Reversible & Irreversible

In December 15 issue of Nature, a team of US scientists reported a very interesting observation : that, under certain circumstances, liquids that are stirred together--like cream into a cup of black coffee--can be unstirred, with the particles of each fluid moving precisely back to their former positions, as if time had reversed.

The well-known 2nd law of thermodynamics essentially states that disorder increases. In the long term, this means that the universe will inevitably fall apart. In the short term, it means that phenomena like the stirring of coffee, billowing of smoke, flow of heat, the decay of body and buildings are all "irreversible". Remember though that the basic physics laws of the universe do not have the notion of 'irreversibility' included in them; They do not demand the 2nd law.

David J. Pine of New York University and Jerry P. Gollub of Haverford College in Pennsylvania showed with their device that there is a sharp transition between reversible and irreversible flows. Their device has two concentric cylinders with a gap of about a tenth of an inch wide in between them, which they filled up with a liquid (of the viscosity of honey) and hundreds of thousands of tiny beads (of the same density as the liquid). Because of having same density, the beads floated within the liquid without rising or falling. David and Jerry traced the motion of about 60 such beads which they dyed black.

Then they rotated the inner cylinder - dragging along the liquid and the beads - and then turned it back to its original position. When the amount of turning was small, the balls all returned to virtually the same positions from where they started. For a slightly greater degree of turning, then the balls started moving around ending up in a completely different formation.

Study of such complex phenomena is important not only for understanding limits and scope of a fundamental law of Physics but also for its application in manufacturing drugs, refining oil and getting an insight into the movement of the Earth's interior.