It had been thought that there might be some truth in the adage, "a watched pot never boils", in the world of quantum mechanics, where watching has some strange consequences. But now two Israeli scientists report that the watched quantum pot may actually boil faster.

The new research will surprise physicists because the effect is the reverse of what they have come to expect. The prevailing view has been that changes in the quantum world take place more slowly if they are monitored. This is known as the 'quantum Zeno effect', after the Greek philosopher Zeno of the fourth century BC, who specialized in devising puzzles and paradoxes.

One of the revelations of quantum theory, which was developed in the early twentieth century, is that at very small scales -- for objects the size of atoms, say -- the act of observation inevitably affects what is observed.

To 'see' an object we must bounce at least one 'quantum particle' of light off it, that is, one photon, the smallest possible 'portion' of light. But if the object is a single atom or molecule, this is like trying to find someone by firing a water cannon at them. For events at the quantum scale, making a measurement involves changing what is measured.

One of the outcomes of this entanglement between observer and observed is that the 'decay' of a quantum system from one state to another of lower energy -- which normally happens spontaneously -- can be slowed down by making frequent measurements of which state the system is in. If the measurements are continuous, the decay never happens at all. It is rather like stopping a ball from falling to the ground simply by looking at it.

Now Gershon Kurizki and Abraham Kofman of the Weizmann Institute of Science in Rehovot, Israel, argue that the converse can also be true, and more common. They call this the 'anti-Zeno effect'.

They have looked again at the theory behind the quantum Zeno effect. In Nature1 they explain that it applies only to a restricted class of quantum decay processes. For many common kinds of 'decay' -- for example, when a radioactive atomic nucleus emits a beta particle, or when an energetic molecule gives out 'fluorescent' light -- frequent measurements help the system make the transition from the initial to the decayed state. The change happens more quickly, in other words.

The researchers have not yet put their predictions to the test, although they say that this should be relatively easy to do. But they point out that their theoretical work ties in with another of Zeno's paradoxes -- perhaps the most famous one of all.

Zeno argued that it should be impossible for the swift-footed Achilles to overtake a lumbering tortoise that starts first in a race. To catch up with the tortoise, Achilles must first pass the point where the tortoise started out from. But the tortoise, however slowly it is moving, will have moved beyond this point. So how can Achilles ever arrive at a point before the tortoise gets there?

The paradox stems from the assumption that we can go on dividing time into ever smaller steps, through which both Achilles and the tortoise move with a constant speed. Kurizki and Kofman point out that one of the conceptual problems with the suppression of decay by constant measurement -- the quantum Zeno effect -- is that it assumes time is infinitely divisible, so that measurement can indeed be continuous.

But in the quantum world, the shorter the interval over which an event happens, the greater the uncertainty about how much energy it involves. An infinitely brief measurement risks blowing the observed system to pieces.