nanotech won't work. Each of them has an answer that clearly shows that it's not a major problem.

Problem 1: The atoms move around; quantum mechanics says you can't tell exactly where an atom is.

Solution 1: Your body makes precise chemicals all the time. Atoms' positions are predictable enough to position them very reliably. Atoms are much heavier than photons, so their positional uncertainty is much less. It was only recently that a very intricate physics experiment managed to show an interference pattern between sodium atoms.

Problem 2: It's impossible for anything to build something more complicated than itself.

Solution 2: One fertilized egg can "build" a whole human. Nanotech only requires a factory to be able to build another factory of roughly the same complexity.

Problem 3: Not all chemical products may be buildable with nanotech.

Solution 3: Not all chemical products need to be buildable. If we can get just a few dozen reactions working, and build diamond in a variety of shapes, we'll be able to build self-reproducing factories; that's all we need for most of the amazing advantages I listed. (Food and biotech will require other reactions; however, those are already done by cells, so we know they're possible.)

Problem 4: What about supplying energy, and getting rid of waste heat?

Solution 4: This is an engineering problem, not a proof that it can't work at all. Energy can be delivered electrically, chemically (it can be converted in a space smaller than a mitochondrion) or mechanically (for example, with high-frequency sound waves). Nanotech will be much more efficient than today's machines, so it will be able to do at least as much work per volume without overheating. Also, the ability to build more intricate structures means we can use fractal cooling channels.

Problem 5: How can such tiny atomic-scale robots build the human-scale or planet-scale devices you talked about?

Solution 5: There are several proposals. My favorite involves many robots fastened down in a factory. The products of hundreds of robots are fastened together by a second-level robot; then hundreds of second-level products are fastened together by a third-level robot, and so on. Surprisingly, it doesn't take many levels to build a pound of product atom-by-atom. (Analysis may be found in _Nanosystems_ by Drexler.)

If you know of any other objections, email me at cphoenix at crnano.org and I'll add them and their solutions to the list.