Not all the universe is falling apart all of the time.

An experiment by scientists in Australia has shown that a small patch of disorder can momentarily lurch into order, akin to Humpty Dumpty's magically putting himself back together again.

That would appear to violate the second law of thermodynamics, which states that entropy, a measure of disorder, rises inexorably unless an outside energy source maintains things in order. A billow of smoke always disperses, never contracts.

The experiment confirms a theory from 1993 that reconciled a longstanding paradox, that the laws of physics do not run fine forward and backward in everyday life, but they do at the atomic level, where subatomic particles collide.

That spontaneous order effect, the creation of order from disorder, is small and short-lived. But it may prove to be important in the emerging field of nanotechnology, where it could bungle future molecular-size machines by making them run backward, the researchers said.

''I think nature already does that,'' said Dr. Denis J. Evans, a professor of chemistry at the Australian National University. Living organisms may take advantage of the effect to kick around proteins and other molecules until they bind together properly, Dr. Evans said. So the effect could be useful.

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In the everyday world, the second law prohibits the flow of heat energy from a cooler reservoir to run a motor or do other work. Otherwise, one could imagine a machine running on the thermal energy in a glass of water -- the jiggling of individual water molecules -- and leaving behind a chunk of ice as the only waste.

But at a much smaller scale, the researchers have demonstrated, it is possible essentially to do just that, at least under certain circumstances.

In the experiment, reported in the current issue of Physical Review Letters, Dr. Evans and other scientists at the Australian National University in Canberra and Griffith University in Brisbane suspended a transparent bead one four-thousandths of an inch wide in a small puddle of water. A laser shining through the water held the bead in place. The bead's curved surface, acting like a lens, bent the laser light, exerting a force that kept it at the center of the beam.

''It just goes zip, and it quickly goes to the focal point, and it sits there,'' said Dr. Edith M. Sevick, a chemist at the Australian National University on the research team.

The team lowered the laser power so that it barely kept its hold onto the bead. ''We can actually seeing it jiggling around right around the focal point,'' Dr. Sevick said.

The team used the laser to drag the bead through the puddle at a leisurely pace of one-seventh of an inch an hour, as if it were a tugboat pulling a barge.

Usually, the water exerted a slowing force. But occasionally enough water molecules bounced off the bead at the same time, reflecting a more orderly arrangement of the molecules, to push the bead ahead, as if a barge suddenly jumped in front of the tugboat pulling it.

''That's the violation,'' Dr. Sevick said.

The violation lasted two seconds at most, and it occurred only because the force of the laser light was minuscule, almost as slight as the force of the water molecules bouncing off the bead. Over longer periods of time or if the laser power was turned up, the effect disappeared.

''You cannot get perpetual motion machines,'' Dr. Sevick said. ''You always get back to the second law.''

Until recently, scientists could not fully explain how the second law arises. In the basic equations of motion, both those devised by Isaac Newton and the later ones of quantum mechanics, time is said to be reversible. The equations remain true even when time flowed backward.

But the equations of thermodynamics, which describe the collective random motion of many trillions of particles, do contain a definite direction of time. Heat always flows from warm to cold, never the other way around. Entropy rises, never falls.

As early as 1876, a physicist, Josef Loschmidt, pointed out that paradox. If the motion of each individual particle is reversible, why is their collective behavior irreversible?

Dr. Evans finally figured out the answer in 1993. The irreversibility arises from causality, that events in the future cannot affect the present. From that, he showed that ordered systems became exponentially less likely while the probability of disorder rose.

Computer simulations verified that the ideas worked. Still, scientists like Dr. Peter T. Cummings of the Oak Ridge National Laboratory in Tennessee said it was surprising to find a clear example in the real world of the hazy zone between very small systems and very large ones.

''It's unexpected there could be an experimental verification of this theorem,'' Dr. Cummings said. The Australian experiment, he said, ''puts it squarely in the realm where it may have practical significance.''

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