Australian researchers have experimentally shown that microscopic
systems (a nano-machine) may spontaneously become more orderly for short
periods of time--a development that would be tantamount to violating the
second law of thermodynamics, if it happened in a larger system. Don't
worry, nature still rigorously enforces the venerable second law in
macroscopic systems, but engineers will want to keep limits to the
second law in mind when designing nanoscale machines. The new experiment
also potentially has important ramifications for an understanding of the
mechanics of life on the scale of microbes and cells.
There are numerous ways to summarize the second law of
thermodynamics. One of the simplest is to note that it's impossible
simply to extract the heat energy from some reservoir and use it to do
work. Otherwise, machines could run on the energy in a glass of water,
for example, by extracting heat and leaving behind a lump of ice. If
this were possible, refrigerators and freezers could create electrical
power rather that consuming it. The second law typically concerns
collections of many trillions of particles--such as the molecules in an
iron rod, or a cup of tea, or a helium balloon--and it works well
because it is essentially a statistical statement about the collective
behavior of countless particles we could never hope to track
individually. In systems of only a few particles, the statistics are
grainier, and circumstances may arise that would be highly improbable in
large systems. Therefore, the second law of thermodynamics is not
generally applied to small collections of particles.
The experiment at the Australian National University in Canberra and
Griffith University in Brisbane (Edith Sevick, sevick@rsc.anu.edu.au,
011+61-2-6125-0508) looks at aspects of thermodynamics in the hazy
middle ground between very small and very large systems. The researchers
used optical tweezers to grab hold of a micron-sized bead and drag it
through water. By measuring the motion of the bead and calculating the
minuscule forces on it, the researchers were able to show that the bead
was sometimes kicked by the water molecules in such a way that energy
was transferred from the water to the bead. In effect, heat energy was
extracted from the reservoir and used to do work (helping to move the
bead) in apparent violation of the second law.
As it turns out, when the bead was briefly moved over short
distances, it was almost as likely to extract energy from the water as
it was to add energy to the water. But when the bead was moved for more
than about 2 seconds at a time, the second law took over again and no
useful energy could be extracted from the motion of the water molecules,
eliminating the possibility of micron-sized perpetual motion machines
that run for more than a few seconds. Nevertheless, many physicists will
be surprised to learn that the second law is not entirely valid for
systems as large as the bead-and-water experiment, and for periods on
the order of seconds. After all, even a cubic micron of water contains
about thirty billion molecules. While it's still not possible to do
useful work by turning water into ice, the experiment suggests that
nanoscale machines may have to deal with phenomena that are more bizarre
than most engineers realize. Such tiny devices may even end up running
backwards for brief periods due to the counterintuitive energy flow. The
research may also be important to biologists because many of the cells
and microbes they study comprise systems comparable in size to the
bead-and-water experiment. (G.M. Wang et
al., Physical Review Letters, 29 July
2002.)
