By Dr David Whitehouse BBC News Online science editor

This result has profound consequences for any chemical or physical process that occurs over short times and in small regions

ANU team

Scientists at the Australian National University (ANU) have carried out an experiment involving lasers and microscopic beads that disobeys the so-called Second Law of Thermodynamics, something many scientists had considered impossible.

The finding has implications for nanotechnology - the design and construction of molecular machines. They may not work as expected.

It may also help scientists better understand DNA and proteins, molecules that form the basis of life and whose behaviour in some circumstances is not fully explained.

No discussion

Flanders and Swann wrote a famous song entitled The First And Second Law about what entropy meant and its implications for the physical world. It has become a mantra for generations of scientists.

The law of entropy, or the Second Law of Thermodynamics, is one of the bedrocks on which modern theoretical physics is based. It is one of a handful of laws about which physicists feel most certain.

So much so that there is a common adage that if anyone has a theory that violates the Second Law then, without any discussion, that theory must certainly be wrong.

The Second Law states that the entropy - or disorder - of a closed system always increases. Put simply, it says that things fall apart, disorder overcomes everything - eventually. But when this principle is applied to small systems such as collections of molecules there is a paradox.

Human scales

This Second Law of Thermodynamics says that the disorder of the Universe can only increase in time, but the equations of classical and quantum mechanics, the laws that govern the behaviour of the very small, are time reversible.

A few years ago, a tentative theoretical solution to this paradox was proposed - the so-called Fluctuation Theorem - stating that the chances of the Second Law being violated increases as the system in question gets smaller.

This means that at human scales, the Second Law dominates and machines only ever run in one direction. However, when working at molecular scales and over extremely short periods of time, things can take place in either direction.

Now, scientists have demonstrated that principle experimentally.

Fraction of a second

Professor Denis Evans and colleagues at the Research School of Chemistry at the Australian National University put 100 tiny beads into a water-filled container. They fired a laser beam at one of the beads, electrically charging the tiny particle and trapping it.

The container holding the beads was then moved from side to side a thousand times a second so that the trapped bead would be dragged first one way and then the other.

The researchers discovered that in such a tiny system, entropy can sometimes decrease rather than increase.

This effect was seen when the researchers looked at the bead's behaviour for a tenth of a second. Any longer and the effect was lost.

Emerging science

The scientists say their finding could be important for the emerging science of nanotechnology. Researchers envisage a time when tiny machines no more than a few billionths of a metre across surge though our bodies to deliver drugs and destroy disease-causing pathogens.

This research means that on the very small scales of space and time such machines may not work the way we expect them to.

Essentially, the smaller a machine is, the greater the chance that it will run backwards. It could be extremely difficult to control.

The researchers said: "This result has profound consequences for any chemical or physical process that occurs over short times and in small regions."

The ANU work is published in Physical Review Letters.