You think Sisyphus had it bad, having to roll that boulder up a hill? Consider the opposite challenge faced by nano-engineers as they try to build microscopic electronic circuits. Using single atoms as building blocks, they need to push with barely measurable forces lest they send their Lilliputian Legos hurtling off the workbench. But how much force is just enough to move an atom?

Now scientists at IBM have become the first to find out, and the answer is little. Very little.

For years, chipmakers and materials scientists have been honing matter to ever smaller dimensions and etching increasingly finer circuits onto silicon. But at the smallest scales it makes sense to build circuits from the bottom up, an atom at a time.

The researchers used a tool that has at its heart a tiny quartz tuning fork vibrating thousands of times per second. measuring how much those vibrations decreased as the tool was used to nudge individual atoms along a surface, the team was able to calculate how much force was required.

The answer depends on the atom. To push a cobalt atom over a copper surface, it turns out, takes 17 piconewtons, or about 0.000000002 (2 billionths) the amount of force it takes to lift a penny. contrast, it takes 210 piconewtons to push a cobalt atom over a platinum surface, which bonds more strongly to cobalt.

The result "could pave the way for new data storage and memory devices," said Andreas J. Heinrich of the IBM Almaden Research Center in California. who helped lead the research with others from IBM and the University of Regensburg in Germany. Their work appeared in the Feb. 22 issue of Science.